Tues. 6pm section with Alexis

1. While working as a field assistant for Barry and Gwynne, you observe a female snake-eating bird solicit copulations from two different males. Barry informs you that she mates with both males so that they will both provide parental care to the chicks in her nest, thereby increasing her fitness. Gwynne argues that the solicitation behavior is really just a result of increased day length (photoperiod) which results in an increase in the estrogen levels of the female. Who is right?? What kind of explanation did each of them provide. (25 pts.).

Niether Barry nor Gwynne is wrong. Actually their answers complement eachother to give a full understanding of the process at hand. (5 pts) One can break down their arguments into explainations falling into one or more of Tinbergen’s four questions for the study of ethology. These are Function (why a behavior occurs), Evolution (significance of the function to fitness), Causation (the underlying mechanisms), and Development (genetic architecture). (2 pts each, but may be included in your explanation of each person’s argument). The first two of these are ultimate arguments while the second two are proximate. Barry gives an ultimate argument for the observed behavior. For example the function of the female’s solicitation behavior is in order to gain increased fitness through male parental care (function and evolution) (6 pts). Gwynne on the other hand gives a proximate explanation. She says that the cause of the behavior is due to hormonal increases (causation). (3 pts) One might argue that if the hormonal response is heritable, then it will increase in frequency due to increased fitness of females exhibiting this behaivor (see Barry’s argument).

2. One of the following two (15 pts.):

1. Using your knowledge of different levels of selection, explain why cheaters prevent the evolution of altruism, and show what other levels of selection might allow for the stable evolution of altruism.

Altruism involves giving behavior that benefits another individual to the giver’s own detriment (cost). (4 pts) In a world full of altruists, where each individual incurs the cost of giving, and also gains the benefit, an invading cheater gains the benefit from giving altruists, but does not pay the costs associated with giving behavior. If this cost and benefit influence fitness, the cheater will increase in frequency in the next generation compared to the altruists. (4 pts) Possible levels of selection that may allow for the stable evolution of altruism include: kin selection, group selection, and genic selection (called a green beard). (3 pts) You must explain how this level allows for the evolution of altruism (4 pts).

b. Using a real or hypothetical example, explain how reinforcement (i.e., reproductive character displacement) contributes to speciation, making sure to point out what features are necessary for this process to occur.

Reinforcement is a mechanism through which female choice of male secondary sex characters causes some males, those with more a more extreme version, to have increased fitness. This leads to a change in frequency of that trait so that the population becomes more extreme with respect to that male trait. If this mechanism occurs in a hybrid zone, and females choose male characters that are species specific, it may lead to a divergence in male character between the two species. (5 pts) In order for reinforcement to occur there must be: hybrid unfitness and female choice of a heritable male trait (3 pts each). 4 pts given for an example illustrating how the process occurs (see notes on collard fly-catchers).