Answer Key- Retest Essays

(20 points) Answer questions a & b on in the space provided on the front of this page. Answer questions c on the back of this page

a) Draw the graph that shows how the population growth rate varies over time in logistic growth when the initial population size is much smaller than the carrying capacity. 5 points


b) describe, in words, all of the information held in the graph you drew in part a. 5 points


Over time the population growth rate increases until the population growth rate reaches its maximum then the population growth rate decreases until the population growth rate equals zero where it remains.

c) Diagram the demographic transition and explain why patterns of population growth are so different between developed and developing countries. 10 points



The patterns of human population growth are quite different. Population sizes in develop nations have stabilized so there is no further growth. However, population sizes are still increasing rapidly in most developing nations. If human populations were regulated by the same factors that regulate other populations, then we might expect population growth rates to be lower in developing than developed countries because medical and agricultural facilities are not as well developed in developing countries. What explains the difference in population growth rates between the two types of nations? The demographic transition, a predictable pattern of changes in per capita birth and death rates that occurs as nations become more developed might help to explain this difference.

In pre-industrial societies, such as the Native Americans living on the Llano Estacado 2000 years ago, the difficulty of obtaining food and the lack of medical care made the per capita death rates quite high. Not surprisingly, people in pre-industrial societies responded to these high death rates by having lots of children (high per capita birth rates). Because the high per capita birth rates equaled the high per capita death rates, there was very little change in population sizes in pre-industrial societies.

As societies begin the transition to becoming fully developed, the initial improvement in food production, public health, and medical facilities causes a decline in the per capita death rates. As societies develop, death rates continue to decline until they stabilize at a minimal level. However, people continue to have high birth rates. Thus, over time in the transitional stage the difference between b and d increases which results in per capita growth rates increasing over time. As population sizes grow and the per capita growth rate increases, population growth rates increase dramatically.

Eventually during the transitional period the per capita birth rates begin to decline until they they reach a minimum value at the end of the transitional period. The reasons for the decline in birth rates is not well understood, but demographers have observed that birth rates decline with increasing education, economic opportunity, and access to family planning. In fully industrial nations, the low per capita birth rates equal the low per capita death rates so the population growth rate in developed countries is equal to zero.

Thus, the patterns of population growth are different between developed and developing countries arises because they are at different points along the demographic transition.

 
(20 points) You may use both sides of this sheet.

Why, at first glance, is the production of altruistic acts difficult to understand? Discuss three possible hypotheses to explain the production of altruistic traits.

Because natural selection causes genes that code for traits that make organisms better at surviving and reproducing in an environment to become more common (selfish traits), at first glance it is difficult to understand the presence of altruistic traits. Altruistic traits decrease the fitness of the actor, so it is difficult to see how those traits can become more common in a population over time. Although altruistic acts decrease the fitness of the actor, they increase the fitness of recipients of the act. Here are three hypotheses that might explain why we see altruistic acts.

Typically, organisms pass on their genes by reproducing. However, close relatives share some genes in common because of descent from a common ancestor. The kin selection theory suggests that organisms might be able to pass on their genes by helping their relatives to reproduce more than they would have wit out their help. Whether or not an organisms should be altruistic depends on the number of relatives helped, the relationship to the helped individuals, the cost of the act, and the benefit of the act. W. D. Hamilton developed Hamilton’s rule and predicted that animals should be altruistic whenever (add equation).

Sometime altruistic acts occur between non-related individuals so kin selection could not produce these behaviors. Reciprocal altruism, the notion that “I will be altruistic towards you now if you are altruistic towards me later on”, can help us to understand altruism among non-relatives. An individual might be willing to pay a cost now in hope of receiving a larger benefit sometime in the future. Reciprocal altruism requires long-term associations among individuals, the ability to recognize individuals, and the ability to remember who owes you and who doesn’t. Thus, reciprocal altruism should be limited to long lived, “smart” animals. Not surprisingly, reciprocal altruism is common in humans.

Some human altruistic acts actually results in the death of the altruistic (e.g., a soldier falling the grenade or a police officer dying to prevent a robbery). Because these acts are not limited to relatives and the act cannot be reciprocated (you can’t receive a benefit if you are dead) the neither kin selection or reciprocal altruism can explain these behaviors. Cultural selection has been proposed as a mechanism to explain these types of altruistic behaviors.

Gene have the ability to replicate themselves; is there anything else that is capable of self -replication. Ideas (memes) can replicate themselves and they differ in their survival and transmission ability. Thus, the differential survival and transmission of ideas can form an analogous process to natural selection. Cultural selection may explain self-sacrificing behavior in humans as well as other behaviors that apparently do not maximize the transmission of genes.