Chapter 15 How Organisms Evolve 15.1 How are populations, genes, and evolution related? 15.2 What causes evolution? 15.3 How does natural selection work?

Figure: 15-CO Title: How Organisms Evolve Caption: We think of hospitals as places in which to seek protection from disease, but they also foster the evolution of drug-resistant supergerms.

15.1 How Are Populations, Genes, and Evolution Related? Genes and the environment interact to determine traits The gene pool is the sum of the genes in a population

The Gene Pool For example, coat color in hamsters: A population of 25 hamsters contains 50 alleles of the coat color gene (hamsters are diploid) If 20 of those 50 alleles code for black coats, then the frequency of the black allele is 0.40 or 40% [20/50 = 0.40]

FIGURE 15-2 A gene pool In diploid organisms, each individual in a population contributes two alleles of each gene to the gene pool.

15.1 How Are Populations, Genes, and Evolution Related? Evolution is the change over time of allele frequencies within a population The equilibrium population is a hypothetical population that does not evolve

The Hardy-Weinberg Principle A mathematical model (1908) proposed independently by Godfrey H. Hardy (English mathematician) Wilhelm Weinberg (German physician)

Hardy-Weinberg Principle (no evolving) There must be no mutation There must be no gene flow The population must be large All mating must be random There must be no natural selection

Section 15.2 Outline 15.2 What Causes Evolution? Mutations Are the Source of Genetic Variability Gene Flow Between Populations Changes Allele Frequencies Allele Frequencies May Drift in Small Populations Mating Within a Population Is Almost Never Random All Genotypes Are Not Equally Beneficial

Causes of Evolution Five factors contribute to evolutionary change: Mutation Gene flow Small population size Nonrandom mating Natural selection

Source of Genetic Variability Mutations are rare changes in the base sequence of DNA in a gene Usually have little or no immediate effect Are the source of new alleles Can be passed to offspring only if they occur in cells that give rise to gametes Can be beneficial, harmful, or neutral Arise spontaneously, not as a result of, or in anticipation of, environmental necessity

FIGURE 15-3 Mutations occur spontaneously This experiment demonstrates that mutations occur spontaneously and not in response to environmental pressures. When bacterial colonies that have never been exposed to antibiotics are exposed to the antibiotic streptomycin, only a few colonies grow. The observation that these surviving colonies grow in exactly the same positions in all dishes shows that the mutations for resistance to streptomycin were present in the original dish before exposure to the environmental pressure, streptomycin.

15.2 What Causes Evolution? Gene flow between populations changes allele frequencies Immigration adds alleles to a population Emigration removes alleles from a population

FIGURE 15-4 Pollen can be an agent of gene flow Pollen, drifting on the wind, can carry alleles from one population to another.

15.2 What Causes Evolution? Genetic drift is the random change in allele frequencies over time, brought about by chance alone Has minor impact in very large populations Occurs more rapidly and has bigger effect on small populations

FIGURE 15-5 Genetic drift If chance events prevent some members of population from reproducing, allele frequencies can change randomly.

Population Size Matters Population size affects genetic drift

FIGURE 15-6 The effect of population size on genetic drift Each colored line represents one computer simulation of the change over time in the frequency of allele A in a (a) large or (b) small population in which two alleles, A and a, were initially present in equal proportions, and in which randomly chosen individuals reproduced.

Causes of Genetic Drift There are two causes of genetic drift Population bottleneck Founder effect

Population Bottleneck A population bottleneck is a drastic reduction in population size brought about by a natural catastrophe or over-hunting A population bottleneck can change allele frequencies and reduce genetic variation

FIGURE 15-7a Population bottlenecks reduce variation (a) A population bottleneck may drastically reduce genetic and phenotypic variation because the few organisms that survive may carry similar sets of alleles.

Population Bottleneck Northern elephant seal Hunted almost to extinction in the 1800s By 1890s, only 20 individuals remained Hunting ban allowed population to increase to 30,000 Biochemical analysis shows that present-day northern elephant seals are almost genetically identical

FIGURE 15-7b Population bottlenecks reduce variation (b) The northern elephant seal passed through a population bottleneck in the recent past, resulting in an almost total loss of genetic diversity.

Founder Effect The founder effect occurs when a small number of individuals leave a large population and establish a new isolated population By chance, allele frequencies of founders may differ from those of original population Over time, new population may exhibit allele frequencies that differ from original population

10-18a Title: The effects of genetic drift. Caption: (a) Its founders were not representative of the original population.

10-18b Title: The effects of genetic drift. Caption: (b) A short-lived drop in population size caused a change in allele frequency in one of the populations.

10-18c Title: The effects of genetic drift. Caption: (c) One population is so small that low-frequency alleles are lost by chance.

Mating Is Almost Never Random Nonrandom mating can change the distribution of genotypes in a population Organisms within a population rarely mate randomly

Mating Is Almost Never Random Most animals are likely to mate with nearby members of their species Certain animals, such as snow geese, exhibit assortative mating, where there is a preference for mates that are similar in appearance

15.2 What Causes Evolution? Mating Within a Population Is Almost Never Random Nonrandom mating among snow geese Competition between males favors evolution of structures for ritual combat Peahens are attracted to the peacock’s showy tail

FIGURE 15-9 Nonrandom mating among snow geese Snow geese, which have either white plumage or blue-gray plumage, are most likely to mate with other birds of the same color.

Figure: 15-7 Title: Peahens are attracted to the peacock's showy tail Caption: The ancestors of today's peahens were apparently picky when deciding on males with which to mate, favoring males with longer and more colorful tails.

Endangered Habitat Destruction Figure: E15-1 Title: Endangered by habitat destruction Caption: The bighorn sheep is among the species designated “endangered” by the U.S. government. Endangered Habitat Destruction

15.2 What Causes Evolution? All Genotypes Are Not Equally Beneficial Antibiotic resistance evolves by natural selection Case study of chapter on penicillin resistance illustrates key points about evolution

All Genotypes Are Not Equal Evolution is change in allele frequencies of a population, owing to unequal reproductive success among organisms bearing different alleles Penicillin-resistant bacteria had greater fitness (reproductive success) than non-resistant bacteria

Penicillin Resistance Illustrates Key Points about Evolution Natural selection does not cause genetic changes to individuals Natural selection acts on individuals, but it is populations that are changed Evolutionary changes are not “good” or “progressive” in any absolute sense

A compromise between opposing pressures Figure: 15-8 Title: A compromise between opposing pressures Caption: (a) A male giraffe with a long neck is at a definite advantage in combat to establish dominance. (b) But a giraffe's long neck forces it to assume an extremely awkward and vulnerable position when drinking. Thus, drinking and male–male contests place opposing evolutionary pressures on neck length.

Table 15-1 Causes of Evolution

Section 15.3 Outline 15.3 How Does Natural Selection Work? Natural Selection Stems from Unequal Reproduction Natural Selection Acts on Phenotypes Some Phenotypes Reproduce More Successfully Than Others Selection Can Influence Populations in Three Ways

Natural Selection Natural selection is often associated with the phrase “survival of the fittest” The fittest individuals are those that not only survive, but are able to leave the most offspring behind

Natural Selection The selection on phenotypes affects genotypes present in the population If environmental conditions favored tall pea plants, then tall pea plants would leave more offspring These offspring would carry tall alleles

Success of Phenotypes Successful phenotypes are those that have the best adaptations to their present environment Adaptations are characteristics that help an individual survive and reproduce

Success of Phenotypes Adaptations arise from the interactions of organisms with both the nonliving and living parts of their environments

The Environment Nonliving (abiotic) components include: Climate Availability of water Concentration of minerals in the soil Living (biotic) components include: Other organisms

The Environment Interactions with other organisms include: Competition Coevolution Sexual selection

Some Phenotypes Reproduce More Successfully Than Others Competition acts as an agent of selection Both predator and prey act as agents of selection Coevolution Predation Sexual selection favors traits that help an organism mate

Agents of Selection Coevolution is the evolution of adaptations in two species due to their extensive interaction e.g. predator-prey relationships

Examples of Coevolution

Some Phenotypes Reproduce More Successfully Than Others Sexual selection favors traits that help an organism mate Female preference for males with a certain phenotype Conspicuous features (bright colors, long feathers or fins, elaborate antlers) Bizarre courtship behaviors Loud, complex courting songs

Figure: 15-6 Title: Competition between males favors evolution of structures for ritual combat Caption: Two male bighorn sheep spar during the fall mating season. In many species, the losers of such contests are unlikely to mate, while winners enjoy tremendous reproductive success. Question Imagine that you studied a population of bighorn sheep and were able to identify the father and mother of each lamb born. Would the difference in number of offspring between the most reproductively successful adult and the least successful one be greater for males or for females?

Selection Influences Populations Natural selection and sexual selection can affect populations in three ways: Directional selection Stabilizing selection Disruptive selection

FIGURE 15-13 Three ways that selection affects a population over time A graphical illustration of three ways natural and/or sexual selection, acting on a normal distribution of phenotypes, can affect a population over time. In all graphs, the beige areas represent individuals that are selected against•that is, they do not reproduce as successfully as do the individuals in the purple range.

Selection Can Influence Populations in Three Ways Directional selection shifts character traits in a specific direction Favors extreme values of traits Stabilizing selection acts against Individuals who deviate too far from the average Favors average values of traits Disruptive selection adapts individuals within a population to different habitats Favors both extreme values of traits

A balanced polymorphism HH Hh hh dies of malaria lives and reproduces dies of sickle-cell anemia A balanced polymorphism HH Hh hh Figure: 15-10 Title: A balanced polymorphism Caption: Natural selection may preserve two or more phenotypes in a population. In some African human populations, selection for malaria-resistant heterozygotes ensures that the alleles for both normal (H) and sickle-cell (h) hemoglobin persist in the population. Question What will happen to the h allele in malaria-free environments? dies of malaria lives and reproduces dies of sickle-cell anemia HH Hh hh