O.k., here's a little adaptationist story that popped into my head today as I was reading an article on the evolution of human skin color:

The primary explanation that's accepted for the adaptive advantage of melanin in skin and the strong correlation between the amount of melanin in the skin of indigenous peoples and the level of UV radiation is protection against UV-induced photolysis of folate, because of all the types of UV-induced damage, this has the most direct negative effect on "fitness" (used in the evolutionary sense). Folate is a metabolite essential for normal development of the embryonic neural tube, and deficiencies in folate are a significant cause of pre- and post-natal infant death.

Ok, here's my story: peoples of northern European descent in Australia (outside the low-UV region to which they are adapted) have adopted and become dependent upon Vegemite (unconsciously?) in order to compensate for the UV-caused loss of folate via their pale skins.

Whaddaya think?

~ Becky in Tucson
where it's also too sunny

"Since records have been kept in developed countries, anencephalus and spina bifida have been found to be particularly common in ight-skinned populations. These defects accounted for as much as 15% of all perinatal and 10% of all postperinatal mortality in the worst-affected populations prior to the introduction of preventative nutritional supplementation (Elwood & Elwood, 1980). Since the advent of prenatal diagnosis, the prevalence of NTDs in relation to all conceptions has been shown to be significantly higher than birth prevalence (Velie & Shaw, 1996; Forrester et al., 1998), partly because of the high rate of early miscarriage in the case of the most serious defects (C. Bower, personal communication)."

"In addition to its important role in ensuring embryonic survival through proper neurulation, folate has been shown to be critical to another important process central to reproduction, spermatogenesis. In both mice (Cosentino et al., 1990) and rats (Mathur et al., 1977), chemically induced folate deficiency resulted in spermatogenic arrest and male infertility, findings which prompted investigations of antifolate agents as male contraceptives in humans.

"Thus, through folate's roles in the survival of embryos through normalization of neurulation and maintenance of male fertility, and its involvement in a range of other physiological processes dependent on nucleotide biosynthesis, regulation of folate levels appears to be critical to individual reproductive success. Folate levels in humans are influenced by dietary intake of folic acid and by destructive, exogenous factors such as UV radiation. Therefore, the solution to the evolutionary problem of maintaining adequate folate levels in areas of high UV radiation involved the ingestion of adequate amounts of folic acid in the diet and protection against UV radiation induced folate photolysis. The latter was accomplished by increasing the concentration of the natural sunscreen, melanin, in the skin. The low prevalences of severe folate deficiency (Lawrence, 1983; Lamparelli et al., 1988) and NTDs (Carter, 1970; Elwood & Elwood, 1980; Buccimazza et al., 1994; Wiswell et al., 1990; Shaw et al., 1994) observed among native Africans and African Americans, even among individuals of marginal nutritional status, are probably due to a highly melanized integument, which protects against folate photolysis."

Nina G. Jablonski and George Chaplin, The evolution of human skin coloration. Journal of Human Evolution (2000) 39, 57:106