IB31Lecture+outline+4+Feb+6++Sp08

IB31Lecture+outline+4+Feb+6++Sp08 - IB 31 Sp 2008 Feb 6...

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IB 31, Sp. 2008 Feb. 6, 2008 Caldwell, Evolution and Natural Selection III V. I want to discuss a couple of other aspects of evolution. First, GROUP SELECTION as it applies in Good of the Species arguments. You know the story about lemmings, arctic rodents that periodically become very numerous (population explodes) followed by mass mortality as huge numbers jump into fjords. Common explanation is mass suicide so that the population will not destroy its food source. The few remaining will survive. The group or population benefits through the individual's sacrifice. This view of the evolution of social behavior to regulate population numbers was popularized by V. C. Wynne-Edwards in his 1962 book Animal Dispersion in relation to Social Behaviour. Similar explanations often crop up in discussions of behavior: A. Territoriality (defending a piece of property) evolves to space out individuals and limit numbers so that resources won't be over-exploited - good for the species. B. Aggression is often ritualized and escalation avoided (explain using examples of rattlesnake wrestling among males and red deer stags progression of displays from roaring to parallel walk to fighting) so that too many individuals won't be killed off. Reduced aggression is for the good of the species! C. In 1966, GEORGE C. WILLIAMS published Adaptation and Natural Selection. Basic premise was to re-assert that traits, including behavior, evolve because they aid in the propagation of an individual's genes - not because they contribute to the survival of the group, population or species. (There are exceptional examples of Group Selection that we will discuss in this course, but they are unusual situations.) 1. Let's go back to our lemming example. A Far Side Gary Larson cartoon neatly sums up the problem with the group selection argument. It shows a long line of lemmings rushing toward the sea, but one is carrying an inner-tube! A gene (mutation) that comes along and causes individuals to reproduce rather than commit suicide would be favored and would quickly spread at the expense of genes promoting death. The individual selectionist hypothesis states that lemmings disperse because conditions at home are becoming crowded and the chances of reproducing are poor. It is better to try to find some place else not so crowed. The chances aren't very good, but they are better than where the lemmings are now, and if they do find an empty world, the payoff would be tremendous. Dispersal is
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often times characterized by persistent, straightened out movement. They jump into Fjords trying to cross, just as they would a small stream. However, their eyesight isn't that good and they can't tell a stream from an ocean! 2. Territories serve many functions which we will discuss later. However, they are often a hedge against bad times. They are sufficiently large to contain adequate resources to successfully reproduce even in lean years. If you cannot be assured of adequate resources to breed, it is better not to try and waste effort, which in turn may increase future success.
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