My guess is that under certain conditions a shorter lived life cycle might be better due to the ability to put all energy into one large brood during optimum conditions, thus passing on these genes (its all about passing on genes). However, it is likely that every once in a while the environment is bad during this initial spawn leaving only the few late spawners to carry on. This would cause genes for this behavior to be maintained in the population. There are also differences in reproductive strategy that effect the expression of genes, so genetically identical individuals will respond differently depending on the conditions they experience. Finally, it is even possible that genes can be turned on or off in one generation and this can be passed down to offspring (epigentics). This last situation has only recently been understood. Biology is really cool.
