PLEASE NOTE:
*
Date sent: Wed, 08 Oct
1997 15:44:20 -0400 (EDT)
From:
HUMBPEIS <B.J.PEISER@livjm.ac.uk
Subject:
PUNCTUATED EQUILIBRIUM
To:
cambridge-conference@livjm.ac.uk
Priority: NORMAL
NEW PAPERS ON PUNCTUATED EQUILIBRIUM
M. C. Wilson: LATE QUATERNARY VERTEBRATES AND THE OPENING OF
THE
ICE-FREE CORRIDOR, WITH SPECIAL REFERENCE TO THE GENUS BISON.
QUATERNARY INTERNATIONAL, 1996, Vol.32, pp.97-105
The ice-free iorridor opened physically in early deglaciation,
after
14 ka BP, but at first was not necessarily passable in an
ecological
sense. One way to gauge the timing of its 'ecological opening' is
to
establish a chronology for the arrival of immigrant animal
species.
The bison have excellent potential because of the abundance of
their
remains. Late Pleistocene and Early Holocene bison finds from
western
Canada north to the Peace River area extend the known database
and
provide new insights. Bison of 'southern' appearance (referable
to B.
bison antiquus) were present as far north as the Peace River
region
until about 10 ka BP. Bison populations in western Canada
apparently
underwent a rapid change at that time, such that barely 500 years
later, bison of 'northern' appearance (referable to B. bison
occidentalis) were established. The rapidity and pervasiveness of
this change seem to defy an evolutionary explanation rooted in
punctuated equilibrium or phenotypic change, and could indicate a
sudden population influx through the newly opened corridor. Thus
far,
no late-glacial bison in western Canada have been dated in excess
of
11.5 ka BP, with the possible exception of material from Empress,
Alberta. If a steppe-tundra was established before that time, the
absence of bison with either southern or northern affinities
seems
inexplicable. Outburst floods have undoubtedly removed part of
the
record by scouring gravel fills from valleys, but it is doubtful
that
they completely obliterated the fossil record of a gregarious
herbivore such as the bison.
-------------------
S. V. Mezhzherin: Gradualism or punctualism: Evidence on
genetic
differentiation of small mammals from the holarctic region.
GENETIKA,
1997, Vol.33, No.4, pp.518-523
Genetic differentiation of taxa from three Holarctic and three
originally Afrotropical phyla of small mammals was analyzed. Its
extrapolation to the time scale revealed the following trends:
(1)
Distributions of fixed gene differences along the scale are
relatively independent. (2) Extrapolation of these distributions
to
the time scale shows that the speciation process is
discontinuous; It
consists of relatively stable periods interrupted by speciation
bursts. (3) Periods of speciation activity in different phyla
coincide. In general, these results are consistent with the
concept
of punctuated equilibria and suggest that the speciation process
is
temporally discontinuous.