LECTURE 05 EVOLUTION Read CH6: 113-123; see also parts of CH 13 below
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 sources of genetic variation are 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.
6) Adaptations result from natural selection on heritable variation in traits that affect
evolutionary fitness.
I. LEARN WELL: be able to use words in sentences describing the evolution of any
trait, e.g. beak size in Darwin’s finches during climate change; see pg. 113-114; 6.4
Phenotypic variation
Genetic variation
Natural selection
(Relative) fitness
Evolution
Adaptation
Pre-adaptation – an evolved trait that becomes useful for another purpose after
environment changes
II. Phenotype is the outward expression of an individual’s genotype. 115
Environment influences genotype expression.
Phenotype = genotype + environmental influences (P = G + E)
Phenotypic variation exists among individuals in a population.
Results from combined effects of genes and environment.
III. Genetic variation 115-6
Required for evolution by natural selection
Important in changing environments
Humans bring rapid change; exceed ability of population to evolve extinction
IV. Sources of genetic variation 116
A. Mutation – ultimate source 13.2
Most are harmful; natural selection weeds out deleterious mutations 13.1
Neutral mutation likely beneficial (pre-adaptation) when environment changes
B. Meiosis and fertilization recombine genes to yield more genetic variation.
Sexual reproduction does not change genotype frequency in a population.
LEARN WELL (BE ABLE TO DESCRIBE HOW) THESE 4 FORCES CAUSE
EVOLUTION
V. Forces that cause change in allele/genotype frequency (= evolution):
A) Natural Selection differentiates subpopulations 117-9; 121-123
Evolutionary change in a population may result from a change in the environment
6.2, 6.8
Selective forces (in environment) act on phenotypic variation in population.
Change in frequency of traits through differential survival and reproduction 6.4
If phenotypic variation reflects underlying genetic variation, then 
change in proportion of genotypes over time because genotypes have unequal
fitness. 6.9
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 119-120
Stabilizing (most common) 6.6
Directional
B) Small population size 267; 277-278
Lose genetic variation by:
1. Genetic drift: due to random variation in fecundity and mortality;
alleles become fixed (no variation); common in small populations
2. Founder event: small number found new population and carry only partial
sample of gene pool of parent population 13.9
3. Bottleneck: period of very small population size when genetic drift can
cause loss of genetic variation 13.9
C) Assortative mating: changes genotype frequency, not allele frequency 273-4
1. Negative: mates differ genetically increases proportion of heterozygotes;
avoids inbreeding
2. Positive (includes inbreeding): mate with close relativesincreases proportion
of homozygotes 13.5, 13.6
Unmasks deleterious recessive alleles-->inbreeding depression (less fit
offspring) 13.7, 13.8
Optimal outbreeding distance 13.15
D) Gene flow (migration/dispersal) 267; 271
Mixes alleles between subpopulations homogenizes differences between
subpopulations
Adds new gene combinations  adds genetic variation to subpopulation