Chapter 17: Evolution of
Populations
Big Idea
How can populations evolve and even form new species?
Population - group of
individuals of the same species that live in the same area (at the same time).
A species is a group of
organisms that can breed and produce fertile offspring.
Carrying Capacity
Populations evolve due to environmental
pressures.
Not enough resources to go around, so not all
individuals will survive.
Environments have a carrying capacity-
maximum number of
individuals of a species that a particular
environment can
support.
1. What is the carrying capacity for the above population?
2. What individuals do you think will die off first?
Genotype and Phenotype in Evolution
Genotype – individual’s combination of alleles for a trait An individual’s genotype, together with environmental conditions, produce its phenotype.
Phenotype includes all physical, physiological, and behavioral characteristics
Natural selection acts on phenotypes, NOT genotypes.
Natural selection NEVER acts directly on genes. Why?
Because it is an entire organism - not a single gene - that either survives and reproduces or dies without
reproducing.
Populations and Gene Pools
Gene pool - all the genes, including all the different alleles for each gene, that are present in a population at any one time
Allele frequency - number of times that an allele
occurs in a gene pool
compared with the total
number of alleles in the
pool for the same gene
Evolution, in genetic terms, involves a change in
the frequency of alleles in a population over time.
Evolution Versus Genetic Equilibrium
Genetic Equilibrium – when allele frequencies remain constant between generations, no evolution is occurring
Hardy-Weinberg Principle - states that allele frequencies in a
population remain constant unless one or more factors cause those frequencies to change - see equations on page 837
p = the frequency (%) of the dominant allele q = the frequency (%) of the recessive allele
p2 + 2pq + q2 = 1 (you won’t have to do this but recognize it) If p and q change from one generation to the next, evolution is occurring.
If they do not change, the population is in genetic equilibrium.
Know what p
and q mean!
Allele Frequency
White = rrTan = RrGray =
What alleles are there more or less of?
RRGenotypes # of each allele
% allele
frequencies
Allele Frequency
We said the allele frequency was p = R =
q = r =
If one of these values changes over time, the population is evolving.
IMPORTANT:
• Evolution occurs in populations. When allele frequencies change, or new alleles are introduced, the population is
evolving.
• Natural selection acts on individual organisms (they either live or die). But when that organism lives or dies, it affects the allele frequency of the population.
Sources of Genetic Variation
Populations need to have variations for natural selection to occur.
There are three main sources of genetic variation:
1. mutation
2. genetic recombination during sexual reproduction
3. lateral gene transfer
Mutations
Any change in the genetic
material of a cell or organism is a mutation
Neutral mutations don’t change an organism’s phenotype
Mutations that do produce a
change in the phenotype of an
organism may or may not affect
its fitness.
Genetic Recombination in Sexual Reproduction
Organisms that
reproduce sexually inherit different
combinations of alleles/traits from their parents.
Crossing-over during meiosis allows for
genetic
recombination and
leads to different
combinations of
alleles.
Lateral Gene Transfer
When organisms pass genes to another
individual that is not their offspring.
Can be same or different species.
This only happens in single-celled
organisms (like
bacteria).
Single-Gene and Polygenic Traits
The number of phenotypes produced for a trait depends on how many
genes control the trait.
Single-gene trait - trait controlled by one gene with two or more alleles
Often just two or three distinct phenotypes (no “in-betweens”) Remember: Some alleles may be dominant, others may be recessive.
See figure 1.4 on page 826 - read the caption
Example: bands
on a snail’s shell
Polygenic trait - trait controlled by two or more genes, for
example height, skin color
Polygenic trait often forms a bell curve, where fitness may vary
from one end to the other.
See page 827 - the photograph of the people - read the caption
below
Natural Selection on Single-Gene Traits
Natural selection on single-gene traits can lead to
changes in allele frequencies and, thus, to change in
phenotype frequencies.
Natural Selection on Polygenic Traits
Natural selection on polygenic traits can affect the relative fitness of phenotypes.
The range of phenotypes changes, rather than “one goes up, the other goes down”
Three types of selection on polygenic traits:
1. directional selection 2. stabilizing selection 3. disruptive selection
Each type of selection affects the shape of the bell curve.
Directional Selection
Individuals at one end of curve have higher fitness than individuals in the middle or at the
opposite end of the curve.
See figure 2.3 on page 833
Shifts the curve to the right
or left.
Stabilizing Selection
Individuals near
the center of curve have a higher
fitness than
individuals at
either end of the curve
Makes the curve
narrower
Disruptive Selection
Individuals at the upper and lower ends of the curve have higher fitness
than individuals near the middle of the
curve
Can create two
distinct phenotypes Turns the curve into two
humps.
Evolving By Chance
Natural selection and
mutations are not the only ways evolution can happen.
Sometimes, evolution can happen by chance.
This is called genetic drift.
Genetic Drift
Genetic drift - random change in allele frequency caused by a series of chance occurrences that cause an allele to become more or less common in a
population.
Two types of genetic drift:
• Bottleneck effect
• Founder effect
Genetic Bottlenecks
Sometimes a disaster or disease will kill many individuals in a population. The few individuals that remain could have limited allele frequencies. They are left to re-populate.
Bottleneck
effect - a change in allele
frequency following a dramatic
reduction in size
of a population
The Founder Effect
Founder effect - change in allele frequencies as a result of the migration (movement) of a small
subgroup of a population.
See figure 2.5 on page 836
Hardy-Weinberg Principle
Five conditions can disturb genetic equilibrium and cause evolution to occur:
1) nonrandom mating
2) small population size
3) immigration or emigration 4) mutations
5) natural selection
Nonrandom Mating
Sexual selection - when
individuals select mates based on heritable traits; i.e. - size, strength, coloration
Small Population Size
Evolutionary change due to genetic drift happens
more easily in small populations.
Gene Flow Through Immigration or Emigration
Genes can flow between populations of the same species when individuals move.
Immigration - individuals moving into a population
Emigration - individuals moving out of a population
Summary: Main Mechanisms of Evolution
1. Mutations: Introduce new versions of genes (alleles) and, therefore, new traits
2. Recombination: During sexual reproduction and
crossing-over, causes new combinations of alleles/traits 3. Natural (or Artificial or Sexual) Selection: Acts on variation to “select” for certain traits
4. Genetic Drift: Change of allele frequency due to random chance
5. Gene Flow: Individuals moving into and out of
populations causes allele frequencies to change
Speciation
So far we have talked mostly about changes within
species, but evolution can also cause new species to be created.
Speciation – the formation of a new species
The five mechanisms of evolution (natural selection,
mutations, etc.) change allele frequency, but, alone, aren’t enough to cause new species to be created.
For speciation to occur, there also has to be reproductive isolation.
Reproductive isolation is the separation of a species or
population so that they can no longer interbreed.
Isolating Mechanisms
There are three types of isolation that can lead to
reproductive isolation (and speciation):
• Behavioral isolation
• Geographic isolation
• Temporal isolation
Behavioral Isolation
Differences in behavior can
prevent mating.
Differences in
courtship rituals or other behaviors
prevent
populations from
breeding.
Geographic Isolation
Populations are separated by geographic barriers such as rivers, mountains, or larger bodies of
water.
Temporal Isolation
Populations reproduce at different times
Molecular Clock
Molecular clock - uses mutation rates in DNA to estimate the length of time that two species have been evolving independently. This helps scientists determine how closely related two organisms are.
The more differences there are, the longer it has been since they shared a common ancestor.
Often uses “neutral” DNA because it doesn’t affect phenotype and tends to accumulate mutations in different species at about the same rate.
