The Evolution of Populations


I.         Population genetics considers how populations change genetically over time. The basic ideas are:

            A.        The importance of populations as the units of evolution.

            B.        The central role of natural selection as the most important mechanism of adaptive evolution.

            C.        The idea of gradualism to explain how large changes can evolve as an accumulation of small changes over long periods of time.

            D.        A population’s gene pool is defined by its allele frequencies.

                        1.         A population is a localized group of individuals that belong to the same species.

                        2.         One definition of a species is a group of natural populations whose individuals have the potential to interbreed and produce fertile offspring.

                        3.         Populations of a species may be isolated from each other and rarely exchange genetic material.

                        4.         Members of a population are far more likely to breed with members of the same population than with members of other populations.

                        5.         The total aggregate of genes in a population at any one time is called the population’s gene pool.

                                    a.         It consists of all alleles at all gene loci in all individuals of a population.

                                    b.         If only one allele exists at a particular locus in a population, that allele is said to be fixed in the gene pool, and all individuals will be homozygous for that gene.

                                    c.         If there are two or more alleles for a particular locus, then individuals can be either homozygous or heterozygous for that gene.

                        6.         Each allele has a frequency in the population’s gene pool.

II.        The Hardy-Weinberg theorem describes the gene pool of a nonevolving population.

            A.        This theorem states that the frequencies of alleles and genotypes in a population’s gene pool will remain constant over generations unless acted upon by agents other than Mendelian segregation and recombination of alleles.

            B.        The shuffling of alleles by meiosis and random fertilization has no effect on the overall gene pool of a population.

                        1.         Suppose that the individuals in a population not only donate gametes to the next generation at random, but also mate at random. In other words, all male-female matings are equally likely.

                        2.         The allele frequencies in this population will not change from one generation to the next. Its genotype frequencies, which can be predicted from the allele frequencies, will also remain unchanged.

                        3.         We say that the population’s genetic structure is in a state of Hardy-Weinberg equilibrium.

            C.        The repeated shuffling of a population’s gene pool over generations does not increase the frequency of one allele over another.

            D.        The general formula is p2 + 2pq + q2 = 1.0, where p = the frequency of one allele and q = the frequency of the other allele

                        1.         Using this formula, we can calculate frequencies of alleles in a gene pool if we know the frequency of genotypes, or the frequency of genotypes if we know the frequencies of alleles.

            E.        Five conditions must be met for a population to remain in Hardy-Weinberg equilibrium. The Hardy-Weinberg theorem describes a hypothetic population that is not evolving. However, real populations do evolve, and their allele and genotype frequencies do change over time. That is because the five conditions for nonevolving populations are rarely met for long in nature.

                        1.         The five conditions are:

                                    a.         Extremely large population size. In small populations, chance fluctuations in the gene pool can cause genotype frequencies to change over time. These random changes are called genetic drift.

                                    b.         No gene flow. Gene flow, the transfer of alleles due to the migration of individuals or gametes between populations, can change the proportions of alleles.

                                    c.         No mutations. Introduction, loss, or modification of genes will alter the gene pool.

                                    d.         Random mating. If individuals pick mates with certain genotypes, or if inbreeding is common, the mixing of gametes will not be random.

                                    e.         No natural selection. Differential survival or reproductive success among genotypes will alter their frequencies.

                        2.         Evolution usually results when any of these five conditions are not met.

                        3.         Although natural populations are rarely, if ever, in true Hardy-Weinberg equilibrium, the rate of evolutionary change in many populations is so slow that they appear to be close to equilibrium.

III.       New genes and new alleles originate only by mutation.

            A.        A mutation is a change in the nucleotide sequence of an organism’s DNA.

            B.        Most mutations occur in somatic cells and are lost when the individual dies. Only mutations in cell lines that form gametes can be passed on to offspring, and only a small fraction of these spread through populations and become fixed.

            C.        A new mutation that is transmitted in a gamete to an offspring can immediately change the gene pool of a population by introducing a new allele.

            D.        Point mutations can have a significant impact on phenotype, however, most point mutations are harmless because much of the DNA in eukaryotic genomes does not code for protein products.

                        1.         Because the genetic code is redundant, some point mutations in genes that code for proteins may not alter the protein’s amino acid composition.

                        2.         On rare occasions, a mutant allele may actually make its bearer better suited to the environment, increasing reproductive success.

                        3.         This is more likely when the environment is changing.

            E.        Chromosomal mutations that delete or rearrange many gene loci at once are almost always harmful. In rare cases, chromosomal rearrangements may be beneficial.

                        1.         For example, the translocation of part of one chromosome to a different chromosome could link genes that act together to positive effect.

            F.        Gene duplication is an important source of new genetic variation.

                        1.         Small pieces of DNA can be introduced into the genome through the activity of transposons.

                        2.         Such duplicated segments can persist over generations and provide new loci that may eventually take on new functions by mutation and subsequent selection.

            G.        New genes may also arise when the coding subsections of genes known as exons are shuffled within the genome, within a single locus or between loci.

IV.      Sexual recombination also produces genetic variation.

            A.        On a generation-to-generation timescale, sexual recombination is far more important than mutation in producing the genetic differences that make adaptation possible.

            B.        Sexual reproduction rearranges alleles into novel combinations every generation.

V.        Natural selection, genetic drift, and gene flow can alter a population’s genetic composition.

            A.        Natural selection is based on differential reproductive success.

                        1.         Individuals in a population vary in their heritable traits.

                        2.         Those with variations better suited to the environment tend to produce more offspring than those with variations that are less well suited.

                        3.         As a result of selection, alleles are passed on to the next generation in frequencies different from their relative frequencies in the present population. Such differences in survival and reproductive success would disturb the Hardy-Weinberg equilibrium. The frequency of the some alleles would decline while the frequency of others would increase.

            B.        Genetic drift results from chance fluctuations in allele frequencies in small populations.

                        1.         Genetic drift occurs when changes in gene frequencies from one generation to another occur because of chance events (sampling errors) that occur in small populations.

                        2.         In a large population, allele frequencies will not change from generation to generation by chance alone. However, in a small population genetic drift can completely eliminate some alleles.

                        3.         Genetic drift at small population sizes may occur as a result of two situations: the bottleneck effect or the founder effect.

                                    a.         The bottleneck effect occurs when the numbers of individuals in a large population are drastically reduced by a disaster.

                                                (1)       By chance, some alleles may be overrepresented and others underrepresented among the survivors.

                                                (2)       Some alleles may be eliminated altogether.

                                                (3)       Genetic drift will continue to change the gene pool until the population is large enough to eliminate the effect of chance fluctuations.

                                    b.         The founder effect occurs when a new population is started by only a few individuals who do not represent the gene pool of the larger source population.

                                                (1)       Genetic drift would continue from generation to generation until the population grew large enough for sampling errors to be minimal.

            C.        A population may lose or gain alleles by gene flow.

                        1.         Gene flow is genetic exchange due to migration of fertile individuals or gametes between populations.

                        2.         Gene flow tends to reduce differences between populations.

                        3.         If extensive enough, gene flow can amalgamate neighboring populations into a single population with a common gene pool.

VI.      Natural selection is the primary mechanism of adaptive evolution and genetic variation is the substrate for natural selection.

            A.        Of all the factors that can change a gene pool, only natural selection leads to adaptation of an organism to its environment. Natural selection accumulates and maintains favorable genotypes in a population.

            B.        Geographic variation results from differences in phenotypes or genotypes between populations or between subgroups of a single population that inhabit different areas.

                        1.         Natural selection contributes to geographic variation by modifying gene frequencies in response to differences in local environmental factors.

                        2.         Genetic drift can also lead to variation among populations through the cumulative effect of random fluctuations in allele frequencies.

                        3.         Geographic variation can occur on a local scale, within a population, if the environment is patchy or if dispersal of individuals is limited, producing subpopulations. This is termed spatial variation.

            C.        Fitness is defined as the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals.

                        1.         Population geneticists define relative fitness as the contribution of a genotype to the next generation compared to the contribution of alternative genotypes for the same locus.

                        2.         Although population geneticists measure the relative fitness of a genotype, it is important to remember that natural selection acts on phenotypes, not genotypes. The whole organism is subjected to natural selection.

                        3.         The relative fitness of an allele depends on the entire genetic and environmental context in which it is expressed.

                        4.         Survival alone does not guarantee reproductive success. Relative fitness is zero for a sterile organism, even if it is robust and long-lived. On the other hand, longevity may increase fitness if long-lived individuals leave more offspring than short-lived individuals.

                        5.         In many species, individuals that mature quickly, become fertile at an early age, and live for a short time have greater relative fitness than individuals that live longer but mature later.

            D.        Natural selection can alter the frequency distribution of heritable traits in three ways, depending on which phenotypes in a population are favored.

                        1.         Directional selection is most common during periods of environmental change or when members of a population migrate to a new habitat with different environmental conditions.

                                    a.         Directional selection shifts the frequency curve for a phenotypic character in one direction by favoring individuals who deviate from the average.

                        2.         Disruptive selection occurs when environmental conditions favor individuals at both extremes of the phenotypic range over those with intermediate phenotypes.

                                    a.         Disruptive selection can be important in the early stages of speciation.

                        3.         Stabilizing selection favors intermediate variants and acts against extreme phenotypes.

                                    a.         Stabilizing selection reduces variation and maintains the status quo for a trait.

            E.        Diploidy and balancing selection preserve genetic variation.

                        1.         The tendency for natural selection to reduce variation is countered by mechanisms that preserve or restore variation, including diploidy and balanced polymorphisms.

                        2.         Diploidy in eukaryotes prevents the elimination of recessive alleles via selection because recessive alleles do not affect the phenotype in heterozygotes.

                                    a.         Even recessive alleles that are unfavorable can persist in a population through their propagation by heterozygous individuals.

                                    b.         Recessive alleles are only exposed to selection when both parents carry the same recessive allele and combine two recessive alleles in one zygote.

                                    c.         This happens only rarely when the frequency of the recessive allele is very low.

                                    d.         The rarer the recessive allele, the greater the degree of protection it has from natural selection.

                        3.         Heterozygote protection maintains a huge pool of alleles that may not be suitable under the present conditions but may become beneficial when the environment changes.

                        4.         Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypes in a population, a state called balanced polymorphism.

                                    a.         One mechanism producing balanced polymorphism is heterozygote advantage.

                                                (1)       In some situations, individuals who are heterozygous at a particular locus have greater fitness than homozygotes. In these cases, natural selection will maintain multiple alleles at that locus.

                                    b.         A second mechanism promoting balanced polymorphism is frequency-dependent selection.

                                                (1)       Frequency-dependent selection occurs when the fitness of any one morph declines if it becomes too common in the population.

            F.        Some genetic variations, neutral variations, have negligible impact on fitness, and thus natural selection does not affect these alleles.

                        1.         Pseudogenes, genes that have become inactivated by mutations, accumulate genetic variations.

                        2.         Over time, some neutral alleles will increase and others will decrease by the chance effects of genetic drift.

VII.     Sexual selection may lead to pronounced secondary differences between the sexes.

            A.        Sexual selection results in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics not directly associated with reproduction.

                        1.         Males and females may differ in size, coloration, and ornamentation.

                        2.         In vertebrates, males are usually the larger and showier sex.

            B.        It is important to distinguish between intrasexual and intersexual selection.

                        1.         Intrasexual selection is direct competition among individuals of one sex (usually males) for mates of the opposite sex.

                                    a.         Competition may take the form of direct physical battles between individuals. The stronger individuals gain status.

                                    b.         Evidence is growing that intrasexual selection can take place between females as well.

                        2.         Intersexual selection or mate choice occurs when members of one sex (usually females) are choosy in selecting their mates from individuals of the other sex.

                                    a.         Because females invest more in eggs and parental care, they are choosier about their mates than males. A female tries to select a mate that will confer a fitness advantage on their mutual offspring.

                                    b.         In many cases, the female chooses a male based on his showy appearance or behavior. Some male showiness does not seem to be adaptive except in attracting mates and may put the male at considerable risk.

                                    c.         Even if these extravagant features have some costs, individuals that possess them will have enhanced fitness if they help an individual gain a mate.

                                    d.         Every time a female chooses a mate based on appearance or behavior, she perpetuates the alleles that caused her to make that choice. She also allows a male with that particular phenotype to perpetuate his alleles.

            C.        Sex is an evolutionary enigma.

                        1.         As a mechanism of rapid population growth, sex is far inferior to asexual reproduction.

                        2.         Yet, sex is maintained in the vast majority of eukaryotic species, even those that also reproduce asexually. Sex must confer some selective advantage to compensate for the costs of diminished reproductive output. Otherwise, migration of asexual individuals or mutation permitting asexual reproduction would outcompete sexual individuals and the alleles favoring sex.

                        3.         How can the genetic variation promoted by sex be advantageous in the short term, on a generation-to-generation timescale?

                                    a.         Genetic variability may be important in resistance to disease.

                                                (1)       Parasites and pathogens recognize and infect their hosts by attaching to receptor molecules on the host’s cells.

                                                (2)       There should be an advantage to producing offspring that vary in their resistance to different diseases.

                                                (3)       One offspring may have cellular markers that make it resistant to virus A, while another is resistant to virus B. This hypothesis predicts that gene loci that code for receptors to which pathogens attack should have many alleles.

                                    b.         In humans, there are hundreds of alleles for each of two gene loci that give cell surfaces their molecular fingerprints.

                                                (1)       At the same time, parasites evolve very rapidly in their ability to use specific host receptors.

                                                (2)       However, sex provides a mechanism for changing the distribution of alleles and varying them among offspring.

VIII.    Natural selection cannot fashion perfect organisms. There are at least four reasons natural selection cannot produce perfection.

            A.        Evolution is limited by historical constraints.

                        1.         Evolution does not scrap ancestral features and build new complex structures or behavior from scratch.

                        2.         Evolution co-opts existing features and adapts them to new situations.

            B.        Adaptations are often compromises.

                        1.         Each organism must do many different things.

            C.        Chance and natural selection interact.

                        1.         Chance events affect the subsequent evolutionary history of populations.

                        2.         For example, founders of new populations may not necessarily be the individuals best suited to the new environment, but rather those individuals that were carried there by chance.

            D.        Selection can only edit existing variations.

                        1.         Natural selection favors only the fittest variations from those phenotypes that are available.

                        2.         New alleles do not arise on demand.

                        3.         Natural selection works by favoring the best variants available.

                        4.         The many imperfections of living organisms are evidence for evolution.