The Evolution of Populations

Transcription

1 Microevolution The Evolution of Populations C H A P T E R 2 3 Change in allele frequencies over generations Three mechanisms cause allele frequency change: Natural selection (leads to adaptation) Genetic drift Gene flow In contrast to macroevolution Genetic variation makes evolution possible Some phenotypic variation is environmental and is not heritable Genetic variation within a population Measuring variability Variation can be discrete Mendel s pea flower color Controlled by one locus Either/or trait Variation and be quantitative Most variation is quantitative Eye color Controlled by two or more genes Continuous variation in trait A locus is fixed if all individuals in a population are homozygous for the same allele Genetic diversity is measured in terms of average heterozygosity The average percentage of loci that are heterozygous 1

2 Generation of genetic diversity Populations share a gene pool Mutation is the ultimate generation of genetic diversity Point mutation Chromosomal rearrangements Mutation rate is higher in rapidly reproducing organisms and in organisms with RNA genome Sexual reproduction shuffles genetic diversity Crossing over, fertilization, independent sorting of chromosomes Allele frequency Genotype frequency In two-allele system, designated as p and q N = 20 N = 20 Hardy Weinberg Equilibrium is used to determine if evolution is occurring For a given p and q, the genotype frequency will be p 2 + 2pq + q 2 if a population is in Hardy Weinberg equilibrium (i.e., a population is not evolving) p 2 and q 2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci In a study of enzyme variation in a species of grasshopper, you find 15 A1A1, 65 A1A2 and 20 A2A2 in a sample of 100 moths. What is the allele frequency of each allele? What is the genotype frequency of each genotype? What is the expected genotype frequency under Hardy- Weinberg Equilibrium? Is the population in Hardy Weinberg Equilibrium? What can you conclude about this population in terms of evolution? 2

3 In the pea plant, red individuals may be either homozygous (R1R1) or heterozygous (R1R2), whereas white flowers are homozygous (R2R2). In a sample of plants, there are 35 red flowers and 65 white flowers. Assume that the population is not evolving. What are the allele frequencies? What proportion of red flowers is heterozygous (i.e, what percentage of the red flowers are heterozygous)? A population not in Hardy-Weinberg equilibrium indicates that one or more of the five evolutionary agents are operating in a population Natural selection Natural selection causes adaptive evolution (i.e., the evolution of traits that make organisms better matched to their environment) Genetic drift Random fluctuation in allele frequencies over time by chance Important in small populations Leads to loss of genetic variation, causing alleles to become fixed (even maladaptive alleles) Two types Founder effect Bottleneck effect 3

4 Founder effect Founder effect Few individuals found new population (small allelic pool) Bottleneck effect Bottleneck effect Gene flow Isolation effects A movement of alleles from one population to another Powerful agent of change Tends to homogenize allele frequencies 4

5 Mutation Non-random mating A random change in a cell s DNA Mutation rates have little effect on H-W equilibrium Ultimate source of genetic variation Mating with specific genotypes Shifts genotype frequencies but has little change on allele frequencies Assortative mating increases the proportion of homozygous individuals Disassortative mating produces excess of heterozygotes Natural selection Directional selection Natural selection is not evolution Differential survival and/or reproduction Differences in survival and/or reproduction are not due to chance, but due to a heritable trait that increases fitness Selection against one extreme Shifts the mean of a population Negative phototropism in Drosophila Disruptive selection Selection against mean Increases variation 5

6 Black-bellied seedcracker finch of west Africa Stabilizing selection Selection against extremes Reduces variation but does not change mean Birth weight in humans Sexual selection A type of natural selection driven by interactions between the sexes Leads to sexual dimorphism Intrasexual selection Intersexual selection 6

7 Maintenance of genetic variation Oscillating selection Directional and stabilizing selection reduce genetic variation Balancing selection (heterozygote advantage, oscillating selection* and frequency dependent selection) maintain variation *not in text Selection favors one phenotype at one time and a different phenotype at another time Fitness of a phenotype does not depend on its frequency Environmental changes lead to oscillation in selection Oscillating selection Heterozygote advantage Galapagos Islands ground finches Wet conditions favor smaller bills Dry conditions favor larger bills Heterozygotes may exhibit greater fitness than homozygotes Keeps deleterious alleles in a population Malaria Anemia/malaria Malaria caused by Plasmodium falciparum that infects red blood cells Heterozygote advantage arises from balance of opposing selective factors anemia and malaria Genotype AA AS SS Phenotype Relative fitness Normal blood, susceptible to malaria Slight anemia, less susceptible to malaria Severe anemia 1 s 1 1 t 7

8 Distribution of sickle cell allele and malaria Frequency dependent selection Depends on how frequently a phenotype occurs in a population Why aren t all organisms perfectly matched with their environment? Must be existing variation Evolutionary constraints Trade-offs (i.e., compromises) occur 8