FIRST SET OF QUESTION, EVOLUTION 2006

            Due Thursday 27 March: You can hand in answers in class, OR email them.

GENERAL NOTE: All of these questions are somewhat open-ended; there is no single, definitive ‘short answer’. However, there is a definite point to each, and each can be answered entirely satisfactorily in a two or three sentences. SO: FIRST, think carefully about what is being asked; THEN, make sure your answer addresses the main point of the question carefully and concisely. Always take pains to use language carefully and accurately.

ANSWER TWO OF NUMBERS 1-3:

1. For a single population, the average rate of evolution (loss of heterozygosity) due to genetic drift is a direct function of effective population size (small populations drift to fixation faster). Compare the case of a single population (of any arbitrary size) with the case of a collection of smaller, separate populations whose combined population is the same as that of the single population in the first case. All else equal, in which case is genetic diversity more likely to be maintained over the long term (if you were a conservation biologist interested in maintaining genetic diversity within the species, which would be the most desirable circumstance)? Explain your reasoning.

2. Many flowering plants are hermaphroditic, but some have mechanisms inhibiting self-pollination (and so encouraging out-crossing) -- e.g., flower structures that make self-pollination extremely difficult or unlikely. Develop a selective argument (hypothetical selective regime) that would favor such mechanisms (make sure you put it in terms of effects on fitness of reproducing individual). What kinds of evidence would support this being the actual reason for existence of these mechanisms? (Note that self-pollination still involves genetic recombination, but offspring’s genome can, of course, only include alleles present in parent plant.)

3. Although gene flow is a non-selective evolutionary mechanism, the potential for gene flow can be acted on by selection. What are some conditions that might lead to selection for mechanisms inhibiting gene flow between local populations (i.e., why might a propensity for keeping one’s genes close to home be likely to produce more descendants)? Give an example of the form such mechanisms might take in animals.

ANSWER TWO OF NUMBERS 4-6

4. Darwin said that, "with all beings there must be much fortuitous destruction, which can have little or no influence on the course of natural selection." The genotype of a particular plant, no matter how well-designed it is for competitive purposes, may have no influence on whether or not it is killed by a meteorite (or a trampling cow); the plant is simply either lucky or unlucky. What are the implications for such "non-selective" mortality for natural selection? (Consider the rate and predictability of selection, fitness of different genotypes, etc.)

5. Predators often have a large effect on fitnesses in their prey population. However, the selection imposed by predators can be either ‘directional' or ‘disruptive'. Propose a scenario for each (make sure you put it in terms of how predation would effect fitness of different variants/genotypes in prey population). Why is predation unlikely to exert ‘stabilizing' selection?

6. We have discussed how competitive interactions among organisms may shape their adaptations to reduce overlap in resource use. One result of this is that the niches (range of resources used) of organisms within the same ‘ecological guild’ tend to be separated, or ‘spaced out’ along a gradient of resource quality or type (e.g., seed size). You can ‘place’ more organisms along such a gradient or axis if niches are narrower; conversely, broader niches will intensify competition and tend to eliminate some species. Imagine a guild of ant species using a range of seed sizes. The total range of seed sizes available does not change over time, but other properties of the environment might change. Suggest two ways you might change environmental circumstances to ‘permit’ more species to persist in coexistence – i.e., ways to allow tighter ‘packing’ of niches.

ANSWER BOTH 7 and 8

7. A population biologist made the following observations regarding two forms of the banded land snail, Cepaea nemoralis, inhabiting a field in the English countryside (NOTE: these snails are hermaphroditic, so don’t have to worry about males and females):

                                                                                          Banded                          Solid

Population Size                                                                 3,600                             3,000

proportion surviving from hatching to adulthood                12%                               15%

Probability of adult mating                                                 0.72                               0.85

Avg. Fecundity (eggs/mated adult)                                    500                                300

proportion of eggs hatching                                               5%                                 5%

Calculate an estimate of the relative fitnesses of the two phenotypes. Explain your reasoning.

8. In some fish (for example, some bluegills) there are two types of males. One type grows large and attracts females (females choose mates) by fancy colors, extensive display behavior, and building and maintaining nest sites which they aggressively defend against other males. The second type is smaller, and has size and markings similar to those of female fish. Small males hang around the periphery of nests while eggs are being laid (because they look like females they aren't chased away by territorial males) and, in a peculiar sort of menage a trois "sneak" their sperm into the nest to fertilize a few (but usually only a small percentage) of the eggs laid. (These fish have external fertilization -- eggs and sperm are released into the water separately.) The two male "strategies" appear to be genetically determined; you can assume that they represent two alternative genotypes. Discuss this situation in terms of the costs and benefits (allocation of resources vs. reproductive success) of the different male "strategies". Discuss relationship here between "normal" individual selection and selection due to mate choice (sexual selection). How can two apparently "competing" genetic traits persist in same population (consider how fitness of each might be influenced by abundance)? Do you think this would be an Evolutionarily Stable Strategy (ESS)?

EXTRA QUESTION: Answer INSTEAD of one other if you wish – OR in addition to others for brownie points:

9. Richard Dawkins says that DNA (the ‘replicator’) is not "for" anything, it just is -- that is, it can't be explained as having a "function" in the same way that we can say that eyes are "for" seeing. What do you think? Is it relevant that the first organisms probably did not possess DNA?