Song – AP Bio
Unit 6: Evolution
Unit 3: EVOLUTION (chapters 19 & 21)
Pre-Darwinian Views
 Linnaeus (1707-1778) – Father of Taxonomy
 Developed system of binomial nomenclature (genus species)
 Hierarchy of taxonomic categories
 King Philip Came Over For Green Spaghetti
 Cuvier (1769-1832) – Founded Palentology – study of fossils (relics of organisms
preserved in rock)
 Cuvier believed that the appearance of new species in the fossil record was due
to sudden catastrophic events (floods, droughts, earthquakes) Catastrophism
 Hutton (1726-1797)—Gradualism: profound change is the cumulative product of slow
but continuous processes
 Lyell (1797-1875) – postulated Uniformitarianism: geological processes of the past are
the same ones operating in the present; incorporated Hutton’s gradualism
 Lamarck – (1744-1829)
 Believed that evolution responded to organism’s felt need to attain
perfection
 Proposed mechanism:
o Use and Disuse – Body organs used to cope with environment
become larger and stronger, those not used deteriorate (E.g.
Giraffe’s neck)
o Inheritance of Acquired Characteristics – acquired modifications
can be passed on to offspring
Darwin’s Mechanism of Evolution
 Charles Darwin (1809-1882)
 Naturalist aboard HMS Beagle (1831)
o Observed great diversity of organisms in South America and
Galapagos Islands
o Geographic distribution unique in some islands (e.g. 13 types
of finches with similar characteristics but different species)
 Wrote long essay on origin of species and natural selection
(1844)
 June 1858, Darwin received manuscript detailing Alfred
Wallace’s own theory of natural selection, similar to Darwin’s.
 1859, Darwin rushed publication of On The Origin of Species
 Darwin is considered the main author of the idea b/c he developed and supported
natural selection much more extensively.
Concepts of Darwinism
I. Descent with Modification
 All organisms related by descent from unknown common ancestor
II. Modification by Natural Selection (the mechanism of evolution)
1. Struggle for existence leads to differential reproductive success
 Influenced by Malthus’ essay on human populations (1789)
 Human population growth rate is faster than that of food supply
 Limitation of resources leads to struggle for survival
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Song – AP Bio
Unit 6: Evolution
2. Those individuals best-suited to the environment will survive and reproduce.
(“Survival and Reproduction of the Fittest”)
 Individual’s “fitness” is measured by survival and reproductive success.
3. Unequal ability to survive and reproduce leads to gradual change in population in
favor of beneficial traits  leads to gradual adaptation of the species to envt.
Example of Natural Selection: Industrial Melanism in the English Peppered Moth

Prior to Industrial Revolution, dark
peppered moths were rare (predation by
birds) – trees were covered by light lichen

Industrial Revolution killed lichen, revealing
the darker bark of trees

Dark colored moths became more
common

POPULATION evolved, not individual
moths (i.e. Black moths were always black,
their frequency changed in the population)
Signs of Evolution
1. Biogeography – geographic distribution of species
 E.g. Islands have many endemic species which are closely related to species on the
nearest mainland or neighboring island
o Even 2 islands with similar environments in different parts of the world are
populated by species that resemble those of mainland rather than resemble
each other
2. Fossil Record – Age and relationships among fossils provide evidence of evolution that is
consistent with biochemical processes and molecular biology (e.g.
prokaryotes have the most primitive genetic and biochemical processes;
they are also the oldest fossils)
3. Taxonomy – Linnaeus taxonomic scheme shows how different taxonomic levels are
related
4. Comparative Anatomy – comparing anatomical structures of different species give clues
to how closely related species are


Homologous structures –structures are similar but
have different functions. This suggests that
evolution has modified an existing structure for a
different function. (e.g. human forearms, bat
wings,cat foreleg, and whale flippers have same
skeletal elements); similar structures due to
common ancestry
http://www.bio.miami.edu/dana/160/160S
13_5.html
Analogous structures – structures are different but
functions are similar;do not have a recent common
ancestor (e.g. humming bird wing is analogous to
humming moth wing)
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fs.fed.us
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Song – AP Bio
Unit 6: Evolution
 Vestigial organs – structures of little or no use to organism
(e.g. whale’s pelvic bone)
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5. Comparative Embryology – Closely related organisms go through similar stages in their
embryonic development
ucsd.tv
6. Molecular Biology
 The closer two species are taxonomically, the higher the percentage of common DNA
 supports common descent
How Populations Evolve
The Modern Evolutionary Synthesis
 A Genetic Basis of Variation and Natural Selection
o Variation is important for evolution (populations cannot change/evolve without it)
o Variation helps ensure the survival of a population (e.g. dramatic climate
change)
o Sources of variation:
 Crossing over – homologous chromosomes exchange pieces of DNA
 Independent Assortment – homologous chromosomes align &
separate independently of other pairs
 Gene Flow – individuals (or gametes) moving between populations
 Mutations – very rare, usually harmful or neutral



This theory emphasizes the importance of populations as the units of evolution, the
essential role of natural selection, and gradualism
In the 1930’s Population Genetics was born, providing a mathematical theory for small
scale evolution (microevolution)
A comprehensive theory of evolution was worked out in the 1940’s called Modern
Synthesis or Neo-Darwinism
The Genetics of Populations
 Important definitions:
 Population – localized group of organisms which belong to the same species
 Species – groups of actually or potentially interbreeding natural populations, which are
reproductively isolated from other such groups
 Gene Pool – the total aggregate of genes in a population at any one time. Within a
gene pool, there are often two or more alleles for a gene. The proportion of these
alleles in the gene pool is called the allele frequency.
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Song – AP Bio
Unit 6: Evolution

Population Genetics – involves the study of changes in allele frequencies
o Microevolution – change in the population’s relative allelic frequencies in
small scale evolutionary change
 The Hardy-Weinberg Theorem
 The frequencies of alleles in the gene pool will remain constant unless acted upon by
other agents
o e.g. the relative frequency of sickle cell anemia should stay the same every
generation unless some force, like natural selection acts to change it.
 Describes the genetics of NON-evolving populations
 The Hardy-Weinberg equation can be used to calculate allele and genotype
frequencies
The Hardy Weinberg Equation

Conditions for Hardy-Weinberg equilibrium (constant, non-evolving populations):
1. Very large population size
2. Isolation from other populations. There is no migration of individuals into or out
of the population
3. No mutations
4. Random mating
5. No natural selection. All genotypes are equal in reproductive success.
Differential reproductive success can alter gene frequencies.

Hardy Weinberg model provides a reference point with which to compare the
frequencies of alleles and genotypes of natural populations whose gene pools may be
changing (microevolution)
o If the population does NOT match expected Hardy-Weinberg frequencies,
then the population is evolving!
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Song – AP Bio
Unit 6: Evolution
Causes of Microevolution
In real populations, several factors can upset Hardy-Weinberg equilibrium and cause
microevolutionary change:
1. Genetic Drift
2. Gene Flow
3. Mutation
4. Nonrandom mating
5. Natural Selection
1. Genetic Drift – changes in the gene pool of a small population due to chance
 If a population is small, catastrophic events have a greater impact on gene frequencies
o i.e. Surviving/Existing gene pool may not accurately reflect overall
population

Reduces overall genetic variability in a population

Two situations for genetic drift to be important:
1) Bottleneck Effect – genetic drift which
results in drastic reduction in
population size
 E.g. 1890’s Population of
northern elephant seals was
reduced to just 20 individuals
by hunters. By chance, some
individuals survive (bottle neck). The small surviving
population is unlikely to represent the genetic makeup of
the original population.
 Reduces overall genetic variability since some alleles may
be entirely absent.
2) Founder Effect – genetic drift which results when a few individuals
colonize a new habitat
 All descendants of those few individuals will be genetically
similar
 E.g. Galapagos finches are all descended from a few S.
American mainland finches
2. Gene Flow – the migration of fertile individuals, or the transfer of gametes between
populations
 Gene flow tends to reduce between-population differences which have accumulated by
natural selection or genetic drift
 Extensive gene flow can eventually group neighboring populations into a single
population
3. Mutation
 Mutation rates – one mutations per 105 or 106 gametes
 A single mutation takes a very long time to affect gene pool of a large population
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Song – AP Bio
Unit 6: Evolution
4. Nonrandom Mating
 Increases the number of homozygous and decreases heterozygous loci in a
population, but does not in itself alter frequencies of alleles in a population’s gene pool.
o
i.e. p2, q2, and 2pq change but not p and q, only the way they are
combined.

Two kinds of nonrandom mating:
1) Inbreeding – mating with closely related individuals
 Self-fertilization (common in plants) is most extreme
example of inbreeding
2) Assortative Mating – individuals mate with partners that are like
themselves in certain phenotypic characters
5. Natural Selection – due to selection, alleles are passed on to the next generation in
disproportionate numbers relative to their frequencies in the present generation.

Variability in a population makes it possible for natural selection to occur.

Natural selection is the basis of adaptive changes in evolution. As the allele
frequency changes, the population as a whole becomes better adapted.
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Song – AP Bio
Unit 6: Evolution
How Genetic Variation is Preserved
Natural selection tends to produce genetic uniformity in a population by eliminating
unfavorable genotypes. This tendency is opposed by several mechanisms that preserve or
restore variation:
1. Diploidy- hides much genetic variation from selection by the presence of recessive
alleles in heterozygotes

The more rare the recessive allele, the greater its protection by heterozygosity.
I.e., a greater proportion are hidden in heterozygotes by a dominant allele

This type of protection maintains a large pool of alleles which may be beneficial
if conditions change
2. Balanced Polymorphism – The ability of natural selection to maintain diversity in a
population. Results from:
1.) Heterozygote advantage (Hybrid Vigor) – heterozygotes have greater
reproductive success

E.g. Heterozygotes for sickle cell anemia are resistant to malaria, much
higher frequency of heterozygotes in Africa where malaria is prevalent.
2.) Balancing selection – patchy environments where different phenotypes are
favored in different subregions of a populations geographic range

E.g. Beak sizes in finches (small and large favored)
3.) Frequency-dependent selection – reproductive success of any one morph
declines if that phenotype becomes too
common in the population

E.g. Batesian mimicry – butterfly has
certain type of protective coloration that
resembles noxious butterfly species. If
there are too many of the mimics, birds
would encounter more good-tasting
mimics and would not associate
particular color patter with bad taste.
Modes of Natural Selection
The frequency of heritable characteristics in a population may be affected in one of 3 different
ways by natural selection, depending on which phenotypes are favored:
1. Stabilizing Selection – occurs when extreme phenotypes are eliminated and the
intermediate phenotype is favored.

E.g. Human birth weights are in the 3-4 kg range. Extreme birth weights
have greater infant mortality.
2. Directional Selection – occurs when extreme
phenotype is favored; the distribution shifts that
direction

E.g. A shift of dark-colored
peppered moths from light-colored
correlated with increasing pollution
3. Diversifying (Disruptive Selection) – occurs
when extreme phenotypes are favored and can
lead to more than one distinct form

E.g. British snails: In forest areas,
predators feed on snails with light
bands, in low-vegetation areas,
feed on snails with dark shell that
lack light bands.
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Song – AP Bio
Unit 6: Evolution
Sexual Selection
Sexual Dimorphism – distinction between the secondary sexual
characteristics of males and females. Usually the male is the “showier”
sex. E.g. peacock
 Darwin viewed sexual selection as a separate selection process
leading to sexual dimorphism. Usually secondary sexual
characteristics have not adaptive advantage other than attracting
mates.
Speciation
Speciation – splitting of one species into two or more species or the transformation of one
species into a new species over time; speciation is the final result of changes in gene pool
allele and genotypic frequencies  leads to Macroevolution: origin of new taxonomic groups
What is a species?
 Morphospecies – species defined by anatomical features (Linnaeus’ hierarchical
classification – King Philip Came Over For Green Spaghetti)
 Biological species – members are reproductively isolated from all other such groups
(alternative definition formulated by Mayr 1942)
Mechanisms of Reproductive Isolation
Reproductive barrier – any factor that impedes two species from producing fertile hybrids,
thus contributing to reproductive isolation
1. Prezygotic Barriers – act to prevent mating or fertilization
1.) Ecological Isolation – Species live in different habitats and rarely come into
contact.
 E.g. Two garter snake species live in same geographical area, but
one is terrestrial and the other is aquatic.
2.) Temporal Isolation – Species breed at different times of day, season, or
year
 E.g. Brown trout and rainbow trout live in same streams but brown
trout breed in the fall and rainbow trout breed in spring.
3.) Behavioral Isolation – Many species use special signals to attract mates.
These tend to be highly specific for a particular species
 E.g. Males of different species of firefly use different flashing
patterns to attract females.
4.) Mechanical Isolation – Structural differences in genitalia or flowers prevent
copulation or pollen transfer
 E.g. Many plant species have flowers which have evolved shapes
that are specific to the insects or birds that pollinate them.
5.) Gametic Isolation – Even when gametes of different species meet, crossfertilization rarely occurs. Molecules on the outside of the eggs and sperm
may function in gamete recognition.
 E.g. Different species of mussels release their eggs and sperm
into the water column for external fertilization. Cross-fertilization
does not occur.
2. Postzygotic Barriers – act after fertilization to prevent the hybrid zygote from developing
into a fertile adult
1.) Hybrid Inviability – Hybrid zygotes fail to develop or reach sexual maturity
 E.g. Frogs of genus Rana occasionally form hybrids, but the zygotes
cannot complete development.
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Song – AP Bio
Unit 6: Evolution
2.)
3.)
Hybrid Sterility – Hybrid zygotes develop but the adults are sterile
 E.g. The mule is a cross between a horse and a donkey. Mules
cannot breed with either species.
Hybrid Breakdown – Hybrid adults are fertile, but the offspring of hybrids are
feeble or sterile.
 E.g. Certain cotton species can form hybrids, but the offspring of
hybrids die as seeds or grow into defective plants.
Modes of Speciation
A new species is formed when a population becomes genetically isolated from other
populations of the parent species. The mechanisms of mutation, genetic drift, natural
selection, etc. cause the isolated population to evolve differences from the parent species.
In geographical terms, the initial genetic isolation can occur in two ways:
1. Allopatric Speciation

Geographical processes may isolate a population from its parent species. A
mountain range may emerge to separate two valleys; a lake may subside to
form several smaller lakes; etc.

E.g. Death Valley California: Isolated springs that used to be connected.
Each spring harbors its own species of pupfish.

Island chains present a diversity of new habitats which are
isolated from one another. Allopatric speciation can occur
repeatedly leading to the formation of many new species. This
is called adaptive radiation.
 E.g. Over 500 species of Drosophila have evolved on
Hawaiian islands
2. Sympatric Speciation

A new species arises within the range of the parent species.
Reproductive isolation evolves without geographic isolation.

Sympatric speciation is likely to involve a special genetic
mechanism. In plants, nondisjunction of all the chromosomes
can produce polyploidy.
Rate of Speciation
There are two models for how evolution proceeds. One model says changes
are steady and gradual; the other says speciation occurs relatively quickly
(punctuated), separated by long periods of no change (stasis).
1. Gradualism – Morphological changes occur at a slow and steady pace.
New speciations form at the same pace.
2. Punctuated Equilibrium – Niles Eldredge and Stephen Jay Gould, 1972.
Speciation occurs relatively quickly (few thousand years). Between
speciations are long periods of little change.
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Song – AP Bio
Unit 6: Evolution

Punctuated equilibrium helps explain the lack of intermediate forms in the
fossil record.
Extinction – death of a species
Extinction is vital to the process of evolution  a niche is left unoccupied that can be filled by
another species. E.g. Mass extinction of dinosaurs left many niches open for mammals to
evolve into.
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