The Evolution of Populations Chapter 23 The Evolution of Populations

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

Where does variation come from? Mutation and sexual recombination produce the variation that makes evolution possible Produce the variation in gene pools that contributes to differences among individuals

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

Altering allele frequencies in a population Three major factors alter allele frequencies and bring about most evolutionary change Natural selection Genetic drift Gene flow

Genetic drift Chance events cause allele frequencies to fluctuate unpredictably from one generation to the next Tends to reduce genetic variation More likely in small populations 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 (one example of genetic drift) In 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 (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 (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 (a second example of genetic drift) Occurs when a few individuals become isolated from a larger population Can affect allele frequencies in a population

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

Natural Selection Natural selection is the primary mechanism of adaptive evolution From the range of variations available in a population natural selection increases the frequencies of certain genotypes, fitting organisms to their environment over generations.

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

Sexual Selection Sexual selection Is natural selection for mating success Can result in 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

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

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

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