The Evolution of Populations

Darwin and Mendel were contemporaries of the 19th century- at the time both were unappreciated for their work

The turning point for evolutionary theory was the development of population genetics- emphasizes genetic variation and recognizes the importance of quantitative characters

A population’s gene pool is defined by its allele frequencies

Population: a localized group of individuals belonging to the same species

Species: individuals that have the potential to interbreed and produce fertile offspring in nature

The total aggregate of genes in a population at any one time is called the population’s gene pool- all the alleles of a gene of all the individuals in a population

Example of allele frequency- population is 500 plants- 20 are white (rr)- 320 are red (RR), 160 are red (Rr)

Allele frequency is .8 or 80%- 320 X 2 (for RR) = 640 + 160 (for Rr) ; 800/1000 = .8

The Hardy-Weinberg theorem describes a nonevolving population- the frequencies of alleles and genotypes in a population’s gene pool remain constant unless acted upon by outside factors- the shuffling of alleles has no effect on a population’s gene pool

This idea was independently discovered by both Hardy and Weinberg in 1908

Uses 2 equations simultaneously- P + Q = 1- p2 + 2pq + q2 = 1

For the HW equation to work, 5 conditions must be met- large population size- no migration- no mutations- random mating- no natural selection

Mutations and sexual recombination generate genetic variation

Only mutations that occur in gametes can be passed along to offspring

A mutation that alters a protein is more likely to be harmful

Mutation: a change in a organism’s DNA- if mutation is in gametes, immediate change can be seen in the gene pool- if the new allele produced by a mutation increases in frequency, it is because the mutant alleles are producing a disproportionate number of offspring by NS or genetic drift

Unique recombinations of existing alleles in a gene pool are produced through meiosis- the effect of crossing over

Microevolution: the generation-to-generation change in a population’s frequencies of alleles

The two main causes of microevolution are genetic drift and natural selection

Genetic drift: a change in a population’s allele frequencies due to chance- the smaller the sample size, the greater the chance of deviation for idealized results- ex. coin toss

Bottleneck effect: genetic drift resulting from the reduction of a population such that the surviving population is not representative of the original population- generally caused by natural disaster

Founder effect: genetic drift in a new colony- a few individuals from a larger population colonize an isolated new habitat- ex. from mainland to island

Natural Selection: the differential success in reproduction- the alleles passed on to the next generation are disproportionate to the frequencies in the present generation- ex. Wildflower population

Gene flow: genetic exchange due to the migration of fertile individuals or gametes between populations- ex. Wildflower population in a windstorm

Genetic variation occurs within and between populations

Both quantitative and discrete characters contribute to variation within a population- quantitative variation indicates polygenic inheritance

- discrete characters can be classified on an either-or basis

Polymorphism: when two or more morphs (variations) are represented in high enough frequencies to be noticeable

Genetic variation can be measured at the level of whole genes (gene diversity) and at the molecular level of DNA (nucleotide diversity)

Gene diversity: the average percent of loci that are heterozygous

Nucleotide diversity: comparing the nucleotide sequence of DNA samples

Geographic variation: differences in gene pools between populations or subgroups. - NS can contribute to geographic variation

Diploidy and balanced polymorphism preserve variation

Genetic variation can be hidden from being selected against by the use of heterozygotes

Balanced polymorphism: the ability of natural selection to maintain stable frequencies of phenotypic forms

- ex. heterozygote advantage as seen in sickle-cell disease- ex. frequency-dependent selection: survival and production of any one morph declines if that phenotype becomes too common in a population

Populations can adapt to the environment in various ways

Directional selection: shifts the frequency curve for variations in one direction by favoring individuals that deviate from the average characterex. size of black bears

Diversifying (disruptive) selection: environmental conditions favor individuals on both extremes of a phenotypic range

Stabilizing selection: acts against the extremes; favors the more common intermediate variants