Factors effecting genetic viability of humans

[Atellus]

It is my understanding that there is a lower limit to the number of individuals required to constitute a genetically viable population. In other words, Adam and Eve alone could not found a successful population. The resulting population would die out due to a lack of genetic variability.

 

I have read something of genetic bottleknecks and the Mount Toba eruption theory which, among other things, seek to explain the morphological differences which characterise so-called human "races". I understand that during the nuclear winter which followed the eruption, which is considered to have taken place about 60,000 years ago, some groups of humanity were sheltered from the worst effects of this climate change in isolated pockets. In these pockets sufficient animal and plant life survived to support viable human populations and it is the lack of interbreeding between these separate populations during this period which led to the development of distinct morphologies in these isolated groups.

 

I would be grateful if someone could help me answer the following questions:

 

1. What is the minimum population size in terms of breeding individuals that is required to sustain a viable population? In effect, if we were to discover a previously unknown island in the middle of an ocean, what is the minimum number of people we must seed it with in order to establish a genetically viable colony?

 

2. What are the deleterious genetic effects caused by having too few reproductive individuals and why do are they harmful? Why couldn't Adam and Eve succeed in breeding a population of thousands or hundreds of thousands?

 

Thank you

[ecoli]

1. What is the minimum population size in terms of breeding individuals that is required to sustain a viable population? In effect, if we were to discover a previously unknown island in the middle of an ocean, what is the minimum number of people we must seed it with in order to establish a genetically viable colony?

 

obviously at least two, but I don't think you can come up with an absolute answer. Afterall, sometimes interbreeding, say between cousins, results in a healtly normal child, but not always.

 

2. What are the deleterious genetic effects caused by having too few reproductive individuals and why do are they harmful? Why couldn't Adam and Eve succeed in breeding a population of thousands or hundreds of thousands?

 

http://en.wikipedia.org/wiki/Inbreeding

[Atellus]

Thanks for the link, I hadn't seen it, but I'm also interested in the molecular mechanisms involved as well. This is related to my question on minimum population size as I was wondering if genetic engineering might not have a solution to the problem for small populations. For instance, if we were to say that the minimum viable human population size would be in the range of 100 - 150, but we only had 50 humans, is there a possibly technological fix, a way of artificially enhancing "effective" variation, perhaps by laboratory recombination of DNA? Is such an idea science or science fiction?

[Mokele]

Minimum size depends on a lot of things, such as the level of genetic variability of the initial group, whether the breeding is controlled or not, etc. If you can hand-pick your individuals, and pick who they mate with (like in species conservation programs for endangered animals), you can get by with a smaller population than you could if a population (possibly all somewhat related if it's a migrating clan/tribe) just shows up and breeds as they will.

 

Mokele

[lucaspa]

It is my understanding that there is a lower limit to the number of individuals required to constitute a genetically viable population. In other words, Adam and Eve alone could not found a successful population. The resulting population would die out due to a lack of genetic variability.

 

That's not true. It turns out that 2 individuals contain 75% of the genetic variability of the entire population. Founder events -- where there is just one breeding pair to start a population -- are relatively rare but not unknown. Drosophila in Hawaii, for instance, are the result of a founder event.

 

"A colony founded by a small number of colonists will suffer some loss of genetic variation: uncommon alleles, in particular are unlikely to be represented. Teh average level of heterozygosity, however, is not greatly reduced in the first generation: it is (1-1/2N)H0, where N is the number of founders and H0 is the heterozygosity in the source population. Thus in a colony founded by one mating pair (N=2), the heterozygosity is, on average, reduced by only 25% in the first generation. Recalling that the genetic variance of a character is proportional to the population's heterozygosity at loci that affect that character, we see that most of the heterozygosity and genetic variance of a large population are, on average, carried over into a colony founded by a few individuals."Evolutionary Biology, D Futuyma, pg 304