The Mechanisms of Evolution • Charles Darwin’s Theory of Evolution • Genetic Variation within Populations • The Hardy–Weinberg Equilibrium • Evolutionary Agents and Their Effects • The Results of Natural Selection • Assessing the Costs of Adaptations • Maintaining Genetic Variation • Constraints on Evolution • Cultural Evolution • Short-Term versus Long-Term Evolution
Charles Darwin’s Theory of Evolution • Darwin was a student at Cambridge University when his botany professor recommended him for a position as the ship’s naturalist on the H.M.S. Beagle, which was preparing to sail around the world. • Observations made on this trip helped Darwin formulate his theory of evolution, which had two major components. • First, species are not immutable, but change, or adapt, over time. • Second, the agent that produces the changes is natural selection.
Charles Darwin’s Theory of Evolution • Darwin did not publish his theory of evolution immediately; he chose to collect more evidence to support his ideas. • Fourteen years after Darwin first made the observations, Alfred Russel Wallace came to similar conclusions independently. • On July 1, 1858, Darwin’s and Wallace’s ideas were presented to the Linnaean Society of London. • A year later Darwin published The Origin of Species.
Charles Darwin’s Theory of Evolution • Darwin observed that slight variations among individuals can significantly affect the chance that a given individual will survive and the number of offspring it will produce. • Darwin called this differential reproductive success of individuals natural selection. • It is likely that Darwin used this term because he was a pigeon breeder and familiar with artificial selection in the breeding of domesticated animals.
Figure 23. Many Types of Pigeons Have Been Produced by Artificial Selection
Charles Darwin’s Theory of Evolution • Darwin clearly understood a fundamental principle of evolution—that populations, not individuals, evolve and become adapted to the environments in which they live. • The term “adaptation” has two meanings in evolutionary biology. • The first meaning refers to the processes by which adaptive traits are acquired. • The second meaning refers to the traits that enhance the survival and reproductive success of their bearers.
Charles Darwin’s Theory of Evolution • When Darwin proposed his theory, he had no examples of selection operating in nature and knew nothing of the mechanisms of heredity. • The rediscovery of Gregor Mendel’s publications gave rise to the study of population genetics which provides a major underpinning for Darwin’s theories. • Population geneticists apply Mendel’s laws to entire populations. • Population geneticists study variation within and among species in order to understand the processes that result in evolutionary changes in species through time.
Genetic Variation within Populations • For a population to evolve, its members must possess heritable, genetic variation, which is the raw material on which agents of evolution act. • We observe phenotypes in nature, the physical expressions of genes. • The genetic constitution that governs a trait is called its genotype. • A population evolves when individuals with different genotypes survive or reproduce at different rates.
Genetic Variation within Populations • Genes have different forms called alleles. • A single individual has only some of the alleles found in the population to which it belongs. • The sum of all the alleles in a population is the gene pool. • The gene pool contains the variation (different alleles) that produces the differing phenotypes on which agents of evolution act.
Genetic Variation within Populations • Natural populations possess genetic variation. • For example, selection for traits in a wild mustard has produced many important crop plants.
Genetic Variation within Populations • Laboratory experiments also demonstrate the genetic variation present in organisms. • Fruit flies (Drosophila melanogaster) with high or low number of bristles on their abdomens were selected and bred for 35 generations. • Numbers of bristles in flies in the two lineages then fell well outside the original range of the population.
Genetic Variation within Populations • The study of the genetic basis of evolution is difficult because genotypes do not uniquely determine phenotypes. • Dominance can lead to a particular phenotype being expressed by more than one genotype. • Different phenotypes can also be produced by a given genotype, depending on environmental conditions encountered during development.
Genetic Variation within Populations • A locally interbreeding group within a geographic population is called a Mendelian population. • The relative proportions, or frequencies, of all alleles in a population are a measure of that population’s genetic variation. • Biologists can estimate allele frequencies for a given locus by measuring numbers of alleles in a sample of individuals from a population.
Genetic Variation within Populations • Measurements of allele frequencies range from 0 to 1, and the sum of all allele frequencies at a locus is 1. • An allele’s frequency (p) is calculated by dividing the number of copies of the allele in a population by the sum of alleles in the population. • If only two alleles (A and a) for a given locus are found among the members of a diploid population, they may combine to form three different genotypes: AA, Aa, and aa.
Genetic Variation within Populations • Allele frequencies can be calculated using mathematics with the following variables: • NAA = the number of individuals that are homozygous for the A allele (AA) • NAa = the number of individuals that are heterozygous (Aa) • Naa = the number of individuals that are homozygous for the a allele (aa) • Note that NAA + NAa + Naa = N, the total number of individuals in a population.
Genetic Variation within Populations • The total number of alleles in a population is 2N because each individual is diploid (in this case, either AA, Aa, or aa). • p = the frequency of allele A. • q = the frequency of allele a. • For each population, p + q = 1.
Genetic Variation within Populations • The two populations in this example have the same allele frequencies for A and a, but they are distributed differently. Therefore, the genotype frequencies of the two populations are different. • Genotype frequency is the number of individuals with the genotype divided by the total number of individuals in the population. • The frequencies of different alleles at each locus and the frequencies of different genotypes in a Mendelian population describe its genetic structure.
The Hardy–Weinberg Equilibrium • A population of sexually reproducing organisms in which allele and genotype frequencies do not change from generation to generation is said to be at Hardy–Weinberg equilibrium. • Five assumptions must be made in order to meet Hardy–Weinberg equilibrium. • Mating is random. • Population size is very large. • There is no migration between populations. • There is no mutation. • Natural selection does not affect the alleles under consideration.
The Hardy–Weinberg Equilibrium • If the conditions of the Hardy–Weinberg equilibrium are met, two results follow. • The frequencies of alleles at a locus will remain constant from generation to generation. • After one generation of random mating, the genotype frequencies will not change. • The second result can be stated in the form of the Hardy–Weinberg equation: p2 + 2pq + q2 = 1.
Figure 23.7 Calculating Hardy–Weinberg Genotype Frequencies (Part 1)
Figure 23.7 Calculating Hardy–Weinberg Genotype Frequencies (Part 2)
The Hardy–Weinberg Equilibrium • The most important message of the Hardy–Weinberg equilibrium is that allele frequencies remain the same from generation to generation unless some agent acts to change them. • The equilibrium also shows the distribution of genotypes that would be expected for a population at genetic equilibrium. • The Hardy–Weinberg equilibrium allows scientists to determine whether evolutionary agents are operating and their identity (as evidenced by the pattern of deviation from the equilibrium).
Evolutionary Agents and Their Effects • Evolutionary agents cause changes in the allele and genotype frequencies in a population. • These are observed as a deviations from the Hardy–Weinberg equilibrium. • The known evolutionary agents are mutation, gene flow, random genetic drift, nonrandom mating, and natural selection.
Evolutionary Agents and Their Effects • The origin of genetic variation is mutation. A mutation is any change in an organism’s DNA. • Most mutations appear to be random and are harmful or neutral to their bearers. • Some mutations can be advantageous. • Mutation rates are low; one out of a million loci is typical. • Although mutation rates are low, they are sufficient to create considerable genetic variation.
Evolutionary Agents and Their Effects • One condition for Hardy–Weinberg equilibrium is that there is no mutation. • Although this condition is never met, the rate at which mutations arise at single loci is usually so low that mutations result in only very small deviations from Hardy–Weinberg expectations. • If large deviations are found, it is appropriate to dismiss mutation as the cause and look for evidence of other evolutionary agents.
Evolutionary Agents and Their Effects • Gene flow results when individuals migrate to another population and breed in their new location. • Immigrants may add new alleles to the gene pool of a population, or they may change the frequencies of alleles already present if they come from a population with different allele frequencies. • No immigration is allowed for a population to be in Hardy–Weinberg equilibrium.
Evolutionary Agents and Their Effects • Genetic drift is the random loss of individuals and the alleles they possess. • In very small populations, genetic drift may be strong enough to influence the direction of change of allele frequencies even when other evolutionary agents are pushing the frequencies in a different direction. • Organisms that normally have large populations may pass through occasional periods when only a small number of individuals survive (a population bottleneck).
Evolutionary Agents and Their Effects • During a population bottleneck, genetic variation can be reduced by genetic drift. • Populations in nature pass through bottlenecks for numerous reasons; for example, predation and habitat destruction may reduce the population to a very small size, resulting in low genetic variation.
Evolutionary Agents and Their Effects • When a few pioneering individuals colonize a new region, the resulting population will not have all the alleles found among members of the source population. • The resulting pattern of genetic variation is called a founder effect.
Evolutionary Agents and Their Effects • Nonrandom mating occurs when individuals mate either more often with individuals of the same genotype or more often with individuals of a different genotype. • The resulting proportions of genotypes in the following generation differ from Hardy–Weinberg expectations. • If individuals mate preferentially with other individuals of the same genotype, homozygous genotypes are overrepresented and heterozygous genotypes are underrepresented in the next generation. • Conversely, individuals may mate preferentially with individuals of a different genotype.
Evolutionary Agents and Their Effects • Self-fertilization (selfing) is another form of nonrandom mating that is common in many organisms, especially plants. • Selfing reduces the frequencies of heterozygous individuals below Hardy–Weinberg expectations and increases the frequencies of homozygotes, without changing allele frequencies.
Evolutionary Agents and Their Effects • For adaptation to occur, individuals that differ in heritable traits must survive and reproduce with different degrees of success. • When some individuals contribute more offspring to the next generation than others, allele frequencies in the population change in a way that adapts individuals to the environments that influenced their success. • This process is known as natural selection.
Evolutionary Agents and Their Effects • The reproductive contribution of a phenotype to subsequent generations relative to the contributions of other phenotypes is called its fitness. • The fitness of a phenotype is determined by the average rates of survival and reproduction of individuals with that phenotype.
The Results of Natural Selection • Most characters are influenced by alleles at more than one locus and are more likely to show quantitative rather then qualitative variation. • For example, the size of individuals in a population is influenced by genes at many loci, and distribution of sizes is likely to be a bell-shaped curve. • Natural selection can act on characters with quantitative variation in three ways: • Stabilizing selection • Directional selection • Disruptive selection
The Results of Natural Selection • Stabilizing selectionpreserves the characteristics of a population by favoring average individuals. • Stabilizing selection occurs when the extremes of a population contribute relatively fewer offspring than the average members to the next generation. • Stabilizing selection operates on human birth weight. Babies that are born lighter or heavier than the population mean die at higher rates than babies whose weights are close to the mean.
Figure 23.12 Natural Selection Can Operate on Quantitative Variation in Several Ways (Part 1)
Figure 23.13 Human Birth Weight Is Influenced by Stabilizing Selection