The Evolution of Populations Chapter 23 The Evolution of Populations

Overview: The Smallest Unit of Evolution One common misconception about evolution is that individual organisms evolve, in the Darwinian sense, during their lifetimes Natural selection acts on individuals, but populations evolve

Genetic variations in populations Contribute to evolution Figure 23.1

Concept 23.1: Population genetics provides a foundation for studying evolution Microevolution Is change in the genetic makeup of a population from generation to generation Figure 23.2

The Modern Synthesis Population genetics Is the study of how populations change genetically over time Reconciled Darwin’s and Mendel’s ideas

The modern synthesis Integrates Mendelian genetics with the Darwinian theory of evolution by natural selection Focuses on populations as units of evolution

Gene Pools and Allele Frequencies A population Is a localized group of individuals that are capable of interbreeding and producing fertile offspring MAP AREA ALASKA CANADA Beaufort Sea Porcupine herd range Fairbanks Whitehorse Fortymile NORTHWEST TERRITORIES YUKON Figure 23.3

The gene pool Is the total aggregate of genes in a population at any one time Consists of all gene loci in all individuals of the population

Mendelian inheritance Figure 23.4 Generation 1 CRCR genotype CWCW Plants mate All CRCW (all pink flowers) 50% CR gametes 50% CW Come together at random 2 3 4 25% CRCR 50% CRCW 25% CWCW Alleles segregate, and subsequent generations also have three types of flowers in the same proportions Mendelian inheritance Preserves genetic variation in a population

Two processes, mutation and sexual recombination Concept 23.2: Mutation and sexual recombination produce the variation that makes evolution possible Two processes, mutation and sexual recombination Produce the variation in gene pools that contributes to differences among individuals

Mutation Mutations Cause new genes and alleles to arise Are changes in the nucleotide sequence of DNA Cause new genes and alleles to arise Figure 23.6

Point Mutations A point mutation Is a change in one base in a gene Can have a significant impact on phenotype Is usually harmless, but may have an adaptive impact

Mutations That Alter Gene Number or Sequence Chromosomal mutations that affect many loci Are almost certain to be harmful May be neutral and even beneficial

Mutation Rates Mutation rates Tend to be low in animals and plants Average about one mutation in every 100,000 genes per generation Are more rapid in microorganisms

In sexually reproducing populations, sexual recombination Is far more important than mutation in producing the genetic differences that make adaptation possible

Differential success in reproduction Natural Selection Differential success in reproduction Results in certain alleles being passed to the next generation in greater proportions

Concept 23.4: Natural selection is the primary mechanism of adaptive evolution Accumulates and maintains favorable genotypes in a population

Genetic Variation Genetic variation Occurs in individuals in populations of all species Is not always heritable Figure 23.9 A, B (a) Map butterflies that emerge in spring: orange and brown (b) Map butterflies that emerge in late summer: black and white

Variation Within a Population Both discrete and quantitative characters Contribute to variation within a population Discrete characters Can be classified on an either-or basis Quantitative characters Vary along a continuum within a population

Phenotypic polymorphism Describes a population in which two or more distinct morphs for a character are each represented in high enough frequencies to be readily noticeable Genetic polymorphisms Are the heritable components of characters that occur along a continuum in a population

Variation Between Populations Most species exhibit geographic variation Differences between gene pools of separate populations or population subgroups 1 2.4 3.14 5.18 6 7.15 XX 19 13.17 10.16 9.12 8.11 2.19 3.8 4.16 5.14 6.7 15.18 11.12 9.10 Figure 23.10

Heights of yarrow plants grown in common garden Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis Figure 23.11 EXPERIMENT Researchers observed that the average size of yarrow plants (Achillea) growing on the slopes of the Sierra Nevada mountains gradually decreases with increasing elevation. To eliminate the effect of environmental differences at different elevations, researchers collected seeds from various altitudes and planted them in a common garden. They then measured the heights of the resulting plants. RESULTS The average plant sizes in the common garden were inversely correlated with the altitudes at which the seeds were collected, although the height differences were less than in the plants’ natural environments. CONCLUSION The lesser but still measurable clinal variation in yarrow plants grown at a common elevation demonstrates the role of genetic as well as environmental differences. Mean height (cm) Atitude (m) Heights of yarrow plants grown in common garden Seed collection sites Sierra Nevada Range Great Basin Plateau

A Closer Look at Natural Selection From the range of variations available in a population Natural selection increases the frequencies of certain genotypes, fitting organisms to their environment over generations

The phrases “struggle for existence” and “survival of the fittest” Evolutionary Fitness The phrases “struggle for existence” and “survival of the fittest” Are commonly used to describe natural selection Can be misleading Reproductive success Is generally more subtle and depends on many factors

Fitness Relative fitness Is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals Relative fitness Is the contribution of a genotype to the next generation as compared to the contributions of alternative genotypes for the same locus

Directional, Disruptive, and Stabilizing Selection Favors certain genotypes by acting on the phenotypes of certain organisms Three modes of selection are Directional Disruptive Stabilizing

Directional selection Favors individuals at one end of the phenotypic range Disruptive selection Favors individuals at both extremes of the phenotypic range Stabilizing selection Favors intermediate variants and acts against extreme phenotypes

The three modes of selection Fig 23.12 A–C (a) Directional selection shifts the overall makeup of the population by favoring variants at one extreme of the distribution. In this case, darker mice are favored because they live among dark rocks and a darker fur color conceals them from predators. (b) Disruptive selection favors variants at both ends of the distribution. These mice have colonized a patchy habitat made up of light and dark rocks, with the result that mice of an intermediate color are at a disadvantage. (c) Stabilizing selection removes extreme variants from the population and preserves intermediate types. If the environment consists of rocks of an intermediate color, both light and dark mice will be selected against. Phenotypes (fur color) Original population Original population Evolved Frequency of individuals

The Preservation of Genetic Variation Various mechanisms help to preserve genetic variation in a population

Diploidy Diploidy Maintains genetic variation in the form of hidden recessive alleles

Balancing Selection Balancing selection Occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population Leads to a state called balanced polymorphism

Heterozygote Advantage Some individuals who are heterozygous at a particular locus Have greater fitness than homozygotes Natural selection Will tend to maintain two or more alleles at that locus

Neutral Variation Neutral variation Is genetic variation that appears to confer no selective advantage

Sexual Selection Sexual selection Is natural selection for mating success Can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

sexual selection Occurs when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex May depend on the showiness of the male’s appearance Figure 23.15

Sexual selection - a “special case” of natural selection. Sexual selection acts on an organism's ability to obtain (often by any means necessary!) or successfully copulate with a mate. Selection makes many organisms go to extreme lengths for sex: peacocks (top left) maintain elaborate tails, elephant seals (top right) fight over territories, fruit flies perform dances, and some species deliver persuasive gifts. After all, what female Mormon cricket (bottom right) could resist the gift of a juicy sperm-packet? Going to even more extreme lengths, the male redback spider (bottom left) literally flings itself into the jaws of death in order to mate successfully. Sexual selection is often powerful enough to produce features that are harmful to the individual’s survival. For example, extravagant and colorful tail feathers or fins are likely to attract predators as well as interested members of the opposite sex.

Selection is a two-way street. Sexual selection usually works in two ways, although in some cases we do see sex role reversals: Male competition Males compete for access to females, the amount of time spent mating with females, and even whose sperm gets to fertilize her eggs. For example, male damselflies scrub rival sperm out of the female reproductive tract when mating. Female choice Females choose which males to mate with, how long to mate, and even whose sperm will fertilize her eggs. Some females can eject sperm from an undesirable mate.

The Evolutionary Enigma of Sexual Reproduction Produces fewer reproductive offspring than asexual reproduction, a so-called reproductive handicap Figure 23.16 Asexual reproduction Female Sexual reproduction Male Generation 1 Generation 2 Generation 3 Generation 4

Mechanisms: The Processes of Evolution

If sexual reproduction is a handicap, why has it persisted? It produces genetic variation that may aid in disease resistance

Why Natural Selection Cannot Fashion Perfect Organisms?

Why Natural Selection Cannot Fashion Perfect Organisms Evolution is limited by historical constraints Adaptations are often compromises Chance and natural selection interact Selection can only edit existing variations Environment, hence selection forces changes