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RE: rate of speciation

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Dr. Darrin T. Milne B.Sc., D.C.
> I have a couple of questions:
> 1. Is the rate of speciation related in some way, to the lifespan of a
> species. (ie. does a species change (diversify) faster if the lifespan is
> shorter?).

By "lifepan of a species" I am assuming that you mean "lifespan of an
individual within that species." Non-Darwinian models of evolution to the
contrary, there is no evidence that species have anything truly analogous to
a "lifespan" (at least in terms of a predetermined, intrinsic duration on
the planet).

Some aspects of evolutionary theory DO predict that species with individuals
with shorter lifespans would have more rapid turnover rates, as their
variation can be sorted more rapidly than in longer-lived forms. However,
there is no good evidence for a nice mathematical relationship here, and
other factors (i.e., tendancy for dispersal, etc.) seem to be more strongly

> 2. If a shark species has not changed, is it possible for a
> species of shark
> from say 250 MYA to produce viable offspring with a shark from today?
A couple of points here:
1) Nearly every popular account of shark history to the contrary, there has
been a HUGE amount of change in shark anatomy during their tenure in the
seas. There are very few sharks cruising the seas today that closely
resemble the sharks of the Paleozoic, or Triassic, or much of the Jurassic.
Indeed, the sharks most familiar to people (tigers, makos, great whites, and
their ilk; and the rays and skates) are evolutionarily very recent (more
recent than mammals, for instance). Of living sharks, cow sharks and Port
Jackson sharks are found in the Early and Middle Jurassic; skates, rays, and
open-sea predatory sharks show up in the Late Jurassic or Cretaceous.

2) Even if morphology didn't change much over long periods of time, we do
not know what sort of genetic changes which are not expressed in hard parts
might accumulate over that amount of time. Therefore, we don't know that a
Permo-Triassic shark and a modern shark would be viable, even if they were
still morphologically identical.

Hope this helps,

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
        Mailing Address:
                Building 237, Room 1117
                College Park, MD  20742

Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796