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Chapter 16
Evolution and Populations
Variation and Gene Pools
Variations are differences between members of a population.
A population is a group of individuals of the same species
that interbreed.
Because members of a population interbreed they share a
common group of genes called a gene pool.
Gene pool: all of the genes including all of the different
alleles in a population.
Relative frequency: the number of times that the allele
occurs in a gene pool, compared with the number of times
other alleles for the same gene occur.
How does variation and gene pools
apply to evolution?
Evolution involves changes in populations
over time.
Evolution is any change in the relative
frequency of alleles in a popultion.
What causes genetic variations
Mutations: a change in the sequence of DNA which
would alter the protein or trait that is produced.
Mutations are random.
Sometimes they are harmful but sometimes they are
The helpful ones are passed on to offspring.
Recombination (Crossing-Over): occurs during
meiosis where gene pairs cross over during Prophase
Single-Gene and Polygenic Traits
Single-Gene traits: controlled by a single gene that
has 2 alleles.
 Ex. Widow’s peak in humans.
Polygenic traits: controlled by 2 or more genes.
 Ex. Height in humans
 Usually a bell curve or normal distribution of
individuals in a population occurs in polygenic
traits (Most people are average height; neither
very tall or very short).
Most human traits are polygenic.
Widow’s Peak
Natural Selection on Single-Gene
Natural selection on single-gene traits can
lead to changes in allele frequencies and thus
to evolution.
Ex. Black versus red lizards
If red lizards are more visible to predators then they
will most likely not live long enough to reproduce.
Black lizards might absorb more sun and warm up
faster on cold days which would allow them to move
faster to feed and avoid predators.
The allele for black color might increase in frequency.
Natural Selection on Polygenic Traits
Natural selection can effect Polygenic traits in
3 ways:
Directional Selection
Stabilizing Selection
Disruptive Selection
Directional Selection
When individuals at one end of the curve have
higher fitness than individuals in the middle
or at the other end, directional selection takes
The range of phenotypes will shift as some
individuals fail to survive and reproduce
while others succeed.
Example of Directional Selection
Beak size in Darwin’s finches.
Birds with bigger beaks can feed more easily on
larger, harder, thicker-shelled seeds.
Suppose a food shortage causes the supply of small
and medium sized seeds to run low, leaving only
larger seeds.
Birds with larger beaks would have better access to
food therefore the average beak size of the
population would increase.
Stabilizing Selection
Occurs when individuals near the center of
the curve have higher fitness than individuals
at either end of the curve.
The situation keeps the center of the curve in
its current position.
Example of Stabilizing Selection
Ex: Infant birth weight.
Human babies born smaller than average are
likely to less healthy and less likely to
Babies that are much larger than average have
more difficulty being born.
The fitness of these larger or smaller
individuals is lower than that of more
average-sized babies.
Disruptive Selection
Occurs when individuals at the upper and
lower ends of the curve have higher fitness
than individuals near the middle.
Selection acts against those individuals of
intermediate type.
Can cause a single curve to split in two or
create 2 distinct phenotypes.
Genetic Drift
Random change in allele frequency in a
In small populations, individuals that carry a
particular allele may leave more descendents
than other individuals, just by chance. Over
time, a series of chance occurrences of this
type can cause an allele to become common in
a population.
Founder effect
Genetic drift may occur when a small group
of individuals colonize a new habitat.
These individuals may carry alleles in
different relative frequencies than did the
larger, original population.
If so, the population the “found” will be
genetically different from the parent (original)
Darwin’s Finches
Hardy-Weinberg Principle
States that allele frequencies in a population
will remain constant unless one or more
factors cause those frequencies to change.
The situation where allele frequencies remain
constant is called genetic equilibrium.
If the allele frequencies do not change the
population will not evolve.
5 Conditions are Required to Maintain
Genetic Equilibrium
Random mating
Large population
No movement into or out of the population
No mutations
No natural selection
The process by which new species are produced.
New species are produced by reproductive isolation,
where members of 2 populations cannot interbreed
and produce fertile offspring.
3 causes of reproductive isolation:
 Behavioral isolation
 Geographic Isolation
 Temporal Isolation
Behavioral Isolation
Occurs when two populations are capable of
interbreeding but have differences in
courtship rituals or other reproductive
strategies that involve behavior.
Ex. Eastern and Western Meadowlarks: use
different songs to attract mates.
Eastern and Western Meadowlark
Geographic Isolation
2 populations are separated by geographic
barriers such as rivers, mountains, or bodies
of water.
2 separate gene pools form where genetic
changes that appeared in one group are not
passed to the other.
Natural selection worked separately on each
group forming distinct subspecies.
Ex. Brown Bear and Polar bear
Brown Bear versus Polar Bear
Temporal Isolation
2 or more species reproduce at different times
therefore they cannot breed with one another.
Ex. Common in plant species where pollination
occurs only at specific times for each
species…preventing cross-pollination or