[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Two new FAQs: Everything You Wanted To Know About Cladistics



Ken Kinman Wrote:

>And finally, true polytomies are probably rare even at species level.
They would be almost non-existent at higher taxonomic levels. Polytomies
in a cladogram are just >an expression of lack of knowledge (or great
uncertainty) about the order in which three or more taxa split from one
another. 
I don't agree that polytomies are that rare, at least among birds which
I know best. There are several evolutionary scenarios where polytomies
would be expected to occur:
Transgressions which separate formerly contiguous landmasses into
separate islands.
Abrupt climatic changes which fragment habitats.
(Mass) extinctions which may be survived by widely separated isolated
propagules of an originally widespread species.
It is easy to find recent examples of at least the two first kinds. Such
are the separation of Maui nui into three islands at the end of the last
glaciation or the isolation of Ptarmigan _Lagopus mutus_  populations in
the Pyrenees, Alps, Scotland and Scandinavia at the same time. At least
in the latter case the fossil record indicates that the separation of
the four populations must have occured within a few centuries of each
other c. 11,000 years ago.
In these cases the isolated taxa are admittedly still only at the
subspecific level (as far as known), but this is simply because not
enough time for speciation has passed yet.
Another quite common scenario which is not a true case of polytomy but a
quite good approximation of it are "multiple invasions" where birds (or
other organisms for that matter) from a widespred mainland species
repeatedly disperse to islands and differentiate into distinct species.
This can even happen on the same island where two taxa which have
invaded at different times and differentiated to a different degree may
coexist as good species. This is admittedly not a true polytomy since
the two species separated at different times and molecular data _might_
be able to resolve the branching pattern. However fossils show that
morphological change in the mainland taxa are much slower, so in the
fossil record it would be indistinguishable from a true polytomy.
A more interesting case is perhaps where the same mainland taxon has
dispersed to many different islands and differentiated there. An extreme
case is the flightless rails of the Polynesia, where _every_ island
which has been investigated had from one three species of  flightless
rails before the Polynesians arrived. These are often morphologically
quite distinct, not to say aberrant, but are all clearly derived from
just a few volant mainland species. These admittedly did not separate at
the same time, but oceanic islands are not long-lived geologically
speaking and it is known that at least some atolls have been completely
drowned during interglacials with high sea levels so the number of
speciation events per time unit (e. g. per million years) must have been
considerable and it is hard to believe that even the most refined
molecular techniques could ever hope resolve the branching order (this
might make an interesting project for somebody working with (sub)fossil
DNA).
I agree that true polytomies would be quite rare at higher levels than
species. However one must remember that _all_ evolution takes place at
the species level and only there (remember that Chordata and Arthropoda
were once just single species of squiggly flatwormy thingies, not that
different from each other). The rarity of polytomies at higher levels is
only due to the fact that most new species do not survive to evolve into
genera, much less families or even higher levels and it would be quite
unlikely that this would happen for more than two species of a polytomy.
Still, I wouldn't be very surprised if it does occasionally happen at
the generic level, though I can't think of a plausible example.
Tommy Tyrberg