21.1. Evolution in a Genetic Context

    A. What Causes Variations?

        1. A population is a group of organisms of the same species occupying a certain area.
        2. Evolution that occurs within a population is called microevolution.
        3. The members of a population vary from one another. Variation is the raw material for evolutionary change.

    B. Microevolution

        1. Population genetics studies the genetic variation in a population.
        2. The gene pool is the total of all the alleles in a population, described in terms of gene frequencies.
        3. Neither dominance nor sexual reproduction changes allele frequencies.
        4. The Hardy-Weinberg Law
            a. This law states an equilibrium of allele frequencies in a gene pool (using a formula p2 + 2pq+q2)
                remains in effect in each succeeding generation of a sexually reproducing population if five conditions are met.
                1) No mutation: no allelic changes occur.
                2) No gene flow: migration of alleles into or out of the population does not occur.
                3) Random mating: individuals pair by chance and not according to the genotypes of phenotypes.
                4) No genetic drift: the population is large so changes in allele frequencies due to chance are insignificant.
                5) No selection: no selective force favors one genotype over another.
            b. In reality, conditions of the Hardy-Weinberg law are rarely, if ever, met, and allele frequencies in the
                gene pool of a population do change from one generation to the next, resulting in evolution.
            c. Any change of allele frequencies in a gene pool of a population signifies that evolution has occurred.
            d. The Hardy-Weinberg law tells us what factors cause evolution -- those that violate the conditions listed.
            e. The Hardy-Weinberg equilibrium provides a baseline by which to judge whether evolution has occurred.
            f. Hardy-Weinberg equilibrium is a constancy of a gene pool frequencies that remains across generations.
        5. Microevolution is the accumulation of small changes in a gene pool over a short period.
            a. Industrial melanism illustrates a change in allele frequencies that resulted in a change in phenotype
                frequencies in a short time.
            b. Light-colored pepper moth populations became dark-colored correlated after increasing pollution.

    C. Genetic Mutations

        1. Natural populations contain high levels of allele variations.
            a. Analysis of Drosphilia enzymes indicates have at least 30% of gene loci with multiple alleles.
            b. Similar results with other species indicates that allele variation is the rule in natural populations.
        2. Gene mutations provide new alleles, and therefore are the ultimate source of variation.
            a. A gene mutation is an alteration in the DNA nucleotide sequence of an allele.
            b. Mutations may not immediately affect the phenotype.
            c. Mutations can be beneficial, neutral, or harmful; a seemingly harmful mutation that requires Daphnia
                to live at higher temperatures becomes advantageous when the environment changes.
            d. Specific recombinations of alleles may be more adaptive than other combinations.

    D. Gene Flow

        1. Gene flow moves alleles among populations by migration of breeding individuals.
        2. Gene flow can increase variation within a population by introducing novel alleles produced by nutation
            in another population.
        3. Continued gene flow decreases diversity among populations, causing gene pools to become similar.
        4. Gene flow among populations can prevent speciation from occurring.

    E. Nonrandom Mating

        1. Random mating involves individuals pairing by chance, not according to genotype or phenotype.
        2. Nonrandom mating involves individuals inbreeding and assortative mating.
        3. Inbreeding is mating between relatives to a greater extent than by chance.
            a. Inbreeding decreases the proportion of heterozygotes.
            b. Inbreeding increases the proportions of both homozygous at all gene loci.
            c. In human populations, inbreeding increases the frequency of recessive abnormalities.
        4. Assortative mating occurs when individuals mate with those that have the same phenotype.
            a. Assortative mating divides a population into two phenotypic classes with reduced gene exchange.
            b. Homozygotes for gene loci that control a trait increase, and heterozygotes for these loci decrease.
        5. Sexual selection occurs when males compete for the right to reproduce and the female selects.

    F. Genetic Drift

        1. Genetic drift refers to changes in allele frequencies of a gene pool due to chance.
        2. Genetic drift occurs in both large and small populations; large populations suffer less sampling error.
        3. Genetic drift causes isolated gene pools to become dissimilar; some alleles are lost and others are fixed.
            a. Separate cypress groves in California show patchy variation.
            b. Because there is no apparent adaptive advantage to the variation, this is due to genetic drift.
        4. Genetic drift occurs when founders start new population, or after a genetic bottleneck with interbreeding.
            a. The bottleneck effect prevents most genotypes from participating in production of the next generation.
                1) Bottleneck effect is caused by a severe reduction in population size due to natural disaster, predation,
                    or habitat reduction.
                2) Bottleneck effect causes severe reduction in total genetic diversity of the original gene pool.
                3) The cheetah bottleneck causes relative infertility because of the intense interbreeding when populations
                    were reduced in earlier times.
            b. Founder effect is genetic drift where rare alleles or combinations occur in higher frequency in a population
                isolated from the general population.
                1) This is due to founding individuals containing a fraction of total genetic diversity of original population.
                2) Which particular alleles are carried by the founders is dictated by chance alone.
                3) Dwarfism is much higher in a Pennsylvania Amish community due to a few German founders.

21.2. Natural Selection

    A. Natural selection is the process that results in adaptation of a population to the environment.

        1. Natural selection requires:
            a. variation (i.e., the members of a population differ from one another),
            b. inheritance (i.e., many of the differences between individual in a population are heritable genetic differences),
            c. differential adaptedness (i.e., some differences affect how well an organism is adapted to its environment),and
            d. differential reproduction (i.e., better adapted individuals are more likely to reproduce).
        2. Fitness is the extent to which an individual contributes fertile offspring to the next generation.
        3. Relative fitness compares the fitness of one phenotype to another.

    B. Types of Selection

        1. Directional selection occurs when extreme phenotype is favored; the distribution curve shifts that direction.
            a. A shift of dark-colored peppered moths from light-colored correlated with increasing pollution.
            b. Increases in insecticide-resistant mosquitoes and resistance of malaria agent to medications are
                examples of directional selection.
            c. The gradual increase in the size of the modern horse, Equus, correlates with a change in the environment
                from forest-like conditions to grassland conditions.
        2. Stabilizing selection occurs when extreme phenotypes are eliminated and the phenotype is favored.
            a. The average human birth weight is near optimum birth weight for survival.
            b. The death rate is highest for infants at the extremes of the ranges of birth weights.
        3. Disruptive selection occurs when extreme phenotypes are favored and can lead to more than one distinct form.
            a. British snails (Cepaea nemoralis) vary because a wide range causes natural selection to vary.
            b. In forest areas, thrushes feed on snails with light bands.
            c. In low-vegetation areas, thrushes feed on snails with dark shells that lack light bands.

    C. Maintenance of Variations

        1. Populations that lack variation may not be able to adapt to new conditions.
        2. How is variation maintained in the face of constant selection pressure?
        3. The following forces promote genetic variation.
            a. Mutation and genetic recombination still occur.
            b. Gene flow among small populations introduces new alleles.
            c. Natural selection, such as disruptive selection, itself sometimes results in variations.

    D. Diploidy and the Heterozygote

        1. Only alleles that cause phenotypic differences are subject to natural selection.
        2. In diploid organisms, a heterozygote shelters of rare recessive alleles that would otherwise be selected out.
        3. Even when selection reduces the recessive allele frequency from 0.9 to 0.1, the frequency in the heterozygote
            remains the same and remains a resource for natural selection in a new environment.

    E. Sickle-Cell Disease

        1. In sickle-cell disease, heterozygotes are more fit in malaria areas because the sickle-cell trait does not
            express unless the oxygen content of the environment is low; but the malaria agent causes red blood cells
            to die when it infects them (loss of potassium).
        2. Some homozygous dominants are maintained in the population but they die at an early age from sickle-cell disease.
        3. Some homozygotes are maintained in the population for normal red blood cells, but they are vulnerable to malaria.

21.3. Speciation

    A. Speciation is the splitting of one species into two or more species or the transformation of one species
        into a new species over time; speciation is the final result of changes in gene pool allele and genotypic
        frequencies.

    B. A biological species is a category whose members are reproductively isolated from all other such groups.

        1. Linnaeus separated species based on morphology.
        2. Reproductive isolation occurs when members of one species can only breed successfully with each other.
        3. Biochemical genetics uses DNA hybridization techniques to determine relatedness of organisms.

    C. Reproductive Isolating Mechanisms

        1. For two species to separate, gene flow must not occur between them.
        2. A reproductive isolating mechanism is any structural, functional, or behavioral characteristic that
            prevents successful reproduction from occurring.
        3. Prezygotic isolating mechanisms are anatomical or behavioral differences between the members of two
            species that prevent mating or make it unlikely fertilization will take place if mating occurs.
            a. Habitat isolation occurs when two species occupy different habitats, even within the same geographic
                range, so that they are less likely to meet and to attempt to reproduce.
            b. Temporal isolation occurs when two species live in the same location, but each reproduces at a different
                time of year, and so they do not attempt to mate.
            c. Behavioral isolation results from differences in mating behavior between two species.
            d. Mechanical isolation is the result of differences between two species in reproductive structures or
                other body parts, so that mating is prevented.
        4. Postzygotic isolating mechanisms prevent successful development after mating has taken place.
            a. Gamete isolation includes incompatibility of gametes of two different species so they cannot fuse to
            form a zygote; an egg may have receptors only for the sperm of its own species or a plant stigma prevents
            completion of pollination.
            b. Zygote mortality is when hybrids (offspring of parents of two different species) do not live to reproduce.
            c. Hybrid sterility occurs when the hybrid offspring are sterile (e.g., mules).
            d. In F2 fitness, the offspring are fertile but the F2 generation is sterile.

    D. Modes of Speciation

        1. Allopatric speciation occurs when new species result from populations being separated by a geographical
            barrier that prevents their members from reproducing with each other.
            a. First proposed by Ernst Mayr of Harvard University.
            b. While geographically isolated, variations accumulate until the populations are reproductively isolated.
        2. Sympatric speciation would occur when members of a single population develop a genetic difference
            (e.g., chromosome number) that prevents them from reproducing with the parent type.
            a. Main example of sympatric speciation is in plants.
            b. Failure to reduce chromosome number produces polyploid plants that reproduce successfully only
            with polyploids.
            c. Backcrosses with diploids are sterile.

    E. Adaptive Radiation

        1. Adaptive radiation is a rapid development from a single ancestral species of many new species.
        2. The case of Darwin's finches illustrates the adaptive radiation of 13 species from one founder mainland finch.
        3. On Hawaiian Islands, a wide variety of honeycreepers descended from one goldfinch-like ancestor.
        4. Hawaii is also the home of the silversword plants that radiated from ancestral tarweeds.