A2 Biology 9700: Founder Effect, Allopatric and Sympatric Speciation

A population of small mammals living on a mainland has a mixture of allele frequencies at many gene loci. A chance storm event carries a very small number of these mammals (fewer than ten individuals) to a previously uninhabited offshore island, where they establish a new, isolated population that grows over many generations.

(a) Explain why the allele frequencies in the new island population are likely to differ, purely by chance, from those in the original mainland population, and name this phenomenon. [3]

(b) Over many thousands of years, the island population becomes physically unable to interbreed successfully with the mainland population, even when individuals from both populations are brought together. State the term for this type of speciation, and explain how the geographical separation of the two populations allowed it to occur. [3]

(c) Elsewhere on the mainland, a separate research group studies two populations descended from the same original mammal species, living in the same area but active at different times: one population is active only at night, and the other only at dawn and dusk. These two populations have also become separate species, without ever being geographically separated from each other. Name this type of speciation, and suggest how the difference in activity times could have led to genetic isolation between the two populations. [3]

(d) Other than showing that two populations can no longer interbreed, suggest how comparing DNA base sequences between individuals from different populations could provide additional evidence about how closely related those populations are. [2]

Part (a): The founder effect

Only a very small, essentially random group of individuals became stranded on the island. Because it is such a small sample, the exact mixture of alleles it carries, purely by chance, is unlikely to match the allele frequencies of the much larger mainland population.

This has nothing to do with any of these particular alleles being advantageous on the island; it is simply a sampling effect: a small sample drawn from a larger population, purely by chance, does not perfectly represent the frequencies found in that larger population. This is called the founder effect, a specific case of genetic drift (random change in allele frequencies due to chance, which has the largest effect in small populations).

Part (b): Allopatric speciation

Because the two populations are separated by a stretch of sea (a geographical barrier), individuals cannot move between them, so there is no gene flow (interbreeding) connecting the two populations. This term for speciation resulting from geographical separation is allopatric speciation.

With gene flow prevented:

Each population accumulates its own, independent mutations.
Each population is subject to its own genetic drift (especially relevant here, given the founder effect already described).
Each population may face different selection pressures, suited to its own local environment (mainland versus island), favouring different alleles in each place.

Over many generations, these independent processes cause the two gene pools to diverge progressively further from each other. Eventually, the accumulated genetic differences are so great that individuals from the two populations can no longer produce fertile offspring together, even if reunited, they have become reproductively isolated, and are recognised as separate species.

Part (c): Sympatric speciation

Here, the two subpopulations share the same geographical area, so there is no physical barrier between them, yet they have still become separate species. This is sympatric speciation.

Instead of a geographical barrier, a behavioural barrier has prevented gene flow: because one subpopulation is active only at night and the other only at dawn and dusk, individuals from the two subpopulations rarely encounter each other at a time when mating could occur. This temporal (behavioural) isolation greatly reduces interbreeding between the two subpopulations, even though they occupy the same physical space.

As in allopatric speciation, once gene flow is reduced or prevented, each subpopulation accumulates its own genetic differences (through independent mutation and selection) over many generations, until the two subpopulations become reproductively isolated and are recognised as separate species, without ever having been geographically separated.

Part (d): Molecular evidence for evolutionary relationships

Beyond testing whether two populations can interbreed, biologists can compare DNA base sequences (or the amino acid sequences of specific proteins, produced from the DNA) between individuals of different populations or species.

Populations or species that share a more recent common ancestor are expected to have had less time to accumulate independent mutations, so their DNA sequences should be more similar.
Populations or species that diverged from a common ancestor longer ago are expected to have accumulated more differences in their DNA sequences.

By counting and comparing the number of sequence differences between populations, biologists can build a picture of how closely related different populations or species are, and roughly how long ago they diverged from a shared ancestor. Providing evidence for evolutionary relationships independent of just observing whether interbreeding is still possible.

Final answers

(a) A small founding sample carries, by chance, allele frequencies that differ from the mainland population, the founder effect (a type of genetic drift).
(b) Allopatric speciation. The sea prevents gene flow, allowing independent mutation, drift and selection to cause divergence until the populations are reproductively isolated.
(c) Sympatric speciation, differing activity times cause behavioural isolation, preventing interbreeding despite no geographical barrier, allowing divergence into separate species.
(d) Comparing DNA base sequences: more similar sequences indicate a more recent common ancestor (closer relationship); more differences indicate a longer period of independent evolution.