• Evolution is now understood to be driven by four processes:

  • Niαtural selection increases the frequency of certain alleles, the ones that contribute to reproductive success in a particular environment

  • Genetic drift causes allele frequencies to change randomly. In some cases, drift may cause alleles that decrease fitness to increase in frequency

  • Gene flow occurs when individuals leave one population, join another, and breed.

  • Mutαtion modifies allele frequencies by continually introducing new alleles.

23.1 Null Hypothesis: The Hardy-Weinberg Principle

• They imagined a hypothetical scenario in which all of the alleles from all the gametes produced in each generation go into a single group called the gene pool

• Hardy-Weinberg principle which serves as a mathematical null hypothesis for the study of evolutionary processes.

• If biologists observe frequencies that conform to 1e Hardy-Weinberg predictions, the frequencies are said to be in Hardy-Weinberg equilibrium.

• The mathematical model underlying the Hardy-Weinberg principle is based on five important assumptions about how populations and alleles behave:

  • Random mating

  • No natural selection

  • No genetic drift(random allele frequency changes)

  • No gene flow

  • No mutation

23.2 Nonrandom Mating

• The most intensively studied form of nonrandom mating is called inbreeding, the mating between relatives.

• Deleterious alleles are alleles that lower fitness

• Inbreeding depression is the decline in average fitness that may take place when homozygosity increases and heterozygosity decreases in a population.

• Sexual selection is fundamentally different from inbreeding because it does lead to changes in allele frequencies in 1e population and increases fitness, and us it is a form of natural selection

23.3 Natural Selection

• Traits that enable organisms to survive in their physical and biological environments, are sometimes called ecological ( or environmental) selection.

• Genetic variation is the number and relative frequency of alleles that are present in a particular population.

• Natural selection occurs in four main patterns, or modes, each affecting genetic variation in different ways:

  • Directionαl selection changes the average value of a trait.

  • Stabilizing selection reduces variation in a trait.

  • Disruptive selection increases variation in a trait.

  • Bαlαncing selection maintains variation in a trait.

• When directional selection occurs, the average phenotype of a population changes in one direction.

• When deleterious alleles decline in frequency, purifying selection is said to occur.

• Disruptive selection eliminates phenotypes near the average value and favors extreme phenotypes

• Stabilizing selection favors phenotypes near the average value and eliminates extreme phenotypes

• Balancing selection occurs when no phenotype has a distinct advantage.

• Of the various mechanisms of balancing selection, the following are the most common:

  • Heterozygote advantage occurs when heterozygous individuals have higher fitness than homozygous individuals do.

  • Frequency-dependent selection occurs when certain alleles are favored when they are rare, but not when they are common

• Intersexual selection is the selection of an individual of one sex for mating by an individual of the other sex.

• Individuals of the same sex sometimes compete with one another to obtain mates is referred to as intrasexual selection.

• A territory is an actively defended area where the owner has exclusive or semi“ exclusive use.

• Both intersexual and intrasexual selection help to explain why some aits differ between the sexes of the same species, a difference called sexual dimorphism

• Natural selection is the only evolutionary process that results in adaptation, but it is not the only evolutionary process that violates the Hardy-Weinberg assumptions.

23.4 Genetic Drift

• Genetic drift is defined as a change in allele frequencies in a population that is due to chance.

• Sampling error occurs when the allele frequencies of a chosen subset of a population (the sample) are different from those in the total population, by chance

• Key information to remember about genetic drift are:

  • Genetic drift is random with respect to fitness

  • Genetic drift is most pronounced in small populations

  • Over time, genetic drift can lead to the random loss or fixation of alleles

• A genetic bottleneck is a sudden reduction in the diversity of alleles in a population

23.5 Gene Flow

• Gene flow is the movement of alleles between populations

• If a population has lost alleles due to genetic drift, then the arrival of new alleles via gene flow should increase genetic diversity.

• If increased genetic diversity results in better resistance to infections by bacteria or viruses or other parasites, for example, gene flow would increase the average fitness of individuals.

23.6 Mutation

• Natural selection often favors certain alleles and leads to a decrease in overall genetic variation in a population.

• Genetic drift tends to decrease genetic diversity over time, as alleles are randomly lost or fixed in a population.

• Gene flow increases genetic diversity in a recipient population if new alleles arrive with immigrating individuals. But gene flow may decrease genetic variation in the source population if alleles leave with emigrating individuals.

• Mutations can occur in a number of ways:

  • Point mutation: A change in a single base pair in DNA.

  • Chromosome-level mutation: A change in the number or composition of chromosomes.

  • Lateral gene transfer: The transfer of genetic information from one species to another, rather than from parent to offspring.

• Beneficial allele is an allele that allows individuals to produce more surviving offspring, increasing fitness.

• Neutral allele is an allele with no effect on fitness, as occurs when a point mutation is silent

• Key information about mutations are that

  • Mutation is the ultimate source of genetic variation

  • Mutations are random with respect to fitness

  • If mutation did not occur, evolution would eventually stop.

  • Mutation alone is usually inconsequential in changing allele frequencies at a particular gene.