gen bio - lecture 20: population genetics

evolution (definition)

change in allele frequency in populations over generations

phenotypic variation + gradations

the observable difference in traits among individuals in a population

• may be controlled by a single gene

• some are gradations: controlled by 2+ genes, which causes traits to be a continuum

genetic variation (definition + 3 sources)

• the difference in DNA sequences between individuals within a species

• sources:

  • mutation

  • recombination

  • rapid reproduction

gene pool

all alleles of all genes in a population

• in diploid species, each locus presents twice in each individual

allele frequency

proportion of a specific allele occurring in a population

• alleles are not altered by inheritance alone

• allele frequencies in the gene pool stay constant until altered by an outside force

fixed alleles

• allele @ a frequency of 1 in a population

  • the only allele @ a locus = NO ROOM FOR VARIATION

genetic equilibrium occurs when…

no change in allele frequency from generation to generation

• populations @ equilibrium are NOT EVOLVING!!

• extremely rare occurrence

idea developed by Godfrey Hardy + Wilhelm Weinberg

hardy-weinberg equilibrium (definition, equation, what the variables mean)

1) describes a population that is not evolving

2) a baseline to study evolutionary change

frequency of organisms in a population: p² + 2pq + q² = 1

frequency of alleles: p + q =1

• p = probability of W (dominant) allele

• p² = probability of WW offspring

• 2pq = probability of Ww offspring

• q = probability of w (recessive) allele

• q² = probability of ww offspring

5 conditions that must be met to maintain hardy-weinberg equilibrium (just list)

1. no mutations

2. random mating

3. no natural selection

4. a very large population

5. no gene flow between populations

**if any of these are not true, allele frequencies ARE CHANGING

mutations

• changes in DNA sequence that are heritable

  • source of genetic variation!

  • new genes and alleles can arise — if silent, this does NOT alter function

random mating

• each individual has an equal chance to mate with any individual of the opposite sex

  • leads to random mixing of gametes/alleles

• non random mating: no mixing of gametes

• consanguineous mating: incest = causes problems + alleles are lost

natural selection

• nonrandom changes in allele frequency based on fitness

  • better adapted alleles increase in frequency + less beneficial alleles decrease in frequency

  • REDUCES GENETIC DIVERSITY!!

  • for selection to affect population, there must be at least 2 phenotypes (polymorphic inheritance/genetics)

    • changes in gene frequency increases adaptation

    • phenotypic variation results from diff alleles

THERE CAN ONLY BE EVOLUTION IF THERE IS GENETIC VARIATION!!

directional selection vs disruptive selection vs stabilizing selection

• directional selection

  • favors individuals @ one extreme of a trait

  • population shifts towards that extreme over time

• disruptive selection

  • favors individuals @ both extremes of a trait versus the average

  • population may split into two distinct groups

• stabilizing selection

  • favors the average/medium trait versus the extremes

  • population becomes less variable and stays near average

genetic drift (bottleneck + founder effect)

• random evolutionary changes in a SMALL breeding population can cause alleles to get lost

  • random events can drive who survives + who breeds

• 2 ways a population can end very small:

  • bottleneck effect: many individuals die + few survivors limits the number of alleles in the next generation

  • founder effect: a few individuals become isolated from the larger population + establish their own

    • few colonists limits # of alleles in next generation

gene flow (+ what happens if there is no gene flow)

• movement of alleles among populations through the movement of fertile individuals (ex. birds) or gametes (ex. pollen)

  • gene flow tends to reduce differences between populations over time

no gene flow = isolated populations = differences accumulate

microevolution

shifts in allele frequencies over generations; small gradual changes

macroevolution

large scale changes that result in changes that are large enough to place as different taxonomic groups