From: Kevin W. Williams
To: All Nov-09-93 08:55AM
Subject: Punctuated Equilibrium
Organization: This variable implies that we ARE organised
From: firstname.lastname@example.org (Kevin W. Williams)
I've been noticing a number of posts on punctuated equilibria
recently, but haven't seen a clear concise explanation on
how PE works without having to invoke the Magic Mutation
Rate Fairy. This post will try to summarise my understanding
of the theory for those who don't understand it. Corrections
to my understanding are greatly appreciated.
Punctuated Equilibrium basically states that there are
periods of effective statis in the gene pool, briefly
punctuated with periods of rapid shift. While there are
many mechanisms that move a population to the point of
rapid change, they center around one phenomenon: it is very
improbable that a single mutation of single allele will convey
a strong survival advantage to the host. Multiple mutations
are normally necessary to achieve a significant survival
For illustration, let's assume a hypothetical wolf population.
This population of wolves has hit a stable point: it is large
enough to kill prey efficiently, and has sufficient endurance
that it wins enough chases when a long-term chase is necessary
to bring down prey.
One wolf in the population mutates such that its size is
larger. Now, it is better equipped to kill prey on the first
try. But ... it requires more food to fuel its larger body
mass, and tires faster on long chases. For the sake of the
example, assume that this balances out as either neutral
or slightly positive.
Another wolf at some distant point mutates so that its muscle
tissue is more efficient. At its current size, this is not
much of an advantage, because the wolves were sustaining
themselves without the greater efficiency. Still, the allele
will slowly spread due to the slight survival advantage.
These genes will spread slowly through the population. Natural
selection for either allele is simply not strong enough to
cause the gene pool to shift. Eventually, the genes will meet
up: an individual will possess both mutations. Now, you have
a larger wolf that doesn't tire quickly, and needs no more food
than the rest of the population. Now, you have a strong advantage.
This combination of alleles will quickly spread through the
population, as the large efficient wolves beat out all other
combinations in the quest for survival.
Net result: the population sat relatively stable for a very
long time, followed by an rapid shift.
Many other combinations have similar behavior. Delicate
sharp claws dull quickly without a retraction mechanism:
dull claws work about the same whether they are retractable
or not. Improvements in focusing capability may not be
particularly useful without improved light sensitivity on
the retina. A better leveraging point for a muscle/bone
connection may not be useful without a lengthening of the
bone to take advantage of it.
In essence, the basis of the shift is not a shift in
the mutation rate of the gene pool: it is that the
survival advantage conveyed by a group of genes may be much
larger than that conveyed by any single member of the
group. Random drift will convey the single members: once
grouped, natural selection will rapidly shift the population.
Small founder populations are frequently invoked for PE.
This is a different physical mechanism which produces the
same effect. Take our example above, but assume both genes
are recessive. In any large population, a recessive which
is not being actively selected for will be relatively
dormant: very few members would be homozygous for either
allele. Now, if one pair of wolves each possessing one
copy of one of the recessives gets isolated, there is
a subpopulation where each of the good alleles has been
artificially boosted to 25% penetration. The chances of
an individual appearing which is homozygous for both
recessive alleles within a few generations of inbreeding
is very good. Once that happens, the same effect will
take over. Same genetic mechanics, just a different
set of population mechanics to act as a trigger.
Kevin Wayne Williams