Chapter 23 Evolution of Populations

Common misconception 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

Population genetics  foundation for studying evolution Microevolution Chg. in genetic makeup of a population from generation to generation Figure 23.2

The Modern Synthesis Population genetics  how populations change genetically over time

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

Population 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

Gene pool Total aggregate of genes in a population at any one time  all gene loci in all individuals of the population

The Hardy-Weinberg theorem Describes a population that is not evolving Frequencies of alleles remain constant from generation to generation provided that only Mendelian segregation and recombination of alleles are at work

Mendelian inheritance Preserves genetic variation in a population 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

Where gametes contribute to the next generation randomly, allele frequencies will not change

Hardy-Weinberg equilibrium Random mating Allele frequencies do not change

A population in Hardy-Weinberg equilibrium Figure 23.5 Gametes for each generation are drawn at random from the gene pool of the previous generation: 80% CR (p = 0.8) 20% CW (q = 0.2) Sperm CR (80%) CW (20%) p2 64% CRCR 16% CRCW 4% CWCW qp Eggs pq If the gametes come together at random, the genotype frequencies of this generation are in Hardy-Weinberg equilibrium: q2 64% CRCR, 32% CRCW, and 4% CWCW Gametes of the next generation: 64% CR from CRCR homozygotes 16% CR from CRCW homozygotes + = 80% CR = 0.8 = p 16% CW from CRCW heterozygotes 20% CW = 0.2 = q With random mating, these gametes will result in the same mix of plants in the next generation: 64% CRCR, 32% CRCW and 4% CWCW plants 4% CW from CWCW homozygotes

If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then p2 + 2pq + q2 = 1 p2 and q2  frequencies of homozygous genotypes 2pq  frequency of heterozygous Also p + q = 1 (allele frequency)

The Hardy-Weinberg theorem Hypothetical population Real populations Allele and genotype frequencies do change over time

5 conditions for non-evolving populations are rarely met in nature Extremely large population size No gene flow No mutations Random mating No natural selection

Mutation and sexual recombination  variation in gene pools, contributes to differences among individuals, makes evolution possible

Mutations Cause new genes and alleles to arise Chgs in nucleotide sequence of DNA Cause new genes and alleles to arise Figure 23.6

Point mutation Chg in 1 base in a gene Significant impact on phenotype Usually harmless, but may have an adaptive impact

Chromosomal mutations that affect many loci Usually harmful, but may be neutral or even beneficial

Mutation rates Low in animals and plants ~ 1 mutation in every 100,000 genes per generation Rapid in microorganisms

In sexually reproducing populations, sexual recombination far more important than mutation f/ genetic differences that make adaptation possible

3 major factors alter allele frequencies  most evolutionary change Natural selection Genetic drift Gene flow

Differential success in reproduction Natural Selection Differential success in reproduction Certain alleles pass to the next generation in greater proportions

Genetic Drift The smaller a sample, the greater the chance of deviation from a predicted result

Genetic drift Allele freq can fluctuate unpredictably from one generation to the next Reduces 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

Bottleneck effect Sudden change in the environment may drastically reduce the size of a population Gene pool no longer reflective of the original population Figure 23.8 A (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

Bottleneck effect Can increase understanding of how human activity affects other species, e.g. hunting Figure 23.8 B (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.

Founder effect A few individuals become isolated f/ a larger population Can affect allele frequencies in a population

Gene flow Causes a population to gain or lose alleles Results f/ movement of fertile individuals

Natural selection (primary mechanism of adaptive evolution) Accumulates and maintains favorable genotypes in a population Reduces differences between populations

Genetic variation Occurs in individuals in populations of all species 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

Quantitative characters Variation Discrete characters Can be classified on an either-or basis Quantitative characters Vary along a continuum within a population

Phenotypic polymorphism Two or more distinct morphs in high enough freq.to be noticeable Genetic polymorphisms Heritable components of characters that occur along a continuum

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 e.g. a cline, 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

Phrases “struggle for existence” and “survival of the fittest” Used to describe natural selection, but can be misleading Reproductive success Is generally more subtle

Fitness Relative fitness Contribution to the gene pool of the next generation Relative fitness Contribution of a genotype to the next generation

Selection Favors certain genotypes by acting on the phenotypes of certain organisms

Directional selection Favors individuals at one end of the phenotypic range Disruptive selection Favors extremes Stabilizing selection Favors intermediates

Phenotypes (fur color) Frequency of individuals 3 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

Diploidy Maintains genetic variation hidden recessive alleles

Balancing selection Natural selection maintains stable frequencies of diff. forms  balanced polymorphism

Heterozygote Advantage Sometimes heterozygous at a particular locus  greater fitness Natural selection Will maintain two or more alleles

The sickle-cell allele Causes mutations in hemoglobin but also confers malaria resistance  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 Fitness of any morph declines if it becomes too common in the population

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

Sexual selection Natural selection for mating success sexual dimorphism, differences between the sexes

Intrasexual selection Direct competition f/ mates

Intersexual selection One sex (usually females) are choosy in selecting mates  showiness of the male’s appearance Figure 23.15

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

Why has it persisted?  genetic variation

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