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CHAPTER 22: Descent With Modification
-In 1859, Charles Darwin published On the Origin of Species by Means of Natural Selection and the study of Biology
would never be the same.
Two key points were made by Darwin:
1. Evolution: Life on Earth has been transformed from its earliest forms to the vast diversity that characterizes it
today. Modern species are descendants of ancestral species.
2. Natural Selection: A population of organisms can change over time as a result of individuals with certain
heritable traits leaving more offspring than other individuals. “Survival of the Fittest”
-Evolutionary Adaptations: An accumulation of inherited characteristics that enhance an organism’s ability to survive
and reproduce in a particular environment.
-Evolution: Change over time in the genetic composition of a population. Eventually, a population may accumulate
enough change that it constitutes a new species.
The Darwinian revolution challenged traditional views of a young Earth inhabited by unchanging species
-The origin of species challenged prevailing scientific views and shook the roots of Western culture.
-Before Darwin (See Figure 22.2, page 439):
 384-322 B.C. = Aristotle: viewed life as unchanging
 Early 1700’s = Carolus Linnaeus: classified life’s diversity for the greater Glory of God. Founder of
 Late 1700’s-early 1800’s = The study of fossils (remains or traces of organisms from the past) was largely
developed by Georges Cuvier. Cuvier looked at the strata of fossils and developed paleontology, the study of
fossils. Cuvier advocated for catastrophism, which speculated that each boundary between strata represents a
catastrophe, such as a flood or drought. (See Figure 22.3, page 440)
 1795 = George Hutton proposed gradualism, the idea that profound change can take place through the
cumulative effect of slow but continuous processes.
 Early 1800’s = Charles Lyell created the theory of uniformitarianism and proposed that the same geologic
processes are operating today as in the past, and at the same rate. He wrote a book called Principles of Geology
which would greatly influence Darwin in the future.
 1809 = Jean Baptiste de Lamarck published his theory of evolution. His work is notable because he
accounted for the known fossil record and the diversity of life, he recognized the great age of the Earth, and he
emphasized that organisms adapt to the environment and can ‘move’ along the Linnaen/Aristotle ladder of life.
He also believed in “use and disuse”, the idea that the parts of the body that an organism uses the most will get
larger and stronger while those that are not used will deteriorate. Lamarck believed that organisms have an
innate drive to become more complex.
In The Origin of Species, Darwin proposed that species change through natural selection
-Charles Darwin (1809-1882) had an interest in nature. He studied medicine (and quit) and finally earned is B.A. in
theology from Cambridge. He was encouraged by his botany professor to continue his education on the HMS
Beagle. (See page 442)
-Darwin set sail on the Beagle in 1831 and gathered detailed information on the flora and fauna of the world. He
specifically noticed the differences and similarities of Finches on the Galapagos Islands.
-Darwin gathered data and after many years, he wrote an essay on the origin of species. Alfred Wallace was also
working on a theory of natural selection and his work was published in 1958. Darwin revised and compiled
substantial amounts of information and published his work in 1959. Darwin’s well-supported book changed
the face of Biology.
-In his book, Darwin proposed two main ideas:
1. Evolution explains life’s unity and diversity
2. Natural selection is a cause of adaptive evolution
-Descent with modification: Darwin’s phase for evolution which states that all organisms are descendants of one
ancestor. Over millions of years, organisms adapted to their environment and evolved into different species.
(The history of life is like a tree, with a trunk, branches and new twigs) See Figure 22.7, page 444.
-Natural Selection & Adaptation: When the environment favors a specific trait, that trait, and the organisms that
possess it, will be ‘selected’ and survive. The more adapted an organism is to its environment, the more likely
it will produce offspring and pass on its genetic content.
Darwin Based his conclusions on the following Observations:
1. All species have a great potential to reproduce, and populations would increase exponentially if all individuals
went on to reproduce.
2. Populations tend to remain stable in size
3. Environmental resources are limited (there is not enough to go around)
Result: There is a struggle to exist between individuals in a population
4. Individuals are different because of traits. . .no two individuals are the same.
5. This variation is usually inherited.
Results: Individuals who have inherited characteristics that best fit the environment will leave behind more
offspring. This unequal survival will lead to the gradual change in populations over time.
-Darwin also used artificial selection (humans modifying other species by selecting and breeding individuals that
possess a desired trait) to support his theory (Think livestock, gardens, etc).
-Quirky stuff:
1. Evolution can only be measured as changes in a population over successive generations, not by changes in
2. Natural selection can amplify or diminish only inherited variations. Adaptations an organism acquires by its own
actions are different from those that evolve as a result of natural selection.
3. Environmental factors vary from place to place and time to time. A trait that is favorable in one situation can be
detrimental in another. Natural selection is always operating, but which traits are favored depends on the
Darwin’s theory explains a wide range of observations
Recent Examples of Research that support Natural Selection
1. Differential Predation and Guppy Populations: The size of guppies when they reach sexual maturity depends
upon the predator most active on that population. The smaller the predator, the smaller the guppy. See Figure
22.12, pg. 447.
2. The Evolution of Drug-Resistant HIV: Researchers have developed numerous drugs to combat HIV, but using
these medications selects for viruses resistant to the drugs. This is a medical nightmare! Figure 22.13, page
Evidence for evolution comes from the following areas:
1. Biogeography: The geographical distribution of species. Some similar species can be found in distinctly
different regions, while very distinct species can be found in similar regions. Figure 22.17, page 450.
2. The Fossil Record: The fossil record provides evidence of ‘links’ between older fossils and modern species.
3. Comparative Anatomy: Using similar anatomical structures in very different ways. Homologous structures:
Anatomically similar characteristics that result from common ancestry. Fig. 22.14, pg. 449.
4. Comparative Embryology: Closely related organisms go through similar stages in their embryonic
development (Ex: Gills). As the embryos grow, they become more distinct, so looking at the fully developed
individual does not reflect the similarity in embryo development. Figure 22.15, page 449.
5. Molecular Biology: Similarity in species based on their DNA and proteins. Figure 22.16, page 450.
CHAPTER 23: The Evolution of Populations
-Microevolution: The change in the genetic makeup of a population from generation to generation
-Macroevolution: The origin of new taxonomic groups over extended periods of time.
Population genetics provides a foundation for studying evolution
-Population Genetics: The study of how populations change genetically over time. It is based upon the work of
Mendel and Darwin
-Modern Synthesis: A comprehensive theory of evolution that came to be in the 1940’s. It included
aspects of paleontology, taxonomy, genetics, biogeography, and population genetics.
-Population: A localized group of individuals belonging to the same species. They must be capable of interbreeding
and producing fertile offspring.
-Species: A group of populations whose individuals have the potential to interbreed and produce fertile offspring in
nature. See Figure 23.3, page 455
-Gene Pool: The total collection of genes in a population at any one time.
-Genetic Structure: A population’s frequencies of alleles and genotypes. If a population only has one allele, the allele
is said to be ‘fixed’ in the gene pool.
-Hardy-Weinberg Theorem: States that the frequency of alleles and genotypes in a population’s gene pool remain
constant from generation to generation, provided that only Mendelian segregation and recombination are at
Hardy-Weinberg equation (See page 457):
p = frequency of dominant allele
q = frequency of recessive allele
p + q = 1 (The frequency of dominant and recessive alleles must account for 100% of the alleles)
p2 = Frequency of a homozygous dominant genotype
2pq = Frequency of heterozygous genotype
q2 = Frequency of homozygous recessive genotype.
SO: p2 + 2pq + q2 = 1 (100% of all genotypes)
What if a population has a different frequency of alleles then expected from the HWE? Then we experience
The five conditions of microevolution/Hardy Weinberg Equilibrium:
1. Extremely Large population Size: The smaller the population, the greater the role played by chance
fluctuations in allele frequencies from one generation to the next, known as genetic drift.
2. No Gene flow: The transfer of alleles between populations (migration of fertile individuals) can alter allele
3. No Mutations: By introducing or removing genes from chromosomes or by changing one allele into another,
mutations can immediately modify the gene pool.
4. Random Mating: If individuals preferentially choose mates with certain genotypes, including close relatives
(inbreeding) and assortative mating (individuals select partners that are like themselves in certain phenotypic
characters) random mixing of gametes does not occur.
5. No Natural Selection: Differential survival and reproductive success of individuals carrying different
genotypes will alter allele frequencies (See Chapter 22)
Mutation and sexual recombination produce the variation that makes evolution possible
A flashback
 MUTATION: Remember that mutations (changes in the nucleotide sequence of DNA) are random and on very
rare occasions they can be both beneficial and inherited. There are point mutations (usually harmless) and any
mutations that delete, disrupt, or rearrange many loci at once are most certain to be harmful (duplication, etc)
 SEXUAL RECOMBINATION: Provides nearly all of the genetic differences in sexually reproducing
populations. Crossing over, independent assortment, etc. create new combinations of alleles every generation.
Nearly all phenotypic variations based on genetic differences result from reombinational shuffling of the
existing alleles in the gene pool.
Natural selection, genetic drift, and gene flow can alter a population’s genetic composition
**These are the three most important factors that alter allele frequencies and cause most evolutionary change
1. Natural Selection results in alleles being passed to the next generation in proportions different from their relative
frequencies in the present
2. Genetic Drift results in skewed allelic frequencies. The smaller the sample, the greater the chance of deviation from
the predicted result.
A. The Bottleneck Effect: Occurs when the genetic makeup of the surviving population does not represent
the makeup of the original generation. Usually this is found after some time of natural disaster (fire, flood,
etc). The survivors pass through a restrictive “bottleneck” See Figure 23.8, page 461
The Founder Effect: Genetic drift in a new colony. Usually occurs when only a few individuals colonize
an area. The smaller group may establish a new population whose gene pool is not reflective of the source
3. Gene Flow results when genetic additions to and/or subtractions from a population occurs, resulting from the
movement of fertile individuals or gametes. This reduces differences between populations, and can even cause
neighboring populations to mold into one single gene pool.
Natural selection is the primary mechanism of adaptive evolution
Of all the factors that can change a gene pool, only natural selection is likely to adapt a population to its environment.
Genetic Variation:
-Variation within populations:
1. Quantitative characters: traits that vary along a continuum within a population. Results from Polygenic
inheritance (the effect of two or more genes on a single phenotypic character, like height)
2. Discrete characters: traits that are determined by a single gene locus with different alleles (Either-or).
-Phenotypic Polymorphism: the existence of polymorphic characters. Polymorphic characters are those that have
two or more distinct morphs (contrasting forms) of a characteristic that are represented in noticeable amounts.
Blood type is an example of a polymorphic character in humans.
Measuring Genetic Variation: the number of polymorphisms in a population can be determined by the amount of
heterozygosity at both the level of whole genes and the molecular level of DNA
-Variation between populations:
-Geographical variation: differences in gene pools of separate populations or population subgroups. (pg. 463).
-Cline: A graded change in some trait along a geographic axis (Fig. 23.11, pg 464)
A Closer Look at Natural Selection
-Fitness (Darwinian fitness): The contribution an individual makes to the gene pool of the next generation, relative to
the contributions of other individuals.
-Relative fitness: The contribution of a genotype to the next generation compared to the contributions of
alternative genotypes for the same locus. It is 0 for a sterile plant or animal and 1 for the most reproductively
Modes of selection: Fig. 23.12, page 465
1. Stabilizing selection: Acts against extreme phenotypes and favors the more common intermediate variants.
2. Directional selection: Occurs during periods of change or migrations. It shifts the frequency curve for variations in
some phenotypic character in one direction or the other by favoring one extreme character.
3. Disruptive selection: Favors individuals on both extremes over intermediate phenotypes.
4. Sexual selection leads to differences between the sexes. It is selection based on variation in secondary sex
characteristics, leading to the enhancement of Sexual dimorphism (The distinction between the secondary sex
characteristics of males and females). Traits such as size, plumage, manes, antlers, etc. are attributes that may
be used for reproductive competition. Figure 23.15, page 468
 Intrasexual selection occurs when individuals of the same sex are directly competing for mates of the
opposite sex. This is most common in males.
 Intersexual selection “mate choice” occurs when individuals of one sex (usually females) are choosy in
selecting their mates from the other sex.
The Preservation of Genetic Variation
1. Diploidy: The diploid nature of eukaryotes can hide genetic variation (recessive alleles) in the heterozygous
2. Balanced Selection: The ability of natural selection to maintain diversity in a population. Occurs by:
A. heterozygote advantage (Heterozygotes have a greater chance to survive than the homozygotes, so two or
more alleles will be maintained). Ex. Sickle-Cell anemia and hybrid vigor in crops.
B. Patchy environment: Places where natural selection favors different phenotypes in subregions of a
population’s existence.
C. Frequency-dependent selection: The reproductive success of any one morph declines if that phenotypic
form becomes too common in the population.
3. Neutral Variation: Characteristics (like fingerprints) that seem to have no selective advantage. They have little
or no impact on reproductive success, and thus natural selection does not affect these alleles. These genes
provide an enormous pool of material for natural selection in the future.
There are two distinct theories for macroevolution:
1. Gradualism: Supported by Darwin, it is a theory which states that evolution occurs
as a result of the gradual accumulation of small changes over time.
2. Punctuated Equilibrium: States that evolutionary history consists of geologically long periods of time with
little or no evolution that are interrupted by short periods of rapid evolution.
Natural Selection Cannot Fashion Perfect Organisms:
1. Evolution is limited by historical constraints: Ancestral anatomy is not always lost
2. Adaptations are often compromises: Each organism must do different things
3. Chance and Natural Selection Interact: Change events affect the history of populations
4. Selection can edit only existing variations: New alleles do not arise on demand.