Essay Questions Answer Key

Graph Question

the graphs have been a pain to post so I will show them in class.

b) Over time, the population size increases until the population size reaches the carrying capacity (100 ind) where it remains.

Initially the population growth rate is at its maximum. Over the time the population growth rate decreases until the population growth rate equals zero where it remains.

c)



d) Over time the population size decreases until the population size reaches its carrying capacity where it remains.

Initially, the population growth rate is very negative. Over time the growth rate becomes less negative until the population growth rate reaches zero where it remains.











If you answered the questions about lyrebirds

Male lyrebirds have two distinctive characteristics. First, they produce an elaborate lyre-shaped tailed that they use in mating displays. Second, male lyrebirds are uncanny mimics. In addition, to being able to mimic the sounds of other birds living in the forest, they are also able to mimic human-produced sounds that they hear. It is possible that female lyrebirds examine both the tail and mimicking ability when choosing the mate.

Many females choose to mate with males with extreme traits. Thus, female lyrebirds might be selected to mate with the male with the longest tail. There are two hypotheses that explain why females should choose to mate with males with extreme traits- (1) the handicap principle and (2) the sexy sons hypothesis.

According to the handicap principle, the long tail of the lyrebird makes it more difficult for the males to move about the forest and increases their likelihood of dieing. Thus, a male with an extremely long tail is carrying a large handicap. A male with a long tail who has been able to survive in spite of that handicap must have genes that code for particularly vigorous survival skills. Thus, by choosing to mate with a male with the longest tail the female ensures that she is mating with a male with good survival skills and hopefully those survival skills get passed on to both her sons and her daughters who in turn should have higher survival.

According to the sexy sons hypothesis, if females prefer to mate with males with extreme traits, then any female who mates with a male with less extreme traits will produce sons with less extreme traits so her son will not be attractive to other females. Thus, female lyrebirds may choose to mate with males with the longest tails because in doing so they will produce sons with long tails who will thus be more effective at mating with females.

Females often prefer to mate with older males because older males have proven that they are good at surviving. If male lyrebirds learn to copy sounds over time, then it is possible that older birds are able to mimic more sounds than younger birds. Thus, females who chose to mate with males who mimicked more sounds might benefit because they pass on good genes for survival to both their sons and their daughters who should therefore have higher survival ability.

It is likely that female lyrebirds would use both tail length and number of songs sung as a way of choosing mates. Two signals of males genetic quality would allow females to make more effective choices than if they relied on only one trait.


If you answered the question in general

Females use a variety of means to determine who is the “best male”. What it means for a male to be “best” depends upon whether or not males provide resources to the female before mating or help the female to care for the baby after it is born. In these cases, the best male is the male that provides the most resources because the female can use these resources to make more or stronger babies. For example, in scorpionflies females prefer to mate with the male who provides them the largest food gift at the time of mating. Studies have shown that females that eat a larger food gift are able to produce more eggs. Similarly, this might explain why human females are attracted to wealthier men (e.g., Anna Nicole and the billionaire).

In most cases, the only thing that a male provides to the female is DNA, so the best male is the male that has the “best genes”. Good genes are those that code for traits that make males good at surviving or reproducing. Thus, if a female mates with males with good genes, the desireable traits should end up in her offspring. Experiments, for example in mice, have shown that females are able to choose to mate with the male with the best genes.

The cues that females use to identify the male with the best genes vary across species. Females would like to mate with a male that is good at survival (the result of finding food, resisting predators and disease). The best way for a male to show he is good at survival is for them to have lived for a long time so females of different species use a variety of cues to determine which males is oldest (e.g., deep croak in frogs, song repertoire in mockingbirds). Females would also like to be able to determine if males are generally good at foraging (e.g., female guppies choosing males with more red spots). Females would like to be able to choose males that are healthy. One indication of general health is symmetry so females prefer to mate with symmetric males. Another indication of health is skin coloration in some species of birds and mammals that use patches of skin in mating displays. Skin coloration is influenced by the health of the male; diseased males have duller skin than healthy males so females should choose to mate with males with brighter skin. A male might also show that he is healthy by carrying around some sort of an extreme trait that acts as a handicap to his survival. Thus, by choosing to mate with this male the female is passing on good survival genes to her sons and daughters (handicap principle).