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Chapter 18 & 19
Evolutionary Change in
Populations & Speciation
Ch. 18: Population Genetics
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Evolution occurs in populations, not
individuals
Evolutionary change is inherited from one
generation to the next.
Darwin recognized that evolution occurs in
populations, but did not understand how
traits are passed on
Populations
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Population = all the individuals of the same
species that live in a particular place at the
same time
Members of a population vary in many
traits.
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Ex. p.390
Some variation is due to environment and
some is due to heredity.
Population Genetics
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The study of genetic variability within a population
and of the forces that act on it.
We will discuss the 5 factors responsible for
evolutionary change:
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Nonrandom mating
Mutation
Genetic Drift
Gene Flow
Natural Selection
Then, we consider genetic variation as the raw
material for evolution.
Calculating Frequencies
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Each population has a gene pool:
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All the alleles for all the genes present in the
population
Genotype frequency: the proportion of a
particular genotype in the population
Phenotype frequency: the proportion of a
particular phenotype in a population
Allele frequency: the proportion of a
specific allele in a population
Hardy-Weinberg Principle
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This law states an equilibrium of allele
frequencies in a gene pool (using a formula
p2 + 2pq + q2=1) remains in effect in each
succeeding generation of a sexually
reproducing population if five conditions are
met.
5 conditions:
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No mutation: no allelic changes occur.
No gene flow: migration of alleles into or out of
the population does not occur.
Random mating: individuals pair by chance and
not according to their genotypes or phenotypes.
No genetic drift: the population is large so
changes in allele frequencies due to chance are
insignificant.
No selection: no selective force favors one
genotype over another.
Hardy-Weinberg Principle
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These conditions are rarely met, so allele
frequencies in the gene pool of a population
do change from one generation to the next,
resulting in evolution.
Hardy-Weinberg Principle
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p2 + 2pq + q2 = 1
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p2 = Frequency of AA
2pq = Frequency of Aa
q2 = Frequency of aa
1 = all the individuals in a population
Always begin by determining the
frequency of the homozygous recessive
genotype.
Microevolution
Evolution within a population
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Small changes over a few generations
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Result from 5 microevolutionary
processes:
1.
2.
3.
4.
5.
nonrandom mating
mutation
genetic drift
gene flow
natural selection
Nonrandom Mating
1.
Random mating involves individuals pairing by
chance, not according to genotype or
phenotype.
2. Nonrandom mating involves inbreeding and
assortative mating.
3. Inbreeding is mating between relatives to a
greater extent than by chance.
a. However, inbreeding decreases the
proportion of heterozygotes.
b. In human populations, inbreeding increases
the frequency of recessive abnormalities.
Nonrandom Mating
4. Assortative mating occurs when
individuals mate with those that have
the same phenotype.
5. Sexual selection occurs when males
compete for the right to reproduce and
the female selects males of a particular
phenotype. (guppies, lions)
Mutation
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Many traits in organisms are polymorphic, i.e., two
or more distinct phenotypes are present in the
population due to mutated genes.
Analysis of Drosophila enzymes indicates they
have multiple alleles at least at 30% of their gene
loci.
In humans, the ABO blood types are an example
of polymorphism.
Mutations can be beneficial, neutral, or harmful; a
seemingly harmful mutation that requires Daphnia
to live at higher temperatures becomes
advantageous when the environment changes.
Genetic Drift
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Genetic drift refers to changes in allele
frequencies of a gene pool due to chance,
more often in small populations
Genetic drift occurs when founders start a
new population, or after a genetic bottleneck
with interbreeding.
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The bottleneck effect prevents most
genotypes from participating in production of
the next generation.
1) The bottleneck effect is caused by a severe
reduction in population size due to a natural
disaster, predation, or habitat reduction.
2) The bottleneck effect causes a severe
reduction in the total genetic diversity of the
original gene pool.
3) The cheetah bottleneck causes relative
infertility because alleles were lost due to
intense inbreeding when populations were
reduced in earlier times.
Genetic Drift
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The founder effect is an example of genetic drift
where rare alleles or combinations occur in
higher frequency in a population isolated from
the general population.
1) This is due to founding individuals containing
a fraction of total genetic diversity of the original
population.
2) Which particular alleles are carried by the
founders is dictated by chance alone.
3) As an example, dwarfism is much higher in a
Pennsylvania Amish community due to a few
German founders.
Gene Flow
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Gene flow (gene migration) is the movement of
alleles among populations by migration of
breeding individuals.
Gene flow can increase variation within a
population by introducing novel alleles
Continued gene flow decreases diversity among
populations, causing gene pools to become
similar.
Gene flow among populations can prevent
speciation from occurring.
Natural Selection
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Natural selection requires
a. variation (i.e., the members of a population differ from
one another),
b. inheritance (i.e., many of the differences between
individuals in a population are heritable genetic
differences),
c. differential adaptedness (i.e., some differences affect
how well an organism is adapted to its environment), and
d. differential reproduction (i.e., better adapted
individuals are more likely to reproduce).
Fitness is the extent to which an individual contributes
fertile offspring to the next generation.
Relative fitness compares the fitness of one phenotype to
another.
Types of Selection
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Directional selection occurs when an extreme
phenotype is favored; the distribution curve shifts that
direction.
a. A shift to dark-colored peppered moths from lightcolored correlated with increasing pollution.
b. Drug-resistant strains of bacteria are a serious health
threat and represent this type of selection.
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c. Increases in insecticide-resistant
mosquitoes and resistance of the malaria
protozoan Plasmodium to medications are
also examples of directional selection.
d. The gradual increase in the size of the
modern horse, Equus, correlates with a
change in the environment from forest-like
conditions to grassland conditions.
Types of Selection
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Stabilizing selection occurs when extreme
phenotypes are eliminated and the intermediate
phenotype is favored.
a. The average number of eggs laid by Swiss
starlings is four or five.
b. If the female lays more or less than this
number, fewer survive.
c. Genes determining the physiology of yolk
production and behavior are involved in clutch
size.
Types of Selection
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Disruptive selection occurs when extreme
phenotypes are favored and can lead to more
than one distinct form.
a. British snails (Cepaea nemoralis) vary
because a wide range causes natural
selection to vary.
b. In forest areas, thrushes feed on snails
with light bands.
c. In low-vegetation areas, thrushes feed on
snails with dark shells that lack light bands.
Chapter 19
Macroevolution and Speciation
Macroevolution
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Macroevolution refers to any evolutionary
change at or above the species level.
Speciation is the splitting of one species
into:
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two or more species or
the transformation of one species into a new
species over time
What is a species?
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Linnaeus separated species based on morphology,
i.e., their traits differed
Darwin saw that similar species are related by
common descent.
Ernst Mayr (1942) developed the biological species
concept: a species is a group of interbreeding
populations that are reproductively isolated from
other such groups.
*The biological definition of a species - members
of one species interbreed and have a shared gene
pool, and each species is reproductively isolated
from every other species.
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Species are based on interfertility, not
physical similarity.
For example, the eastern and western meadowlarks may
have similar shapes and coloration, but differences in
song prevent interbreeding between the two species.
In contrast, humans have
considerable diversity,
but we all belong to the
same species because of
our capacity to interbreed.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 24.2
Speciation
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The key to speciation is reproductive isolation.
Two types:
 Anagenesis – small, progressive evolutionary changes over
long periods
 Cladogenesis – 2 or more populations of an ancestral
species split and diverge, also called divergent speciation
Anagenesis
Anagenesis
Cladogenesis
Allopatric Speciation
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Allopatric speciation occurs when new species
result from populations being separated by a
geographical barrier that prevents their members
from reproducing with each other.
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While geographically isolated, variations accumulate until
the populations are reproductively isolated.
Examples: rivers shifting their courses, glaciers melting,
mountain ranges forming, large lakes diminishing into
several smaller, geographically separated pools
Ex. Kaibab squirrel – p.413
Sympatric Speciation
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Sympatric speciation would occur when
members of a single population develop a
genetic difference (e.g., chromosome
number) that prevents them from
reproducing with the parent type.
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The main example of sympatric speciation is
in plants.
Failure to reduce chromosome number
produces polyploid plants that reproduce
successfully only with polyploids.
Evolutionary Change Can
Occur Rapidly or Gradually
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Punctuated Equilibrium – history of a species,
long periods of stasis (little or no evolutionary
change) are punctuated, or interrupted, by short
periods of rapid speciation
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i.e. evolution proceeds in “spurts”
Gradualism – evolution proceeds continuously
over long periods
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Difficult to observe because fossil record is not always
complete
Maintains that populations evolve slowly by accumulating
adaptations due to selective pressures
Adaptive Radiation
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The evolutionary diversification of many
related species from one or a few
ancestral species in a short time period.
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Ex. Hawaiian honeycreepers p.422
Ex. Finches of Galapagos Islands
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13 species from 1 founder mainland species
Extinction
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End of a lineage – occurs when the last
individual of a species dies
Permanent
Only 1 species is alive today for every
2000 that have become extinct
Chapter 18 & 19 Review
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Chapter 18: p. 404-405
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Post-Test: 1, 4, 5, 7, 9-15
Review: 1, 2, 6
Chapter 19: p. 426
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Post-Test: 1, 7, 9, 11, 15
Review: 4, 6, 9