Bio 16-2

a mode of natural selection in which an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype

Directional Secection

the reduced genetic diversity that results when a population is descended from a small number of colonizing ancestors

Founder Effect

variation in the relative frequency of different genotypes in a small population, owing to the chance disappearance of particular genes as individuals die or do not reproduce

Genetic Drift

describes the condition of an allele or genotype in a gene pool (such as a population) where the frequency does not change from generation to generation

Genetic Equilibrium

How does genetic drift lead to a change in a population's gene pool?

Genetic drift describes changes in the allele frequency of a population. Small populations are most susceptible because individual that carries an allele may leave more offspring than the other individuals just by chance, thus changing the frequency of alleles in the next generation

states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences

Hardy-Weinberg Principle

Describe how natural selection can affect traits controlled by single genes.

Natural selection on a single-gene trait can change the allele frequencies, leading to evolution.

How does natural selection affect single-gene and polygenic traits?

Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Natural selection on polygenic traits can affect the distributions of phenotypes in any of three ways: directional selection, stabilizing selection, or disruptive selection.

What is genetic drift?

Random change in allele frequencies that occurs in small populations

a type of natural selection in which the population mean stabilizes on a particular non-extreme trait value

Stabilizing Selection

Describe three patterns of natural selection on polygenic traits. Which one leads to two distinct phenotypes?

Three modes of natural selections are directional selection where one extreme trait is favored over others, disruptive selection where two extreme traits are favored over average trait, and stabilizing selection where average trait is favored over extreme traits. Disruptive selection leads to two distinct phenotypes.

What is the Hardy-Weinberg principle?

states that allele frequency of a population remains constant when all the following conditions are met: large population size, no mutation, no gene flow, no natural selection, and random mating.

What five conditions are needed to maintain genetic equilibrium?

1) random mating 2) large population 3) no movement into or out of population (isolation) 4) no mutations 5) no natural selection

How are directional selection and disruptive selection similar? How are they different?

Directional selection occurs when nature favors one extreme trait over other traits. Disruptive selection occurs when nature favors two extreme traits and disfavors average traits. Directional selection shifts the curve to the left or right, while disruptive selection creates a grove in the middle.

describes changes in population genetics in which extreme values for a trait are favored over intermediate values. In this case, the variance of the trait increases and the population is divided into two distinct groups

Disruptive Selection