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Chapter 23-The Evolution of Populations Populations Evolve Individuals do not. • A population is defined as a group of individuals belonging to the same species for which there are no restrictions to random mating among its members. • gene pool, the total of all alleles in the population • Allele frequencies- the relative abundance of different alleles carried by the individuals in that population There is a stability in the allele frequencies of a population. In the absence of disturbing factors, the frequency of different genotypes in a population will reach an equilibrium and will remain stable from generation to generation. This observation is based on the Hardy Weinberg Principle. Nonevolving Populations • Nonevolving populations are said to be in HardyWeinberg equilibrium. The Hardy-Weinberg theorem is a theory maintaining that the sexual shuffling of genes alone cannot alter the overall genetic make-up of a population. • The five conditions that are required for a population to be in Hardy-Weinberg equilibrium are: 1. A very large population size 2. Isolation from other populations 3. No mutations 4. Random mating 5. No natural selection Hardy Weinberg Principle p2 + 2pq + q2 = 1 p=frequency of dominant allele q=frequency of recessive allele p2=frequency of homozygous dominant genotype 2pq=frequency of the heterozygous genotype q2=frequency of the homozygous recessive genotype. ALSO, p+q=1 HWP can be used to detect if random mating is taking place and if evolutionary changes are taking place in a given population. Wildflower Population in Hardy-Weinberg Equilibrium Figure 23.3a The Hardy-Weinberg theorem • One of the HardyWeinberg equations is p + q = 1, where p is the frequency of the dominant allele and q is the frequency of the recessive allele. • Here is the genetic structure of the parent population of a population of wildflowers that are in Hardy-Weinberg equilibrium. Notice that p (.8) + q (.2) = 1. Wildflower Population in Hardy-Weinberg Equilibrium (continued) • Another equation used in when populations are in Hardy-Weinberg equilibrium is p2+ 2pq + q2 = 1. Here,p2 is the frequency of the homozygous dominant genotype, 2pq is the frequency of the heterozygous genotype, and q2 is the frequency of the homozygous recessive genotype. • Notice that the frequencies of the alleles of these offspring are the same as their parents (p = .8 and q = .2). Figure 23.3b The Hardy-Weinberg theorem Why does evolution take place? Evolution occurs when a population moves away form HWE. This is due to changes in the allele frequencies in a gene pool. What causes these allele frequencies to change? MICROEVOLUTION! http://glencoe.mcgrawhill.com/sites/9834092339/student_view0/chapter 20/animation_-_mechanisms_of_evolution.html Causes of Microevolution Figure 23.4 Genetic drift https://www.youtube.com/watch?v =PQuZkMjQn6c • Microevolution is a change in the gene pool of population over a succession of generations. • There are five causes of microevolution: 1. Genetic drift 2. Gene flow 3. Mutation 4. Nonrandom mating 5. Natural selection • The picture at the left illustrates genetic drift, which is a random change in the gene frequency of a small breeding population. • Genetic drift can lead to extinction because it reduces variation making organisms susceptible to a changing environment. • Ex. The really fast zebra gene. Special Cases of Genetic Drift • One type of genetic Ex. Old order Amish of drift is termed the Lancaster founder effect. The founder effect occurs Started out with a population of a when a small population few dozen. This population of organisms colonizes shows a large percentage of a new area. The gene dwarfism and polydactylism. frequency of a population will change because this small population most likely http://highered.mcgrawwill not represent the hill.com/sites/dl/free/0072835125/ gene pool of the larger 126997/animation45.html population •A second type of genetic drift is the bottleneck effect. The bottleneck effect occurs when there is a sudden decrease in the number of individuals of a population due to some natural disaster. A decrease in the # of individuals will also decrease variation. Ex. Elephant seal Hunted to near extinction, the elephant seal population was put under protection. Today we have a large population but very little variation. Figure 23.5 The bottleneck effect: an analogy Bottleneck Example Nene Goose Gene Flow • Gene flow is the loss or gain of alleles from a population due to emigration or immigration of fertile individuals. • Result-can introduce new alleles (increase variation) or change existing allele frequencies (decrease variation) • Ex. Human movement around the globe gene flow became an important agent of microevolutionary change in isolated populations. http://nortonbooks.co • Ex. Bird Migration m/college/biology/ani mations/ch17a01.htm Mutation Any change in the genetic material that alters the DNA. 3 types A change in the nucleotide base pairs Rearrangement of genes on a chromosome Change in the chromosome Mutations are the source of variation in a population. Unfavorable mutations are usually not selected. Non Random Mating- Inbreeding or selection of mates for specific phenotypes. (reduces the frequency of heterozygous individuals) •A common type of nonrandom mating is assortative mating, which occurs when individuals select partners with similar phenotypic characters. OR Inbreeding- causes the relative frequencies of genotypes to deviate from the expected HWE. (more chances for recessive traits to come together) Natural Selection- A process of interaction between organisms and their environment that results in a differential rate of reproduction. Can result in a change in allele frequency and genotypes in a population. Of all the mechanisms of microevolution, natural selection is most likely to adapt a population to its environment. Some examples of adaptive natural selection. Cryptic Coloration (protective coloration) color or scheme that permits an organism to blend into its environment. Ex Insects, Preying Mantis, Katydid, Walking Sticks Aposematic Coloration (Warning Coloration) Any brightly colored species that usually contains some type of toxin. Mimicry- Two different organisms look alike. Ex. 1 Batesian mimicry- A harmless or edible species resembles one that is dangerous. Viceroy and Monarch butterfly. Ex. 2 Mullerian mimicry- Two or more unpalatable, aposematic colored species resemble each other. Advantage- Predators learn quickly to avoid brightly colored prey Insects that feed on milkweed resemble the warning color of the monarch. http://www.youtube.com/watch?v=FvDpbjSRpuc VICEROY MONARCH The Three Modes of Natural Selection • Stabilizing selection favors intermediate phenotypes over extreme phenotypes. • Directional selection favors one extreme phenotype over the other phenotypes. • Diversifying selection occurs when individuals on both extremes of a phenotypic range are favored over intermediate Figure 23.12 Modes of selection Directional Selection Diversifying Selection Other Important Facts Over Chapter 23 • Polymorphism is the condition where two or more contrasting forms of a particular trait are represented in relatively high frequencies in a population. •Heterozygote advantage means that individuals who are heterozygotes for a particular trait are more likely to survive than homozygotes of the same trait. •Sexual selection will select for traits that enhance an individual’s chance of mating. Sexual Dimorphism