22. The Mechanisms of Evolution

The Mechanisms of Evolution22 The Mechanisms of Evolution 22.1 What Facts Form the Base of Our Understanding of Evolution?22.2 What Are the Mechanisms of Evolutionary Change?22.3 What Evolutionary Mechanisms Result in Adaptation?22.4 How Is Genetic Variation Maintained within Populations?22.5 What Are the Constraints on Evolution?22.6 How Have Humans Influenced Evolution?22.1 What Facts Form the Base of Our Understanding of Evolution?The young Charles Darwin was passionately interested in geology and natural science.In 1831, he was recommended for a position on the H.M.S. Beagle, for a 5-year survey voyage around the world.Figure 22.1 Darwin and the Voyage of the Beagle (Part 1)Figure 22.1 Darwin and the Voyage of the Beagle (Part 2)Figure 22.1 Darwin and the Voyage of the Beagle (Part 3)22.1 What Facts Form the Base of Our Understanding of Evolution?Darwin often went ashore to study rocks and collect specimens, and make observations about the natural world.In the Galapagos Islands he observed that species were similar to, but not the same as, species on the mainland of South America. He also realized that species varied from island to island.22.1 What Facts Form the Base of Our Understanding of Evolution?Darwin postulated that species had reached the islands from the mainland, but then had undergone different changes on different islands.Part of the puzzle was determining what could be a mechanism for such changes.22.1 What Facts Form the Base of Our Understanding of Evolution?These observations, and many others, led Darwin to propose an explanatory theory for evolutionary change based on two propositions:Species change over time.The process that produces the change is natural selection.22.1 What Facts Form the Base of Our Understanding of Evolution?Darwin continued to amass evidence to support his ideas until 1858, when he received a letter from another naturalist, Alfred Russel Wallace.Wallace proposed a theory of natural selection almost identical to Darwin’s.A paper with the work of both men was presented in 1858 to the Linnean Society of London.22.1 What Facts Form the Base of Our Understanding of Evolution?Darwin published his book, The Origin of Species in 1859.The book provided exhaustive evidence from many different fields to support evolution and natural selection.22.1 What Facts Form the Base of Our Understanding of Evolution?Darwin and Wallace were both influenced by economist Thomas Malthus, who published An Essay on the Principle of Population in 1838.Populations of all species have the potential for rapid increase.But this does not occur in nature, so death rate must also be high.22.1 What Facts Form the Base of Our Understanding of Evolution?Darwin observed that, though offspring tended to resemble their parents, they are not identical.He suggested that slight variations among individuals affect the chances of surviving and producing offspring: natural selection.22.1 What Facts Form the Base of Our Understanding of Evolution?Natural selection:Differential contribution of offspring to the next generation by various genetic types belonging to the same population.22.1 What Facts Form the Base of Our Understanding of Evolution?Darwin had observed variation and artificial selection of certain desirable traits in plants and animals by breeders.Darwin himself bred pigeons.Figure 22.2 Many Types of Pigeons Have Been Produced by Artificial Selection22.1 What Facts Form the Base of Our Understanding of Evolution?Individuals do not evolve. Populations do.Population: A group of individuals of the same species that live and interbreed in a particular geographic area.Members of a population become adapted to the environment in which they live.22.1 What Facts Form the Base of Our Understanding of Evolution?Adaptations: The processes by which useful characteristics evolve; and the characteristics themselves.An organism is considered to be adapted to a particular environment when it can be demonstrated that a slightly different organism survives and reproduces less well in that environment.22.1 What Facts Form the Base of Our Understanding of Evolution?For a population to evolve, its members must possess heritable genetic variation.The phenotype is the physical expression of an organism’s genes.Features of a phenotype are the characters (e.g., eye color), specific form of a character is a trait (e.g., blue).22.1 What Facts Form the Base of Our Understanding of Evolution?A heritable trait is at least partly determined by genes.Genetic makeup of an organism is the genotype.22.1 What Facts Form the Base of Our Understanding of Evolution?Population genetics has three main goals:Explain the origin and maintenance of genetic variationExplain patterns and organization of genetic variationUnderstand mechanisms that cause changes in allele frequencies22.1 What Facts Form the Base of Our Understanding of Evolution?Different forms of a gene are called alleles.The gene pool is the sum of all copies of all alleles at all loci in a population.Figure 22.3 A Gene Pool22.1 What Facts Form the Base of Our Understanding of Evolution?Populations have genetic variation for many characters.Artificial selection for different characters in a single species of wild mustard produced many crop plants.Figure 22.4 Many Vegetables from One Species (Part 1)Figure 22.4 Many Vegetables from One Species (Part 2)22.1 What Facts Form the Base of Our Understanding of Evolution?In laboratory experiments with Drosophila, researchers selected for high or low numbers of body bristles from an initial population with intermediate numbers.After 35 generations, numbers for both high-bristle and low-bristle fell outside the range of the original population.Figure 22.5 Artificial Selection Reveals Genetic Variation22.1 What Facts Form the Base of Our Understanding of Evolution?Locally interbreeding groups are called Mendelian populations.Allele frequencies, or their proportion in the gene pool, are estimated by counting alleles in a sample of individuals.22.1 What Facts Form the Base of Our Understanding of Evolution?Allele frequency:If a locus has two alleles, A and a, there could be three genotypes: AA, Aa, and aa. The population is polymorphic at that locus.Figure 22.6 Calculating Allele Frequencies22.1 What Facts Form the Base of Our Understanding of Evolution?If p is the frequency of allele A, and q is the frequency of allele a, p + q = 1 q = 1 – p If there is only one allele at a locus, its frequency = 1. The population is monomorphic at that locus; the allele is said to be fixed.22.1 What Facts Form the Base of Our Understanding of Evolution?Genotype frequencies may not be the same as allele frequencies.Frequencies of different alleles at each locus and the frequencies of genotypes in a Mendelian population make up the genetic structure of the population.22.1 What Facts Form the Base of Our Understanding of Evolution?If certain conditions are met, the genetic structure of a population does not change over time.If an allele is not advantageous, its frequency remains constant.The Hardy-Weinberg equilibrium describes a model situation in which allele frequencies do not change.22.1 What Facts Form the Base of Our Understanding of Evolution?Conditions that must be met:Mating is random.Population size is infinite. Large populations aren’t affected by genetic drift.No gene flow—no migration into or out of the population.No mutation.Natural selection does not affect survival of any genotypes.22.1 What Facts Form the Base of Our Understanding of Evolution?If these conditions hold:Allele frequencies remain constant;after one generation, genotype frequencies occur in these proportions: Genotype AA Aa aa Frequency p2 2pq q2Figure 22.7 Calculating Hardy–Weinberg Genotype Frequencies (Part 1)Figure 22.7 Calculating Hardy–Weinberg Genotype Frequencies (Part 2)22.1 What Facts Form the Base of Our Understanding of Evolution?For generation 1, probability of two A alleles coming together is:Probability of two a alleles:22.1 What Facts Form the Base of Our Understanding of Evolution?There are two ways of producing a heterozygote:The Hardy-Weinberg equation:22.1 What Facts Form the Base of Our Understanding of Evolution?Populations in nature never fit the conditions for Hardy-Weinberg equilibrium.But, it is useful in predicting genotype frequencies from allele frequencies; and, because the model describes conditions that would result in no evolution, patterns of deviation from the model help identify specific mechanisms of evolution.22.2 What Are the Mechanisms of Evolutionary Change?Hardy-Weinberg equilibrium is a null hypothesis that assumes evolutionary forces are absent.Known evolutionary mechanisms:MutationGene flowGenetic driftNonrandom matingNatural selection22.2 What Are the Mechanisms of Evolutionary Change?Mutation is the origin of genetic variation.Mutation is any change in DNA; it appears to be random with respect to the adaptive needs of an organism.Most mutations are harmful or neutral, but if conditions change, could become advantageous.22.2 What Are the Mechanisms of Evolutionary Change?Mutations can also restore alleles that other processes remove.Mutation rates are low—about one per locus in a million zygotes.Creates a lot of variation because of the number of genes that can mutate, chromosome rearrangements that can change many genes simultaneously, and large numbers of individuals.22.2 What Are the Mechanisms of Evolutionary Change?Because mutation rate is low, mutations in themselves result in only minor deviations from Hardy-Weinberg equilibrium.If large deviations are found, it is appropriate to look for other mechanisms.22.2 What Are the Mechanisms of Evolutionary Change?Gene flow is a result of the migration of individuals and movements of gametes between populations.New alleles can be added to the gene pool, or allele frequencies changed.22.2 What Are the Mechanisms of Evolutionary Change?Genetic drift results from random changes in allele frequencies.In large populations, genetic drift can influence frequencies of alleles that don’t affect survival and reproduction.If populations are reduced to a small number of individuals—a population bottleneck, genetic drift can reduce the genetic variation.Figure 22.8 A Population Bottleneck22.2 What Are the Mechanisms of Evolutionary Change?A population forced through a bottleneck is likely to lose much genetic variation.Example: Greater prairie chickens in Illinois were reduced to about 50 birds in the 1990s; California fan palms are now restricted to a few oases in southern California.Figure 22.9 Species with Low Genetic Variation (Part 1)Figure 22.9 Species with Low Genetic Variation (Part 2)22.2 What Are the Mechanisms of Evolutionary Change?Genetic drift also effects small populations that colonize a new region.Colonizing population is unlikely to have all the alleles present in the whole population.Founder effect—equivalent to a bottleneck.22.2 What Are the Mechanisms of Evolutionary Change?Example of founder effect:Populations of European fruit fly D. subobscura began in Chile, and then in Washington state.Both populations grew and expanded their ranges.These populations have very similar genetic structure, and much less variation than the European populations.Figure 22.10 A Founder Effect22.2 What Are the Mechanisms of Evolutionary Change?Nonrandom mating occurs when individuals choose mates with particular phenotypes.If individuals choose the same genotype as themselves, homozygote frequencies will increase.22.2 What Are the Mechanisms of Evolutionary Change?Nonrandom mating in primroses (Primula):Two flower types—pin and thrum. Pollen from one type can fertilize only flowers of the other type.Figure 22.11 Flower Structure Fosters Nonrandom Mating22.2 What Are the Mechanisms of Evolutionary Change?Selfing, or self-fertilization, is a common form of nonrandom mating.Selfing reduces the frequency of heterozygotes, and increases homozygotes, but does not change allele frequencies in the population.22.3 What Evolutionary Mechanisms Result in Adaptation?Adaptation occurs when some individuals in a population contribute more offspring to the next generation.Allele frequencies change in a way that adapts individuals to the environment that influenced that reproductive success.This is natural selection.22.3 What Evolutionary Mechanisms Result in Adaptation?Natural selection acts on phenotype.Fitness is the reproductive contribution of a phenotype to subsequent generations.Changes in the relative success of different phenotypes in a population leads to change in allele frequencies.22.3 What Evolutionary Mechanisms Result in Adaptation?Fitness of a phenotype is determined by the average rates of survival and reproduction of individuals with that phenotype.22.3 What Evolutionary Mechanisms Result in Adaptation?Most characters are influences by alleles at more than one locus.Such characters often show quantitative variation instead of qualitative.Example: The distribution of body size of individuals in a population is likely to resemble a bell-shaped curve.22.3 What Evolutionary Mechanisms Result in Adaptation?Natural selection can act on characters with quantitative variation in three ways:Stabilizing selection preserves average phenotype.Directional selection favors individuals that vary in one direction.Disruptive selection favors individuals that vary in opposite directions from the average.Figure 22.12 Natural Selection Can Operate on Quantitative Variation in Several Ways (Part 1)Figure 22.12 Natural Selection Can Operate on Quantitative Variation in Several Ways (Part 2)Figure 22.12 Natural Selection Can Operate on Quantitative Variation in Several Ways (Part 3)22.3 What Evolutionary Mechanisms Result in Adaptation?Stabilizing selection reduces variation in a population, but does not change the mean. Rates of evolution are slow because natural selection is usually stabilizing.Example: human birth weightsFigure 22.13 Human Birth Weight Is Influenced by Stabilizing Selection22.3 What Evolutionary Mechanisms Result in Adaptation?Directional selection occurs when individuals at one extreme are more successful.If directional selection operates over many generations, an evolutionary trend occurs.Example: resistance to tetrodotoxin (TTX) in garter snakesFigure 22.14 Resistance to TTX Is Associated with the Presence of Newts22.3 What Evolutionary Mechanisms Result in Adaptation?Disruptive selection: Variation is created when individuals at either extreme are more successful than average individuals.Example: bill size in black-bellied seed crackersFigure 22.15 Disruptive Selection Results in a Bimodal Distribution22.3 What Evolutionary Mechanisms Result in Adaptation?Sexual selection is a special type of natural selection, which acts on characters that determine reproductive success.If an individual survives but does not reproduce, it makes no contribution to the next generation.Sexual selection favors traits that increase the chances of reproduction.22.3 What Evolutionary Mechanisms Result in Adaptation?Traits such as bright colors, long horns, and elaborate courtship displays, may improve ability to compete for mates (intrasexual selection);or to be more attractive to the opposite sex (intersexual selection).Such traits are costly, but reliably demonstrate the fitness of the bearer to the choosing sex.22.3 What Evolutionary Mechanisms Result in Adaptation?This has been shown experimentally in long-tailed widowbirds.Male tails were shortened or lengthened.Both were able to successfully defend their territories, but males with lengthened tails attracted more females.Long tails indicate the health and vigor of the male.Figure 22.16 The Longer the Tail, the Better the Male (Part 1)Figure 22.16 The Longer the Tail, the Better the Male (Part 2)22.3 What Evolutionary Mechanisms Result in Adaptation?Sexual selection has also been shown in zebra finches.Brightness of male’s bill is an indicator of health.Color is due to carotenoids, which are also important in the immune system. A brighter bill indicates more carotenoids and greater overall health.Figure 22.17 Bright Bills Signal Good Health (Part 1)Figure 22.17 Bright Bills Signal Good Health (Part 2)Figure 22.17 Bright Bills Signal Good Health (Part 3)22.4 How Is Genetic Variation Maintained within Populations?Many mutations do not affect the function of the resulting proteins.An allele that does not affect fitness is a neutral allele. They tend to accumulate in a population.Molecular techniques allow neutral alleles to be identified and used to estimate rates of evolution (see Chapter 24).22.4 How Is Genetic Variation Maintained within Populations?Sexual reproduction results in new combinations of genes through crossing over and independent assortment, and the combination of gametes.Sexual recombination produces genetic variety that increases evolutionary potential.22.4 How Is Genetic Variation Maintained within Populations?But sexual reproduction has disadvantages:Recombination can break up adaptive combinations of genes.Reduces rate at which females pass genes to offspring.Dividing offspring into genders reduces the overall reproductive rate.22.4 How Is Genetic Variation Maintained within Populations?How did sexual reproduction arise?Possible advantages:Sexual reproduction facilitates repair of damaged DNA. Damage on one chromosome can be repaired by copying intact sequence on the other chromosome.22.4 How Is Genetic Variation Maintained within Populations?Permits elimination of deleterious mutations.In asexually reproducing species, deleterious mutations can accumulate; only death of the lineage can eliminate them—Muller’s ratchet.22.4 How Is Genetic Variation Maintained within Populations?Sexual recombination produces some individuals with many deleterious mutations, some with few. The individuals with few deleterious mutations are more likely to survive.22.4 How Is Genetic Variation Maintained within Populations?The variety of genetic combinations possible in sexually reproducing species may be especially valuable in defense against pathogens and parasites.22.4 How Is Genetic Variation Maintained within Populations?Sexual recombination does not affect the frequency of alleles, but generates new combinations of alleles on which natural selection can act.22.4 How Is Genetic Variation Maintained within Populations?Frequency-dependent selection: A polymorphism can be maintained when fitness depends on its frequency in the population.Example: a scale-eating fish in Lake Tanganyika. “Left-mouthed” and “right-mouthed” individuals are both favored; the host fish can be attacked from either side.Figure 22.18 A Stable Polymorphism22.4 How Is Genetic Variation Maintained within Populations?Environmental variation also helps to preserve genetic variation.Example: Colias butterflies live in an environment with temperature extremes. The population is polymorphic for an enzyme that influences flight at different temperatures.Heterozygotes are favored because they can fly over a larger temperature range.Figure 22.19 A Heterozygote Mating Advantage (Part 1)Figure 22.19 A Heterozygote Mating Advantage (Part 2)22.4 How Is Genetic Variation Maintained within Populations?Subpopulations in different geographic regions maintain genetic variation.The subpopulations may be subjected to different environmental conditions and selective pressures.Example: populations of white clover that produce cyanide as defense against herbivores. Plants that produce cyanide are more likely to be killed by frost.Figure 22.20 Geographic Variation in a Defensive Chemical22.5 What Are the Constraints on Evolution?Lack of genetic variation can prevent evolution of potentially favorable traits.If the allele for a given trait does not exist in a population, that trait cannot evolve, even if it would be favored by natural selection.22.5 What Are the Constraints on Evolution?Evolution must work within the boundaries of universal constraints such as:Cell size, constrained by surface area-to-volume ratiosProtein folding, constrained by types of bonding that can occurLaws of thermodynamics that constrain energy transfers22.5 What Are the Constraints on Evolution?Developmental processes also constrain evolution.All evolutionary innovations are modifications of previously existing structures.22.5 What Are the Constraints on Evolution?Example: two lineages of bottom-dwelling fishesSkates and rays evolved from a common ancestor with sharks. They started with a flattened body plan, and can swim along the ocean floor.Sole and flounder evolved from laterally flattened bony fishes. They can’t swim well, but lie still on the bottom. The eyes gradually shifted to one side.Figure 22.21 Two Solutions to a Single Problem (Part 1)Figure 22.21 Two Solutions to a Single Problem (Part 2)22.5 What Are the Constraints on Evolution?Adaptations involve both fitness costs and benefits.Benefit must outweigh cost if adaptation is to evolve—the trade-offs must be worthwhile.Example: Garter snake resistance to TTX only occurs where poisonous newts are common.22.5 What Are the Constraints on Evolution?Conspicuous features used by some males to compete with other males are a trade-off with reproductive success.Species in which males have multiple mates are polygynous. Males are usually larger than females and often have weapons—horns, antlers, etc.Dramatic differences between sexes are known as sexual dimorphism.22.5 What Are the Constraints on Evolution?In polygynous species, males must defend their mates against other males.Defensive structures require a lot of energy, but the male has a lot of reproductive success.22.5 What Are the Constraints on Evolution?Short-term changes in allele frequencies can be observed and manipulated to demonstrate the processes by which evolution occurs.But patterns of long-term evolutionary change can be influenced by infrequent events (e.g., meteorites) or very slow processes (e.g., continental drift). Other types of evidence are used to study these long-term changes.22.6 How Have Humans Influenced Evolution?As humans have changed their environments, selective forces that act on human populations have changed also.Today, survival and reproductive success are related to genes that confer defense against diseases such as malaria and AIDS, and stresses of modern industrial life such as hypertension.22.6 How Have Humans Influenced Evolution?Humans have also influenced the evolution of other species:Efforts to control populations of “pests” make humans agents of evolutionary change.Sport hunters that seek large trophy animals are removing the biggest and healthiest animals from populations.Figure 22.22 Trophy Hunting Selects for Smaller Males22.6 How Have Humans Influenced Evolution?Humans also move species around the globe, and modify species through breeding and biotechnology.Humans are also changing the climate; and have caused the extinction of many species.