Worksheet #7 - Evolutionary Mechanisms - Natural Selection and Genetic Drift

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© UBC BIOL121 – NOT FOR RESALE (author E. Jeffery) WORKSHEET #7 – EVOLUTIONARY MECHANISMS 1 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. Evolution due to natural selection Genetic drift Characteristic: (Is it a characteristic of this type of evolution? Yes/No) 1. Changes in the frequency of an allele are due to differences in the survival and/or reproductive success of individuals 2. The individuals that survive/reproduce are a non-representative sample of the previous generation 3. The survival/reproduction of the allele in the population is related to the phenotype associated with that allele 4. It appears that all alleles/individuals are equally likely to survive and reproduce (i.e., we can’t predict what will happen) 5. Can occur because of a drastic reduction in population size 6. Can reduce the amount of genetic variation in a 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 BIOL121

© UBC BIOL121 – NOT FOR RESALE (author E. Jeffery) WORKSHEET #7 – EVOLUTIONARY MECHANISMS 2 2. Watch the following video: https://www.youtube.com/watch?v=-h8I3cqpgnA a. Which of the four evolutionary mechanisms must have given rise to this trait (i.e., orchids mimicking females bees/wasps)? Explain in one sentence. I would say non-random mating or natural selection gave rise to this trait due to the fact that back when this genetic variation first occurred, plants with this certain trait were most likely much more likely to reproduce than those who do not, thus allowing them to produce more offspring with this potential trait than other plants who did not poses it. b. Do you think this trait (i.e., orchids mimicking females bees/wasps) could be considered an adaptation for the orchids? Justify your answer. Hint: An adaptation can be defined as a heritable trait that is associated with higher fitness in the current environment. Does this trait fit that definition? What information would you need to make up your mind? In order to make up my mind I would need to know if there were any environmental changes in the past that made it more difficult for the orchid plants to reproduce, thus giving a precedent for this heritable trait. If that were the case, I could definitely see this as an adaptation in order to better survive their environments. If there was not change, I would see it more as a positive mutation that caused those affected to better reproduce, thus giving rise to this trait in the later generations.

© UBC BIOL121 – NOT FOR RESALE (author E. Jeffery) WORKSHEET #7 – EVOLUTIONARY MECHANISMS 3 c. Do you think the male bees’/wasps’ behaviour (i.e., pollinating the bee orchids) could be considered an adaptation for the male bees/wasps? Briefly explain your answer. - I do not think so. An adaptation is something that is developed in order for a population to become better suited with their environments, most likely due to an environmental change. This trait however, (pollinating bee orchids) most likely causes less reproduction in the bee population due to the fact that they think they are mating and reproducing when they aren’t. On the other hand, it could be an adaptation due to the fact that bees need pollen to survive and would benefit from there being more orchids in their environment. Thus, through helping the orchid reproduce, they are also helping themselves by creating a steady food source. d. Check out this web comic: https://xkcd.com/1259/. In the case of this orchid species ( Ophrys apifera ), could mimicking a female bee be considered an adaptation for the orchid? Briefly explain your answer. Just for fun: This comic also contains mouse-over text. - Now that we see this comic, yes mimicking a female bee would be considered an adaptation for the orchid. We can see from the comic that ever since the bee went extinct, the orchid was no longer able to reproduce and pollinate. This shows us that the only means of pollination for the orchid was through the male bees attempting to reproduce with their “fake female”. It was most likely an adaptation used in order to help the orchid continue to live and reproduce.

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© UBC BIOL121 – NOT FOR RESALE (author E. Jeffery) WORKSHEET #7 – EVOLUTIONARY MECHANISMS 4 e. Why do you think the male bees/wasps are so easily fooled by the orchid? In other words, why haven’t these insect species evolved in such a way that males can tell the difference between a real female vs. an orchid pretending to be a female? - The only reason for this that I can think of would be because it does not affect the reproduction of offspring for the male. I.E., the male will keep on attempting to reproduce until it actually lays its seed with a real live wasp. It would either be that it doesn’t matter or that it is seriously affecting the bee population, but it is too early in the evolution/adaptation of the bees to see any results.

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