> Where did Johnson get his impression? Perhaps from reading Stanley:
>
> "Let us suppose that we wish, hypothetically, to form a bat or a whale...
> [by a] process of gradual transformation of established species. If an
average
> chronospecies lasts nearly a million years, or even longer, and we have
> at our disposal only ten million years, then we have only ten or fifteen
> chronospecies to align, end to end, to form a continuous lineage
> connecting our primitive little mammal with a bat or a whale. This is
clearly
> preposterous... A chain of ten or fifteen of these might move us from
> one small rodent like form to a slightly different one, perhaps
representing
> a new genus, but not to a bat or a whale!"
This thought experiment assumes 1) that new species arrive on average every
million years and 2) that the old species must be replaced by the new
species. If instead we assume that new species can be contemporaneous with
old species, and that speciation can occur on average once every 100,000
years, that would give us 100 "chronospecies", whereas a speciation rate of
once every 10,000 years would give us 1000 "chronospecies". If 10
"chronospecies" is enough to produce a new genus, 100 should be more than
enough to get from "rodent-like" to a bat or even a whale; if it isn't, 1000
certainly is.
So, the question becomes, what is the best estimate for the rate of
speciation.
Kevin L. O'Brien