Evolutionary Change in Population - Microevolution
Microevolution is defined as changes in the frequency of a gene in a population.
These are subtle changes that can occur in very short periods of time, and may not be visible to a casual observer.
Mathematically, we can determine whether microevolution is occuring by assessing whether a population is
in Hardy-Weinberg Equilibrium.
There are five "forces" that can cause shifts in gene frequency. : nonrandom mating, mutation, genetic drift, gene
flow, and natural selection.
When one or more of these processes acts on a population, allele or genotype frequencies change from one
generation to the next.
Nonrandom mating changes genotype frequencies
When individuals select mates on the basis of phenotype (thereby selecting the corresponding genotype), they bring
about evolutionary change in the population.
Examples: inbreeding and assortative mating.
Inbreeding - The mating of genetically similar individuals that are more closely related than if they had been chosen
at random from the entire population.
 the frequency of homozygous genotypes increases with each successive generation of inbreeding

inbreeding does not change the overall allele frequency
 Example: Self-fertilization in plants
 inbreeding depression - inbred individuals have lower fi tness than those not inbred.
In inbreeding depression, as evidenced by fertility declines and high juvenile mortality, is thought to be caused by
the expression of harmful recessive alleles as homozygosity increases with inbreeding.
Assortative mating - individuals select mates by their phenotypes
Example: biologists selected two phenotypes—high bristle number and low bristle number—in a fruit fly (Drosophila
melanogaster) population.
Observation: flies preferentially mated with those of similar phenotypes
 positive assortative mating - same phenotype are selected
 negative assortative mating -opposite phenotypes are selected
Mutation increases variation within a population
Mutations occur unpredictably and spontaneously. Mutation may result from:
 a change in the nucleotide base pairs of a gene
 a rearrangement of genes within chromosomes so that their interactions produce different effects
 a change in chromosome structure.
Mutation by itself causes small deviations in allele frequencies from those predicted by the Hardy–Weinberg
principle. Mutation is usually negligible as an evolutionary force, but it is essential to the evolutionary process
because it is the ultimate source of genetic variation
In genetic drift, random events change allele frequencies
 The production of random evolutionary changes in small breeding populations is known as genetic drift
 Genetic drift results in changes in allele frequencies in a population from one generation to another
 genetic drift decreases genetic variation within a population, although it tends to increase genetic
differences among different populations.
The Effect of Genetic Drift is Inversely Related to Population Size
Gene flow generally increases variation within a population
The migration of breeding individuals between populations causes a corresponding movement of alleles, or gene
flow, that has significant evolutionary consequences.
 As alleles flow from one population to another, they usually increase the amount of genetic variability within
the recipient population
 If sufficient gene flow occurs between two populations, they become more similar genetically.
 it tends to counteract the effects of natural selection and genetic drift
Natural selection changes allele frequencies in a way that increases adaptation
 Natural selection operates on an organism’s phenotype
 Three kinds of selection cause changes in the normal distribution of phenotypes in a population: stabilizing,
directional, and disruptive selection.
 Although we consider each process separately, in nature their influences generally overlap.
Stabilizing selection- The process of natural selection associated with a population well adapted to its environment
 Stabilizing selection selects against phenotypic extremes
 individuals with average, or intermediate, phenotypes are favored
 Example: human birth weight, which is under polygenic control and is also influenced by environmental
factors
Directional Selection. If an environment changes over time, directional selection may favor phenotypes at one of the
extremes of the normal distribution
 Over successive generations, one phenotype gradually replaces another
 Example: Darwin’s Galápagos finches provide an excellent example of directional selection.
Disruptive Selection- a special type of directional selection in which there is a trend in several directions rather than
just one.
 It results in a divergence, or splitting apart, of distinct groups of individuals within a population. Disruptive
selection, which is relatively rare, selects against the average, or intermediate, phenotype.