22.2- What Are the Mechanisms of Evolutionary Change?
• Hardy-Weinberg equilibrium is a null hypothesis that assumes evolutionary forces are
• Known evolutionary mechanisms:
• Mutation • Nonrandom mating
• Gene flow • Natural selection
• Genetic drift
Mutations Generate Genetic Variation
• Origin of genetic variation is mutation; mutation is any change in an organism’s DNA
• Most mutations are harmful to their bearers or are neutral, but if environmental conditions
change, previously harmful or neutral alleles may become advantageous
• Mutations can restore to populations alleles that other evolutionary processes have
• Most mutations appear to be random and are harmful or neutral to their bearers.
• Some mutations can be advantageous.
• Mutation rates are low; one out of a million loci is typical.
• Although mutation rates are low, they are sufficient to create considerable genetic
• Rates as high as one mutation per locus in a thousand zygotes per generation are rare; one
in a million is more typical
• One condition for Hardy–Weinberg equilibrium is that there is no mutation.
• Although this condition is never met, the rate at which mutations arise at single loci is
usually so low that mutations result in only very small deviations from Hardy–Weinberg
• If large deviations (from H-W expectations) are found, it is appropriate to dismiss mutation
as the cause and look for evidence of other evolutionary agents.
Gene flow may change allele frequencies
• Gene flow results when individuals migrate to another population and breed in new
• No immigration is allowed for a population to be in Hardy–Weinberg equilibrium.
Genetic drift may cause large changes in small populations
• Genetic drift is the random loss of individuals (and their alleles)-may produce large
changes in allele frequencies from one generation to the next
• In very small populations, genetic drift may be strong enough to influence the direction of
change of allele frequencies even when other evolutionary agents are pushing the
frequencies in a different direction.
• Organisms that normally have large populations may pass through occasional periods
when only a small number of individuals survive (a population bottleneck). Genetic
variation can be reduced by genetic drift.
o Population bottlenecks occur when only a few individual survive a random event,
resulting in a shift in allele frequencies within the population
• Founder effect- random changes in allele frequencies resulting from establishment of a
population by a very small number of individuals
o When a few pioneering individuals colonize a new region, the resulting population
will not have all the alleles found among members of the source population.
Nonrandom Mating Changes Genotype Frequencies
• Nonrandom mating occurs when individuals mate either more often with individuals of
the same genotype or more often with individuals of a different genotype.
• The resulting proportions of genotypes in the following generation differ from Hardy–
• If individuals mate preferentially with other individuals of the same genotype, homozygous
genotypes are overrepresented and heterozygous genotypes are underrepresented in the
• Conversely, individuals may mate preferentially with individuals of a different genotype • Self-fertilization (selfing) is another form of nonrandom mating that is common in many
organisms, especially plants.
• Selfing reduces the frequencies of heterozygous individuals below Hardy–Weinberg
expectations and increases the frequencies of homozygotes, without changing allele
frequencies, and thus not result in adaptation
• Sexual selection- is a particularly important form of nonrandom mating that does change
allele frequencies and often results in adaptations
22.3- What Evolutionary Mechanisms Result in Adaptation?
• Recall: for adaptation (and evolution) to occur, individuals that differ in heritable traits
must survive & reproduce with different degrees of success.
• When some individuals contribute more offspring to the next generation than others, allele
frequencies in the population change in a way that adapts individuals to the environments
that influenced their success: natural selection.
• The reproductive contribution of a phenotype to subsequent generations, relative to the
contributions of other phenotypes, is called its fitness.
• 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
• Most characters (traits) are influenced by alleles at more than one locus and are more
likely to show quantitative rather then qualitative variation.
• For example, the body size of individuals in a population is influenced by genes at many
loci, and distribution of body sizes is likely to be a bell-shaped curve. Quantitative
variation: avg body size of a population may increase or decrease as a result of selection.
• Natural selection can act on characters with quantitative variation in three ways:
Stabilizing selection-preserves the average characteristics of a population by
favoring average individuals
Directional selection-changes the characteristics of a population by favoring
individuals that vary in one direction from the mean of the population
Disruptive selection- changes the characteristics of a population by favoring
individuals that vary in opposite directions from the mean of the population
• Stabilizing selection favors average individuals.
the extremes of a population con