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 23.2: 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 Gene duplication Duplicates chromosome segments

Mutation Rates Mutation rates Tend to be low in animals and plants Average about one mutation in every 100,000 genes per generation Mutations are spread more rapidly in microorganisms because of short generation times

Mutations Can only be passed on to offspring only if they occur in the germ line Are the ultimate source of genetic variation (new genes and alleles) But are NOT considered a significant source of genetic change, especially in slowly reproducing plants and animals

Sexual Recombination In sexually reproducing populations, sexual recombination is far more important than mutation in producing the genetic differences that make adaptation possible Most variation is produced by genetic differences that result from recombination of existing alleles Recombination may affect genotype frequencies but usually has no effect on allele frequencies

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

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

Genetic Drift Genetic drift is chance changes in the gene pool Chance changes have more of an effect on a small gene pool Statistically, the smaller a sample the greater the chance of deviation from a predicted result

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 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 The founder effect 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

Concept 23.4: Natural selection is the primary mechanism of adaptive evolution 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) Requires genetic variation

Variation Phenotypic variation Occurs between individuals in populations of all species Is not always heritable Only the genetic component can have evolutionary consequences 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

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

Polymorphisms 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

Measuring Genetic Variation Population geneticists Measure the number of polymorphisms in a population by determining the amount of heterozygosity at the gene level and the molecular level Average heterozygosity Measures the average percent of loci that are heterozygous in a population

Variation Between Populations Most species exhibit geographic variation in the 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 Cline Some geographic variations 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

Five Causes of Microevolution Two processes, mutation and sexual recombination produce the variation in gene pools that contributes to differences among individuals and that makes evolution possible Three major factors alter allele frequencies and bring about most evolutionary change Natural selection Genetic drift Gene flow