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3/29/17 Section 1: Genetic Variation Chapter 17: Population Genetics and Speciation Population Genetics: The study of the frequency and interaction of alleles and genes in populations Normal Distribution: A line graph showing the general trends in a set of data of which most values are near the mean I. Population Genetics A. Charles Darwin Knew 1. Heredity influences characteristics 2. Did not know about genes B. We now study and predict genetic variation and change C. Microevolution 1. Evolution at the level of genetic change in populations 2. Can be studied by observing changes in the numbers and types of alleles in populations, called population genetics Microevolution D. The study of genetics and evolution are advancing together E. Speciation 1. Link from microevolution to macroevolution 2. Now can study in detail 1 3/29/17 II. Phenotypic Variation Examples of Polygenic A. Variety of phenotypes that exists for a given characteristic depend on how many genes affected B. Polygenic characters are influenced by several genes 1. Example a. Human eye color b. Height C. Biologists study polygenic phenotypes by measuring each individual in the population and then analyzing the distribution of the measurements 1. Distribution is an overview of the relative frequency and range of a set of values 2. Some values in a range are more common than others III. Measuring Variation and Change D. A normal distribution or bell curve 1. Tends to cluster around the average value in the center of the range A. Particular combination of alleles in a population at any one point in time makes up a gene pool. B. Genetic variation and change are measured in terms of the frequency of alleles. 2 3/29/17 Example of Gene Pool C. A frequency is the proportion or ratio of a group that is of one type 1. To study genetic change a. frequency in a population is tracked over time Gene Frequency IV. Sources of Genetic Variation A. Evolution cannot proceed if there is no variation B. Major source of new alleles in natural populations is mutation in germ cells C. Mutation generates new alleles at a slow rate D. Only mutations in germ cells (egg and sperm) are passed on to offspring Section 2: Genetic Change Genetic Equilibrium: A state in which the allele frequencies of a population remain in the same ratios from one generation to the next. I. Equilibrium and Change A. Population in which no genetic change occurred would be in a state of genetic equilibrium B. Genetic change in a population can be measured 1. Change in genotype frequency 2. Change of allele frequency 3. Change in one doesn’t necessarily mean a change in other 3 3/29/17 Example of Genetic Equilibrium C. The Hardy-Weinberg principle predicts that the frequencies of alleles and genotypes in a population will not change unless at least one of five forces acts upon the population. Hardy-Weinberg D. The forces that can act against genetic equilibrium 1. Gene flow 2. Nonrandom mating 3. Genetic drift 4. Mutation 5. Natural selection Hardy Weinberg Forces E. Gene Flow 1. Occurs when genes are added or removed from a population 2. Can be caused by a. migration b. movement of individuals from one population to another 4 3/29/17 Example of Gene Flow F. Nonrandom Mating 1. In sexually reproducing populations, any limits or preferences of mate choice Example Genetic Drift 1. Chance events can cause rare alleles to be lost from one generation to the next, especially when population are small. Example of Genetic Drift H. Mutation • 1. Introduces New Alleles to the Population 5 3/29/17 Mutation Example I. Natural Selection 1. Acts to eliminate individuals with certain traits from a population 2. As individuals are eliminated, the alleles for those traits become less frequent 3. Both allele and genotype frequencies change Example of Natural Selection II. Sexual Reproduction and Evolution A. Creates chances to recombine alleles and increase variation in a population B. Creates the possibility that mating patterns or behaviors can influence the gene pool. 1. Example, in animals, females sometimes select mates based on the male’s size, color, ability to gather food, or other characteristics a. Behavior is called sexual selection and is an example of nonrandom mating b. Another example inbreeding 1) individuals either self-fertilize or mate with others like themselves 2) more likely to occur if a population is small 3) all members are likely to be closely related 6 3/29/17 III. Population Size and Evolution IV. Natural Selection and Evolution A. Strongly affects the probability of genetic change in a population B. Allele frequencies are more likely to remain stable in large populations than in smaller populations C. Genetic drift is a strong force in small populations and occurs when a particular allele disappears A. Natural selection is a result of the following facts: 1. All populations have genetic variation 2. Individuals tend to produce more offspring than the environment can support 3. Populations depend upon the reproduction of individuals B. How Selection Acts C. Genetic Results of Selection 1. Causes evolution in populations by acting on individuals 2. Acts when individuals survive and reproduce (or fail to do so) 3. Less “fit” individuals are less likely to pass on their genes D. Why Selection is Limited 1. Key lesson that scientists have learned about evolution by natural selection is that the environment does the selecting. 2. Natural selection is indirect a. Acts only to change the relative frequency of alleles b. Acts on genotypes by removing unsuccessful phenotypes 1. Each allele’s frequency may increase or decrease depending on the allele’s effects on survival and reproduction 3. The role of Mutation a. Only characteristics that are expressed can be targets of natural selection. b. If a mutation results in rare recessive alleles, selection cannot operate against it. 1) For this reason, genetic disorders (cystic fibrosis) can persist in populations 7 3/29/17 V. Patterns of Natural Selection A. Three major patterns are possible in the way that natural selection affects the distribution of polygenic characters over time. 1. These patterns are: a. directional selection b. stabilizing selection c. disruptive selection Directional Selection B. Directional Selection 1. The “peak” of a normal distribution moves in one direction along its range a. Selection acts to eliminate an extreme from a range of phenotypes making them less common C. Stabilizing Selection 1. The bell-curve shape becomes narrower a. Selection eliminated individuals that have alleles for any extreme type b. Very common in nature Stabilizing Selection D. Disruptive Selection 1. The bell curve is “disrupted” and pushed apart into two peaks 2. Selection acts to eliminate individuals with average phenotype. 8 3/29/17 Disruptive Selection Section 3: Speciation Reproductive Isolation: A state in which a population can no longer interbreed with other populations to produce future generations Subspecies: A taxonomic classification below the level of species, refers to populations that differ from, but can interbreed with, other populations of the same species I. Defining Species A. Scientists may use more than one definition for species B. Definition depends on organisms and field of science being studied C. Generally defined as a group of natural populations that can interbreed and produce fertile offspring D. Instead of, or in addition to, the biological species concept, species may be defined based on 1. Physical features 2. Ecological roles 3. Genetic relatedness II. Forming New Species A. Each population of a single species lives in a different place B. Natural selection acts upon each population and tends to result in offspring that are better adapted to each specific environment C. If the environments differ, adaptations may differ 1. This is called divergence and can lead to formation of a new species 9 3/29/17 E. Reproductive Isolation D. Speciation 1. Process of forming new species by evolution from preexisting species 2. Has occurred when the effects of evolution result in population with unique features and is reproductively isolated 1. State in which two populations can no longer interbreed to produce future offspring 2. Groups may be subject to different forces a. will tend to diverge over time 3. Populations of the same species may differ enough to be considered subspecies Example of Subspecies F. Mechanisms of Isolation 1. Any of the following mechanisms may contribute to the reproductive isolation of populations a. Geography 1) physical barrier that prevents interbreeding Geographical Barriers b. Ecological Niche 1) populations use different niches causes divergence c. Mating Behavior and Timing 1) two populations have different mating times 10 3/29/17 Example of Different Niches • Spiny Mice- Avoid competition • One Population Feeds at Night • One Population Feeds during Day d. Polyploidy 1) because of 2 sets of chromosomes cannot match chromosomes 2) becomes genetically isolated Polyploidy in Plants e. Hybridization 1) two species are so different have a hybrid that is not fertile 2) horse + donkey= mule (sterile) 3) lion + tiger= liger (sterile) Mule Liger 11 3/29/17 G. Extinction: The End of a Species Extinct Animals 1. Occurs when a species fails to produce any more descendants a. Can only be detected when complete 2. More than 99% of all of the species that have ever lived have become extinct 3. Many cases of extinction are the result of environmental change 4. If a species cannot adapt fast enough to changes, the species may be driven to extinction 12