Eco-Vocab: Founder Effect
The concept of the “founder effect” is one that comes up commonly in ecology, particular in population ecology where you’re looking at how groups and species change over time. The founder effect can cause speciation and genetic change within a species to occur much more quickly than it would under normal conditions. Basically, the founder effect describes what happens when you take a small number of individual out of a large population and isolate them from that parent population so those few individuals can only breed with each other. This can happen in any number of ways: a small group emigrates away looking for new resources and it is too far to return to the main group to breed, a physical barrier like a river or a sudden rockfall isolates them from the parent group, or a catastrophic event wipes out most of the species leaving only a few members (this is also called a genetic bottleneck).
So what happens then? Well, normally in a large population you have a pretty even distribution of genes that cause certain traits (eye color, tail length, you name it). As long as there is no major selective pressure being exerted for or against that trait (ie, as long as blue eyes don’t lower your chances of survival and reproduction), the distribution of those genes within the population is going to stay roughly the same. Now, if you take a small sample of individuals out of that group, the distribution of genes that cause different traits might be the same as in the large population, but there is a much greater chance of gene distribution being skewed one way or the other.
It’s like how when you flip a coin 1,000 time it will almost certainly be heads about 500 times and tails about 500 times, but out of those flips it’s possible and even likely to get a run of, say, ten heads in a row. So out of a population of individuals where maybe half are spotted and half are striped, you could end up with a subpopulation of all striped individuals. In fact, in a population where most are spotted and only a few are striped (maybe striping is a recessive trait in this species) you could still end up with a subpopulation of all striped individuals.
It’s when they start to breed that things get interesting. These individuals suddenly have a much smaller genetic pool to paddle around in, and that can cause change to happen very quickly. As we know, a small population can be trouble because genetic diseases thrive on homozygosity. So one of the first things that is likely to happen is that genetic problems that were relatively rare in the large population (because if you carried the gene for it you were not very likely to mate with someone else who also had it) become more commonplace. Sometimes this may actually end up eliminating the trait from the gene pool entirely because all the individuals with the genes for it produce only offspring that die and it doesn’t get passed on, so you’re left with an even smaller pool containing less muck. But sometimes it sticks around and becomes more common than ever – like the sad case of Tay-Sach’s disease in the Ashkenazi Jewish population.
Other traits may be more common throughout the population that never would have been in a larger population, because they were not particular advantageous (or disadventageous either). And since events that cause founder effects are often associated with a change in conditions or location for the smaller group, a change in selective pressure can mean that a trait that used to be rare and evolutionarily neutral can quite suddenly become common and desirable. Natural selection takes over and speciation from the parent group can happen much more quickly than under other conditions. We see this on islands especially, when a small group has emigrated there – new genetic distribution + new environmental conditions = evolution has a party.
The founder effect is especially strong in species that have slow reproduction rates, particularly large mammals, because the population stays small for a long time after the event that separated them. Species that reproduce copiously are less effected. If you have a million beetles, and you separate 100 of them… it won’t be that long before you have another million beetles. Of course there will still be a change in genetic distribution from the separating event, there’s no getting around that, but it won’t continue to compound for as many generations because they’ll be back up to a large breeding population that much quicker.
One of the most commonly cited examples of the founder effect is cheetahs (Acinonyx venator), particularly the cheetahs in southern and eastern Africa, which are distinct populations. And frankly, neither of them is doing that well. You see, cheetahs are so genetically close to each other that they can actually accept skin grafts from one another. Scientists think that there was a catastrophic event around the end of the last Ice Age that killed most of the existing cheetah population and caused a genetic bottleneck with the existing cheetah population. The few that were left could only breed with each other and the founder effect has a created an extremely homozygous population even thousands of years later. Another event maybe a hundred years ago, most likely hunting and human settling, caused them to split into two geographically distinct populations and decreasing the breeding pool of each even further.
The south African population is the worse off, having undergone more inbreeding resulting in such genetic problems that it can barely reproduce any longer, but both populations suffer from a low sperm count, low sperm motility, and deformed flagella, making it highly vulnerable to extinction and not all robust. (How do scientist know about the sperm? Capture and electro-ejaculation sampling. It is exactly what is sounds like.) Cheetahs also often throw up unusual or strange color morphs more often than other felids due to increased likelihood of individuals with the same recessive trait mating with each other. The only reason cheetahs may have survived as species this long is that most of the fatal genetic illnesses were probably rapidly bred out after the event that reduced the population, so although they have trouble reproducing, when they do the offspring are generally healthy (or at least not unusually likely to be unhealthy). Putting the two populations back together may be a start towards reclaiming a little diversity and giving them better odds on species survival.
Humans have been uniquely shaped by the founder effect, in a case of what is called a serial founder effect. Basically, we evolved in Africa and migrated to the rest of the world. But it didn’t happen all at once. A small group left the larger group and went north and settled down in a likely spot. They were subject to the founder effect, the population changed in response to the new conditions and the new genetic distribution, and grew larger. Then a subset of that population got tired of the crowds and set out for a new spot to settle in, where they didn’t have to share resources, and they too were subject to the founder effect, and so on. This can actually be traced by genetic studies done today. In general, the people in Africa today, descendants of the original large gene pool have the highest genetic diversity in their population, followed by people on the Indian subcontinent (one of the first places humans settled when they left Africa) and so on. Places like Iceland, which was settled late and is also an isolated area, have some of the least genetic diversity on the planet and a high incident of recessive traits like blonde hair and blue eyes.
So as you can see, the founder effect can have a pretty strong effect, good, bad, or just different, on populations. And it has probably been the initial starting push for the speciation of many species we know today. But it is important not to assume that all speciation events are the result of a founder effect – natural selection doesn’t necessarily need any help to develop new species. For instance for a long time it was thought that the royal spoonbill (Platalea regia) in Australasia and the black-faced spoonbill (Platalea minor) in eastern Asia had speciated as a result of the ancestors of the black-faced spoonbill being separated physically from the parent population and undergoing a founder effect combined with new selective pressure in their new home. But recent studies have shown that there was almost certainly genetic mixing between the two populations long after the black-faced spoonbill ancestors had taken up residence in their new digs. It’s more likely speciation occurred slowly as one part of the group began to take advantage of new resources in a new area while others did not, and they slowly began to preferentially mate with others who did the same.
Founder effect is a vital concept in the study of populations and of evolution, but it should not be given credit for all genetic change. There are other mechanism that can effect selection and evolution, and we will look at this in future eco-vocab posts.
Yeung, C. K. L., Tsai, P.-W., Chesser, R. T., Lin, R.-C., Yao, C.-T., Tian, X.-H., et al. (2011). Testing founder effect speciation: divergence population genetics of the spoonbills Platalea regia and Pl. minor (Threskiornithidae, Aves). Molecular biology and evolution, 28(1), 473-82. doi: 10.1093/molbev/msq210.
Deshpande, O., Batzoglou, S., Feldman, M. W., & Cavalli-Sforza, L. L. (2009). A serial founder effect model for human settlement out of Africa. Proceedings. Biological sciences / The Royal Society, 276(1655), 291-300. doi: 10.1098/rspb.2008.0750.
Ramachandran, S., Deshpande, O., Roseman, C. C., Rosenberg, N. a, Feldman, M. W., & Cavalli-Sforza, L. L. (2005). Support from the relationship of genetic and geographic distance in human populations for a serial founder effect originating in Africa. Proceedings of the National Academy of Sciences of the United States of America, 102(44), 15942-7. doi: 10.1073/pnas.0507611102.
OʼBrien, S. J., Wildt, D. E., Bush, M., Caro, T. M., FitzGibbon, C., Aggundey, I., et al. (1987). East African cheetahs: evidence for two population bottlenecks? Proceedings of the National Academy of Sciences of the United States of America, 84(2), 508-11. Retrieved from http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=304238&tool=pmcentrez&rendertype=abstract.