Answer key to Short-answer questions on Exam 2 -- Summer 1999

NOTE: these are (mostly) actual answers written by your fellow students.

 1. Natural selection may have occurred if the surviving crows survived due to some trait that made them more able to withstand the effects of the natural catastrophe. Perhaps the surviving crows had a genetic predisposition to nest in places that were somehow resistant to the effects of tornadoes (which may be common in the area during particular months), or were better at detecting and responding to changes in the weather associated with the occurrence of tornadoes. One would have to demonstrate that behaviors or physiological differences exist between the surviving crows and those that perished in order to argue that natural selection had taken place.

OR - If the crows that survived the tornado did so because they were "more fit" or better suited to deal with this type of catastrophe because of a genetic difference that gave them a n advantage over the crows that died, then when these crows reproduce, the level of adaptation of the population to dealing with tornadoes would increase in the population, and you could conclude that natural selection has occurred. If, however, it was just by chance that these crows survived (perhaps they had access to shelter), then natural selection did not occur.

 2. Deserts are areas that receive very little precipitation and that experience sharp temperature differences, either seasonal, between day and night or both; both temperature extremes and low moisture limit the growth of plants. Plant adaptations include very thick leaves or stems (to store moisture), deep roots for better access to soil moisture, etc. Animals in deserts are often nocturnal or live in burrows, both of which limit exposure to the hottest and driest conditions, and may excrete concentrated urine or extremely dry feces in order to conserve water. (And a similar format for any other biome chosen.)

3. Liebig's law of the minimum states that survival of a particular species in a particular place is limited by the single most limiting factor. In other words, if a species lives in an environment in which 9 out of 10 characteristics are favorable to it, if the 10th factor reaches the tolerance limit for the species, it will be unable to survive (regardless fo the suitability of the other nine). Besides implying that a species wil only be able to survive in habitats with specific conditions, the law also implies that the factors that limit a species presence or abundance are likely to be different in different parts of its range.

4. Predators often have acute sensory abilities, such as the eyesight of hawks and the hearing of owls, which lets them detect and locate prey. Also, the ability to respond to chemical cues to alert them to the presence of near-by food. Prey often have morphological adaptations such as spines and thorns on plants, or behavioral adaptations such as flocking in birds, making individuals less vulnerable to being eaten or picked out of a group.

5. If an area that once constituted the core of a forest is suddenly brought near the edge of a forest (as by logging) the area will undergo changes in temperature, exposure to light and species composition among other possibilities, which can adversely affect the organisms that rely on the core habitat. Core habitat can also be eliminated completely from even relatively large chunks of habitat if edge effects penetrate deeply enough from the edge, making the shape as well as the size of preserves important considerations.

6. Environmental resistance refers to factors that cause the growth rate of a population to slow (from exponential) as the size of the population approaches the carrying capacity of the environment. These factors work in a density-dependent way, such that as population size increases, birth rates decline, death rates increase or both; the net effect is that the size of the population can be regulated to remain within certain limits. Factors that are part of "environmental resistance" include competition for resources that become more limited as population density increases, predation (which may increase as population increases and it becomes easier for predators to find their prey), pathogens or parasites (which may be more easily transmitted as popultaion density increases), and stress-related disease or hormonal changes that increase as population size increases.