|MadSci Network: Evolution|
Dear Johnny, Bergmann's rule states that warm-blooded animals in colder climates are larger than their relatives in warmer climates. One reasoning behind this is that larger animals have relatively smaller external surface areas and thus, lose relatively less body heat. There are many examples of this rule. For example, in Europe, larger mammals, including deer, bear, wolf and wild boar, increase in size towards the north-east & decrease in size towards the south-west. (However, there are also many exceptions to this rule.) A corollary of Bergmann's rule is Allen's rule, which states that protruding body parts, such as tails, ears and bills are relatively shorter in the cooler parts of the range of a species than in the warmer parts. These rules are briefly discussed with examples at: http://www.micro.utexas.edu/courses/levin/bio304/evolution/macroevolution.html http://anthro.palomar.edu/adapt/adapt_2.htm In general, the shells of cold-water marine mollusks are plainer than the shells of tropical mollusks. The latter usually have spines, knobs and thick lips on their shells that seem to protect them from predation. The structural differences between the shells of tropical and cold-water marine mollusks have been attributed in part to decreased intensity of predation as the water temperature decreases. Also, calcium carbonate that the mollusks use to build their shells is more soluble at low than at high temperatures. Therefore, it may be more costly for cold-water mollusks to obtain and retain calcium carbonate. The effects of these and other climatic factors on the evolution of mollusk shells have been discussed by Geerat Vermeij in A Natural History of Shells (Princeton University Press, 1993). The variation in human skin colors is another example of climate induced changes. Humans who receive more direct and intense solar radiation (for example those who live in the tropical Africa and Asia) have skins genetically darker than those who have less solar exposure (for example, northern Europeans). However, the exact adaptive advantage of having more or less skin pigment (melanin) in relation to the intensity of solar radiation is not known. You can read more about this at: http://biology.uindy.edu/Biol345/HUMAN%20STRATEGY/24pigmentation.htm I hope these ideas will be helpful. Aydin Orstan
Try the links in the MadSci Library for more information on Evolution.