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.  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.  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.

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  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

Disruptive Selection

Stabilizing Selection

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 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.  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.  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.  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.  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.  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.  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.  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.