Assigned Reading: Processes of Evolution
I. Definition: The central tenet of the theory of evolution is that the diversity of life is the product of descent with modification. Ancestors give rise to descendants that inherit the features of their ancestors. Occasionally, a feature is modified. When organisms bearing the modification reproduce, their offspring inherit it. Over the last 4 billion years, the process of descent with modification has produced the diversity of life.
II. Evolutionary Processes:
Although scientists had been considering the possibility of organismal evolution for some time prior to 1852, Charles Darwin was the first to propose a mechanistic evolutionary process that did not invoke some sort of incorporeal "driving natural force." Since Darwin's time, other natural processes of evolution have been recognized, and scientists have debated their relative importance. This does not mean, as the popular media would sometimes have it, that scientists are reconsidering the validity of evolution as the generator of biological diversity. It means that they are arguing over how to fine tune our understanding of exactly how it works.
III. Evolutionary Patterns:
- Natural Selection: With each new generation, variation is generated. Offspring are slightly different from one another and from their parents. Sometimes, a variant posesses an advantage over its siblings that allows it to produce more offspring in turn. Because it has more offspring, its variation will be more heavily represented in the next generation. Other variants may be at a special disadvantage and produce fewer, or no offspring. Their type of variation will be de-emphasized in the next generation. This, of course, was the process proposed by Charles Darwin.
- Sexual Selection: Organisms do not mate indiscriminantly. Their mate selection preferences will heavily influence the reproductive success of their prospective mates. Thus, individuals with traits that are desired by the opposite sex will contribute more offspring to the next generation. This process can result in the evolution of behaviors and structures that, in other ways, pose disadvantages. For example, despite being pretty, the male peacock, with his gaudy, cumbersome tail, is at a distinct disadvantage when it's time to hide or flee from a predator. Nevertheless, it is a disadvantage he must live with, at least for a while, if his genes are to make it into the next generation.
- Heterochrony: Each individual organism has a growth pathway, called its ontogeny. Heterochrony is evolutionary change in one's ontogeny. For example, most salamanders go through a water breathing larval stage before becoming air breathing reproductive adults. The axolotl, a salmander from Mexico, has evolved a new ontogeny in which it does not reach its adult stage, but instead reproduces as a water-breathing larva.
- Pleiotropy: Suppose a gene for blond eyes is strongly favored by sexual selection. Right next to it on the chromosome is a gene for blue hair that is not favored. Genetics dictate that because the blue-hair gene is located next to the blond-eye gene, it is likely to accompany it. Thus, by favoring mates with blond eyes, individuals will also, inadvertantly, be facilitating the spread of the blue-hair gene. When a selectively neutral gene rides the coat-tails of a selectively advantageous one, it is called pleiotropy.
- Drift: The result of random chance. Suppose we had a group of humans of all physical types and selected from it 500 individuals to be isolated on a desert island for 20 generatons. Their descendants would probably look like composites of the original sample. If one of the original 500 were albino, for example, the chances would be small that the gene for albinism would spread throughout the resulting population. Different random 500 person samples would probably yield similar results. If we isolated an original sample of only four people; however, the final product would greatly depend on the attributes of the specific individuals. If one of the four were albino, there would be a much greater chance that the resulting population would contain many albinos. Thus, drift is a powerful source of evolution in small populations.
The processes of evolution yield three significant patterns of ancestry and descent.
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- Distribution of organisms in time. (The domain of paleontology.)
- Distribution of organisms in space. (The domain of biogeography.)
- Distribution of organismal form. (The domains of comparative anatomy, embryology, molecular biology, and genetics.)