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Evolution
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Evolution is the progressive change in a kind (not one individual, but a population)
of organism over time.
 Specifically, evolution is the change over time of the genetic composition of
populations.
 It is the process by which modern organisms have descended from ancient
organisms.
Evolutionary History
 The Ancient Greek philosopher Anaxiamander (611-547 B.C.) and the Roman
philosopher Lucretius (99-55 B.C.) coined the concept that all living things were
related and that they had changed over time.
 Another ancient Greek philosopher, Aristotle developed his Scala Naturae, or
Ladder of Life, to explain his concept of the advancement of living things from
inanimate matter to plants, then animals and finally man.
 Aristotle’s concept of man as the "crown of creation" still plagues modern
evolutionary biologists and has lead to many of the misconceptions about evolution.
 “Scientists" were constrained by the prevailing thought patterns of the Middle Ages the inerrancy of the biblical book of Genesis and the special creation of the world in
a literal six days of the 24-hour variety.
 Archbishop James Ussher of Ireland, in the mid 1600's, calculated the age of the
earth based on the geneologies from Adam and Eve listed in the biblical book of
Genesis, working backward from the crucifixion. According to Ussher's
calculations, the earth was formed on October 22, 4004 B.C. Ussher's ideas were
readily accepted, in part because they posed no threat to the social order of the
times.
 Geologists had for some time doubted the "truth" of a 5,000 year old earth.
 Leonardo da Vinci calculated the sedimentation rates in the Po River of Italy, and
concluded it took 200,000 years to form some nearby rock deposits.
 Galileo studied fossils (evidence of past life) and concluded that they were real and
not inanimate artifacts.
 James Hutton, regarded as the Father of modern Geology, developed (in 1795) the
Theory of Uniformitarianism, the basis of modern geology and paleontology.
According to Hutton's work, certain geological processes operated in the past in
much the same fashion as they do today, with minor exceptions of rates, etc. Thus
many geological structures and processes cannot be explained if the earth is only
5000 years old.
 British geologist Charles Lyell refined Hutton's ideas during the 1800s to include
slow change over long periods of time. Scientists recognized that Earth is many
millions of years old. Lyell’s book Principles of Geology had profound effects on
Charles Darwin and Alfred Wallace.
 Georges-Louis Leclerc, Comte de Buffon (1707-1788) a French naturalist,
mathematician and biologist. Best remembered for his Historie Naturelle, a 44
volume encyclopedia describing everything known about the natural world, wrestled
with the similarities of humans and apes and even talked about common ancestry of
Man and apes. Although Buffon believed in organic change, he did not provide a
coherent mechanism for such changes. He thought that the environment acted
directly on organisms through what he called "organic particles." Buffon also
published Les Epoques de la Nature (1788) where he openly suggested that the
planet was 75,000 years old - much older than the 6,000 years proclaimed by the
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church. An idea for which he was condemned by the Catholic church. In Les
Epoques de la Nature he discussed concepts very similar to Charles Lyell's
"uniformitarianism" which were formulated 40 years later. Buffon, however,
vacillated as to whether or not he believed in evolutionary descent, and professed to
believe in special creation and the fixity of species.
Erasmus Darwin (1731-1802) grandfather of Charles Darwin; a British physician
and poet in the late 1700's, proposed that life had changed over time. His writings
on both botany and zoology contained many comments that suggested the
possibility of common descent based on changes undergone by animals during
development, artificial selection by humans, and the presence of vestigial organs.
He also talked about how competition and sexual selection could cause changes in
species: "The final course of this contest among males seems to be, that the
strongest and most active animal should propogate the species which should thus
be improved."
William “Strata” Smith (1769-1839), a civil engineer who was employed as a
surveyor in England. Being employed as a surveyor required detailed knowledge of
the rocks through which canals were to be dug. This led Smith to examine the local
rocks very carefully. While doing this, Smith observed that the fossils found in a
section of sedimentary rock were always in a certain order from the bottom to the
top of the section. This order of appearance could also be seen in other rock
sections, even those on the other side of England. As Smith described it…. The
layers of sedimentary rocks in any given location contain fossils in a definite
sequence; the same sequence can be found in rocks elsewhere, and hence strata
can be correlated between locations. In essence Smith fathered the science of
stratigraphy, the correlation of rock layers based on (among other things) their fossil
contents.
Baron Georges Cuvier (1769-1832) almost single-handedly founded vertebrate
paleontology as a scientific discipline and created the comparative method of
organismal biology, an incredibly powerful tool. It was Cuvier who firmly established
the fact of the extinction of past lifeforms. He believed that the Earth was
immensely old, and that for most of its history conditions had been more or less like
those of the present. However, periodic "revolutions", or catastrophes had befallen
the Earth; each one wiped out a number of species. An idea known as
Catastrophism. He regarded these "revolutions" as events with natural causes, and
considered their causes and natures to be an important geological problem.
Although he was a lifelong Protestant, Cuvier did not explicitly identify any of these
"revolutions" with Biblical or historical events. The idea of catastrophism was a
comfortable one for the times and thus was widely accepted. Cuvier eventually
proposed that there had been several creations that occurred after catastrophies.
Louis Agassiz (1807-1873) the “Father of Glaciology” and promoter of Cuvier’s
catastrophism he proposed 50-80 catastrophies and creations. His finding of
parallels between ontogeny, paleontology, and morphology was rapidly adopted by
biologists like Haeckel and used by Darwin to support evolution. He was no
evolutionist; in fact, he was probably the last reputable scientist to reject evolution
outright for any length of time after the publication of The Origin of Species. Agassiz
saw the Divine Plan of God everywhere in nature, and could not reconcile himself to
a theory that did not invoke design. He defined a species as "a thought of God."
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Jean-Baptiste Lamarck (1744 – 1829) a French naturalist just before Darwin.
Proposed “by selective use or disuse of organs, organisms acquired or lost certain
traits during their lifetime. These traits could then be passed on to their offspring.
Over time, this process led to change in a species.” Lamarck’s Theory of Evolution:
The Inheritance of Acquired Traits
o All organisms have an innate tendency toward complexity and perfection.
 Ancestors of birds acquired an urge to fly, kept trying to fly, eventually
their wings increased in size and now they are suited to flying.
o Organisms could alter the size or shape of particular organs by using their
bodies in new ways.
 Birds try to use their front limbs to fly and they grew wings.
 If a winged animal did not use its wings they would decrease in size
and disappear.
o Acquired characteristics could be inherited.
 If a bird spent its life trying to fly and developed larger wings, then its
offspring would inherit larger wings.
Lamarck's theory of evolution is incorrect in several ways:
o He did not know how traits are inherited.
o He did not know that an organism's behavior has no effect on its inheritable
characteristics.
However, Lamarck was one of the first to develop a scientific theory of evolution
and realize that organisms are adapted to their environments. In this way, he paved
the way for the work of later biologists.
Thomas Malthus (1766-1834). A political economist who was concerned about,
what he saw as, the decline of living conditions in nineteenth century England. He
blamed this decline on three elements:
o The overproduction of young;
o The inability of resources to keep up with the rising human population; and
o The irresponsibility of the lower classes.
To combat this, Malthus suggested the family size of the lower class ought to be
regulated such that poor families do not produce more children than they can
support. In Essay on the Principle of Population (1798), Malthus concluded that
unless family size was regulated, man's misery of famine would become globally
epidemic and eventually consume man. His view that poverty and famine were
natural outcomes of population growth and food supply was not popular among
social reformers who believed that with proper social structures, all ills of man could
be eradicated. Although Malthus thought famine and poverty natural outcomes, the
ultimate reason for those outcomes was divine institution. He believed that such
natural outcomes were God's way of preventing man from being lazy. Both Darwin
and Wallace independently arrived at similar theories of Natural Selection after
reading Malthus. unlike Malthus, they framed his principle in purely natural terms,
both in outcome and in ultimate reason. By so doing, they extended Malthus' logic
further than Malthus himself could ever take it. Both Darwin and Wallace realized
that producing more offspring than can survive establishes a competitive
environment among siblings, and that the variation among siblings would produce
some individuals with a slightly greater chance of survival.
Alfred Russel Wallace (1823-1913) - English naturalist, evolutionist, geographer
and anthropologist. He spent many years in South America, publishing salvaged
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notes in Travels on the Amazon and Rio Negro in 1853. In 1854, Wallace left
England to study the natural history of Indonesia. While in Indonesia he contracted
Malaria, but influenced by Malthus’ ideas on population growth he managed to write
down his ideas on natural selection in an essay titled “On the Tendency of Varieties
to Depart Indefiniely from the Original Type.” He sent his essay to Darwin in hopes
that he would share it with Lyell. Seeing that Wallace shared his own ideas
regarding the “species question,” Darwin presented Wallace’s paper and along with
some of his own writings to the Linnean Society meeting in July of 1858. His essay
also motivated Darwin to publish a shortened version of the work he had been
planning and was years away from completing (a work that Darwin never did
complete). On November 24th, 1859, the shortened book “On the Origin of
Species…” was published and Darwin was forever associated with Evolution.
However, most prominent scientists were well aware of Wallace’s contributions.
Today some think of him as the “codeveloper” of the theory of natural selection.
Charles Darwin (1809-1882) - “Father of Evolution.” He went on a voyage from
1831 – 1836 on the H.M.S. Beagle as a dinner companion to the captain and wound
up becoming the ships’ naturalist. The voyage went around the world and made a
very important stop on the Galapagos Islands (a group of small islands off the west
coast of South America). It was at the Galapagos Islands that Darwin collected
samples and observed the characteristics of many animals and plants which varied
noticeably among the different islands, each with a very different climate. He
focused a lot on finches, marine iguanas and tortoises. He proposed animals on
the different islands had once been members of the same original species which
came from a South American ancestral species. Darwin began assembling his
ideas upon his return from the voyage. He wrote some preliminary manuscripts
while working on other things for the next 20+ years. Darwin finally published On
the Origin of Species in 1859 after he read Wallace’s essay which summarized
Darwin’s own thoughts on natural selection. Why did he wait so long? Because he
was both stunned and disturbed by what he had observed and discovered as it
challenged the fundamental scientific beliefs of that time (not to mention religious
beliefs). Since the day it was published until now, On the Origin of Species is
considered one of the most influential, yet controversial books ever written.
Darwin saw things as “descent with modification.” By this he meant that the species
of organisms inhabiting Earth today descended from ancestral species. This was
due to 5 things he observed:
1. Exponential fertility
2. Stable population size
3. Limited resources
4. Individuals vary
5. Heritable variation.
The 5 observations led to 3 inferences:
1. Struggle for existence
2. Non-random survival
3. Natural selection (differential success in reproduction).
He proposed a mechanism for evolution called the Theory of Natural Selection – the
best adapted individuals in a population survive and produce offspring that are
likewise well adapted and that evolution is a matter of variations and chance.
Another way of understanding Natural Selection is that populations of organisms
can change over the generations if individuals having certain heritable traits leave
more offspring than others (differential reproductive success).
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Summary of Darwin's Theory:
1. Individual organisms in nature differ from one another and this variation is
inherited. (Natural Variation).
2. Organisms in nature produce more offspring than can survive, and many of
those that survive do not reproduce. (Over-reproduction).
3. Because more organisms are produced than can survive members of each
species must compete for limited resources. (Struggle for Existence).
4. Because each organism is unique, each has different advantages and
disadvantages in the struggle for existence. (Fitness Adaptations – any inherited
characteristic that enhance an organism’s chance of survival, which also
increases that organism’s chance of reproducing).
5. Individuals best suited to their environment survive and reproduce most
successfully (Survival of the Fittest). The characteristics that make them best
suited to their environment are passed on to offspring.
6. Individuals whose characteristics are not as suited to their environment die or
leave fewer offspring. (Natural Selection).
7. Species change over time. Over long periods, natural selection causes changes
in the characteristics of a species, such as in size and form. New species arise,
and other species disappear. (Speciation).
8. Species alive today have descended with modifications from species that lived in
the past. (Descent with Modification).
9. All organisms on Earth are united into a single tree of life by common descent.
(Common Descent).
What Darwin needed was a mechanism to explain the theory of natural selection.
How could favorable variations be transmitted to later generations? Ironically the
mechanism was being figured out at almost the same time. The rediscovery of
Mendel’s work along with the explosion of genetics in the 20th century was the
missing mechanism to support Darwin’s theory of natural selection. Darwinian
theory supported by genetics is known as the modern synthesis.
Even with the continuous addition of evidence, Darwin’s ideas concerning evolution
are ridiculed. Mainly due to misunderstanding and misconceptions.
Adaptations
1. An adaptation is an inherited characteristic that increases an organism’s chance of
survival. There are three main types of adaptations.
2. Structural Adaptations: anatomical adaptation by an organism that promotes the
fitness in its environment – for example: wood peckers having a tough pointed beak
for “drilling” holes in trees to get their prey or the shell of a turtle.
a. Mimicry can be an example of a structural adaptation – for example: a
pueblan milksnake (harmless) looking like a coral snake (poisonous). This is
also known as deceptive coloration.
b. Aposmatic coloration can be an example of a structural adaptation – a
coral snake displaying bright colors to indicate that it is poisonous or the
unrealistically brilliant colored eye opening of a red-eyed tree frog that
provides just enough time to jump to another tree to avoid the startled
predator.
c. Camouflage is a structural adaptation that enables a species to blend with
their surroundings – for example: a walking stick. This is also known as
cryptic coloration.
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3. Behavioral Adaptations: results from the response of an organism to its external
environment – for example: frog mating “songs” or a squirrel storing acorns for the
winter.
4. Physiological Adaptations: adaptation with a chemical basis that is associated
with an organism’s function – for example: the proteins that make a spider’s web or
the venom of a rattlesnake.
Evidence of (Species) Evolution
1. Fossils: preserved remains or evidence of an organism that lived long ago (bones,
casts, frozen in ice or amber, etc…).
 Incomplete records: There are “gaps” in the fossil record. It is estimated that 99%
of all species that have ever lived on Earth are now extinct. The possibility that a
representative of each species was preserved, or has been found, is unlikely.
 Found in layers of sedimentary rock (unless distorted by geological forces) with the
upper layers having the newest, “more complex” organisms and the lower layers
containing older, “less complex” organisms.
 Transitional links are intermediate species between major groups of organisms.
Fossil links combined with modern comparative anatomy allows us to deduce
vertebrate descent:
 Eusthenopteron is fish ancestral to amphibians.
 Seymouria is amphibian ancestral to reptiles.
 Archeopteryx has features intermediate between primitive reptiles and birds.
 Therapsids are reptiles ancestral to mammals.
2. Anatomy: Many organisms share a unity of plan;
 Homologous Structures: body parts which are similar in structure and/or in function
– for example: a human’s arm, a cat’s forelimb , a whale’s flipper and a bat’s wing.
 Vestigial Organs: body parts (structures) once useful for an organism’s lifestyle, but
now having no apparent function – for example: the wings of a flightless bird, the
hindlimb bones of a snake or whale, a human’s appendix, etc….
3. Comparative Embryology: the study of developing organisms that shows a
number of relationships not obvious in the adult organisms – for example:
pharyngeal pouches and “tails” as embryos in a variety of chordates.
 All vertebrates exhibit notochord during development. All vertebrates, including
humans, exhibit paired pharyngeal pouches. In fishes and amphibians, these
become functioning gills. In humans, they become the eustachian tubes, middle ear
cavity, tonsils, and thyroid and parathyroid glands. Simplest explanation is that fish
notochord and pharyngeal pouches are primitive fish features and fish are ancestral
to other vertebrates.
4. Comparative Biochemistry: Study of an organism on a biochemical level – for
example: the similarities of amino acids in hemoglobin of the blood of various
vertebrates. Almost all living organisms use the same basic biochemicals: DNA,
ATP, many identical enzymes, DNA triplet code, 20 amino acids, introns, and
hypervariable regions. (Prokaryotes i.e. true bacteria (Domain Eukarya), do not
have introns. This points to a long period of time since all living things shared
common ancestory. Similarity of biochemistry is explained by descent from
common ancestor.
 DNA base sequences differences in DNA between a number of organisms
shows less difference the more closely related they are; for example, 2.5%
difference between humans and chimpanzees but 42% difference between
humans and lemurs.
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Amino acid sequences of cytochrome c show similarity between human and
monkey, distance from human to duck and greater distance to Candida yeast.
 Data are understandable assuming humans and chimpanzees share a more
recent common ancestor than do humans and lemurs, ducks, or yeast.
 Biochemical evidence is generally consistent with anatomical similarity of
organisms.
5. Genetic Evidence: for example: DNA similarities between people in certain parts of
the world compared to people in other parts of the world. This is often considered a
subset of comparative biochemistry.
6. Direct Observations: observations of evolutionary changes that occur rapidly – for
example: penicillin-resistant bacteria, Industrial Melanism (see Peppered Moth
Survey Lab).
 Population: a localized group of individuals belonging to the same species.
 Species: a group of organisms that can interbreed and produce fertile offspring.
 Speciation: the evolution of a new species.
The Process of Speciation:
 It is theorized that different species developed because of:
1. Reproductive Isolation: the separation of species or populations so that they
cannot interbreed and produce fertile offspring.
a. Temporal Isolation: different species of plants produce their flowers at different
times so they are not cross-breeding.
b. Behavioral Isolation: different species of songbirds could interbreed, but don’t
as different “songs” are used to attract mates, so they do not end up crossbreeding.
c. Mechanical Isolation: mechanical barriers prevent the copulatory organs of
closely related species from fitting together.
2. Geographical Isolation: two populations of a species are separated by
geographical barriers (such as rivers, mountains, canyons, etc…) that prevent them
from reproducing with one another. (A type of Reproductive Isolation).
a. Habitat isolation: two organisms use different habitats within the same
geographical area. This is exemplified by plants in a salt marsh vs. those that
live in the same terrestrial area.
b. Ring Species: provide unusual and valuable situations in which we can
observe two species and the intermediate forms connecting them. A ring of
populations encircles an area of unsuitable habitat. At one location in the ring of
populations, two distinct forms coexist without interbreeding, and hence are
different species. Around the rest of the ring, the traits of one of these species
change gradually, through intermediate populations, into the traits of the second
species. A ring species, therefore, is a ring of populations in which there is only
one place where two distinct species meet. Ernst Mayr called ring species "the
perfect demonstration of speciation" because they show a range of intermediate
forms between two species.
3. Polyploidy: a chromosomal alteration in which the organism possesses more than
two complete sets of chromosomes. Usually lethal in animals, but can lead to
hardier plant species.
Population Genetics:
 Individuals don’t evolve, populations do.
 Gene pools: entire collection of genes among a population; populations evolve as
the relative frequencies of different alleles (genes) change.
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Natural selection acts on the phenotypes (physical traits) of a population, not
directly on the genes. But it can change the relative frequencies of the alleles in a
gene pool (population) over time.
 Gene Flow: the loss or gain of alleles from a population due to the emigration or
immigration of fertile individuals, or the transfer of gametes between populations.
This reduces the differences between populations.
It is theorized that different species developed because of:
 Genetic Drift: in small populations, individuals that carry a particular allele may
leave more descendants than other individuals, just by chance, not selection. Over
time, a series of chance occurrences of this type can cause an allele to become
common in a population. For example, Amish of Lancaster, Pennsylvania have a
recessive allele causing dwarfism and higher proportion of polydactylism (1-in-14
compared to 1-in-1,000). There are two common types of genetic drift:
1. The Founder Effect: Where individuals may not be representative of the
population they came from, yet start a new population where their
phenotypes come to be expressed frequently. The new Atlantic population of
Scorpaenidae Lionfish (Pterois volitans ), may have originated from only 10
individuals
2. The Bottleneck Effect: Disasters such as earthquakes, floods or fires may
reduce the size of a population drastically, killing victims rather unselectively.
the result is that the small surviving population is unlikely to be representative
of the original population in its genetic makeup. Populations subjected to
near extinction (like Cheetahs) endure a bottleneck. Northern Elephant Seal Reduced to 20 individuals in 1896, Now 30,000 individuals, with no
detectable genetic diversity,
It is theorized that different species developed because of:
• Non-Random Mating: inbreeding and assortive mating (both shift frequencies of
different genotypes). Both homozygotes increase in frequency, but heterozygotes
decrease.
Sources of Genetic Variation:
• Mutation:
change(s) in the DNA of an organism. This is the original source of genetic
variation (raw material for natural selection). Provides new alleles. Seemingly harmful
mutations can be source of variation for better adaptation to a new or changing
environment.
• Genetic shuffling that takes place during meiosis prior to sexual reproduction.
Both
the independent assortment of chromosomes during meiosis and the possibilities of
“crossing over.”
• Natural Selection on Single-Gene Traits: when there are only two possible
phenotypes, natural selection can lead to changes in allele frequency which don’t
match typical expectations determined by punnett squares.
• Natural Selection on Polygenic Traits:
polygenic traits are those controlled by more
than one set of genes. The phenotypes displayed by these traits often are displayed in
a “normal distribution,” also known as a bell-shaped curve. When there are more than
two variations of possible phenotypes, natural selection causes shifts in the distribution
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of traits. There are three ways natural selection can effect the distribution of
phenotypes.
1. Directional Selection: the entire curve shifts as the character trait changes.
Occurs when one extreme of the distribution has an adaptation that could
become favorable. When an environmental change or species migrate – the
new environment favors the extreme phenotype and the population evolves.
For Example: woodpeckers that have shorter-lengthed beaks struggling to
compete with woodpeckers that have longer-lengthed beaks leading to the
natural selection of the woodpeckers with longer-lengthed beaks. Directional
Selection: the entire curve shifts as the character trait changes.
2. Stabilizing Selection: the distribution curve narrows as individuals in the
middle of the distribution are favored. Average phenotypes (individuals) are
favorable and extremes are not. Operates most of the time in most
populations. Limits evolution as allele frequencies remain constant and the
average individuals continue to dominate the population. For example: larger
spiders are easily seen and eaten by predators. While smaller spiders find it
difficult to find food. Therefore, average-sized spiders are favored by natural
selection. Stabilizing Selection: the distribution curve narrows as individuals
in the middle of the distribution are favored.
3. Disruptive Selection: the distribution curve begins to split in as the
extremes are favored over the average phenotypes. (Also known as
Diversifying Selection). Two opposite phenotypes are favored and the
average phenotype is not. Two subpopulations begin to form. If these
subpopulations do not interbreed they can form two distinct species. For
example: in a beach environment light-colored limpets are favored against
light-colored rocks and dark-colored limpets are favored against a darkcolored background. Intermediate-colored limpets are not favored on either
background and are eaten by birds. As the light- and dark-colored limpets
continue to survive in slightly different areas they will interact less and may
become two separate species. The distribution curve begins to split in as the
extremes are favored over the average phenotypes. (Also known as
Diversifying Selection).
Hardy-Weinberg Principle:
• The Hardy-Weinberg Principle states that allele frequencies in a population will
remain constant unless one or more factors cause those frequencies to change. This
state of unchanging allele frequencies is known as genetic equillibrium.
• Five conditions required to maintain genetic equilibrium (limit evolution of a population):
1. There must be random mating, in other words all individuals have the
opportunity to produce offspring – rarely happens in nature.
2. There must be a large population size.
Genetic drift has less effect on large
populations than on smaller populations.
3. There must not be movement of individuals into or out of the population – no
immigration or emigration.
4. There must not be genetic mutations – no new alleles can be introduced.
5. There must not be natural selection – meaning no traits are favored over
other traits.
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• Hardy-Weinberg Equation:
p=frequency of one allele (A);
q=frequency of the other allele (a);
p + q = 1.0 (p = 1 - q & q = 1 - p)
P2=frequency of AA genotype;
2pq=frequency of Aa plus aA genotypes;
q2=frequency of aa genotype;
p2 + 2pq + q2 = 1.0
• The five conditions of the Hardy-Weinberg principle are rarely met, so allele
frequencies in the gene pool of a population do change from one generation to the
next, resulting in evolution. We can now consider that any change of allele frequencies
in a gene pool indicates that evolution has occurred.
• The Hardy-Weinberg principle proposes those factors that violate the conditions listed
cause evolution. A Hardy-Weinberg equilibrium provides a baseline by which to judge
whether evolution has occurred.
• Hardy-Weinberg equilibrium is a constancy of gene pool frequencies that remains
across generations, and might best be found among stable populations with no natural
selection or where selection is stabilizing.
Patterns of Evolution:
•Adaptive Radiation: the process by which a single species or small group of species
evolves into several different forms that live in different ways. This is also referred to as
divergent evolution. This is what Darwin saw on the Galapagos Islands with finches that
contained far greater variation, 13 different-sized finches with different bills adapted to
particular food-gathering methods.
•Convergent Evolution: the process by which unrelated organisms independently evolve
similarities when adapting to a similar environment. Dolphins and sharks both having
adaptations (flippers and fins) for swimming. Birds, insects and bats all having the
adaptation (wings) of being capable of flying. Structures that are not similar but function in
similar ways are called analogous structures. Two similar species filling similar ecological
roles (niches) having evolved in distinctly different parts of the world.
•Coevolution: the process by which two species evolve in response to changes in each
other. This is also known as mutual adaptation or mutualism. A clown fish living with sea
anemones. Insects that pollinate flowers. Bacteria Nodules on the roots of plants
(legumes).
•Phyletic Gradualism: the pattern of evolution in which evolutionary change is slow and
steady. This is an older model of evolution/speciation. This pattern makes the existence
of transitional fossils unlikely.
•Punctuated Equilibrium: a pattern of evolution in which long stable periods are
interrupted by brief periods of more rapid change. This pattern often follows periods of
mass extinction. Tempo of speciation: gradual vs. divergence in rapid bursts; Niles
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Eldredge and Stephen Jay Gould (1972); helped explain the non-gradual appearance of
species in the fossil record.
•Mass Extinctions: extinctions (the death of an entire species) occur all the time.
More
so now due to man than by natural events. However, in geological history there have been
periods of mass extinction, caused by natural events, where numerous species disappear
which greatly effect the survival of the species that remain leading to “explosions” of
evolution as these species adapt to the new circumstances.
•Five mass extinctions in fossil record define end of:
1.
2.
3.
4.
5.
Ordovician
Devonian
Permian
Triassic
Cretaceous
•Following extinctions, remaining groups expand to fill habitats vacated by extinct species.
Marine animal fossil record indicates mass extinctions occur every 26 million years;
corresponds to movement of solar system within Milky Way galaxy.
•Extinction of dinosaurs at end of Cretaceous.
•Proposed in 1977 that Cretaceous extinction was caused by asteroid impact.
•Cretaceous-Tertiary border has high level of iridium, rare in earth's crust but
common in meteorites.
•Calculations of effects of nuclear bomb explosions ("nuclear winter") compare with
worldwide climate cooling expected from large asteroid impact.
•Worldwide layer of soot also defines iridium layer.
•Huge meteorite crater of correct age found in Caribbean Ocean and Yucatan
peninsula; suspected site of impact of meteor that resulted in dinosaur extinction.
Population Variation:
•Polymorphism: coexistence of 2 or more distinct forms of individuals (morphs) within the
same population.
•Geographical Variation:
differences in genetic structure between populations (clines).
Prevention of natural selection’s reduction of variation.
•Diploidy a 2nd set of chromosomes hides variation in the heterozygote.
Variation Preservation:
•Balanced polymorphism
•Heterozygote advantage or hybrid vigor; i.e., malaria/sickle-cell anemia);
A
favored Heterozygote keeps homozygotes in the population. Two alleles for sickle
cell cause severe problems; two alleles for normal blood make person susceptible
to serious malaria infections. Heterozygous individual for sickle-cell trait has
adaptive advantage but will continue to produce two homozygotes that are not
adaptive.
•Frequency Dependent Selection (survival & reproduction of any 1 morph declines if it
becomes too common; i.e., parasite/host).
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Natural Selection:
•Fitness: the contribution an individual makes to the gene pool of the next generation.
•Sexual Selection:
•Sexual dimorphism: secondary sex characteristic distinction between males and
females.
•Intersexual Selection:
Occurs when individuals of one sex (usually females) are
choosy in selecting their mates from individuals of the other sex.
The Origin of Species.
•Macroevolution: the origin of new taxonomic groups.
•Speciation: the origin of new species.
•Anagenesis (phyletic evolution): accumulation of heritable changes.
•Cladogenesis (branching evolution): budding of new species from a parent
species that continues to exist (basis of biological diversity).
What is a species?
•Biological species concept (Mayr): a population or group of populations whose members
have the potential to interbreed and produce viable, fertile offspring (genetic exchange is
possible and that is genetically isolated from other populations).
•Reproductive Isolation:
•Prezygotic Barriers: impede mating between species or hinder the fertilization of the ova
1. Habitat (snakes; water/terrestrial).
2. Behavioral (fireflies; mate signaling).
3. Temporal (salmon; seasonal mating).
4. Mechanical (flowers; pollination anatomy).
5. Gametic (frogs; egg coat receptors).
•Postzygotic Barriers: fertilization occurs, but the hybrid zygote does not develop into a
viable, fertile adult
1. Reduced hybrid viability (frogs; zygotes fail to develop or reach sexual maturity).
2. Reduced hybrid fertility (mule; horse x donkey; cannot backbreed).
3. Hybrid breakdown (cotton; 2nd generation hybrids are sterile).
Modes of speciation (based on how gene flow is interrupted):
•Allopatric Speciation: populations segregated by a geographical barrier; if enough
variation arises between the separate populations due to independent mutations, drift,
selection, etc., they will be reproductively isolated when the populations are reunited and
will result in adaptive radiation. Island species often follow this pattern.
•Sympatric Speciation: if a population separates into two reproductively isolated groups
without geographic isolation, as with doubled chromosome numbers (polyploidy) in some
plants or cichlid fishes in the same lake.
•Phylogeny: the evolutionary history of a species.
•Systematics: the study of biological diversity in an evolutionary context.
•The Fossil Record: the ordered array of fossils, within layers, or strata, of sedimentary
rock.
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•Paleontologists: scientists that study the fossil record.
•Comparing Dating Methods:
•Relative Dating: can tell the age of something as it relates to something else of to
the geological period.
•Absolute Dating: can tell the age of something more precisely:
•Radiometric dating; age using half-lives of radioactive isotopes.
•Isotopes each have particular half-life or time it takes for half of isotope to
decay and become nonradioactive.
•Carbon-14 (14C) used to date organic matter; half decays to 14N each 5,730
years; limited to about last 50,000 years.
•Half of potassium-40 (40K) decays to argon-40 (40Ar) each 1.3 million years;
estimates age of younger rocks.
•Uranium-238 decays to lead-207; estimates age of older rocks.
Phylogenetics:
•The tracing of evolutionary relationships (phylogenetic tree).
•Linnaeus. Binomial nomenclature. Genus, specific epithet. Homo sapiens.
•Taxon (taxa).
Phylogenetic Trees
•Cladistic Analysis:
taxonomic approach that classifies organisms according to the
order in time at which branches arise along a phylogenetic tree (cladogram).
•Clade: each evolutionary branch in a cladogram. Three types:
1. Monophyletic: single ancestor that gives rise to all species in that taxon and to no
species in any other taxon; legitimate cladogram.
2. Paraphyletic: lacks the common ancestor that would unite the species; does not
meet cladistic criterion.
3. Polyphyletic: members of a taxa are derived from 2 or more ancestral forms not
common to all members; does not meet cladistic criterion.
•Constructing a Cladogram:
•Sorting homology vs. analogy...
•Homology: likenesses attributed to common ancestry.
•Analogy: likenesses attributed to similar ecological roles and natural
selection.
•Convergent Evolution: species from different evolutionary branches that resemble one
another due to similar ecological roles.
Earth’s Evolutionary History
•Summary
o Earth’s history up to 543 million years ago (Almost 90% of the Earth’s history
occurred during this time.)
o The Earth forms.
o One-celled organisms (prokaryotes) arise.
o Eukaryotic cells evolve.
o The atmosphere becomes enriched in oxygen.
o Complex multicellular organisms, including the first animals evolved.
Precambrian Time: Hadean Time:
o Before 3.8 billion years ago.
o No geologic record.
o Sometime during the first 800 million or so years of its history, the surface of
the Earth changed from liquid to solid. Once solid rock formed on the Earth,
its geological history began.
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o No known life.
Precambrian Time Archaen Eon:
• billion to 2.5 billion years ago.
• Continent, atmosphere and oceans form.
• The atmosphere was made of methane, ammonia, hydrogen cyanide, carbon
dioxide, carbon monoxide, hydrogen sulfide, (most of which would be toxic to
most life on our planet today) and water vapor.
Precambrian Time: Archaen Eon:
o Earth cools enough for water to remain liquid.
o The primitive oceans were brown because of all the iron they contained.
o One theory of life on Earth is the “Primordial Soup Theory.” This states that
all the elements for life were available. Then, with the addition of electricity
from lightning, the first organic molecules were formed.
o In the early 1950’s two scientists, Stanley Miller and Harold Urey, created
an experiment meant to mimic early Earth environment and see if they could
create organic molecules (Oparin’s Hypothesis).
o Several amino acids were formed, but this experiment has been shown to be
set up atmospherically incorrectly.
o However, Miller did an experiment in 1995 that actually produced cytosine
and uracil, 2 of the bases found in RNA.
How did life Begin?
 The first organic compounds on Earth may have been synthesized near submerged
volcanoes and deep-sea vents.
 Some of the organic compounds from which the first life on Earth arose may have
come from space (extraterrestrial).
o Carbon compounds have been found in some of the meteorites that have
landed on Earth.
o The possibility that life is not restricted to Earth is becoming more accessible
to scientific testing.
 Abiotic Synthesis of Polymers: Small organic molecules polymerize when they are
concentrated on hot sand, clay, or rock.
 Protobionts – are aggregates of abiotically produced molecules surrounded by a
membrane or membrane-like structure. Laboratory experiments demonstrate that
protobionts could have formed spontaneously from abiotically produced organic
compounds. For example, small membrane-bounded droplets called liposomes can
form when lipids or other organic molecules are added to water.
 The RNA World: RNA molecules called ribozymes have been found to catalyze
many different reactions, including self-splicing; making complementary copies of
short stretches of their own sequence or other short pieces of RNA
 Early protobionts with self-replicating, catalytic RNA would have been more
effective at using resources and would have increased in number through natural
selection
Precambrian Time: Archaen Eon:
o One-celled organisms known as prokaryotes arise, the ancestors of presentday bacteria and cyanobacteria.
o All life during the more than one billion years of the Archaean was bacterial.
Precambrian Time: Proterozoic Eon:
o 2.5 billion to 543 million years ago.
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o Atmospheric oxygen increases as a byproduct of photosynthetic bacteria.
o While the increase in atmospheric oxygen was a global catastrophe as it
limited a lot of bacterial life, it allowed for the evolution of eukaryotes (cells
that posses a cellular nucleus, a nuclear membrane and membrane-bound
organelles).
o Evolution of eukaryotes:
Endosymbiotic Theory: eukaryotic cells arose
as heterotrophic prokaryotic cells engulfed smaller prokaryotes and
incorporated them into their cell.
o By the end of this eon multicellular organisms, including the animals, have
evolved.
Paleozoic Era

Summary
 543 million to 248 million years ago.
 Multi-cellular life explodes in the ocean at the beginning of this era, proliferates and
colonizes land with amphibians, then nearly dies out due to the largest mass
extinction ever (95% of species).
Paleozoic Era: Cambrian Period
 543 million to 490 million years ago.
 Explosion of Multicellular life.
 The Cambrian Period marks an important point in the history of life on earth; it is the
time when most of the major groups of animals first appear in the fossil record. This
event is sometimes called the "Cambrian Explosion", because of the relatively short
time over which this diversity of forms appears.
Paleozoic Era: Ordovician Period
 490 million to 443 million years ago.
 The Ordovician is best known for the presence of its diverse marine invertebrates.
 Primitive life on land (evidence that plants began invading the land), vertebrates
(primitive fish) in the ocean.
Paleozoic Era: Silurian Period
 443 million to 417 million years ago.
 The Silurian witnessed a relative stabilization of the earth's general climate.
 Coral reefs made their first appearance during this time, and the Silurian was also a
remarkable time in the evolution of fishes. Not only does this time period mark the
wide and rapid spread of jawless fish, but also the highly significant appearances of
both the first known freshwater fish as well as the first fish with jaws.
 It is also at this time that our first good evidence of life on land is preserved,
including relatives of spiders and centipedes, and also the earliest fossils of
vascular plants (indicating the diversity radiation of plants).
Paleozoic Era: Devonian Period
 417 million to 354 million years ago.
 Two major animal groups colonized the land. The first tetrapods (land-living
vertebrates) appeared, as did the first terrestrial arthropods.
Paleozoic Era: Carboniferous Period
 354 million to 290 million years ago.
o (Mississippian 354 to 323 million years ago).
o (Pennsylvanian 323 to 290 million years ago).
 First true reptiles appear.
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
The further exploitation of the land by certain tetrapods was due to the amniotic
egg, which allowed the ancestors of birds, mammals, and reptiles to reproduce on
land by preventing the desiccation of the embryo inside.
 Coal begins to form as plant materials did not decay as they became covered with
sea water and eventually exposed to tremendous pressure.
Paleozoic Era: Permian Period
 290 million to 248 million years ago.
 By the beginning of the Permian, the motion of the Earth's crustal plates had
brought much of the total land together, fused in a supercontinent known as
Pangea.
 90% of all organisms (especially marine invertebrates) go extinct at the end of this
era.
 Reptiles inherit the earth in what is termed “the age of the dinosaurs.”
Mesozoic Era
 Summary
•248 million to 65 million years ago.
•Dinosaurs establish their empire.
•Angiosperms evolve.
•“Sudden impact” brings it all crashing down.
Mesozoic Era: Triassic Period
 248 million to 206 million years ago.
 Following the largest extinction event in the history of life, it is a time when the
survivors of that event spread and re-colonized the Earth.
 Small dinosaurs, ichthyosaurs, plesiosaurs, first true mammals arise.
Mesozoic Era: Jurassic Period
 206 million to 144 million years ago.
 Huge dinosaurs, flying pterosaurs, oldest known birds.
 Pangea begins to break up.
Mesozoic Era: Cretaceous Period
 144 million to 65 million years ago.
 Global warming encourages spread of dinosaur domain.
 Sudden mass extinction, perhaps due to an asteroid impact, wipes out the
dinosaurs. 70% of all organisms die out.
 The end of the Cretaceous brought the end of many previously successful and
diverse groups of organisms. The Cretaceous was thus the time in which life as it
now exists on Earth came together.
Cenozoic Era
 Summary
 65 million years ago to present.
 The Cenozoic is sometimes called the Age of Mammals, because the largest
land animals have been mammals during that time. This is a misnomer for
several reasons. First, the history of mammals began long before the Cenozoic
began. Second, the diversity of life during the Cenozoic is far wider than
mammals. The Cenozoic could have been called the "Age of Flowering Plants"
or the "Age of Insects" or the "Age of Teleost Fish" or the "Age of Birds" just as
accurately.
 Humans make their entrance.
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Cenozoic Era: Tertiary Period - Paleocene Epoch
 65 million to 55 million years ago.
 Mammals inherit the Earth.
 Chief among the early mammals are marsupials, insectivores, lemuroids, creodonts
(the carnivorous stock ancestral to all cats and dogs), and primitive hoofed animals.
Cenozoic Era: Tertiary Period - Eocene Epoch
 55 million to 34 million years ago.
 Ancestral forms of the horse, rhinoceros, camel and other modern groups such as
bats, primates, and squirrel-like rodents appear simultaneously in Europe and North
America.
 Mammals adapt to marine life.
Cenozoic Era: Tertiary Period - Oligocene Epoch
 34 million to 24 million years ago.
 Rhinoceroses rank as the largest land mammals of any age.
 Making their debut on the stage of life: elephants, cats, dogs, monkeys and the
great apes.
 The appearance of many grasses - plants that would produce vast tracts of
grasslands in the following epoch.
Cenozoic Era: Tertiary Period - Miocene Epoch
 24 million to 5 million years ago.
 Global climate cools, fostering the reestablishment of the Antarctic ice sheet.
 Racoons and weasels make their first appearance.
 Large apes roam Africa and southern Europe.
Cenozoic Era: Tertiary Period - Pliocene Epoch
 5 million to 1.8 million years ago.
 Climate becomes cooler and drier.
 Mammals are well-established as the dominant terrestrial life form, and the rapid
evolution of one group, the primates, produces species considered direct ancestors
of modern humans.
Cenozoic Era: Quarternary Period - Pleistocene Epoch
 1.8 million to 10 thousand years ago.
 Most recent global ice age.
 Glacier ice spreads over more than one-fourth of Earth’s land surface.
 Modern humans arise and begin their migrations.
Cenozoic Era: Quarternary Period - Holocene Epoch
 10 thousand years ago to present.
 Global climate moderates.
 The last continental ice sheets rapidly retreat from Europe and North America.
 Sea levels rise.
 All of recorded human history occurs during the holocene – the Age of Man.
 Humanity has greatly influenced the Holocene environment; Habitat destruction,
pollution, and other factors are causing an ongoing mass extinction of plant and
animal species; according to some projections, 20% of all plant and animal species
on Earth will be extinct within the next 25 years.