LECTURE 05B EVOLUTION CH9:182-3; CH16: 311-12.5;314.5-319;322-323.5;327
MAJOR CONCEPTS
1) Organisms facing a changing environment can evolve only if genetic variation exists
in population and natural selection favors alleles suitable for new environment.
2) The source of genetic variation is mutation and sexual recombination.
3) Forces that influence evolution include: natural selection, gene flow (migration), small
population size + chance (loss of genetic variation by random processes), and
assortative mating.
4) Natural selection acts on phenotypic variation in population and results from
differences in survival and reproduction among phenotypes.
5) The extent to which phenotypic variation is due to genetic variation determines the
potential for evolution by natural selection.
DEFINE: Be able to use words when describe evolutionary process
Phenotypic variation
Genetic variation
Natural selection
(Relative) fitness
Evolution
Adaptation
Preadaptation
Phenotypic variation exists among individuals in a population.
Results from combined effects of genes and environment.
Genetic variation
Required for evolution by natural selection
Important in changing environments
Humans bring rapid change; exceed ability of population to evolve extinction
Sources of genetic variation
Mutation – ultimate source
Most are harmful; natural selection weeds out deleterious mutations
Neutral mutation likely beneficial (pre-adaptation) when environment changes
Meiosis and fertilization recombine genes to yield more genetic variation.
Sexual reproduction does not change genotype frequency in a population.
Forces that cause change in genotype frequency (= evolution)
1) Natural Selection differentiates subpopulations
Acts on phenotypic variation in population.
Change in frequency of traits through differential survival and reproduction
If phenotypic variation reflects underlying genetic variation, then 
change in proportion of genotypes over time because genotypes have unequal
fitness.
More phenotypic variation due to genetic variation (greater heritability) 
greater potential for evolution by natural selection.
Fitness: genetic contribution by an individual to future generations
Relative fitness: Maximum = 1 = most fit in population
Types of natural selection  remove genetic variation
Stabilizing (most common)
Directional
2) 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
3) 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)
Optimal outbreeding distance
4) Gene flow (migration/dispersal)
Mixes alleles between subpopulations homogenizes differences between
subpopulations