Selective Pressures

Most hypotheses offered to explain why live bearing has evolved in animals invoke agents affecting offspring viability as the selective pressure for an evolutionary shift in reproductive mode. Costs that accrue to mothers then either facilitate or constrain the transition. These may include reduced maternal mobility, with consequences for foraging efficiency and predator evasion, reduced fecundity, and the increased metabolic demands of carrying offspring throughout their development (Shine, 1985; Goodwin et al., 2002, among others). It is difficult, however, to use present-day characteristics of ovoviviparous or viviparous organisms as evidence for hypotheses on the evolution of these traits, as current habitats may be different from the habitats in which the reproductive modes first evolved (Shine, 1989). It is also important to note that each strategy has its benefits and liabilities in a given environment. Oviparity is not inherently inferior to ovoviviparity or viviparity just because it is the ancestral state. The problem of water balance in cockroaches, for example, is handled by each reproductive mode in different ways, each of which may be optimal in different habitats. Egg desiccation can be minimized if: (1) the ootheca is deposited in a moist environment, (2) the ootheca has a waterproofing layer, or (3) the female dynamically maintains water balance while the egg case is externally attached or housed in a brood sac (Roth, 1967d).

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