Worksheet - Evolutionary Mechanisms Answer Key

Worksheet #5 - Evolutionary Mechanisms – Answer Key 1. The left hand column of the table below lists several possible characteristics of evolution due to natural selection vs. evolution resulting from genetic drift. For each characteristic, please indicate whether or not it accurately describes the two different types of evolution. (7 marks) Characteristic: 1. Changes in the frequency of an allele are due to differences in the survival and/or reproductive success of individuals Explanation: Remember that differences in survival and/or reproductive success between individuals are not considered to be the effects of natural selection unless they are caused by the way phenotype interacts with the environment. Random (i.e., unpredictable) differences in which individuals survive and which ones pass on their alleles to the next generation can result in genetic drift. 2. The individuals that survive/reproduce are a non-representative sample of the previous generation Explanation: In the case of natural selection, the individuals in the next generation are non-representative of the previous generation because they have traits (or their parents had traits) that were more successful in the current environment. If the next generation is non-representative of the previous generation for unpredictable reasons (i.e., not related to how phenotype interacts with environment), then we say that genetic drift has occurred. If we describe genetic drift as a change in allele frequencies due to sampling error, then we can describe evolution by natural selection as a change in allele frequencies due to sampling bias. If the next generation is representative of the previous generation in terms of its allele frequencies, then allele frequencies haven’t changed (= no evolution), and neither natural selection nor genetic drift have occurred. 3. The survival/reproduction of the allele in the population is related to the phenotype associated with that allele Explanation: If there is no way we can predict fitness based on phenotype (i.e., there is no apparent pattern or causal relationship between Evolution due to natural Genetic drift selection (Is it a characteristic of this type of evolution? Yes/No) Yes Yes Yes Yes Yes No phenotype and fitness), then natural selection is not occurring. Genetic drift is still possible, though! 4. It appears that all alleles/individuals are equally likely to survive and reproduce (i.e., we can’t predict what will happen) Explanation: Same as above – if there is no way we can predict fitness based on phenotype (i.e., there is no apparent pattern or causal relationship between phenotype and fitness), then natural selection is not occurring. Genetic drift is still possible, though! 5. Can occur because of a drastic reduction in population size Explanation: A drastic reduction in population size could possibly be the result of a large number of deaths (or sudden widespread inability to reproduce) in the population. If there is some relationship between the alleles an individual carries and their chances of survival/reproduction, then the small number of surviving individuals might be carrying specific alleles that were selected for (i.e., natural selection has caused a change in allele frequencies). If there is no relationship between genotype and fitness, a drastic reduction in population size might still result in a change in allele frequencies, via genetic drift (i.e., even though the sample is random, it is non-representative). 6. Can reduce the amount of genetic variation in a population Explanation: If natural selection is strong (i.e., specific alleles, and only those alleles, make it into the next generation), then it’s possible that only a few alleles might survive (in other words, the majority of genetic variation would disappear from the population). Similarly, if genetic drift leads to the random loss of certain alleles, this would also reduce genetic variation in the population. 7. Is more likely to happen if the population is large (i.e., contains a large number of individuals), and stays large over multiple generations Explanation: Genetic drift is more likely to occur in small populations, because random differences in survival and reproduction between individuals will have a relatively bigger impact on allele frequencies in a small population. Why would natural selection be more likely to result in evolution if the population is large? Because if the population is large it means that allele frequencies are less likely to be affected by genetic drift. Remember that genetic drift can cancel out the effects of evolution due to natural selection. No Yes Yes Yes Yes Yes Yes No 2