Rates of evolution are highly variable[#](Simpson 1944).
Many species remain stable over hundreds of thousands of generations
and more than a million years. Crocodiles have survived for 200
million years without substantial change[#](Myers, et al. 2006).
Species durations in the
fossil record range from 2 million years or so in mammals to more
than 20 million years in conifers[#](Kutschera & Niklas 2004). However, examples of more rapid
speciation events are also well-documented. In cichlid fishes and
some flowering plants, reproductive isolation - speciation - has
occurred within a few hundred generations[#](Soltis, et al. 2007)(Kocher 2004). Speciation via
polyploidy The condition of having more than two homologous sets of chromosomes in each cell. is instantaneous.
In his Origin of Species[#](Darwin 1859), Darwin emphasized the gradual
nature of evolution: new species (and higher
taxaAny unit used in the science of biological classification, or taxonomy. (sing. taxon) such as family
or order) are the products of accumulated small changes over
hundreds of thousands to millions of years. How can the gradual
process described by Darwin explain rapid speciation events?
In fact, the gradual nature of evolution and the extent to which
macroevolution is the product of microevolutionary modifications
is still debated. Most evolutionary scientists accept the basic
tenet of the synthetic
theory that small-scale evolutionary changes in populations,
when continued for longer periods, can explain large-scale phylogenetic
changes including novel body plans (such as wings, evolved from
arms)[#](Kutschera & Niklas 2004).
And the continuity
between micro- and macroevolution is documented in many fossil
lineages. However, they also acknowledge that exceptions exist,
even though macroevolutionary changes in body plans by means of
many small evolutionary steps can occur rapidly when viewed in
the perspective of geological time scales.
Perhaps the most well-known theory that proposes to explain the
rapid periods of speciation is the “punctuated equilibrium”
theory of Eldredge and Gould[#](Eldredge & Gould 1972)
. Punctuated equilibrium is
a modification to Darwin's gradualist theory, not a replacement
for it. According to this theory, evolution tends to be characterized
by long periods of morphological stasisA period of little or no discernible evolutionary change.
(“equilibrium”), “punctuated”
by episodes of rapid phenotypic change. Punctuated equilibrium
is the hare to Darwin’s tortoise.
Speciation under punctuated equilibrium is usually a special
case of
allopatric (Meaning "different place"), speciation that occurswhen a population becomes geographically isolated from otherpopulations and adapts to local conditions.
speciation, and the speciation event takes place over periods of as little as fifty thousand years.
But are these two theories really different? The concept of differential
rates of evolution is not new[#](Simpson 1944).
Both punctuated equilibrium and
the synthetic theory explain how large, stable populations can
produce new species. The large population itself doesn't change,
but small isolated populations might, resulting in branching
tree rather than an evolutionary ladder. They both account
for the relative scarcity of transitional forms in the fossil
record.
What is different, however, is that punctuated equilibrium proposes
that species remain in stasis nearly all of the time, even when
the environment is changing. Darwin did not, however, see
an active mechanism for stasis. Instead he felt that stasis is
merely a passive occurrence, that natural selection merely "stops"
(rather than having its effects blocked by an active mechanism).
Punctuated equilibrium attributes stasis to a phenomenon
Eldredge calls “habitat tracking”:
as long as a suitable habitat can be found, a species will move
rather than stay put while natural selection adapts the species
to the new conditions[#](Eldredge 1995). If this happens, little or no adaptive
change would take place. Of course, stable conditions will also result in no evolutionary
change, and can account for stasis. And Richard Dawkins offers
a mechanism of "buffered gene pools" as an active mechanism
for stasis[#](Dawkins 1982).
Finally, the cause
of evolution may determine its rate: genetic driftA change in the genetic makeup of a small population that takes placestrictly by chance.
or polyploidy can cause evolution more quickly than natural
selection. Numerous studies have shown that evolutionary transitions
are gradual, although the rates may vary. It appears that evolution
is both gradual and occasionally “punctuated.”
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