Mechanisms of Evolution Microevolution Population Genetics Key Concepts 23.1: Population genetics provides a foundation for studying evolution 23.2: Mutation and sexual recombination produce the variation that makes evolution possible 23.3: Natural selection, genetic drift, and gene flow can alter a population s genetic composition 23.4: Natural selection is the primary mechanism of adaptive evolution Concept 23.4: Natural selection is the primary mechanism of adaptive evolution Natural selection Accumulates and maintains favorable genotypes in a population Is the only deviation from the Hardy-Weinberg that leads to adaptation (of the population to the environment) 1
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 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 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 2
Frequency of individuals 3 Modes of Selection 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 Original population Original population Evolved population Phenotypes (fur color) 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. 3
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 4
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 Distribution of malaria caused by Plasmodium falciparum (a protozoan) 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% Frequency-Dependent Selection In frequency-dependent selection The fitness of any morph declines if it becomes too common in the population 5
Example of frequencydependent selection Parental population sample 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). Experimental group sample Phenotypic diversity 0.06 0.05 0.04 0.03 Frequencyindependent control Plain background Figure 23.14 Patterned background 0.02 0 20 40 60 80 100 Generation number 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 6
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 Sexual reproduction Produces fewer reproductive offspring than asexual reproduction, a so-called reproductive handicap Asexual reproduction Sexual reproduction Female Generation 1 Female Generation 2 Male Generation 3 Generation 4 Figure 23.16 7
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 Adaptations are often compromises Chance and natural selection interact Selection can only edit existing variations 8