LECTURE 07 CH 16 POPULATION GENETICS AND EVOLUTION
Genetic variation
Required for evolution by natural selection
Important in changing environments
Humans bring rapid change; exceed ability of population to evolve extinction
Mutation – ultimate source of genetic variation
Change in nucleotide in DNA change in amino acid (enzyme) phenotype change
Most are harmful; natural selection weeds out deleterious mutations
Mutation likely beneficial when environment changes
Meiosis and fertilization recombine genes in new combinations
No evolutionary change via sexual reproduction alone
Hardy-Weinberg Law: frequencies of genes and genotypes remain constant
through time (unless acted on by forces outlined below)
Gene pool: all genes in an interbreeding population
Forces that cause change in genotype frequency (= evolution)
Small population size
Lose genetic variation by:
Genetic drift: due to random variation in fecundity and mortality;
alleles become fixed (no variation); common in small populations
Founder event: small number found new population and carry only partial
sample of gene pool of parent population
Bottleneck: period of very small population size when genetic drift can cause
loss of genetic variation
Assortative mating: changes genotype frequency, not allele frequency
Negative: mates differ genetically increases proportion of heterozygotes;
avoids inbreeding
Positive (includes inbreeding): mate with close relativesincreases proportion
of homozygotes
Unmasks deleterious recessive alleles-->inbreeding depression (less fit
offspring
Mechanisms to avoid inbreeding
Inbreeding sometimes can be beneficial but usually has negative effect on
fitness
Optimal outbreeding distance
Gene flow (migration/dispersal)
Mixes alleles between subpopulations homogenizes differences between
subpopulations
Genotypes vary geographically
Random changes
Differences in natural selection factors
Differences large if major barriers to gene flow (geographic isolation)
begin to speciate
Natural Selection differentiates subpopulations
Change in frequency of traits through differential survival and reproduction
Change in proportion of genotypes over time
Fitness: genetic contribution by an individual to future generations
Relative fitness: Maximum = 1 = most fit
Types of natural selection
Stabilizing (most common)
Directional
Disruptive dimorphism
Which is stronger force acting on populations?
Is natural selection or gene flow stronger force in plants? NS Animals? GF
What maintains vs. removes genetic variation?
Natural selection tends to remove genetic variation via stabilizing selection
Mutation, gene flow, varying selection in time and space maintain genetic variation
Frequency-dependent selection: alleles selected for when rare and against when
common maintains genetic variation