The Real Founder Effect
The Founder Effect
The Founder Effect stems from an idea of Ernst Mayr:
The reduced variability of small populations is not
always due to accidental gene loss, but sometimes to the
fact that the entire population was started by a single
pair or by a single fertilized female. These "founders"
of the population carried with them only a very small
proportion of the variability of the parent population.
This "founder" principle sometimes explains even the
uniformity of rather large populations, particularly if
they are well isolated and near the borders of the range
of the species. The reef heron (Demigretta sacra) occurs
in two color phases over most of its range, a gray one
and a white one, of which the white comprises about 10 to
30 percent of the individuals. On the Marquesas Islands
and in New Zealand, two outposts of the range, only gray
birds occur, while the white birds comprise 50 percent on
the Tuamotu Islands, another marginal population (Mayr
and Amadon 1941). The differences in the composition of
these populations is very likely due to the genetic
composition of the original founders. The same
explanation probably covers most of the cases in which
isolated populations of polymorphic species have
much-reduced variability.
- Mayr 1942.
Mayr refers to the loss of genetic diversity caused by
the founders becoming isolated from their ancestral
population.
However, there are a number of other effects associated with
founding members of new populations.
For one thing, there is The Founder Selection
Effect - which suggests that the founders may
represent a non-random sample of the original population
- probably with exceptional stamina and strength - since
they successfully penetrated a barrier to migration
before any other members of their species.
Founders may faced reduced competition - since their new
environment is likely surrounded by geographical barriers
that prevent the influx of new species by migration.
Founders may often find themselves on islands - in which
case, all the environmental
conditions associated with islands may affect them.
The reduced variability Mayr describes is only one of the
many effects which are commonly associated with
founders - so Mayr's founder effect might be seen as a
subset of the collection of effects which are associated
with founders in the real world.
Misconceptions
A narrow conception of the founder effect
may have led to historical problems in interpreting
real biological phenomena.
Ernst Mayr himself provides an example:
The potentiality for rapid divergent evolution in small
populations explains also why we have on islands so many
dwarf or giant races, or races with peculiar color
characters (albinism, melanism), or with peculiar
structures (long bills in birds), or other peculiar
characters (loss of special male plumage in birds)
- Mayr 1942, p. 236.
Here he attributes the common observation of diverse
adaptive radiation on islands to a small initial population
size.
However a more rational interpretation of these
observations is that species on islands face less
competition than their ancestors did on the mainland -
and therefore face less opposition when migrating into
adjacent niches - an effect which would not be strongly
dependent on the diversity of the initial population.
It seems possible that an understanding of the breadth of
The Real Founder Effect would have avoided this
problem.
References
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