Mechanisms of evolutionary change
22.2 What are the Mechanisms of Evolutionary Change?
Mutations generate genetic variation
• Origin of genetic variation is mutation: any change in an organism’s DNA.
• Mutations appear to be random with respect to the adaptive needs of
• Most are harmful to their bearers or neutral, but if environmental conditions
change, previously harmful or neutral alleles may become advantageous.
• Can restore to populations alleles that other evolutionary processes have
• Create and help maintain genetic variation within populations.
• Mutation rates are very low for most loci: one in a million is a typical chance.
Gene flow may change allele frequencies
• Migration of individuals and movements of gametes between populations,
gene flow, are common.
• If the arriving individuals or gametes survive and reproduce in their new
location, they may add new alleles to the gene pool of the population, or they
may change the frequencies all alleles already present if they come from a
population with different allele frequencies.
Genetic drift may cause large changes in small populations
• Genetic drift—random changes in allele frequencies—may produce large
changes in allele frequencies from one generation to the next.
• Populations that are normally large may pass through occasional periods
when only a small number of individuals survive population bottlenecks;
genetic variation can be reduced by genetic drift.
• Genetic drift can have similar effects when a few pioneering individuals
colonize a new region.
o B/c of its small size, the colonizing population is unlikely to have all the
alleles found among members of its source population.
o The resulting change in genetic variation, founder effect, is
equivalent to that in a large population reduced by a bottleneck.
Non-random mating changes genotype frequencies
• Mating patterns may alter genotype frequencies if individuals in a population
choose other individuals of particular genotypes as mates (non-random
o For example, if females mate with males of the same genotype, then
homozygous genotypes will be overrepresented and heterozygous
• Self-fertilization (selfing), another form of non-random mating.
• Sexual selection is a particularly important form of non-random mating that
DOES change allele frequencies and often results in adaptations.
22.3 What Evolutionary Mechanisms Result in Adaptation?
• Adaptation occurs when some individuals in a population contribute more
offspring to the next generation that others allele frequencies in the population change in a way that adapts individuals to the environment that
influenced such reproductive success (natural selection).
• Natural selection acts on the phenotype.
• The reproductive contribution of a phenotype to subsequent generations
relative to the contributions of other phenotypes is called its fitness.
• Changes in absolute numbers of offspring are responsible for increases and
decreases in the size of a population, but only changes in the relative success
of different phenotypes within a population lead to changes in allele
frequencies from one generation to another.
• The fitness of a phenotype is determined by the average rates of survival and
reproduction of individuals with that phenotype.
Natural selection produces variable results
• Natural selection can act on charact