Chapter 8 Population Genetics: How do Genes Move through Time and Space?

Transcription

1 Chapter 8 Population Genetics: How do Genes Move through Time and Space? 4/29/2009 Chun-Yu Chuang How Do We Characterize Variation? Variation can be smooth or discontinuous. Two views of biology Naturalists Supported Darwin s ideas. Experimentalists Supported Mendelian Ideas. 1

2 Naturalists Viewpoint Saw most traits in a population exhibited a continuum of forms. Believed the ability to survive and reproduce might depend on having traits that fall within some range of a spectrum. Believe that traits within populations change or evolve as features of environmental change. Experimentalists Viewpoint Rejected naturalists view of evolution. Viewed variation as a sudden change due to mutations. Maintained evolution progressed by leap and bounds by sudden random mutations. Evolution could not be a gradual process 2

3 Stalemate Broken Herman Nilsson-Ehle (1909) Using wheat kernels proved that traits that appear in populations as a continuous spectrum of forms with a genetic basis. Stalemate Broken Nilsson-Ehle showed a cross between true-breeding red- and white-kerneled plants produced all light redkerneled plants Cross between light red-kerneled plants yielded 7 categories of color. 3

4 Frequency Diagrams Illustrate Variation Useful graphing tool for illustrating variation in population X-axis:range of different forms that a trait can exhibit Y-axis number of individuals in population that exhibit each form of the trait Frequency Diagram of Human Height Graphing human height creates a bell-shaped curve. So many different forms that the categories blend. 4

5 Frequency Diagram of Wheat Kernels Frequency diagrams of Nilsson-Ehle F 2 variation. Plants grown in controlled laboratory vs. those grown in the wild Proved that genes can be responsible for seven different forms. Continuous Variation is Determined by Two or More Genes Polygenic (quantitative) traits: Influenced by two or more genes residing at different loci on the same or on different chromosomes. 5

6 How Do Populations Differ? Brachydactyly Human trait in which the terminal bones of the fingers and toes do not grow their normal length Populations are Collections of Alleles Populations: Group of interbreeding organisms of the same species that exist together in both time and space. Gene pools: All of the alleles found in the population. Think of a beanbag Beans are analogous to alleles and the entire bag of beans is the population s gene pool. 6

7 Alleles Occur at Certain Frequencies Example: gene pool for sickle cell anemia Possible alleles humans could have: HBA or HBS If we let p = HBA and q = HBS, the sum should equal 100 % of the alleles in the gene pool. This could be rewritten p + q = 1 Hardy Weinberg Principle Makes it possible to calculate allele frequencies (p, q) based on phenotypes. Can calculate the sum of the genotypes: p 2 + 2pq + q 2 = 1 p 2 = frequency of homozygous dominant genotype 2pq = frequency of heterozygous genotype q 2 = frequency of homozygous recessive genotype 7

8 Hardy Weinberg Principle States allele frequencies for a population will remain the same from generation to generation as long as specific conditions are met. Populations in which p and q do not change are said to be in a genetic equilibrium. Hardy Weinberg Principle Required conditions for genetic equilibrium: 1. Populations are large. 2. Individuals mate randomly. 3. Populations do not gain or lose individuals. 4. Natural selection is not occurring in the population. 5. Mutation is not occurring at a high enough rate to influence genetic variation. 8

9 Hardy Weinberg Principle Power of this principle: Allows us To calculate what would happen if natural selection were not occurring To compare what does happen in the real world Also allows us to calculate the proportions of individuals in the population that have each of the three possible genotypes. Microevolution Definition: Change in allele frequencies in a gene pool over time Factors that contribute to microevolution: Natural selection Genetic Drift Founder effect Bottleneck effect Mutation Gene flow 9

10 Natural Selection Example of natural selection: industrial melanism Rapid shift in the color of peppered moth populations during the 19 th century in England Natural Selection Color of moth due to pair of alleles: Carbonaria = M Speckled = m Before industrialism: Speckled moths had advantage because their coloring served to camouflage them 10

11 Natural Selection After industrialism: Lichens on the trees died. Made speckled moths visible. Darker moths were more likely to survive. Resulted in change in the allele frequency of the population. Natural Selection Heterozygote advantage: Tendency of red blood cells to sickle makes these cell resistant to penetration by the parasite that causes malaria. Heterozygotes can survive disease and have immunity against malaria. 11

12 Types of Selection Directional selection Selection that acts on one extreme of the range of variation for a particular characteristic. Example: Frog tongue length. Types of Selection Stabilizing selection Selection that operates against the extremes in the distribution of a particular trait in a population. Example: human birth weight 12

13 Disruptive selection Types of Selection Selection that favors the extremes and disfavors the middle range of particular traits in a population. Example: bird beak size Some Changes in Allelic Frequency Are Random Genetic Drift: Random change in allelic frequencies as a result of chance alone. Seen in small populations Two types: Founder effect Bottlenecks Often referred to as neutral selection Occurs independent of natural selection 13

14 Founder effect Eventual genetic difference between an isolated offshoot population and the original population from which it came. Example: Pennsylvania Amish, settlers of Tristan da Cunha 14

15 Bottlenecks A drastic decrease in the size of a population with a resulting decrease in the genetic variability within a population. Usually due to a catastrophe (drought, hunting, flood etc.) Mutation A permanent change in the genetic material of a cell or organism. Can be inherited from generation to generation. Introduces new alleles into the population Effects can be lethal, neutral or advantageous in a population. 15

16 A shift in the allelic frequencies within a population and between populations resulting from migration. Either immigration or emigration Example: DDT and mosquitoes Gene Flow 16