The Founder Selection 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.

Modern interpretations of the founder effect often portray it as an extreme example of genetic drift - e.g.:

The founder effect is actually an accute occurrence of genetic drift, where changes in the frequencies of alleles in a population or sub-population occur for reasons other than natural selection.

- ISCID encyclopedia

The founder effect is a particular example of the influence of random sampling.

- Mark Ridley's Evolution textbook

However there is another effect which is caused when a small number of individuals establish a new population. This effect is not based on sampling error.

The Founder Selection Effect

The founders may represent a non-random sample of the original population. This is very likely to be true when the founders have succeeded in penetrating a challenging barrier which normally acts to prevent migration.

In that case the founders may be individuals with exceptional capabilities - since they penetrated the barrier before any of the other members of their species managed to do so.

This effect is known as The Founder Selection Effect.


Examples are commonplace - if considering all species. Founders are likely to be birds and rodents - since they have the best migration abilities. However, The Founder Selection Effect also applies within species.

In the common case of island speciation, founders may have better swimming capabilities than randomly selected members of the population - since they successfully survived the journey to the island.

They may be fatter, have more fuel reserves and better thermal insulation.

They may be more likely to go swimming in the first place.

Founders may tend to be extremely physically fit - and possess more stamina and strength than most members of the population.


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