Ch 11
The Evolution of Populations
Objectives
• 7B analyze and evaluate scientific explanations concerning any data of sudden
appearance, stasis, and sequential nature of groups in the fossil record;
• 7C analyze and evaluate how natural selection produces change in populations,
not individuals;
• 7D analyze and evaluate how the elements of natural selection, including
inherited variation, the potential of a population to produce more offspring than
can survive, and a finite supply of environmental resources, result in differential
reproductive success
• 7E analyze and evaluate the relationship of natural selection to adaptation and
to the development of diversity in and among species;
• 7F analyze and evaluate the effects of other evolutionary mechanisms, including
genetic drift, gene flow, mutation, and recombination
Section 11.1 Genetic Variation within Populations
1. Genetic variation increases the chances of some individuals
surviving
a. In a population there are a wide range of phenotypes (a trait
produced by one or more genes)
b. Natural selection acts on different phenotypes in a population,
and phenotype is determined by genotype
c. More genetic diversity = more phenotypic diversity
Section 11.1 Genetic Variation within Populations
d. Greater variation in phenotype, the more likely some
individuals will survive
a.Penguin example (short and round vs tall and slim in
cold/famine conditions)
e. Gene pool- the combined alleles of all the individuals in a
population
f. Allele frequency- how common a certain allele is
Section 11.1 Genetic Variation within Populations
2. Genetic Variation
a. Two main sources of genetic variation are
i. Mutations- random change in DNA, forming new alleles. Only
works if the mutation is in reproductive (sperm and egg)
cells
ii. Recombination- occurs during meiosis and results in new
gene combinations
Section 11.2 Natural Selection in Populations
1. Natural selection acts on distributions of traits (phenotypes)
a. Normal distribution- common when all phenotypes provide an
equal chance of survival. Occur where the frequency of a trait is
highest around the average
i. But environments can change!
ii. Microevolution- the observable change in allele frequencies
of a population over time (does not lead to a new species)
Section 11.2 Natural Selection in Populations
1. Directional selection- selection that favors one extreme phenotype.
Causes a shift is phenotypic distribution
EX: oysters with thicker shells cannot be eaten by crabs, so thick shells
becomes common
2. Stabilizing selection- selection that favors the intermediate (middle)
phenotype
EX: Both white and black moths are easy for predators to find on grey
bark, so the grey phenotype is selected
3. Disruptive selection- occurs when both extreme phenotypes are
favored
EX: blue birds get mates, brown birds are camouflaged from predators, but
bluish brown birds have neither benefit
Section 11.2 Natural Selection in Populations
directional
stabilizing
disruptive
Section 11.2 Natural Selection in Populations
If all human females suddenly only dated short guys, what
type of selection would occur
What trait would become more common in the human
population?
Section 11.3 Other Mechanisms of Evolution
1. Gene flow- the movement of alleles between populations
a. Gene flow can increase genetic variation of the receiving
population and keeps gene pools similar
Section 11.3 Other Mechanisms of Evolution
2. Genetic drift- changes in allele frequency due to random chance,
usually works only on small populations
a. Two processes can cause populations to become small enough for
genetic drift to occur
i. Bottleneck effect- what occurs after an event that greatly
reduces population size
ii. Founder effect- what occurs after a small number of
individuals colonize a new area
b. Genetic drift can cause problems, including
• Less genetic variety
• Lethal genes can become more common
Section 11.3 Other Mechanisms of Evolution
Bottleneck-a destructive event that
leaves only a few survivors
a. overhunting a population
reducing the variations
Section 11.3 Other Mechanisms of Evolution
Founder-allelles from yellow flowers increase in new small population
Section 11.3 Other Mechanisms of Evolution
3. Sexual selection- when certain traits increase mating success
a. Females tend to be more selective, as eggs are few and
expensive
b. Two types of sexual selection
i. Intrasexual selection- males compete with each other for
females
ii. Intersexual selection- males display to attract a female
Section 11.3 Other Mechanisms of Evolution
c. Traits that increase mating
success are not always
adaptive or advantageous for
the survival of an organism
Section 11.3 Other Mechanisms
of Evolution
4. 5 factors that can lead to evolution
a. Gene flow
b. Genetic drift
c. Mutation
d. Sexual selection
e. Natural selection
Section 11.3 Other Mechanisms of Evolution
Male Irish Elk, now extinct, had 12 foot wide antlers.
Describe what could have lead to this trait
Section 11.5 Speciation through Isolation
1. Isolation of populations can lead to speciation
a. Reproductive Isolation- when members of different populations
(same species) can no longer mate successfully
b. Speciation- the rise of two or more species
from one existing species
Section 11.5 Speciation through Isolation
2. Populations can become isolated in several ways
a. Behavioral isolation- isolation caused by differences in courtship
or mating behaviors
EX: fireflies- different species flash at different intervals
b. Geographic isolation- a physical barrier divides a population into
2 or more groups
EX: a river cuts a rabbit population in half
c. Temporal isolation- when timing prevents reproduction between
populations
EX: some birds mate in winter, some in summer
Section 11.5 Speciation through Isolation
Section 11.6 Patterns in Evolution
1. Evolution through natural selection is not random
a. Individuals with traits that are better adapted for their
environment have a better chance of surviving and
reproducing then individuals without those traits
b. The environment controls the direction of selection, and
environments can change
Section 11.6 Patterns in Evolution
c. Convergent evolutionevolution towards similar
characteristics in unrelated
species. Analogous structures
are common examples- shark
fins vs whale fins
d. Divergent evolution- when
closely related species evolve
in different directions. Kit fox
vs Red fox.
Section 11.6 Patterns in Evolution
2. Species can shape each other over time
a. Coevolution- when two or more species evolve in response to
each other
i. Evolutionary arms race- crab and snail example
Section 11.6 Patterns in Evolution
3. Extinction- the elimination of a species from earth
a. Background extinctions- occur continuously but at low rates
b. Mass extinction- destroy many species quickly. Usually the result
of catastrophic event like an ice age or asteroid impact. There
have been 5 during earth’s history and we are in the middle of a
sixth now
Section 11.6 Patterns in Evolution
4. Patterns of speciation
a. Gradualism- species evolve slowly over long periods of time
b. Punctuated equilibrium- episodes of speciation occur suddenly
and are followed by long periods of little change.
i. Adaptive radiation- the rapid evolution of many diverse
species from ancestral ones. Usually follows a catastrophic
event or after a species colonizes a new area
Section 11.6 Patterns in Evolution