The Evolution of Populations
Populations
A group of organisms of the same species
living in the same area at the same time
A population of water buffalo
Population Genetics
• Each individual has two copies
of a gene, one from each
parent. These copies of genes
are called alleles.
• Natural selection on traits can
lead to changes in allele
frequencies, or how common
a version of a gene is in gene
pool of the population.
• If the allele frequencies of a
population are changing, then
the population is evolving.
Genetic Equilibrium
• Condition where no evolution is occurring
Genetic = Allele Frequencies
Equilibrium= Stability
• If a population is not evolving, the allele frequencies
in a population remain stable and genetic equilibrium
occurs.
This population of beetles
is NOT in equilibrium. The
allele frequencies
changed from generation
to generation.
Hardy-Weinberg Equilibrium
5 conditions must be met for the population to
remain stable and not evolve
1) No Natural Selection
2) No Mutations
3) No Migration
4) Random Mating
5) Large Population
Hardy Weinberg Equilibrium
1. No Natural Selection.
•
•
Differential survival and reproductive success will
alter allele frequencies.
Will only occur if the environment is stable and
resources are abundant, which isn’t very common.
Hardy Weinberg Equilibrium
• 2. There are no mutations.
• Mutations change allele frequencies in a
population.
• Some mutations are beneficial and will provide a
selective advantage. Most have no affect or are
harmful to the organism.
Hardy Weinberg Equilibrium
3. No Migration
• individuals entering (immigrating) or leaving
(emigrating) the population can add or take away
alleles from a population.
Hardy Weinberg Equilibrium
4. Mating is random.
• Most mating is nonrandom, meaning the individuals
chose their sexual partners.
• Individuals may chose a mate based on characteristics
or proximity.
Hardy-Weinberg Equilibrium
5) Large Populations
Having a large population decreases the
likelihood that allele frequencies change due to
chance events.
A person stepping on some
of the beetles in this small
population has a big effect.
It eliminated almost all the
green beetles! If the
population was larger, the
loss of a few green beetles
would not have such a big
effect on allele frequencies.
Is genetic equilibrium common?
NO!
• There are selective pressures in the
environment and mutations
naturally occur.
Remember
• Only populations and species (which are groups of
populations) can evolve. Individuals do NOT
evolve.
• Any changes within an organism’s lifetime are due
to environmental mutations or acquired
characteristics due to behavior. Remember only
genetic changes in the egg and sperm will be
passed on to offspring.
• It is the changes that accumulate over generations
in populations or species that lead to changes in
organisms over time.