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.


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.


Tim Tyler | Contact |