The question was,
"Why does an early high death rate change the rate of evolution?"
A late high death rate means that the people who die have already brought children into the world. In this case, there is no change in the composition of the gene pool; the deaths do not lead to a change in the gene pool.
An early high death rate means that the people who die have no children. In this case, the gene pool changes; it contains only those genes that belonged to those who did not die.
In the first case, the deaths cause no change in the information content of the gene pool; in the second case, the deaths cause a change. That change in the information content is evolution. (By information content, I mean, not only specific genes, but their frequency, etc.; and, of course, the `change in information' is not necessarily better or more complex.)
Suppose that 95% die before reproducing; only 5% of the genetic information is passed on.
On the other hand, suppose 95% survive until after reproducing (and bringing up children to adulthood). Most of the genetic information is passed on.
(In the real world, differential death rates may be well below 1%; hence, evolution is often gradual. Also, most people -- even educated people -- have no intuitive sense of the lengths of time available or the sizes of populations. People unconsciously think of `a million years' as some some relatively small multiple of one lifetime, rather than as a large multiple. It is hard to imagine vastness accurately.
(So, as a practical matter, it is usually best to speak in terms of how slow evolution is, rather than how rapid it may sometimes be.)
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