Notes

ch 20 evolution

core concepts:
patterns of genetic variation can be described by allele frequencies
evolution - change in frequency of alleles or genotypes over time
natural selection leads to adaptations - enhance the fit btwn an organism & its environment
mechanisms of evolution: genetic drift, migration, mutation, nonrandom mating


natural selection - depends on differential success (surviving & reproducing) of variants in a population

species - group of individuals that can exchange gnetic material thru interbreeding or share alleles thru reproduction
gene pool - consists of all alleles present in all indiviudals of a species
population - an interbreeding group of organisms of of the same species living in the same geographic area

genetic variation - refers to differences in DNA sequences
a. mutation - generate new variation; somatic (in body tissues) or germ-line (in reproductive cells passed to next gen); can be harmful (deleterious), neutral (neither good/bad), or advantageous (improve chances of survival)
b. recombination - shuffles mutations to create new combinations of mutations
both result in new alleles

patterns of genetic variation can be described by allele frequencies:
allele frequency - # of copies of allele/total # of alleles in population
ex: 100 pea pea plants: 25% AA (yellow) 25% Aa (yellow) 50% aa (green)
frequency of a:
50 aa indiviudals & 25 Aa individuals = 50 * 2 + 25 = 125 a copies
total alleles = 100 * 2 = 200
frequency of a = 125/200 *100 = 62.5%
frequency of A = 100 - 62.5 = 37.5%

measuring genotype/allele frequnecies:
1. observable traits: count # of individuals /w a trait (doesn't consider traits are encoded by more than 1 gene, effects of environment, or heterozygous individuals)
2. gel electrophoresis: wells in gel loaded with samples & an electric current passes thru; rate of movement determined by charge & size (differences in amino acid sequence); homozygous = 1 band, heterozygous - 2 bands; doesn't show variation at DNA levels
3. DNA sequencing: calculating allele frequencies; in a sample of n diploid individuals, allele frequency is the # of occurances of the allele dividied by 2n

evolution - change in allele or genotype frequency from 1 gen to next (populations evolve, not individuals; allele frequencies can stay the same but genotype frequencies can change)

hardy-weinberg equilibrium - specifies relationship btwn allele frequencies & genotype frequencies under these conditions:
1. no differences in survival & reproductive success of individuals in a population
2. population size isn't changed by movement of indiviudals
3. no mutations in gene being studied in population
4. population is large to prevent sampling errors
5. indiviudals in population mate at random
when met, no evolution is occurring in population
allele frequencies: p + q = 1
genotype frequencies: p^2 + 2pq + q^2 = 1
-frequency of heterozygotes for a population in HW equilibrium can't exceed 50%

natural selection - change in an allele's frequency over time based on that particular allele's impact on survival & reproduction conditions:
1. must be variation in population
2. variation in population must be heritable
3. competition for resources among individuals in population must occur
4. individuals in population that are better able to obtain resources must be more likely to survival & reproduce

fitness - measure of the extent to which the indiviudal's genotype is represented in the next gen; natural selection acts to increase overall fitness of population bc individuals /w advantageous alleles reproduce more

modern synthesis - extended mendel's theory to include multiple genes per trait to account for patterns of continuous variation

types of selection:
positive - increases frequency of a favorable allele
negative - decreases frequency of a deleterious allele
balancing - keeps an allele at an intermediate frequency (heterozygote advantage -

patterns of natural selection:
stabilizing - negative selection against extremes; results in increase in indiviudals with intermediate values
directional - positive selection against one of two extremes; shifts distribution of values so the intermediate value increases or decreases
disruptive - selects against the intermediate value

artificial selection - form of directional selection; successful genotypes are selected by breeder)
sexual selection - preference by one sex for certain characteristics in other sex
intrasexual selection - members of one sex compete /w one another for access to other sex
intersexual selection - memebers of one sex choose members of other sex

nonadaptive mechanisms of evolution:
genetic drift - random change n allele frequencies from generation to generation; frequencies can go up or down by chance; impact depends on population size (smaller populations more strongly impacted)
population bottleneck - population drastically reduces in size
founder event - a few indivudals from an orginal population leave to start a new one

change in allele frequencies:
migration - results in movement of alleles from one population to another (gene flow)
gene flow - populations become more similar to one another over time & have less genetic differences
mutation - change in DNA sequence

nonrandom mating: individuals preferntially choose mates according to their genotypes; certain phenotypes increase & others decrease; rearranges alleles instead of adding new ones; inbreeding can result in changei n genotype frequencies

molecular evolution: 2 speciies are genetically isolated from e/o once they have separated from their most recent common ancestor; different mutations accumlate & determine the genetic differentiation btwn the 2 species

molecualr clock: the correlation btwn the time 2 species have been evolutionarily separated & the amount of genetic divergence btwn them