Population Genetics Population genetics refers to the

study of the numbers and interaction of alleles within a population.

From this, phenotypic variation can be studied.

Phenotypic Variation Phenotypic variation refers to the

different varieties of traits that appear within a population.

Most traits are not as simple as an either/or phenotype.

Most characteristics are polygenic--meaning that they controlled by many genes.

Polygenic Characters Often when you are looking at

polygenic characteristics, you can graph the information and get a bell curve.

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Patterns of Natural Selection

Again, most characteristics are polygenic and produce a bell curve when graphed.

When natural selection acts on polygenic characters, it acts to remove parts of the curve. Directional Selection Disruptive Selection Stabilizing Selection

Directional Selection

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Disruptive Selection

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Stabilizing Selection

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Genetic Equilibrium Ideally, the frequency of alleles

and the ratios of the genotypes within a population should not change from one generation to the next.

The Hardy-Weinberg Equilibrium

The Hardy-Weinberg Principle serves as a model for most populations because they are often evolving so slowly that they appear to be in equilibrium.

Additionally, population sizes are often large and this too would make allele frequencies appear to be in equilibrium.

The Hardy-Weinberg Equilibrium

In reality, populations are subject to outside pressures and are constantly undergoing change.

The Hardy-Weinberg Equilibrium

These outside pressures are the following: Gene flow-the movement of genes in

and out of a population. Nonrandom mating-choosy mates alter

the allele frequencies of a population. Genetic drift/population size-chance

fluctuations and small populations often result in large changes in the numbers of alleles within a population.

The Hardy-Weinberg Equilibrium

Outside pressures continued: Mutation-mutations are constantly

occurring and can change allele frequencies.

Natural selection-natural selection selects for fit individuals and eliminates less desirable traits.

Natural Selection and Evolution

Recall, Selection acts on the variation within

the population. Individuals produce more offspring

than the environment can support. All populations depend on the

reproductive success of individuals. As a result, the allele frequencies

within a population change and the equilibrium is disrupted.

A Species Recall in the last chapter we defined

a species as being able to mate and produce viable, fertile offspring.

This is the biological species concept. All populations of a species lives in a

different place/environment. The differing environments lead to

different adaptations among the organisms.

A Species The accumulation of these

differences leads to divergence which can give rise to a new species.

Speciation usually requires a large number of generations to occur, and results in reproductive isolation--the state where two populations can no longer interbreed with one another.

Mechanisms of Isolation Geography-geographical barriers

can divide a population. Ecological niche-when a species

explores a new niche, divergence can occur.

Hybridization-this occurs when two closely related species come into contact, mate, and produce sterile offspring--recall the mule. There are many cases where hybridization leads to a new and successful species.

Mechanisms of Isolation Mating behavior and timing-if

external stimuli cause the breeding habits of different populations of the same species to change, this can lead to divergence.

Polyploidy-chromosomal accidents often lead to the formation of new species--especially in plants. They can no longer mate with

members of the population.