15.1 Natural Selection

• Natural selection is a process that leads to the adaptation of a population to both living and nonliving components of the environment.

• Biotic components of the environment include competition, predation, and parasitism, which organisms respond to in order to acquire resources.

• Abiotic components of the environment include weather conditions, which are dependent on temperature and precipitation.

• Charles Darwin believed that species evolve over time and suggested natural selection as the mechanism for adaptation to the environment.

• Darwin's hypothesis of natural selection is consistent with modern genetics, as stated in Table 15.1.

• Natural selection results in the fittest individuals becoming more prevalent in a population, leading to changes over time.

• The fittest individuals are those that reproduce more than others and are better adapted to the environment in most cases.

Types of Selection

• Multiple alleles can produce a range of phenotypes.

• Frequency distributions of phenotypes in a population often resemble bell-shaped curves.

• Natural selection works to decrease detrimental phenotypes and favors those better adapted to the environment.

• Three types of natural selection: stabilizing, directional, and disruptive.

• Stabilizing selection favors intermediate phenotypes and selects against extreme phenotypes.

• Directional selection favors extreme phenotypes and shifts the frequency distribution curve in that direction.

• Resistance to antibiotics and insecticides is an example of directional selection.

• The human struggle against malaria is an example of directional selection.

• Directional selection was observed in a guppie experiment.

• The environment had two areas: below the waterfall with pike and above the waterfall without pike.

• Natural selection favored small and drab-colored male guppies in the lower area to avoid detection by the pike.

• Male guppies moved to the area above the waterfall showed a change in phenotype towards larger, more colorful guppies.

• Disruptive selection favors two or more extreme phenotypes over intermediate phenotypes.

• Disruptive selection favors polymorphism.

• Example: British land snails found in low-vegetation areas and forests.

• Thrushes feed mainly on snails with dark shells without light bands in low-vegetation areas.

• Thrushes feed mainly on snails with light-banded shells in forest areas.

• Two distinctly different phenotypes, each adapted to its own environment, are found in this population.

Sexual Selection

• Sexual selection refers to adaptive changes in males and females that increase their ability to secure a mate.

• Each sex has a different strategy for sexual selection.

• Females produce few eggs, so the choice of a mate is important.

• Males can father many offspring because they continuously produce sperm in great quantity.

• Sexual selection in males usually results in an increased ability to compete with other males for a mate.

• Sexual selection in females favors the choice of a single male with the best fitness.

• Males often demonstrate their fitness by coloration or elaborate mating rituals.

• By choosing a male with optimal fitness, the female increases the chances that her traits will be passed on to the next generation.

• Sexual selection is considered a form of natural selection by many.

Adaptations Are Not Perfect

• Natural selection does not always produce perfectly adapted organisms to their environment.

• Evolution is constrained by the available variations, and each species must build upon its own evolutionary history.

• The amount of variation that may be acted on by natural selection is limited.

• As adaptations evolve in a species, the environment may also change.

• Most adaptations provide a benefit to the species for a specific environment for a specific time.

• Imperfections are common because of necessary compromises.

• The success of humans is attributable to their dexterous hands, but the spine is subject to injury because the vertebrate spine did not originally evolve to stand erect.

• A feature that evolves has a benefit that is worth the cost.

Maintenance of Variations

• A population always has genotypic variation.

• Variation is beneficial for adaptation to new environments and avoiding extinction.

• Forces that promote variation are always at work: mutation, recombination, independent assortment, and fertilization.

• Gene flow between populations can introduce new alleles.

• Natural selection favors certain phenotypes, but other phenotypes may remain in the population at a reduced frequency.

• Disruptive selection promotes polymorphism in a population.

• Heterozygotes in diploid species help maintain variation by conserving recessive alleles in the population.

The Heterozygote Advantage

• Only expressed alleles are subject to natural selection.

• Heterozygotes can protect recessive alleles from being weeded out.

• Heterozygotes can maintain the possibility of a recessive phenotype with greater fitness in a changed environment.

• Balanced polymorphism occurs when natural selection maintains two different alleles of a gene at a certain ratio.

• Sickle-cell disease is an example of balanced polymorphism.

• HbSHbS genotype leads to sickle-cell disease and early death.

• HbAHbS genotype leads to sickle-cell trait and better survival in low-oxygen environments.

• HbAHbA genotype is the fittest under normal conditions.

• Malaria is prevalent in regions with a higher frequency of the recessive allele (HbS).

• Heterozygotes (HbAHbS) are favored in malaria-prevalent environments because the parasite cannot live in their sickle-shaped red blood cells.

• Homozygotes are selected against, but the recessive allele is maintained in the population.

• Table 15.2 summarizes the effects of the three possible genotypes.

15.2 Microevolution

• Traits can change temporarily in response to a varying environment.

  • Examples include:

    • Arctic foxes change fur color from brown to white in winter.

    • Dog's fur increases in thickness in cold weather.

    • Skin bronzing, when exposed to the sun, lasts only for a season.

  • These are not evolutionary changes because they are not heritable

• Evolution causes a change in a heritable trait within a population, not within an individual, over many generations.

• Darwin observed that populations, not individuals, evolve.

• Genes interact with the environment to determine traits.

• Evolution is about genetic change, specifically the change in allele frequencies in a population over time.

• This type of evolution is called microevolution.

Evolution in a Genetic Context

• Biologists applied genetics principles to populations in the 1930s.

• They developed a way to recognize evolution and measure population changes.

• In population genetics, the gene pool is made up of various alleles at all gene loci in individuals.

• The gene pool is described in terms of genotype and allele frequencies.

• Genotype frequency is the percentage of a specific genotype in a population.

• Allele frequency represents how much a specific allele is represented in the gene pool.

• Peppered moths can be light or dark-colored.

  • Color is controlled by a single set of alleles: D = dark color, d = light color

• In a population of peppered moths in Great Britain before pollution darkened trees:

  • 4% were homozygous dominant (DD)

  • 32% were heterozygous (Dd)

  • 64% were homozygous recessive (dd)

• Allele frequencies can be calculated from genotype frequencies

• The population has a 20% frequency of the D allele and an 80% frequency of the d allele.

• Gametes have a 20% chance of carrying the D allele and an 80% chance of carrying the d allele.

• A punnett square can be used to calculate genotype ratios in the next generation, assuming random mating.

• To produce a DD moth, both parents must contribute the D allele (20% chance each).

• The probability of both events occurring is 0.20 x 0.20 = 0.04 (4%).

• Therefore, 4% of the next generation should be homozygous dominant (DD).

• Genotype frequencies in the next generation are the same as in the previous generation.

• Sexual reproduction alone cannot bring about a change in genotype and allele frequencies.

• Dominance does not cause an allele to become a common allele.

• Allele frequencies of the gene pool remain at equilibrium from one generation to the next.

  • Recognized in 1908 by G.H. Hardy and W. Weinberg

• The developed binomial equation to calculate genotype and allele frequencies

• p = frequency of dominant allele, q = frequency of recessive allele

• Equation: p² + 2pq + q² = 1

• Five conditions must be met for mathematical relationships to remain in effect:

  1. No mutations

  2. No gene flow

  3. Random mating

  4. No genetic drift

  5. No selection

• Microevolution occurs when conditions are not met.

• Deviation from Hardy-Weinberg equilibrium can measure microevolution.

• These conditions are rarely if ever, met in real populations.

• As a result, allele frequencies in real populations do change from one generation to the next.

• This change in allele frequencies is called microevolution.

• Microevolution can be detected and measured by noting the amount of deviation from Hardy-Weinberg equilibrium.

• Natural selection is one of the factors that can cause microevolution.

• In the case of industrial melanism, the frequency of the dark-colored phenotype increased in moth populations because the dark-colored moths were better camouflaged against the darker tree trunks that were caused by pollution.

• This change in phenotype frequency was due to natural selection, and it is an example of microevolution.

Causes of Microevolution

• Any conditions that change the equilibrium of alleles in a population can cause evolutionary change.

• Five factors can cause a divergence from the Hardy-Weinberg equilibrium:

  • Genetic mutation

  • Gene flow

  • Nonrandom mating

  • Genetic drift

  • Natural selection.

Genetic Mutation

• Mutations are permanent genetic changes that are the raw material for evolutionary change.

• Without mutations, there can be no new variations among members of a population on which natural selection can act.

• The rate of mutations is generally very low, on the order of 1 mutation per 100,000 cell divisions.

• Many mutations are neutral, meaning that they are not selected for or against by natural selection.

• Prokaryotes are more dependent than eukaryotes on mutations to introduce variations because they do not reproduce sexually.

• All mutations that occur and result in phenotypic differences can be tested by the environment.

• In sexually reproducing organisms, mutations, if recessive, do not immediately affect the phenotype.

• In a changing environment, even a seemingly harmful mutation that results in a phenotypic difference can be the source of an adaptive variation.

• The water flea Daphnia has a mutation that requires it to live at temperatures between 25°C and 30°C, which is adaptive only under certain environmental conditions.

Gene Flow

• Gene flow is the movement of alleles among populations by migration of breeding individuals.

• Gene flow can increase variation within a population by introducing novel alleles produced by mutation in another population.

• Continued gene flow due to the migration of individuals makes gene pools similar and reduces the possibility of allele frequency differences among populations now and in the future.

• Gene flow among populations can prevent speciation from occurring.

• Due to gene flow, the snake populations featured in Figure 15.10 are subspecies of Pantherophis obsoleta.

• These subspecies can readily interbreed when they come in contact with one another due to enough genetic similarity between the populations.

Nonrandom Mating

• Random mating: individuals select mates and pair by chance, not according to their genotypes or phenotypes.

• Inbreeding: mating between relatives, an example of nonrandom mating.

• Inbreeding does not change allele frequencies but gradually increases the proportion of homozygotes.

• Assortative mating: individuals tend to mate with those that have the same phenotype with respect to a certain characteristic.

• Assortative mating causes the population to subdivide into two phenotypic classes, between which gene exchange is reduced.

• Homozygotes for the gene loci that control the trait in question increase in frequency, and heterozygotes for these loci decrease in frequency.

• Sexual selection favors characteristics that increase the likelihood of obtaining mates and promotes nonrandom mating.

• In most species, males that compete best for access to females and/or have a phenotype that attracts females are more apt to mate and have increased fitness.

Genetic Drift

• Genetic drift refers to changes in allele frequencies of a gene pool due to chance.

• Allele frequencies "drift" over time and can increase or decrease depending on which members of a population die, survive, or reproduce with one another.

• Genetic drift occurs in both large and small populations, but a larger population is expected to suffer less of a sampling error than a smaller population.

• In a small population, random events may reduce the ability of one genotype to produce the next generation, leading to a change in allele frequencies.

• Genetic drift can lead to the loss of one or more alleles, causing other alleles to become fixed in the population over time.

• An experiment involving brown eye color in Drosophila flies showed that the random selection of males and females acted as a form of genetic drift, leading to the fixation of certain alleles in some populations.

• Genetic drift is a random process that produces different results in different populations.

• Cypress groves in California are separate populations with different phenotypes.

• Phenotypes within each grove are more similar to each other than to other groves.

• Environmental conditions are similar for all groves, and variations among populations are due to genetic drift.

• The bottleneck effect occurs when a species is subjected to near extinction, preventing most genotypes from participating in the next generation.

• Cheetahs have extreme genetic similarity due to a bottleneck, causing certain alleles to be lost from the population.

• The founder effect is a mechanism of genetic drift in which rare alleles occur at a higher frequency in a population isolated from the general population.

• The Amish of Lancaster County, Pennsylvania, carries a recessive allele causing an unusual form of dwarfism and polydactylism at a higher frequency due to the founder effect.