Speciation -- i.e., the formation of reproductively isolated groups --
may arguably be necessary for macroevolution to occur, but it is
difficult to see how it could possibly be sufficient. If speciation
were the whole of the macroevolutionary story, the Earth should be
swarming with millions of species of cyanobacteria (or whatever), but
little more than that.
It is fairly easy to find prominent evolutionary theorists who disagree
vigorously with the equation "speciation = macroevolution." Consider,
for instance, the following, from George L.G. Miklos of the Australian
National University:
The modern synthesis has focussed on speciation events as
the blast furnace of evolutionary change. However, are
these terminal twigs of the tree of life to be the source of
our inspiration? Was the Cambrian explosion about
speciation? No -- it was about *body plans*. Evolutionary
change is not particularly concentrated in speciation
events, and as [George] Williams (1992) has concluded,
"speciation in the usual sense (of Mayr) has no special
significance for macroevolution." I would go further....to
concentrate on the processes of speciation is to totally
miss the essence of the emergence of complex forms. Fifty
phyla did not speciate from one (or a number of) unicellular
prokaryotes....available data indicate that emergent
complexity of body plans is not a problem in speciation.
(p. 26; emphasis in original) [...]
Finally, it is necessary to acknowledge that after a century
of the dominant paradigm, the evolution of major
complexities in the history of life has had very little to
do with the origin of species. The seamless moving footway
of neo-Darwinism that was to have transported us from
allelic variation in natural populations to understanding
body plans in different phyla has led to a cul-de-sac. The
origin of phyla is not via speciation 'writ large.' (p. 34)
(George L.G. Miklos, "Emergence of organizational complexities during
metazoan evolution: perspectives from molecular biology, palaeontology
and neo-Darwinism," _Mem. Ass. Australas. Palaeontols. 7 [1993]:7-41)
In their major new textbook on the mechanisms of macroevolution, _Cells,
Embryos, and Evolution: Toward a Cellular and Developmental
Understanding of Phenotypic Variation and Evolutionary Adaptability_
(Blackwell, 1997; 642 pp.), John Gerhart and Marc Kirschner do not
provide an index entry for "speciation," nor could I find much (or any)
discussion of the phenomenon. Rather, entire chapters are dedicated to
such open questions as "Novelty" (chapter 5), "Body Plans" (chapter 7),
and "Developmental Flexibility and Robustness" (chapter 9).
How one gets dozens (or thousands) of species of butterflies, or nematodes,
or langurs, is an interesting problem -- but that is a very different matter
from, and does not explain, how to build a butterfly, or a nematode, or a
langur in the first place.
Paul Nelson