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The Evolution of Populations CHAPTER 23 YOU MUST KNOW • How mutation and sexual reproduction each produce genetic variation. • The conditions for Hardy-Weinberg equilibrium. • How to use the Hardy-Weinberg equation to calculate allele frequencies to test whether a population is evolving. • What effects genetic drift, migration or selection may have on a population, and analyze data to justify your predictions. Genetic Variation makes evolution possible. • Phenotypic variation reflects genetic variation. • Mutations are the only source of new genes and new alleles. • Only mutations in cell lines that produce gametes can be passed to offspring. • Point mutations are changes in one nucleotide bases in a gene. • They can have significant impact on phenotype, as in sickle-cell disease. • Chromosomal mutations delete, disrupt, duplicate, or rearrange many loci at once. • Most of the genetic variations within a population are due to the sexual recombination of alleles that already exist in a population. • Sexual reproduction shuffles existing alleles and deal them at random to produce individual genotypes. • • • Crossing over Independent assortment Fertilization Hardy-Weinberg Principle • Used to describe a population is that not evolving. • *Population- is a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring. • It states that the frequencies of alleles and genes in a population’s gene pool will remain constant over the course of generations unless they are acted upon by forces other than Mendelian segregation and the recombination of alleles. • However, it is unlikely that all the conditions for Hardy-Weinberg equilibrium will be met. Allele frequencies change. Populations evolve. 5 conditions for Hardy-Weinberg Equilibrium • 1. No change in allelic frequency due to mutation. • 2. Random mating. • 3. No natural selection • 4. The population size must be extremely large. (no genetic drift) • 5. No gene flow. (Emigration, Immigration, transfer of pollen, etc.) Equation for Hardy-Weinberg Equilibrium • • • • • • • • • Two allelic forms, A and a Let p= The frequency of A, the dominant allele. And q= the frequency of a, the recessive allele. All dominant alleles (p) and recessive alleles (q) equal 100% p+q=1 p2 + 2pq + q2 =1 p2 = frequency of AA 2pq = frequency of Aa q2 = frequency of aa Practice • Suppose in a plant population that red flowers (R) are dominant to white (r). In a population of 500 individuals, 25% show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous for this trait? Allele frequency • Specific factors that alter allele frequency. • 3 major factors that alter allele frequencies and bring about most evolutionary change. 1. Natural Selection • 2. Genetic Drift • 3. Gene flow • 1. Natural Selection • Alleles being passed on to the next generation in proportions different from their relative frequencies in the present generation. • Individuals with variations that are better suited to the environment will pass on their genes. 2. Genetic Drift • Is the unpredictable fluctuation in allele frequencies from one generation to the next. • The smaller the population, the greater chance is for genetic drift. • This is a random, nonadaptive change in allele frequencies. • Genetic drift can lead to the loss of genetic diversity Founder Effect- a few individuals become isolated from a larger population and establish a new population whose gene pool is not reflective of the source population. • Bottleneck Effect- A sudden change in the environment (earthquake, flood, or fire) drastically reduces the size of a population. The few survivors that pass through the restrictive bottleneck may have a gene pool that no longer reflects the original population. • EX. The population of California condors was reduced to nine individuals. • 3. Gene Flow • Occurs when a population gains or loses alleles by genetic addition or subtraction from the population. • This results from the movement of fertile individuals or gametes. • Gene flow tends to reduce the genetic differences between populations, thus making populations more similar. Adaptive Evolution • Relative fitness refers to the contribution an organism makes to the gene pool of the next generation relative to the contributions of other members. • Fitness does NOT indicate strength or size, it is measured by the reproductive success. • Natural selection acts more directly on the phenotype and indirectly on the genotype and can alter frequency distribution of heritable traits in three ways. 1. Directional selection • 2. Disruptive selection • 3. Stabilizing selection • 1. Directional Selection • Shifts the overall makeup of the population by favoring variants that are 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. 2. 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. 3. 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. Sexual Selection • Is a form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates. • It can result in sexual dimorphism, a difference between the two sexes in secondary sexual characteristics such as differences in size, color, ornamentation, and behavior. • Intrasexual selection- selection within the same sex, individuals of one sex compete directly for mates of the opposite sex. • Intersexual selection- “mate choice”, individuals of one sex (usually females) are choosy in selecting their mates from the other sex. • Showiness of the male’s appearance or behavior How is genetic variation preserved in a population? • Diploidy: Because most eukaryotes are diploid, they are capable of hiding genetic variation (recessive alleles) from selection. • Heterozygote advantage: Individuals who are heterozygous at a certain locus have an advantage for survival. • EX. In sickle-cell disease, individuals homozygous for normal hemoglobin are more susceptible to malaria, whereas homozygous recessive individuals suffer from the complications of sickle-cell disease. Heterozygous benefit from protection from malaria and do not have sickle-cell disease. Why natural selection cannot produce perfect organisms • Selection can only edit existing variations • NS favors only the fittest phenotypes among population, which may not be the ideal traits. New advantageous alleles do not arise on demand! • Evolution is limited by historical constraints Each species has a legacy of descent with modification from ancestral forms. • Evolution does not scrap the ancestral anatomy and build new, but builds on existing structures and adapts them to new situations. • • Adaptations are often compromises. • Seals spend time on land, but if they had legs they could not swim as well. • Change, natural selection, and the environment interact. Chance events can affect the subsequent evolutionary history of populations. • Ex. Storm blows insects to a new island, the wind doesn’t choose those that could best survive on that island. •