Grene, Marjorie. “Hierarchies in biology.” American Scientist 75, no. 5 (1987): 504-510.
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That evolution happened is a fact. Just why and how it happened is, in part, still a matter for debate, and a number of evolutionary biologists are currently attempt-ing to rethink the problem of causality in their discipline (e.g., Eldredge 1985, 1986; Gould 1982, 1986). Contem-porary Darwinism appears to restrict the search for causes to two levels only: the level of changing gene frequencies, and the level of phenotypic characters of organisms—that is, the morphological or behavioral characters that are triggered by underlying genetic determinants but that nevertheless produce effects having significant consequences for the differential reproduction of the organisms in question. At the phenotypic level, in other words, individuals are differently adapted to a given environment, and these differences result in variation in the number of offspring that these individual are likely to leave in the next generation.