Download Diff. Biology Study Guide: Evolution Key Terms 1. Biological

Diff. Biology Stud y Guide: Evolu tion
Key Terms
1. Biological evolution- Evolution of life
2. Jean Baptist de Lamarck- -established early principles associated with evolution by comparing
current species to fossils
1. Use and Disuse- Useful organs are enhanced and non-useful organs are discarded. (Giraffe)
2. Inheritance of acquired characteristics- Useful traits that are enhanced are passed to offspring
-natural inheritance- Giraffe
-man-influenced inheritance- evolution of dog
3. inheritance of acquired characteristics4. James Hutton- his most famous work, "Theory of the Earth". This work was interpreted and
used by many as the basis for geological theory. He desired to trace back the origin of the various
minerals and rocks, and thus to arrive at some clear understanding of the history of the earth. For
many years he continued to study the subject. At last, in the spring of the year 1785, he
communicated his views to the recently established Royal Society of Edinburgh in a paper entitled
Theory of the Earth, or an Investigation of the Laws Observable in the Composition, Dissolution
and Restoration of Land upon the Globe. In this remarkable work the doctrine is expounded that
geology is not cosmogony, but must confine itself to the study of the materials of the earth; that
everywhere evidence may be seen that the present rocks of the earth's surface have been in great
part formed out of the waste of older rocks; that these materials having been laid down under the
sea were there consolidated under great pressure, and were subsequently disrupted and upheaved
by the expansive power of subterranean heat; that
during these convulsions veins and masses of molten
rock were injected into the rents of the dislocated
strata; that every portion of the upraised land, as
soon as exposed to the atmosphere, is subject to
decay; and that this decay must tend to advance until
the whole of the land has been worn away and laid
down on the sea-floor, whence future upheavals will
once more raise the consolidated sediments into new
land. This is the premise to plate tectonics &
continental drift.
The problem with the traditional rock cycle is that it
implies that rocks just cycle endlessly from one to
the other. The drawing above expresses this cyclical
nature (rock cycle illustrations come in endless
variety; this is a very simple stripped down version.) It also expresses the view of the 19th century
Uniformitarian school of thinking, captured best by James Hutton when he said the earth has "no
vestige of a beginning, and no prospect of an end." He envisioned earth processes going round and
round but never getting anywhere, never evolving.
5. catastrophism- Past Cataclysmic Activity
Catastrophism is the idea that many of Earth’s crustal features (strata layers, erosion, polystrate
fossils, etc) formed as a result of past cataclysmic activity. In other words, the Earth’s surface has
been scarred by catastrophic natural disasters.
Empirical Evidence
Catastrophism is supported by actual, recorded history. Nearly 300 ancient flood legends have
survived the ravishment of time. Legends of a worldwide deluge, commonly known as the
"Noachian Flood," are found in Europe, Asia, Africa, Australia, North American and South
America. Furthermore, earth's sedimentary layers with the fossil record seem to suggest a past
marine cataclysm. Sedimentary rock (sandstone, siltstone, shale, limestone, etc) is the result of
moving water, laid down layer upon layer by hydrologic sorting. Animals whose fossil remains
are found within those layers must have been caught in this running water to have been buried
and preserved. The remains, as well as the rocks, would be sorted according to density.
Otherwise, the carcasses would rot or be scavenged. Approximately 95% of all earth's fossil
remains discovered thus far are marine invertebrates. Of the remainder, approximately 4.74% are
plant fossils, 0.25% are land invertebrates (including insects), and 0.0125% are vertebrates (the
majority of which are fish). Roughly 95% of all land vertebrates discovered and recorded to date
consist of less than one bone.
6.uniformitarianism - "The Present is the Key to the Past"
Uniformitarianism is a geological doctrine. It states that current geologic processes, occurring at
the same rates observed today, in the same manner, account for all of Earth's geological features.
Thus, it assumes that geological processes are essentially unchanged today from those of the
unobservable past, and that there have been no cataclysmic events in earth's history. As present
processes are thought to explain all past events, the Uniformitarian slogan is, "the present is the
key to the past."
James Hutton and Sir Charles Lyell
The doctrine of Uniformitarianism was significantly advanced by James Hutton (1726-1797) in
his publication, Theory of the Earth (1785). Hutton influenced Sir Charles Lyell (1797-1875),
who is acclaimed as the father of modern geology with his work, Principles of Geology (18301833, a three volume work). Lyell, in turn, influenced Charles Darwin, who later wrote The
Origin of Species (1859). Lyell is responsible for the general acceptance of Uniformitarianism
among geologists for the past 150 years
7.Charles Lyell - Sir Charles Lyell (1797-1875) was a British geologist. In his Principles of
Geology (3 volumes, 1830-33), Lyell conclusively showed that the earth was very old and had
changed its form slowly, mainly from conditions such as erosion. Lyell was able to date the ages
of rocks by using fossils embedded in the stone as time indicators. Charles Darwin made use of
Lyell's data on fossils for his theory of evolution. Lyell himself had believed that the various
species of plants and animals had remained unchanged since they were created. When confronted
with Darwin's findings, he admitted "I now realize I have been looking down the wrong road." He
became one of Darwin's strongest supporters. Lyell was born in Scotland. He studied geology at
Oxford University and traveled on several geological expeditions in Europe and North America.
8.Alfred Russel Wallace. (January 8, 1823 — November 7, 1913) was a British naturalist,
geographer, anthropologist and biologist. Wallace's independent proposal of a theory of evolution
by natural selection prompted Charles Darwin to reveal his own more developed and researched,
but unpublished, theory sooner than he had intended.
In 1855, Wallace published a paper, "On the Law Which has Regulated the Introduction of
Species" (1855) (http://www.wku.edu/%7Esmithch/wallace/S020.htm), in which he gathers and
enumerates general observations regarding the geographic and geologic distribution of species,
and concludes that "Every species has come into existence coincident both in space and time with
a closely allied species." The paper was a foreshadowing of the momentous paper he would write
three years hence.
Wallace had once briefly met Darwin, and was one of Darwin's numerous correspondents from
around the world, whose observations Darwin used to support his theories. Wallace knew that he
was interested in the question of how species originate, and trusted his opinion on the matter.
Thus, he sent him his essay, "On the Tendency of Varieties to Depart Indefinitely From the
Original Type" (1858) (http://www.wku.edu/%7Esmithch/wallace/S043.htm), and asked him to
review it. In it, Wallace describes a novel theory of what is now known as "natural selection," and
proposes that it explains the diversity of life. It was essentially the same as the theory that Darwin
had worked on for twenty years, but had yet to publish. Darwin wrote in a letter to Charles Lyell:
"he could not have made a better short abstract! Even his terms now stand as heads of my
chapters!" Although Wallace had not requested that his essay be published, Charles Lyell and
Joseph Hooker decided to present the essay, together with excerpts from a paper that Darwin had
written in 1844, and kept confidential, to the Linnean Society of London on 1 July 1858,
highlighting Darwin's priority.
9.Charles Darwin - (12 February 1809 – 19 April 1882) was an English naturalist whose
revolutionary theory laid the foundation for both the modern theory of evolution and the principle
of common descent by proposing natural selection as a mechanism. He published this proposal in
1859 in the book The Origin of Species, which remains his most famous work. A worldwide sea
voyage aboard HMS Beagle and observations on the Galapagos Islands in particular provided
inspiration and much of the data on which he based his theory.
10. Natural Selection - is the primary mechanism within the scientific theory of evolution, i.e. it
alters the frequency of alleles within a population. After a century of obscure and vague
preliminary formulations, it was proposed as the main mechanism of evolution by Charles
Darwin and Alfred Russel Wallace in 1858. Natural selection can be subdivided into two types;
ecological selection, and sexual selection. Natural selection is distinguished from artificial
selection by humans. Other mechanisms of evolution include genetic drift and gene flow.
Mutations create the genetic variation on which natural selection acts.
It is important to note that the term "natural selection" is often used in the inaccurate yet fairly
harmless metaphorical sense as having causal status. To be precise, natural selection is not truly a
"mechanism" in itself, as opposed to something like gravity. Instead, natural selection is the
result of genetic and environmental forces acting upon an organism.
11. survival of the fittest - is a phrase which is a shorthand for a concept relating to competition
for survival or predominance. It codifies the observance of zoologists and animal breeders that
individuals having the highest level of fitness for a particular environment tend to survive longer
in it. Moveover, the longer they survive, the more territory they dominate and the more offspring
they have -- tending to crowd out competitors. The phrase is essentially a metaphor and is often
felt to be unhelpful: it is very close to a tautology, since if "fitness" is measured in the obvious
objective terms - survival - the phrase becomes "survival of the survivors".
The phrase "survival of the fittest" was first used by Herbert Spencer in his 1851 work Social
Statics. It was used by Charles Darwin in the 6th edition of The Origin of Species, in a secondary
header of Chapter 4 about natural selection [1] (http://www.bartleby.com/11/4001.html) and at
several places in the text, mostly using the phrase "Natural Selection, or the Survival of the
Fittest". He gave full credit to Spencer, writing "I have called this principle, by which each slight
variation, if useful, is preserved, by the term natural selection, in order to mark its relation to
man's power of selection. But the expression often used by Mr. Herbert Spencer, of the Survival
of the Fittest, is more accurate, and is sometimes equally convenient.". At this time the word
"fittest" would have primarily meant "most suitable" or "most appropriate" rather than "in the
best physical shape". In the first five editions of The Origin of Species, Darwin used the phrase
natural selection [2]
12. Thomas Malthus - (February, 1766 – December 23, 1834), who preferred to be known as
"Robert Malthus", was an English demographer and political economist best known for his
pessimistic but highly influential views.
Malthus's views were largely developed in reaction to the optimistic views of his father and his
associates, notably Rousseau and William Godwin. In An Essay on the Principle of Population,
published in 1798, Malthus predicted population would outrun food supply, leading to a decrease
in food per person. This prediction was based on the idea that population if unchecked increases
at a geometric rate whereas the food supply grows at an arithmetic rate. (See Malthusian
catastrophe for more information.) Only misery, moral restraint and vice (which for Malthus
included contraception) could check excessive population growth. Malthus favoured "moral
restraint" (including late marriage and sexual abstinence) as a check on population growth.
13. variation - The gene pool of a species or a population is the complete set of unique alleles that
would be found by inspecting the genetic material of every living member of that species or
population. A large gene pool indicates a large genetic diversity, which is associated with robust
populations that can survive bouts of intense selection. Meanwhile, low genetic diversity (see
inbreeding and population bottlenecks) can cause reduced fitness and an increased chance of
extinction.
The phenotypes associated with multiple alleles creates a range of expressions. This helps ensure
certain phenotypic/genotypic progeny are able to adapt to changing environmental stresses and thus
survive.
14. excessive reproduction- all populations have the potential to overpopulate their
environment.
15. evidence for evolution- tools used by biologists to provide evidence for the theory of
evolution.
16. Geologic- utilizing sediment formations and remnants within these layers to address
chronological evolution.
17. Radioactive decay- most elements have unstable, radioactive isotopes which decay at a
specific rate (half-life). The ratios of radioactive elements to their stable decomposed elements
provides a relative time for the existence of an earlier organism.
18. Fossils- Fossils are the mineralized remains of animals or plants or other artifacts such as
footprints. The totality of fossils and their placement in rock formations and sedimentary layers
(strata) is known as the fossil record. The study of fossils is called paleontology.
19. Paleontology- study of life of the past using fossil remnants
20. Biogeography- the study of land forms of the earth and the organisms that inhabit these
locations.
21. Comparative anatomy- comparing anatomical structures of living or nonliving organisms.
22. homologous- Two or more structures are said to be homologous if they are alike because of
shared ancestry. This could be evolutionary ancestry, meaning that the structures evolved from
some structure in a common ancestor (the wings of bats and the arms of humans are homologous
in this sense) or developmental ancestry, meaning that the structures arose from the same tissue in
embryonal development (the ovaries of females and the testicles of males are homologous in this
sense).
23. analogous- Two structures in biology are said to be analogous if they perform a similar
function by a simlar mechanism, but did not arise from a common ancestor performing that
function: for example, the wings of insects and the wings of birds. These similar structures most
likely evolved through different pathways, a process known as convergent evolution.
24. vestigial- A vestigial organ is an organ whose original function has been lost during evolution.
In 1893, Robert Wiedersheim published a list of 86 human organs that had no known function.
Theorizing that they were vestiges of evolution, he called them "vestigial".
Today, the list of human vestigial organs is much smaller, and hotly debated. It still includes the
appendix, and coccyx. Many people maintain that the coccyx is a remnant of a lost tail. Wisdom
teeth are vestigial as well. Also, the plica semilunaris, the small fold of tissue on the inside corner
of the eye, is the vestigial remnant of the nictitating membrane (the third eyelid) in other animals.
The formation of goose bumps in humans under emotional stress is a vestigial reflex; its purpose
in our evolutionary ancestors was to raise hair to make the animal appear bigger and scare off
enemies.
25. Domestication- Domesticated animals, plants, and other organisms are those whose collective
behavior, life cycle, or physiology has been altered as a result of their breeding and living
conditions being under human control for multiple generations. Humans have brought these
populations under their care for a wide range of reasons: for help with various types of work, to
produce food or valuable commodities (such as wool, cotton, or silk), and to enjoy as pets or
ornamental plants.
26. Embryology- Embryology is the subdivision of developmental biology that studies embryos
and their development. The formation and development of an embryo is known as
embryogenesis.
27. Biochemistry- study of similiarities & differences in biomolecules.
28. Molecular Genetics29. Modern Synthesis- The modern evolutionary synthesis (often referred to simply as the
modern synthesis), neo-Darwinian synthesis or neo-Darwinism, brings together Charles
Darwin's theory of the evolution of species by natural selection with Gregor Mendel's theory of
genetics as the basis for biological inheritance. Major figures in the development of the modern
synthesis include Ronald Fisher, Theodosius Dobzhansky, J.B.S. Haldane, Sewall Wright, Julian
Huxley, Ernst Mayr, George Gaylord Simpson and G. Ledyard Stebbins. Essentially, the modern
synthesis (or neo-Darwinism) introduced the connection between two important discoveries; the
units of evolution (genes) with the mechanism of evolution (selection).
George John Romanes coined the term neo-Darwinism to refer to the theory of evolution
preferred by Alfred Russel Wallace et al. Wallace rejected the Lamarckian idea of inheritance of
acquired characteristics, something that Darwin, Huxley et al wouldn't rule out. The mechanism
of inheritance wasn't discovered in Darwin or Wallace's time, however, so the debate was never
settled. Mendelian genetics was rediscovered in 1900. However, there were differences of
opinion as to what was the variation that natural selection acted upon. The biometric school, led
by Karl Pearson followed Darwin's idea that small differences were important for evolution. The
Mendelian school, led by William Bateson, however thought that Mendel's work gave an
evolutionary mechanism with large differences. This issue was finally resolved by Ronald Fisher,
who in 1918 produced a paper entititled "The Correlation Between Relatives on the Supposition
of Mendelian Inheritance", which showed using a model how continuous variation could be the
result of the action of many discrete loci. This is generally regarded as the starting point of the
synthesis.
According to the modern synthesis as established in the 1930s and 1940s, genetic variation in
populations arises by chance through mutation (this is now known to be due to mistakes in DNA
replication) and recombination (crossing over of homologous chromosomes during meiosis).
Evolution consists primarily of changes in the frequencies of alleles between one generation and
another as a result of genetic drift, gene flow and natural selection. Speciation occurs gradually
when populations are reproductively isolated by geographic barriers.
The modern evolutionary synthesis continued to be developed and refined after the initial
establishment in the 1930s and 1940s. The most notable paradigm shift was the so-called
Williams revolution, after George C. Williams presented a gene-centric view of evolution in the
1960s. The synthesis as it exists now has extended the scope of the Darwinian idea of natural
selection, specifically to include subsequent scientific discoveries and concepts unknown to
Darwin such as DNA and genetics that allow rigorous, in many cases mathematical, analyses of
phenomena such as kin selection, altruism, and speciation. A particular interpretation of neoDarwinism most commonly associated with Richard Dawkins asserts that the gene is the only
true unit of selection. Dawkins further extended the Darwinian idea to include non-biological
systems exhibiting the same type of selective behavior of the 'fittest' such as memes in culture.
30. species- In biology, a species is, loosely speaking, a group of related organisms that share a
more or less distinctive form and are capable of interbreeding. As defined by Ernst Mayr, species
are "groups of actually or potentially interbreeding natural populations which are reproductively
isolated from other such groups" (however, see definitions of species below).
31. gene pool- The gene pool of a species or a population is the complete set of unique alleles that
would be found by inspecting the genetic material of every living member of that species or
population. A large gene pool indicates a large genetic diversity, which is associated with robust
populations that can survive bouts of intense selection. Meanwhile, low genetic diversity (see
inbreeding and population bottlenecks) can cause reduced fitness and an increased chance of
extinction. When many alleles exist for a given gene or locus, a population is said to be
polymorphic with respect to that gene or locus. When no variation exists, it is labelled
monomorphic.
32. allelic frequency- a term of population genetics that is used in characterizing the genetic
diversity of a species population, or equivalently the richness of its gene pool. Allele frequency is
defined as follows. Given a) a particular chromosome locus, b) a gene occupying that locus, c) a
population of individuals carrying n loci in each of their somatic cells (e.g. two loci in the cells of
diploid species, which contain two sets of chromosomes) and finally d) a variant or allele of the
gene, then the allele frequency of that allele is the fraction or percentage of loci that the allele
occupies within the population.
Note that for diploid genes, however, the proportion of individuals that carry this allele may be up
to two in five. If the allele distributes randomly, then the binomial theorem will apply: 32% of the
population will be heterozygous for the allele (i.e. carry one copy of that allele and one copy of
another in each somatic cell) and 4% will be homozygous (carrying two copies of the allele). So
all together 36% of diploid individuals would be expected to carry an allele that has a frequency
of 20%. However, alleles distribute randomly only in the absence of selection and under other
assumptions. When these conditions apply, a population is said to be in Hardy-Weinberg
equilibrium.
The frequencies of all the alleles of a given gene often are graphed together as an allele frequency
distribution histogram. Population genetics studies the different "forces" that might lead to
changes in the distribution and frequencies of alleles -- in other words, to evolution. Besides
selection, these forces include genetic drift, mutation and migration.
33. genetic equilibrium- the frequency of alleles, genotypes & phenotypes are relatively stable
within a population exposed to consistent environmental factors
34. genotype frequencies- the frequency of a certain genotypes (homozygous dominant,
homozygous recessive, or heterozygous) within a population.
35. Hardy-Weinberg Law- The Hardy —
Weinberg principle (HWP) (also Hardy —
Weinberg equilibrium (HWE), or Hardy —
Weinberg law) states that, under certain
conditions, after one generation of random
mating, the genotype frequencies at a single
gene locus will become fixed at a particular
equilibrium value. It also specifies that those
equilibrium frequencies can be represented as a
simple function of the allele frequencies at that
locus.
36. genetic drift-Genetic drift is a mechanism of evolution that acts in concert with natural
selection to change the characteristics of species over time. It is a stochastic effect that arises
from the role of random sampling in the production of offspring. Like selection, it acts on
populations, altering the frequency of alleles and the predominance of traits amongst members of
a population, and changing the diversity of the group. Drift is observed most strongly in small
populations and results in changes that need not be adaptive.
37. neutral selection- the idea that many of the changes seen at the molecular level are selectively.
Prior to the 1960's little was known about the details of the molecular basis of heredity or about
the extent of natural variation in proteins and DNA. With the development of techniques to
visualize isozyme variation and to determine the primary sequence of proteins, population
biologists began to examine the amount of variation in proteins both within and between
populations. The amount of variation and the rate at which it was accumulating was surprisingly
high. Based upon population genetic theory, which had been developed during the first half of
the 20th century, the amount of variation was difficult to account for if all of the variation was
under selection and was being maintained by selection.
38. mutation pressure
39. gene flow- (also known as gene migration) is the transfer of genes from one population to
another. Migration into or out of a population may be responsible for a marked change in gene
pool frequencies (the number of individual members with a particular trait). Immigration may
result in the addition of new genetic material to the established gene pool of a particular species
or population, and conversely emigration results in the removal of genetic material. There are a
number of factors that affect the rate of gene flow between different populations. One of the most
significant factors is mobility, and animals tend to be more mobile than plants. Greater mobility
of an individual tends to give it greater migratory potential.
40. bottleneck effect- A population bottleneck (or genetic bottleneck) is an evolutionary event
in which a significant percentage of a population or species is killed or otherwise prevented from
reproducing, and the population is reduced by 50% or more, often by several orders of
magnitude. A graph of this change resembles the neck of a bottle, from wide to narrow; hence the
name. Population bottlenecks increase genetic drift, as the rate of drift is inversely proportional to
the population size. It also changes the relationship of natural selection (see: inbreeding).
41. founder effect- The founder effect is an evolutionary phenomenon. Founder effects arise when
a new and isolated environment is invaded by only a few members of a species, which then
multiply rapidly. In the extreme case, a single fertilised female might arrive in a new
environment. It is a type of population bottleneck. The result of the small number of founders is
that there is a sharp loss of genetic variation compared with the parent population. As a result, the
new population may be distinctively different, genetically and phenotypically, from the parent
population it derived from. In addition, there is a raised probability of inbreeding, resulting in an
unusual number of defects due to recessive genes.
Founder effects are common in island ecology, but the isolation need not be geographical. For
example, the Amish populations in the United States, which have grown from a very few
founders but have not recruited newcomers, and tend to marry within the community, exhibit
founder effects: phenomena such as polydactyly (extra fingers and toes, a symptom of Ellis-van
Creveld syndrome), though still rare absolutely, are more common in Amish communities than in
the US population at large.
In extreme cases, founder effects may lead to the speciation and subsequent evolution of new
species.
42. selection pressure- an environmental factor which influences genotypes/phenotypes in a
population.
43. polymorphism- describes multiple possible states for a single property (it is said to be
polymorphic). In biology multiple alleles of a gene within a population, usually expressing
different phenotypes, are called polymorphism. For instance, human skin color is polymorphic.
See polymorphism (biology). Polymorphism in biology can also refer to single nucleotide
polymorphisms
44. heterozygote advantage- A heterozygote advantage (heterozygous advantage or
overdominance) describes the case in which the heterozygote genotype has a higher relative
fitness than either the homozygote dominant or homozygote recessive genotype. This selection
favoring the heterozygote is one of the mechanisms that maintains polymorphism and helps to
explain some kinds of genetic variability. There are several cases in which the heterozygote is
conveyed certain advantages and some disadvantages while both versions of homozygotes are
only at disadvantages. A well established case of heterozygote advantage is that of the gene
involved in sickle cell anaemia.
Often times the advantages and disadvantages conveyed are rather complicated, due to the fact
that more than one gene exists on any given allele. Major genes almost always have multiple
effects, which can simultaneously convey separate advantageous traits and disadvantageous traits
upon the same organism. In this instance, the state of the organism’s environment will provide
selection, with a net effect either favoring or working in opposition to the gene, until an
environmentally-determined equilibrium is reached.
This sort of selection can be seen in all kinds of populations human and non-human. In the fly
Drosophila melanogaster, there is an autosomal, completely recessive gene that expresses ebony
body-color. When there is a fly with two copies of the recessive allele, this homozygote expresses
the dark ebony color, but is also terribly weakly, and is placed at a harsh reproductive
disadvantage. If this were the only effect of the gene, and only conveyed disadvantages, we
would expect selection to ‘weed out’ this gene until it became extinct from the population.
However, the same gene also conveys some advantages, by providing improved viability. This
advantage is dominant, and conveyed in the heterozygote. So in this case, the homozygote ‘ebony
color’ gene fly will be at a distinct advantage due to weakness; the heterozygote will express
none of the disadvantages of weakness, but will gain from his one allele that improves his
viability; and the homozygote wild-type will be perfectly healthy, but does not possess the
improved viability of the heterozygote, and will be at a disadvantage in survivorship and
reproduction.
Sickle cell anemia (SCA) is a genetic disorder that is caused by the presence of two incompletely
recessive alleles. When a sufferer’s red blood cells are exposed to low oxygen conditions, the
cells lose their healthy round shape and become sickle shaped. This deformation of the cells can
cause them to become lodged in capillaries, depriving other parts of the body from precious
oxygen. When untreated, a person with SCA may suffer from painful periodic bouts, often
causing damage to internal organs, strokes, or anemia. Typically the disease results in premature
death. However, there is convincing evidence indicating that in areas with persistent malaria
outbreaks, individuals with the heterozygous state had a distinct advantage historically. Those
with the benign sickle trait possess a resistance to malarial infection. The pathogen that causes the
disease spends part of its cycle in the red blood cells, and those with sickle cells effectively stop
the pathogen in its tracks, until the immune system destroys the foreign bodies. These individuals
have a great immunity to infection and have a greater chance of surviving outbreaks and going on
to reproduce. However, those with two alleles for SCA may survive malaria but will typically die
from their genetic disease unless they have access to advanced medical care. The homozygous
‘normal’ or wild-type case will have a greater chance of passing on their genes successfully, in
that there is no chance of their offspring suffering from SCA; yet, they are more susceptible to
dying from malarial infection before they have a chance to pass on their genes.
Cystic fibrosis, or CF, is an autosomal recessive hereditary disease of the lungs, sweat glands and
digestive system. The disorder is caused by the malfunction of the CFTR protein, which controls
inter-membrane transport of chloride ions, which is vital to maintaining equilibrium of water in
the body. The malfunctioning protein causes viscous mucus to form in the lungs and intestinal
tract. Historically, a child born with CF would only have a life expectancy of a few years, but
modern medicine has made it possible for these people to life into adulthood. However, even in
these individuals, male and female, CF typically causes sterility. It is the most common genetic
disease among people of European descent. The possession of the CF disease can increase
survivorship of people affected by diseases involving loss of body fluid, typically due to diarrhea.
The most common of these maladies is cholera, which killed many Europeans historically. Those
with cholera would often die of dehydration when they ingested water slower than their body
passed it. Those with CF, or either carriers of the allele where much less likely to suffer this fate.
Due to this increased resistance to disease, the CF allele in low frequencies was beneficial to the
gene pool. Even today, approximately one in twenty-five of European descent is a carrier for the
disease, and one in every 2500 children born is affected by cystic fibrosis.
45. directional selection- In population genetics, directional
selection occurs when natural selection favors a single allele
and therefore allele frequency continuously shift in one
direction. It is in contradistinction to balancing selection
where selection may favor multiple alleles. Directional
selection is a particular mode or mechanism of natural
selection.
A common example is the peppered moth (Biston betularia).
Before the industrial revolution in England (1740?), the
peppered moth was mostly found in a light gray form with
little black speckled spots. The allele for dark-bodied moths
is dominant, while the allele for light-bodied moths is
recessive. The light-bodied moths were able to blend in with
the light colored lichens and tree bark. The less common
black peppered moth was more likely to be eaten by birds.
Therefore, the frequency of the dark allele was about 0.01%.
During the industrial revolution in England, many of the
light-bodied lichens died from sulphur dioxide emissons.
The trees became covered with soot from the new coalburning factories. This led to an increase in bird predation
for the light-colored moths (they no longer blended in as
well). The dark-bodied moths, however, blended in very well with the trees. As a result, during
reproduction, a lot of light-bodied moths were produced, and a few dark-bodied moths. Most of
the light-bodied moths didn't survive, while the black-bodied moths continued to survive.
Gradually, the allele frequency shifted towards the dominant allele, as more and more darkbodied moths survived to reproduce.
46. disruptive selection- Disruptive selection is a type of
natural selection that simultaneously favors individuals at both
extremes of the distribution. When disruptive selection
operates, individuals at the extremes contribute more offspring
than those in the center, producing two peaks in the
distribution of a particular trait.
A hypothetical example of this is where you have a population
of rabbits. The dominant trait being black fur represented by
“B” the recessive trait being white fur represented by “b”. A
rabbit with the genotype of “BB” would have a phenotype of
black fur, a genotype of “Bb” would have gray fur, and a geno
type of “bb” would have a phenotype of white fur. Put this
population of rabbits into an area that had very dark black
rocks as the enviroment and also near by was an area of very
white colored stone. The rabbits with the black fur would be
able to hide from preditors amongst the black rocks and the
white furred rabbits would be able to hide in the white rocks
but the gray furred rabbits would stand out in both of the
habitats and thus not survive.
47. stabilizing selection- Stabilizing selection favors the norm,
the common, average traits in a population. Look at the Siberian
Husky, a dog bred for working in the snow. The Siberian Husky
is a medium dog, males weighing 16-27kg (35-60lbs). These
dogs have strong pectoral and leg muscles, allowing it to move
through dense snow. The Siberian Husky is well designed for
working in the snow. If the Siberian Husky had heavier muscles,
it would sink deeper into the snow, so they would move slower
or would sink and get stuck in the snow. Yet if the Siberian
Husky had lighter muscles, it would not be strong enough to pull
sleds and equipment, so the dog would have little value as a
working dog. So stabilizing selection has chosen a norm for the
the size of the Siberian Husky.
48. balanced polymorphism - A balanced polymorphism is a
situation in which natural selection within a population is able
to maintain stable frequencies of two or more phenotypic
forms. There are two major mechanisms by which natural
selection preserves this variation and consequently produces a
balanced polymorphism. The first mechanism is known as
heterozygote advantage; in which an individual who is
heterozygous at a particular gene locus has a greater reproductive success than a homozygous
individual. This can be seen in human populations with the locus for a certain protein present in
hemoglobin (an important component in blood). Individuals who are homozygous for the
recessive allele at this locus are inflicted with sickle-cell disease,a disease in which blood cells
are grossly misshapen and which often results in a reduced lifespan. An individual heterozygous
at this locus will not suffer from sickle-cell disease but because of slightly irregularly shaped
blood cells they are resistant to malaria. This resistance is favored by natural selection in tropical
regions where malaria is present and so the heterozygote has an evolutionary edge. It is in this
way that natural selection preserves stable frequencies of both the heterozygote and the
homozygote dominant phenotypes. The second important mechanism by which natural selection
can preserve two or more phenotypic forms is known as frequency-dependent selection.
Frequency-dependent selection is a form of selection in which the relative "fitness" of a specific
phenotype declines if the frequency of that phenotype becomes too high. An example of this type
of selection is between parasites and their hosts. An example follows: suppose that a certain
parasite can recognize one of two receptors in its host, receptor Alpha or receptor Beta, if many
parasites with receptor Alpha exist then hosts with receptor Beta will be selected for, and this will
subsequently increase the selective pressure on parasites which use receptor Beta and this
relationship will continue rocking back and forth.
49. adaptation- Adaptation in biology, an anatomical structure, physiological process or
behavioral trait that has evolved over a period of time by the process of natural selection that
increases the likelihood of producing larger numbers of offspring or its reproductive success.
50. fitness- Fitness in biology refers to individual's ability to propagate its genes.
51. sexual selection- Sexual selection is the theory that competition for mates between individuals
of the same sex drives the evolution of certain traits. It is distinct from ecological selection which
is the competition for food within the species' ecological niche. Many traits, e.g. smooth skin or
fur, strong muscles, fluid motions, appear not only to enable hunting or gathering but also to be
important sexual attractors, especially in the more intelligent species. For these, ecological and
sexual selection both operate on a trait.
Traits amenable to sexual selection, which give an individual an advantage over its rivals in
courtship without being directly involved in reproduction, are called secondary sex
characteristics. Sex differences directly related to reproduction and serving no direct purpose in
courtship are called primary sex characteristics. These traits are caused by secondary sexual
characteristics. Secondary sexual characteristics distinguish the sexes of a species, yet have no
direct role in reproduction. As mentioned above, these dimorphic traits sometimes become
overdeveloped relative to other members of a species. Sometimes these exaggerated traits prove
to be a hindrance to the animal, thereby lowering their fitness. These animals are still selected
sexually and are therefore able to reproduce. Through this process, the exaggerated trait is passed
on to future generations. Large antlers such as that of a moose are bulky and heavy and slow the
creature down when running from a predator as well as run the risk of becoming entangled in low
hanging tree branches and shrubs. No doubt this trait has lead to the demise of a creature in more
than one instance. Bright colorations, such as those seen in many male birds, capture both the eye
of a female and that of a predator; when a male peacock spreads it tail, it is beautiful, but very
obvious. These traits also represent a costly energetic investment for the animal that bears them.
52. species- In biology, a species is, loosely speaking, a group of related organisms that share a
more or less distinctive form and are capable of interbreeding. As defined by Ernst Mayr, species
are "groups of actually or potentially interbreeding natural populations which are reproductively
isolated from other such groups" (however, see definitions of species below).
The definition of a species given above as taken from Mayr, is somewhat idealistic. Since it
assumes sexual reproduction, it leaves the term undefined for a large class of organisms that
reproduce asexually. Biologists frequently do not know whether two morphologically similar
groups of organisms are "potentially" capable of interbreeding. Further, there is considerable
variation in the degree to which hybridization may succeed under natural and experimental
conditions, or even in the degree to which some organisms use sexual reproduction between
individuals to breed. Consequently, several lines of thought in the definition of species exist:
•A morphological species is a group of organisms that have a distinctive form: for example, we
can distinguish between a chicken and a duck because they have different shaped bills and the
duck has webbed feet. Species have been defined in this way since well before the beginning of
recorded history. Although much criticised, the concept of morphological species remains the
single most widely used species concept in everyday life, and still retains an important place
within the biological sciences, particularly in the case of plants.
•The biological species or isolation species concept identifies a species as a set of actually or
potentially interbreeding organisms. This is generally the most useful formulation for scientists
working with living examples of the higher taxa like mammals, fish, and birds, but meaningless
for organisms that do not reproduce sexually. It distinguishes between the theoretical possibility
of interbreeding and the actual likelihood of gene flow between populations. For example, it is
possible to cross a horse with a donkey and produce offspring, however they remain separate
species—in this case for two different reasons: first because horses and donkeys do not normally
interbreed in the wild, and second because the fruit of the union is rarely fertile. The key to
defining a biological species is that there is no significant cross-flow of genetic material between
the two populations.
•A mate-recognition species is defined as a group of organisms that are known to recognise one
another as potential mates. Like the isolation species concept above, it applies only to organisms
that reproduce sexually.
•A phylogenetic or evolutionary or Darwinian species is a group of organisms that shares an
ancestor; a lineage that maintains its integrity with respect to other lineages through both time
and space. At some point in the progress of such a group, members may diverge from one
another: when such a divergence becomes sufficiently clear, the two populations are regarded as
separate species.
53. subspecies- In taxonomy, a subspecies is the taxon immediately subordinate to a species.
Members of one subspecies differ morphologically but sometimes only genetically (disputed —
see talk page) from members of other subspecies of the species.
Subspecies are defined in relation to species. It is not possible to understand the concept of a
subspecies without first grasping what a species is. In the context of large living organisms like
trees, flowers, birds, fish and humans, a species can be defined as a distinct and recognisable
group that satisfies two conditions:
1. Members of the group are reliably distinguishable from members of other groups. The
distinction can be made in any of a wide number of ways, such as: differently shaped leaves, a
different number of primary wing feathers, a particular ritual breeding behaviour, relative size of
certain bones, different DNA sequences, and so on. There is no set minimum 'amount of
difference': the only criterion is that the difference be reliably discernable. In practice, however,
very small differences tend to be ignored.
2. The flow of genetic material between the group and other groups is small and can be expected
to remain so because even if the two groups were to be placed together they would not interbreed
to any great extent.
Note the key qualifier above: to be regarded as different groups rather than as a single varied
group, the difference must be distinct, not simply a matter of continuously varying degree. If, for
example, the population in question is a type of frog and the distinction between two groups is
that individuals living upstream are generally white, while those found in the lowlands are black,
then they are classified as different groups if the frogs in the intermediate area tend to be either
black or white, but a single, varied group if the intermediate population becomes gradually darker
as one moves downstream.This is not an arbitrary condition. A gradual change, called a cline, is
clear evidence of substantial gene flow between two populations. A sharp boundary between
black and white, or a relatively small and stable hybrid zone, on the other hand, shows that the
two populations do not interbreed to any great extent and are indeed separate species. Their
classification as separate species or as subspecies, however, depends on why they do not
interbreed.
54. anagenesis- Anagenesis is the progressive evolution of species involving a change in gene
frequency in an entire population rather than a
cladogenetic branching event. When enough
mutations reach fixation in a population to
significantly differentiate from an ancestral
population a new species name may be assigned.
A key point is that the entire population is
different from the ancestral population so that the
ancestral population can be considered extinct. It
is easy to see from the preceding definition how
controversy can arise among taxonomists
regarding when the differences are significant
enough to warrant a new species classification.
Anagenesis may also be referred to as phyletic
evolution
55. cladogenesis- Cladogenesis is an evolutionary
splitting event in which each branch and its
smaller branches is a "clade"; an evolutionary
mechanism and a process of adaptive evolution that leads to the development of a greater variety
of animals or plants.
56. cline- In population genetics, a cline is a gradual change of a character or feature (phenotype)
in a species over a geographical area. The change in phenotype does not result in different species
as long as the geographically spread populations can interbreed with one another. This meaning
of cline was introduced by Sir Julian Huxley
57. subspecies- see 53
58. races- A race is a distinct population of humans distinguished in some way from other humans.
The most widely observed races are those based on skin color, facial features, ancestry, and
genetics. Conceptions of race, as well as specific racial groupings, are often controversial due to
their political and sociological uses and implications.
Since the 1940s, evolutionary scientists have rejected the view of race according to which a
number of finite lists of essential (e.g., Platonic) characteristics could be used to determine a like
number of races. By the 1960s, data and models from population genetics called into question
taxonomic understandings of race, and many have turned from conceptualizing and analyzing
human variation in terms of race to doing so in terms of populations and clines instead. That
being said, many scientists still believe that race is a valid and useful concept. Moreover, since
the 1990s, data and models from genomics and cladistics have resulted in a revolution in our
understanding of human evolution, which has led some to propose a new "lineage" definition of
race. These scientists have made related arguments that races are valid when understood as fuzzy
sets, clusters, or extended families. Currently, opinions differ substantially within and among
academic disciplines.
59. speciation - refers to the appearance of a new species of life on earth, particularly as seen in the
fossil record. There are many ideas about the process leading to the creation of new species, each
typically based on any of the Darwinian theories of biological evolution.
Most of these ideas share the hypothesis that speciation occurs when a parent species (also
referred to as a common ancestor) splits into two (or more) reproductively-isolated populations,
each of which then accumulates changes from sexual reproduction and/or random mutation (in
addition to any other various contributors to genetic change) until the populations are no longer
capable of interbreeding (cladogenesis). (This is one definition of what a species is; see species.)
If a single population of a species changes enough over time to be designated a new species while
the old species dies out, we have a process called anagenesis. Among simpler forms of life, such
as bacteria, single mutations can cause drastic changes (called "saltation") that result in speciation
in a very short time. Speciation is also related to a process known as adaptive radiation.
Ernst Mayr proposed a speciation mechanism called allopatry. Allopatry begins when
subpopulations of a species become isolated geographically (for example, by habitat
fragmentation or migration). The isolated populations are then liable to diverge evolutionarily
over many generations as a) they become subjected to dissimilar selective pressures and b) they
independently undergo genetic drift; particularly when one of the subpopulations is small (a
scenario that leads to the "founder effect"). This kind of speciation is evident in many vertebrates'
taxa. See also the issue of Ring species.
Another proposed mechanism of speciation is sympatry, by which new species emerge alongside
the old. This might occur if, say, subpopulations become dependent on different plants in the
same area; and if variations in mating lead one subpopulation to become reproductively isolated
from the other. Many examples of this kind of speciation are found in the invertebrates,
especially the insects. Polyploidy is also a very common cause of sympatric speciation.
Polyploidy is seen as a mode of speciation in many plants, a well studied example being that of
the wheat species. In asexually reproducing organisms, there are other modes that become
prominent including horizontal gene transfer by viruses and mutations.
A further mechanism is parapatry, where the zones of two species abut but do not overlap. There
is only partial separation afforded by geography, so individuals of each species may come in
contact or cross the barrier from time to time.
60. microevolution- Microevolution is the occurrence of small-scale changes in gene frequencies
in a population over a few generations, also known as change at or below the species level. These
changes may be due to several processes: mutation, gene flow, genetic drift, as well as natural
selection. Population genetics is the branch of biology that provides the mathematical structure
for the study of the process of microevolution. Biologists distinguish between microevolution and
macroevolution, which is the occurrence of large-scale changes in gene frequencies in a
population over a long period of time (and may culminate in the evolution of new species).
61. macroevolution- Macroevolution is the concept that evolution of species and higher taxa is the
result of large-scale changes in gene-frequencies over time.
62. phylogeny- A phylogeny (or phylogenesis) is the origin and evolution of a set of organisms,
usually of a species. A major task of systematics is to determine the ancestral relationships among
known species (both living and extinct), and the most commonly used methods to infer
phylogenies include cladistics, phenetics, maximum likelihood, and Bayesian.
During the late 19th century, the theory of recapitulation, or Haeckel's biogenetic law, was widely
accepted. This theory was often expressed as "ontogeny recapitulates phylogeny", i.e. that the
development of an organism exactly mirrors the evolutionary development of the species. The
early version of this hypothesis has since been rejected as being oversimplified and misleading.
However, modern biology recognizes numerous connections between ontogeny and phylogeny,
explains them using evolutionary theory, and views them as supporting evidence for that theory.
See the article on ontogeny and phylogeny
63. prezygotic isolating mechanisms- A type of reproductive isolation that occurs before the
formation of a zygote can take place. In most cases mating does not even occur. Forms of
prezygotic isolation include spatial, behavioral, mechanical and temporal isolation. (See 64-69)
64. geographic isolation -Geographic isolation is the physical separation of members of a
population. Populations may be physically separated when their original habitat becomes divided,
as, for example, when new land or water barriers form. Also, when part of a population colonizes
anew, remote area such as an island, the colonizers geographically isolated from other
populations of the species. For example, when a group of American finches colonized the
Hawaiian islands, the group became geographically isolated from other populations of the
species. these finches eventually gave rise to the 23 species of Hawaiian honeycreepers.
Geographic isolation of a population may occur as a result of physical changes in an
environment. When a river changes course or even when a highway is built across a field,
populations may become geographically isolated. An example in which geographic isolation may
have led to speciation. The desert of Death Valley, California, has a number of isolated ponds a
formed by springs. Each pond contains a species of fish that lives only in that pond. Scientists
suggest that these species arose through geographic isolation. Geologic evidence from a study of
wave patterns in sedimentary rocks indicates that most of Death Valley was covered by a huge
lake during the last ice age. When the ice age ended, the region became dry. Only small, spring
fed ponds remained. Members of a fish species that previously formed a single population in the
lake may have become isolated in different ponds. The environments of the isolated ponds
differed enough that natural selection and perhaps genetic drift acted on the separate populations.
Eventually the fish in the different ponds may have diverged so much genetically that they could
no longer interbreed even if brought together. In this way geographic isolation of fishes in Death
Valley probably led to the formation of new species. Geographic isolation, plus reproductive
isolation, probably is the usual cause of the formation of new species.
65. ecological(habitat) isolation- For habitat isolation to occur, populations do not need to be
separated by great distance. Instead, they must occupy different habitats, even within the same
area. For example, if two populations of flies exist in the same geographical area, but one group
lives in the soil and another lives on the surface of the water, members of the two populations are
very unlikely to meet and reproduce.
66. temporal(seasonal) isolation- Temporal isolation represents another way in which populations
living in the same area can be prevented from mating. Different populations may be ready to
mate at different times of the year. For example, two populations of plants may produce flowers
in different seasons, making mating between the populations impossible.
67. behavioral isolation- In many animals, courtship displays and rituals are vital to reproduction.
Such behaviors can be very specific, varying between closely related species. Male behaviors
such as courtship calls, songs, and dances will only be recognized by females of the same species.
Some species of crickets are morphologically identical, but can be distinguished by the fact that
females will only respond to the mating songs of males of their own species. Males of other
species are ignored.
68. mechanical isolation- Mechanical isolation occurs when mating is physically impossible.
Often this occurs because the genitalia of different species are incompatible. Bush babies, a group
of small arboreal primates, are divided into several species based on mechanical isolation. Each
species has distinctly shaped genitalia that, like locks and keys, only fit with the genitalia of its
own species.
69. gametic isolation- In some cases, there are no barriers to mating, but the gametes are unable to
fuse and fertilization does not take place. Often this occurs because the female immune system
recognizes sperm as foreign and attacks it. In other cases, the sperm or pollen may not be adapted
to the environment of the female reproductive track.
70. postzygotic isolating mechanisms- In some cases, no barriers exist to mating between
members of different species. In these cases, the zygote formed is called a hybrid. However, even
after a hybrid zygote forms, reproduction may still not be successful. When that reproduction is
successful, living hybrids are usually themselves unable to reproduce. The production of hybrid
offspring is costly: the energy of mating and producing offspring has still been spent by the
parents, with no future inheritance of their genetic material in return.
71. developmental isolation- While gametes from different species can sometimes fuse to produce
a hybrid zygote, these zygotes are frequently abnormal. Most do not survive to birth or
germination. Those that do often do not develop normally and never reach sexual maturity. In
these cases, the mating of different species is said to be unsuccessful even though offspring were
produced because those offspring are incapable of passing on their genes.
72. hybrid inviability- Unlike mules, most hybrid offspring that survive the zygote stage to be
born are not healthy. Most die before reaching reproductive age. As with the production of
hybrids that cannot mature into reproductive adults, reproduction that results in unhealthy hybrids
that die before reaching sexual maturity is considered unsuccessful.
73. hybrid sterility- Among those hybrid offspring that do develop normally and reach sexual
maturity, most are sterile, meaning they do not produce viable gametes. For example, mules
result from the mating of a horse with a donkey. They are born and develop into normal healthy
adult animals, but they cannot produce offspring of their own.
74. allopatric speciation- A type of speciation which occurs when a population becomes
segregated into two populations by some sort of geographic barrier (also called geographic
speciation). This phenomenon is presumed to have been the mechanism whereby many species of
organisms evolved
75. sympatric speciation- one of three theoretical models for the phenomenon of speciation. In
complete contrast to allopatry, species undergoing sympatric speciation are not geographically
isolated by, for example, a mountain or a river. The speciating populations share the same
territory. This speciation phenomenon most commonly occurs through polyploidy, in which an
offspring or group of offspring will be produced with twice the normal number of chromosomes.
Where a normal individual has two copies of each chromosome (diploidy), these offspring may
have four copies (tetraploidy). A tetraploid individual cannot mate with a diploid individual,
creating reproductive isolation.
Sympatric speciation is rare. It occurs more often among plants than animals, since it is so much
easier for plants to self-fertilize than it is for animals. A tetraploidy plant can fertilize itself and
create offspring. For a tetraploidy animal to reproduce, it must find another animal of the same
species but of opposite sex that has also randomly undergone polyploidy.
A number of models have been proposed to account for this mode of speciation. The most
popular, disruptive speciation, was first put forward by John Maynard Smith in 1962. Smith
suggested that heterozygous individuals may, under particular environmental conditions, have a
greater fitness than those with alleles homozygous for a certain trait. Under the mechanism of
natural selection, therefore, heterozygosity would be favoured over homozygosity, eventually
leading to speciation.
76. polyploidy- Polyploid (in Greek: multiple) cells or organisms contain more than two copies
(ploidy) of their chromosomes. The polyploid types are names triploid (3x), tetraploid (4x),
pentaploid (5x), hexaploid (6x) and so on. Where an organism is normally diploid, a monoploid
(1x) may arise as a spontaneous abberation, monoploidy may also occur as a normal stage in an
organisms life cycle.
Polyploidy occurs in animals but is especially common among flowering plants, including both
wild and cultivated species. Wheat, for example, after millennia of hybridization and
modification by humans, has strains that are diploid (two sets of chromosomes), tetraploid (four
sets of chromosomes) with the common name of durum or macaroni wheat, and hexaploid (six
sets of chromosomes) with the common name of bread wheat. Polyploidy can be induced in cell
culture by some chemicals: the best known is colchicine, which causes chromosome doubling.
-Examples
77. evolutionary bottleneck- see #40
78. punctuated equilibrium- a theory of evolution which states that changes such as speciation can
occur relatively quickly, with long periods of little change—equilibria—in between. This theory
is one of the proposed explanations of the evolutionary patterns of species as observed in the
fossil record, particularly the relatively sudden appearance of new species in a geologically short
time period, and the perhaps typical lack of substantial change of species during their existence.
79. gradualism- Gradualism, in biology, holds that evolution occurs through the accumulation of
slight modifications over a period of generations. This is in contrast to saltationism, which holds
that evolution occurs through large changes, possibly in a single generation. Gradualism holds
that every individual is the same species as its parents, and that there is no clear line of
demarcation between the old species and the new species. It holds that the species is not a fixed
type, and that it is the population, not the individual, that evolves.
"Support for gradualism derives from the fact that many, probably most, evolutionary processes
are indeed gradual and incremental....However, it is one thing to believe that gradual processes
predominate in nature and quite another to hold that all evolutionary processes must be gradual.
The issue is, after all, simply an empirical one; even if no nongradual changes were ever
witnessed, one could never exclude the possibility that the next evolutionary process to be
uncovered might be nongradual." Examples include sudden adaptation due to natural disaster,
preadaptations and exaptations (Gould and Vrba 1982), and possibly, syntax and language, which
being hierarchical could not have been gradual, "the difference between flat structure (beads on a
wire) and hierarchyical structure is absolute" (Bickerton 2000, p.159). One example of
preadaptation is insect flight as insects were originally exclusively terrestrial, some with fanlike
structures for cooling which were selected for until they were large and efficient enough to allow
flight (ibid, p.160).
80. Geologic time chart- see geologic time
81. index fossils- Index fossils (or zone fossils) are fossils used to define and identify geologic
periods (or faunal stages). They work on the premise that although different sediments may look
different depending on the conditions under which they were laid down, they may include the
remains of the same species of fossil. If the species concerned were short-lived (in geological
terms, lasting a few hundred thousand years) then you can be sure that the sediments in question
were deposited within that narrow time period. The shorter the lifespan of a species, the more
precisely you can correlate different sediments, and so rapidly evolving types of fossils are
particularly valuable. The best index fossils are common, easy-to-identify at species level, and
have a broad distribution—otherwise the likelihood of finding and recognising one in the two
sediments is low.
Ammonites fit these demands well, and are the best-known fossils that have been widely used for
this. Other important groups that provide index fossils are the corals, graptolites, brachiopods,
trilobites, and echinoids (sea urchins). Conodonts may be identified by experts using light
microscopy such that they can be used to index a given sample with good resolution. Terrestrial
fossils, such as the teeth of mammals, have also been used.
82. radioactive dating- a technique used to date materials based on a knowledge of the decay rates
of naturally occurring isotopes, and the current abundances.
83. phylogeny- A phylogeny (or phylogenesis) is the origin and evolution of a set of organisms,
usually of a species. A major task of systematics is to determine the ancestral relationships among
known species (both living and extinct), and the most commonly used methods to infer
phylogenies include cladistics, phenetics, maximum likelihood, and Bayesian.
A phylogenetic tree is a tree showing the evolutionary interrelationships among various
species or other entities that are believed to have a common ancestor. A phylogenetic tree is a
form of a cladogram. In a phylogenetic tree, each node with descendants represents the most
recent common ancestor of the descendants, and edge lengths correspond to time estimates. Each
node in a phylogenetic tree is called a taxonomic unit. Internal nodes are generally referred to as
Hypothetical Taxonomic Units
(HTUs) as they cannot be
directly observed.
A rooted phylogenetic tree is a
directed tree with a unique
node corresponding to the
(usually imputed) most recent
common ancestor of all the
entities at the leaves of the
tree. Figure 1 depicts a rooted
phylogenetic tree, which has
been colored according to the
three-domain system (Woese
1998).
An unrooted phylogenetic tree
is, loosely speaking, a tree
derived from a rooted phylogenetic tree by omitting the root. More precisely, it is a forest of
rooted phylogenetic trees depicted so that the roots are all linked. Figure 2 depicts an unrooted
phylogenetic tree¹ for myosin, a superfamily of proteins
84. systematics- Systematics is the study of the diversity of organism characteristics. In biology,
systematists are the scientists who classify species and other taxa, which they do with the aim of
defining how they relate evolutionarily.
85a. convergent evolution- In evolutionary biology, convergent evolution describes the process
whereby organisms not closely related independently acquire similar characteristics while
evolving in separate and sometimes varying ecosystems. An example of convergent evolution is
the similar nature of the wings of insects, birds, and bats. All three serve the same function and
are similar in structure, but each evolved independently. Eyes also evolved independently in
various animals.
Convergent evolution is a different phenomenon than evolutionary relay and parallel evolution.
Similar to convergent evolution, evolutionary relay describes how independent species acquire
similar characteristics through their evolution in similar ecosystems, but not at the same time
(dorsal fins of extinct ichthyosaurs and sharks). Parallel evolution occurs when two independent
species evolve together at the same time in the same ecospace and acquire similar characteristics
(extinct browsing-horses and extinct paleotheres). Structures that are the result of convergent
evolution are called analogous structures or homoplasies; they should be contrasted with
homologous structures which have a common origin.
85b. divergent evolution- Pattern of evolution in which two closely related species gradually become
more and more dissimilar. Compare with convergent evolution and parallel evolution. Divergent
evolution, when organisms evolve to fill more diverse ecological niches than ancestral forms is
probably much more common in the history of life than convergent evolution. In the broadest
sense, all living things are the product of divergent evolution. Most scientists believe that all
organisms are in fact descended from one population of early single celled organisms. However,
in this essay we will begin to look at divergent evolution within the scope of smaller taxonomic
groups, mostly orders and families. The way to recognize divergent evolution on any scale is that
at the base taxonomic level the animals share characteristics that link them, but they possess
superficial differences that allow them to exploit different ecological niches. This is most likely
due to differential selection on a relatively short time scale.
86. DNA hybridization- The technique of DNA-DNA hybridization provides a way of comparing
the total genome of two species. Let us examine the procedure as it might be used to assess the
evolutionary relationship of species B to species A:









The total DNA is extracted from the cells of each species and purified.
For each, the DNA is heated so that it becomes denatured into single strands (ssDNA).
The temperature is lowered just enough to allow the multiple short sequences of repetitive
DNA to rehybridize back into double-stranded DNA (dsDNA).
The mixture of ssDNA (representing single genes) and dsDNA (representing repetitive
DNA) is passed over a column packed with hydroxyapatite. The dsDNA sticks to the
hydroxyapatite; ssDNA does not and flows right through. The purpose of this step is to be
able to compare the information-encoding portions of the genome — mostly genes present
in a single copy — without having to worry about varying amounts of noninformative
repetitive DNA.
The ssDNA of species A is made radioactive.
The radioactive ssDNA is then allowed to rehybridize with nonradioactive ssDNA of the
same species (A) as well as — in a separate tube — the ssDNA of species B.
After hybridization is complete, the mixtures (A/A) and (A/B) are individually heated in
small (2°–3°C) increments. At each higher temperature, an aliquot is passed over
hydroxyapatite. Any radioactive strands (A) that have separated from the DNA duplexes
pass through the column, and the amount is measured from their radioactivity.
A graph showing the percentage of ssDNA at each temperature is drawn.
The temperature at which 50% of the DNA duplexes (dsDNA) have been denatured (T50H)
is determined.
As the figure shows, the curve for A/B is to the left of A/A, i.e., duplexes of A/B separated at a
lower temperature than those of A/A. The sequences of A/A are precisely complementary so all the
hydrogen bonds between complementary base pairs (A-T, C-G) must be broken in order to separate
the strands. But where the gene sequences in B differ from those in A, no base pairing will have
occurred and denaturation is easier.
Thus DNA-DNA hybridization provides genetic comparisons integrated over the entire genome. Its
use has cleared up several puzzling taxonomic relationships.
87. molecular clocks
Questions
1. Explain Charles Darwin's contributions to evolutionary ideas. Give the basic
assumptions upon which Darwin's theory rests. Indicate the types of evidence that Darwin
used in formulating his theory.
2. List the processes that can lead to genetic variation.
3. Explain why changes in somatic cells cannot bring about evolution.
4. Briefly contrast Lamarck's theory of inheritance of acquired characteristics with
Darwin's theory of natural selection.
5. Explain the concept of the gene pool. Given the frequencies of two alleles, be able
calculate the ratios of the genotypes produced by them, using a Punnett square or
algebraic method.
6. State the Hardy-Weinberg Law, and discuss its four conditions for maintenance of
genetic equilibrium. Why are these conditions rarely met in nature?
7. Explain how natural selection in one generation can affect the genotype of the next
generation.
8. Using diagrams contrast directional selection, stabilizing selection and disruptive
selection.
9. Describe heterozygote superiority, explain how this process helps maintain
deleterious recessive genes. (Note sickle cell anemia)
10. Give a biological definition of species.
11. Explain the geographic isolation model of speciation, in doing so take into account
the roles of mutation, natural selection, and the gene pool. Distinguish between
extrinsic and intrinsic isolating mechanisms.
12. Explain what is meant by adaptive radiation, and discuss the evidence for this
phenomenon.
13. Compare the hypotheses of gradualism and punctuated equilibrium, and give an
example supporting each hypothesis.