A QUANTITATIVE GENETIC ANALYSIS OF BRAIN AND BODY SIZE ASSOCIATIONS, THEIR ORIGIN AND ONTOGENY: DATA FROM MICE.

Abstract

Numerous studies have been carried out on the origin and significance of evolutionary changes in overall brain size within and between species and higher categories (e.g., Jerison, 1973, 1979; Gould, 1975, 1977; Radinsky, 1977, 1978; Hahn et al., 1979; Lande, 1979; Szarski, 1980; Armstrong, 1983; Atchley, 1984a; Felsenstein, 1984; Riska and Atchley, unpubl.). Speculation abounds concerning the underlying cause of evolutionary variation in brain size (Jerison, 1973; Gould, 1975, 1977; Sacher and Staffeldt, 1974; Lande, 1979; Martin, 1981; Armstrong, 1983; Riska and Atchley, 1984); however, no unambigious explanation has been forthcoming. The most popular hypothesis is that variation in brain size arises as a consequence of a morphogenetic effect with body size. While seldom stated in genetic terms, this morphogenetic scaling hypothesis nevertheless suggests variation in body size is accompanied by covariation in brain size since these two traits are genetically coupled or correlated as a result of shared genetic precursors. The evolutionary result is that selection for change in one trait produces a correlated change in the other (Roderick et al., 1976; Atchley, 1984a). The only general genetic model for brain and body size evolution is that of Lande (1979) who suggests that evolution of brain size within and between closely related species occurs as a correlated response to selection for body size. Based on data from a selection experiment for rate of weight gain, Atchley (1 984a) demonstrated that brain size changes as a result of evolutionary changes in body size and rate of development. Reciprocally, Roderick et al. (1976), Fuller and Geils (1972), and Fuller (1979) show that selection for adult brain weight produces a correlated response in body weight. While these experimental studies validate the major features of the morphogenetic scaling hypothesis, they provide little evidence for the developmental origin of the brain: body size correlation, how it might change