The Questions of Developmental Biology

Ferguson and Joanen (1982) speculate that dinosaurs may have had temperaturedependent
sex determination and that their sudden demise may have been caused by a slight
change in temperature that created conditions wherein only males or only females hatched from
their eggs.
Charnov and Bull (1977) have argued that environmental sex determination would be
adaptive in certain habitats characterized by patchiness a habitat having some regions where it
is more advantageous to be male and other regions where it is more advantageous to be female.
Conover and Heins (1987) provide evidence for this hypothesis. In certain fishes, females benefit
from being larger, since size translates into higher fecundity. If you are a female Atlantic
silverside (Menidia menidia), it is advantageous to be born early in the breeding season, which
allows you a longer feeding season and thus would allow you to grow larger. In the males, size is
of no importance. Conover and Heins showed that in the southern range of Menidia, females are
indeed born early in the breeding season. Temperature appears to play a major role in this pattern.
However, in the northern reaches of its range, the species shows no environmental sex
determination. Rather, a 1:1 ratio is generated at all temperatures (Figure 21.11).
The researchers speculated that the more
northern populations have a very short feeding
season, so there is no advantage for a female to
be born earlier. Thus, this species of fish has
environmental sex determination in those regions
where it is adaptive and genotypic sex
determination in those regions where it is not.
Here again, one sees that the environment can
induce sexual phenotype, or sexual phenotype
can be a property of the genome, as it is with
most mammals.
Temperature isn't the only environmental factor that can affect sex determination in fish.
The sex of the blue-headed wrasse, a Panamanian reef fish, depends on the other fish it
encounters. If the wrasse larva reaches a reef where a male lives with many females, it develops
into a female. When the male dies, one of the females (usually the largest) becomes a male.
Within a day, its ovaries shrink and its testes grow. If the same wrasse larva had reached a reef
that had no males or that had territory undefended by a male, it would have developed into a male
wrasse (Warner 1984).
Polyphenisms for alternative conditions
Most studies of adaptations concern the roles that adult structures play in enabling the
individual to survive in otherwise precarious or hostile environments. However, the developing
animal, too, has to survive in its habitat, and its development must adapt to the conditions of its
existence.