23.3 23.4 Natural selection, genetic drift, and gene flow can alter a population’s genetic composition

Concept 23.3: Natural selection, genetic drift, and gene flow can alter a population’s genetic composition Three major factors alter allele frequencies and bring about most evolutionary change Natural selection Genetic drift Gene flow

Genetic Drift Unpredictable fluctuation in alleles frequency from one generation to the next because of a population finite size. Statistically, the smaller a sample The greater the chance of deviation from a predicted result

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

Genetic drift Describes how allele frequencies can fluctuate unpredictably from one generation to the next Tends to reduce genetic variation Figure 23.7 CRCR CRCW CWCW Only 5 of 10 plants leave offspring Only 2 of Generation 2 p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3

The Bottleneck Effect A sudden change in the environment may drastically reduce the size of a population The gene pool may no longer be reflective of the original population’s gene pool Figure 23.8 A Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population after some environmental disaster. By chance, blue marbles are over-represented in the new population and gold marbles are absent. Original population Bottlenecking event Surviving population

Understanding the bottleneck effect Can increase understanding of how human activity affects other species Figure 23.8 B Similarly, bottlenecking a population of organisms tends to reduce genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction.

The Founder Effect Occurs when a few individuals become isolated from a larger population Can affect allele frequencies in a population

Gene Flow Results from the movement of fertile individuals or gametes Causes a population to gain or lose alleles Tends to reduce differences between populations over time

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

Genetic Variation Seasonal differences in hormones 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 Seasonal differences in hormones

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 The different forms can be called morphs. Quantitative characters Vary along a continuum within a population

Polymorphism Phenotypic polymorphism Genetic polymorphisms 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 Alleles of several loci affect the character height

Measuring Genetic Variation Population geneticists measure the number of polymorphisms in a population by determining the amount of heterozygosity At the gene level Average heterozygosity: measures the average percent of loci that are heterozygous in a population At the molecular level: nucleotide variability Average heterozygosity tends to be greater than nucleotide variability

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 In the island of Madeira, house mice separated by mountains have evolve in isolation of each other. Differences in Karyotypes. The patterns of fused chromosomes differ from one population to another. These mutations leave the genes intact, the effects on the mice appear neutral.

Heights of yarrow plants grown in common garden Some examples of geographic variation occur as a cline: a graded change in a trait along a geographic axis (parallels to the gradient in the environment) 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

Evolutionary Fitness Reproductive success 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

In a more quantitative way: Relative fitness Is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals In a more quantitative way: Relative fitness 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 Directional selection Disruptive selection Stabilizing selection

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.

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.

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.

The Preservation of Genetic Variation Various mechanisms help to preserve genetic variation in a population Diploidy (Eukaryotes are diploid) Maintains genetic variation in the form of hidden recessive alleles

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

The sickle-cell allele Causes mutations in hemoglobin but also confers malaria resistance Exemplifies the heterozygote advantage Figure 23.13 Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% 5.0–7.5% 7.5–10.0% 10.0–12.5% >12.5% Distribution of malaria caused by Plasmodium falciparum (a protozoan)

Frequency-Dependent Selection The fitness of any morph declines if it becomes too common in the population

Parental population sample Experimental group sample An example of frequency-dependent selection Figure 23.14 Parental population sample Experimental group sample Plain background Patterned background On pecking a moth image the blue jay receives a food reward. If the bird does not detect a moth on either screen, it pecks the green circle to continue to a new set of images (a new feeding opportunity). 0.06 0.05 0.04 0.03 0.02 20 40 60 80 100 Generation number Frequency- independent control Phenotypic diversity

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

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

Intrasexual selection Is a direct competition among individuals of one sex for mates of the opposite sex

Intersexual 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

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

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 Evolution is limited by historical constraints Each species has a legacy of descent with modification. (birds 4 legs & wins?) Adaptations are often compromises Seals swim well, walk not as well Chance and natural selection interact Selection can only edit existing variations