It is true, even a slight favorable effect can turn into something over a number of generations, although if it is very slight, I doubt that several will do. In most instances, though, the environment is pretty likely to change before there is a clear selecting out of the trait. Animals are often migratory; weather patterns are uncertain; streams and rivers change course; foliage grows up where it had been bare, or dies and leaves a clearing; and so on.
As for the effect if the parents die to early, well, you are again right, supposing that the period between birth and maturity is lengthy. However, in most creatures, it is fairly brief. Humans are unusual. Also, among some higher order mammals, adoption fills the gap. Anyway, if the incidence of mortality is only slightly differentiated, and does not usually affect the breeding process, well, the trait is indifferent.
But there is an issue we have not touched on, which is that not all traits are strictly genetic, nor always heritable. Even height, one of the most genetically sensitive traits, is very affected by diet, which is the primary cause for contemporary American to tower over their 18th century ancestors, and many mutations are not heritable......... |
