BSC2010 Unit 3 Lecture 3

Mean

Average value

Variance

Measure of the spread of values in the distribution

Fitness functions

Graphs of fitness vs. phenotype

Fitness

Relative contribution of an individual to the gene pool of the next generation

Directional selection

Individuals at one extreme of a character distribution contribute more offspring to the next generation → shifts the mean, does not necessarily change variance

Which animal is a good example of directional selection?

Peppered moth

Stabilizing selection

Reduces variation in trait values in the population but does not change the mean

Example of stabilizing selection

Human birth weight

Disruptive selection

Individuals at the extremes of a character distribution have the highest fitness; increased variance; bimodal distribution of traits

Example of disruptive selection

Black-bellied seedcracker (narrow- and wide-billed individuals)

Heterozygote advantage

Individuals with 2 different alleles for a gene have greater fitness than homozygotes

Example of heterozygote advantage

Colias butterflies (heterozygotes tolerate a wider range of temperatures)

Frequency dependent selection

When the fitness of a phenotype varies depending on how common that phenotype is

Negative frequency dependent selection

Phenotypes have higher fitness when rare, so ultiple phenotypes are maintained.

Positive frequency dependent selection

Common phenotypes have higher fitness; may reduce genetic variation

Example of negative frequency dependent selection

Scale-eating African cichlid fish → lefty vs. righty jaw; rock-paper-scissors in male side-blotched lizards (mating patterns)

Sexual dimorphism

Elaborate traits that are only present in one sex and seem like they should decrease survival

Sexual selection

An organism’s phenotype influences its mating success and thus its fitness

Two forms of sexual selection

Competition for access to mates (intrasexual selection); choices amons phenotypes in potential (intersexual selection)

Typically, males ___ and females ___.

Males compete and females choose

Why are competitions or courtship displays typically seen in males?

Females typically have a greater investment in each reproductive event, starting at the level of the gamete. (anisogamy)

Anisogamy

Unequal-sized gametes

What does anisogamy normally lead to?

Females choose a high-quality partner to make the most of their high-investment eggs. Males fertilize as many eggs as possible.

Intrasexual selection

May involve direct combat or ritualized fighting between males

“Strategies” that intrasexual selection can involve?

Mate guarding and “sneaker” males

Intersexual selection

Involves mate choice based on favored phenotypes

Indirect benefits of intersexual selection

Females may be selecting for traits that indicate health, longevity, “good genes”

Assortative mating

When individuals prefer others of the same phenotypes

Dis-assortative mating

When individuals prefer others of different phenotypes

Habitat selection

When more frequent matings occur between individuals with similar habitat preferences