Wednesday, October 31, 2007

Legitimacy of the Founder Effect

One very pertinent question that always comes to mind when looking at the Founder Effect is the viability of the extant founder population and the subsequent deleterious effects from the lack of genetic diversity in their offspring. How is it possible for the offspring to continue to be viable when their genetic diversity is so severely reduced? We often hear about the likelihood that a population of less then 500 founding individuals is doomed to fail because they lack genetic diversity causing lethal alleles to accumulate through inbreeding. For example, the state of the Cheetah is in peril because their species went through a recent genetic bottleneck which lead to only a few individuals surviving and passing on their limited genetic diversity to future generations. In fact, cheetah's are so similar to each other that one can swap organs between two regionally distant animals and due to their histocompatability, their bodies will not reject the donor organ. This means that the organs of one cheetah are recognized by another cheetah's immune system as though they came from the same body. In essence, the variation between proteins on the surface of the cells (major histocompatability complexes) are small enough that there's a lack of an antibody mediated immune response, minimizing cross-donor rejection. This reduced genetic diversity limits the adaptability of the cheetah and major stressors can severely crash the population. Thus, the question once again can be restated by asking why are certain species able to cope with such limited founding diversity and suddenly flourish, while others are prone to extinction?

To further investigate this question, one must delve deeper into understanding not only the Founder Effect, but the interplay between Genetic Drift and Natural Selection as well. All of these factors occur to help perpetuate speciation. Speciation can occur when one or more daughter groups are isolated from the founding group, or the founding group itself branches into two or more extant populations that are isolated enough from each other to speciate. There are four main methods of speciation, ranging from complete geographic isolation, to mostly geographic, somewhat geographic, and finally no geographic barriers at all. Island populations that suddenly get cut off from the mainland when sea levels rise speciate due to complete isolation. Some species evolve to occupy certain ecological niches within the same geographic area. For example, perhaps a mutation for a slightly thicker bill in an outlying area within the same geographic boundaries results in a finch that has a thicker bill and perhaps the ability to extract insects from dead bark. This group then eventually exploits the rotten wood insect niche throughout the initial geographic range.

Genetic drift then is the random rate of variation that occurs within a given population and depending on the initial numbers and environmental conditions can be either kept in check by Natural Selection or allowed to proceed. Smaller populations are more likely to undergo genetic drift because of the limited genetic diversity, while larger populations intermix well and barring environmental changes that open new ecological niches for certain groups within the population to occupy leave little room for major drift. Natural Selection functions by allowing only those alleles that are biologically or reproductively adaptive to perpetuate, while genetic drift is just the random shuffling of alleles mostly irrespective of the adaptive pressure. Examples of Genetic Drift in Humans can include epicanthic folds in the eyes of Asians, increased hairiness in the bodies of Mediterraneans or Okinawans, Blond Hair and Blue Eyes in Scandinavians, wavy hair in Polynesians, and etc. Examples of Natural Selection in Humans on the other hand include darker or lighter skin, thicker, stockier builds in colder regions, barrel flared chests in Andean and Himalayan people, reduction of perspiration in Australian Aborigines, etc. It must further be noted that not all of these human characteristics are strictly a result of natural selection or genetic drift, some are a product of the interplay between the two forces.

Coming back to the Founder Effect then, we can look at a hypothesis as to how New World Monkeys came to Occupy the Americas when in fact, there was already a large gap between the continents when their ancestors branched from their African counterparts. It has been established, through the fossil record, and mitochondrial analysis that New World Monkeys emerged approximately 35 million years ago (link), during the early oligocene. At the time, the geographic distance between Africa and South America was at least 1300 miles (link) and therefore, quite a barrier for a founding population of pioneering monkeys. Vegetative Rafting has been proposed as a solution to the large oceanic gap that was bridged by these accidental voyagers. Due to the large volume of water discharged in the rainy season within Tropical locales, occasionally large swaths of vegetation get swept out to sea by huge rivers like the Amazon or the Congo. These vegetative rafts then float hundreds of miles out to sea, usually drying up. However, with prevailing winds and currents, some of these rafts can travel quite far, and this is the possible scenario that lead to the ocean voyage of some African Monkeys. This initial founder population then must have been very limited, perhaps a half dozen individuals. The genetic analysis has shown that they all originated from a small group of founders (just like a population bottleneck) but because of the rich geographic and environmental diversity, their populations exploded and speciated. In particular, population drift probably had a very large role to play in the diversity of current New World Monkeys and the ecological niches they occupy. The Question again is how this small founding population, with limited genetic diversity, blossomed into numerous species, irrespective of the initial inbreeding that could have resulted in numerous lethal alleles and population crashes. Several possible theories are likely, but none definitively stand out. Perhaps some animals are better able to cope with limited genetic diversity. Or, perhaps the new ecological niches helped increase the rate of speciation and variation within the population. Given that Genetic Drift had a large part to play with the small founding population, how did that effect the likelihood that lethal alleles were mitigated.

Such questions are the reason why Evolutionary Biology is such a fascinating field of study, and rarely are answers as complete as one would like. However, should anybody reading this have better knowledge on any of these topics or answers to my questions, please e-mail me or let me know in the responses what possible theories or holes in my reasoning can help to further illuminate this subject.

**02/17/2009 Update
The possible role of r/K selection in the founder effect.

The concept of r/K selection describes the selection of certain traits which promote success in particular environments. It comes about from an algebraic equation related to population dynamics. In general, species that are K strategists have increased investment in fewer offspring, while r strategists maximize the amount of offspring with as little investment as possible. Competition is the major key differentiating these two strategies. Since r strategists have little competition, they can focus on increasing their numbers as quickly as possible. Yet, once competition pressures increase as new species come about, other strategies, such as more investment in less offspring, evolve to cope. Ultimately, an ecological balance is reached between r and K strategists. From this, the relevance to the founder effect seems intuitive. Founder species, especially those like the ancestors of New World monkeys must have been r strategists. Finding themselves in an environment with countless niches, they must have reproduced quickly and evolved to fill them. Once enough speciation occurred to sufficiently increase competition, some monkey species may have adapted towards K selection.

Still, even though we can identify the ecological process that r/K selection plays within the founder effect, we must still explain the eventual success after the initial lack of genetic variability. Perhaps r strategists, with their hi fecundity, mutate rapidly enough that their genetic diversity is readily expanded within a few hundred generations, reaching a critical threshold when lethal alleles are balanced with genetic variability. The critical number of founders certainly differs among species, with those that are K-strategists probably requiring a larger gene initial gene pool. It's also possible that some species, regardless of their ecological strategies, are naturally prone to greater mutations; perhaps a less accurate DNA repair machinery, or environmental constraints that increase the adaptive advantage of survival through increased offspring variability.

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