The Evolution of Populations

PopulationsA 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 FrequenciesEquilibrium= 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 evolve1) No Natural Selection2) No Mutations3) No Migration4) Random Mating5) Large Population

Hardy Weinberg Equilibrium1. 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.

• 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

Hardy Weinberg Equilibrium3. No Migration• individuals entering (immigrating) or leaving

(emigrating) the population can add or take away alleles from a population.

Hardy Weinberg Equilibrium4. 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 Equilibrium5) Large PopulationsHaving 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.