chapter 19: the evolution of populations

genes, mutations, and inheritance

what darwin never knew

genes, mutations, and inheritance 1

phenotypic variation is mostly genetic, but environment can influence expression which creates non-heritable variation

genes, mutations, and inheritance 2

genes and the inheritance of genes leads to different types of variation

discrete genetic variation

2 or more alleles at single gene locus

continuous variation

phenotypes produced by combined effects of 2 or more genes

genes, mutations, and inheritance 3

sexual recombination produces genetic diversity among offspring, crossing over, recombinant chromatids, independent assortment, random fertilization

genes, mutations, and inheritance 4

new alleles arise from mutations in DNA, point mutation, chromosomal alterations

genes, mutations, and inheritance 5

most DNA variability does not affect phenotype, base-pair substitutions

genes, mutations, and inheritance 6

most new alleles are harmful, harmful effects may be hidden due to recessive allele, some new alleles may be neutral with regard to selection, if environment changes harmful or neutral alleles may become adaptive

population genetics

the study of how alleles frequencies in populations change overtime

genetic composition of population

gene pool, genotypic frequency, allele and allelic frequency

gene pool

all the alleles of all the genes in a population, many genes have "fixed" alleles (homozygous in all individuals)

genotypic frequency

the proportion of a given genotype within a population

%AA, %Aa, %aa

allelic frequency

the percentage of any specific allele in the gene pool of the population

% A allele and % a allele

population

a group of interbreeding individuals that belong to the same species and live in the same area

Hardy-Weinberg equilibrium

condition that occurs when the frequency of alleles in a particular gene pool remain constant over time

Hardy-Weinburg principle

p + q = 1

p² + 2pq + q² = 1

no mutations, mating is random, no selection, very large population, no gene flow in or out

hardy-weinburg and microevolution

hardy-weinburg helps us detect microevolution, if actual observed ratios do not equal expected H-W ratios, then the population is evolving

natural selection

a natural process resulting in the evolution of organisms best adapted to the environment, acts non-randomly on phenotypes of individuals, changes allelic and genotypic frequencies non-randomly

genetic drift

random change in allele frequencies that occurs in small populations

founder effect

change in allele frequencies as a result of the migration of a small subgroup of a population, founder gene pool differs from original source, better alleles may be lost

bottleneck effect

a change in allele frequency following a dramatic reduction in the size of a population

gene flow

movement of alleles from one population to another, migration of adults, dispersal of gamtes/seeds/larvae, tends to add genetic diversity to population, reduce genetic differences between populations

relative fitness

the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals, how many offspring do you leave

stabilizing selection

form of natural selection by which the center of the curve remains in its current position; occurs when individuals near the center of a distribution curve have higher fitness than individuals at either end

directional selection

form of natural selection in which the entire curve moves left or right of the particular allele; occurs when individuals at one end of a distribution curve have higher fitness than individuals in the middle or at the other end of the curve

diversifying selection

a type of natural selection in which organisms with phenotypes at both extremes of the phenotypic range are favored by the environment

frequency-dependent selection

selection in which the fitness of a phenotype depends on how common the phenotype is in a population

sexual selection

when individuals select mates based on heritable traits

sexual dimorphism

differences in physical characteristics between males and females of the same species

intrasexual selection

selection within the same sex, individuals of one sex compete directly for mates of the opposite sex

intersexual selection

individuals of one sex are choosy in selecting their mates from the other sex

maintaining genetic variation

through diploidy (less successful recessive alleles are hidden in heterozygotes), frequency dependent selection (fitness of a phenotype decreases as its frequency increases in population), diversifying selection (surviving extreme alleles will carry different alleles), and heterozygous advantage (selection favors heterozygote)

limitations on natural selection

acts on phenotype of entire individual, can act only on existing variation (extinction), chance, environment & natural selection interact