Chapter 23: Evolution of Populations

The Hardy-Weinberg Principle tests?

whether or not a population is evolving

Main sources of genetic variation in populations

genetic drift, geneflow, natural selection, and sexual reproduction

How does sexual reproduction increase genetic variation?

By recombining existing alleles

How do new alleles arise?

mutation, a change in the nucleotide sequences of DNA

What creates new combinations of existing alleles?

crossing over, independent assortment, fertilization

Crossing over

exchange of genetic material between homologous chromosomes during meiosis

independent assortment

random distribution of chromosomes into gametes during meiosis

Fertilization

random combination of gametes

population

a group of individuals of the same species that live in the same area and are able to produce viable, healthy offspring

gene pool

all copies of every allele at every locus of members of a population

If the genetic makeup of a population differs from the expectation under Hardy-Weinberg then?

the population may be evolving

If a population is not evolving?

genotype and allele frequencies will be constant from generation to generation

natural selection

based on differential success in survival and reproduction

Adaptive evolution

a process in which traits that enhance survival or reproduction increase in frequency over time

three ways in which natural selection can altar the frequency distribution of heritable traits

Directional selection, disruptive selection, stabilizing selection

directional selection

favors individuals at one extreme end of the phenotypic range

disruptive selection

favors individuals at both ends of the phenotypic range

stabilizing selection

favors intermediate variants and acts against extreme phenotypesGenetic drift

Genetic drift

random fluctuation of allele frequencies from one generation to the next

the founder effect

When a few individuals become isolated from the main population and allele frequencies become smaller

bottleneck effect

when the gene pool does not resemble the parent population because of a drastic drop in population size due to sudden environmental change

Gene flow

the spreading of alleles among multiple populations

the only mechanism of evolution that causes adaptive evolution

natural selection

Natural selection can only

add onto existing traits

population genetics

the study of genetic variation within and among populations

Frequency

Number of things that occurred / Number of things that could have happened

Assumptions of the Hardy-Weinberg Equation in which gene frequencies will remain constant across generations

No gene flow, no selection, no mutation, no genetic drift, infinitely large population size, random mating

Frequency of A within a population

A = p

Frequency of a within a population

a = q

Formula for genotype frequencies of the Hardy-Weinberg equation

p² + 2pq + q² = 1

What forces cause changes in allele frequencies

mutation, selection, drift, migration, non-random mating