26 October, 2006

Pot pourri

This is a typical conversation in my kitchen:

I'm currently watching the first season of Battlestar Galactica, usually while cooking dinner. The premise of the show is that an evil race of robots destroys almost the entire human race, leaving only a small population of 50,000 individuals alive.

This immediately prompted us to pause the movie and launch into a discussion of the population genetics implications of such a crash.

Now, contrary to conventional wisdom, there is a huge difference between the actual and apparent genetic size of a population. That is, even though there are 6 billion people on this planet, human beings are remarkably genetically uniform; in fact, they show the amount of variation one would expect from a much smaller ideal population. This discrepancy leads population geneticists to speak of a quantity called effective population size - in the case of humans, about 10,000 individuals. This is such a small number because, first, there is population structure that prevents truly random mating between all individuals in the population, and, second, the human population has undergone at least a few "bottlenecks" - instances of dramatic population collapses - and the ancestral population was probably considerably smaller than the modern population. Thus, the amount of actual variation in our population is low.

A bit of Googling around led us to a paper by Masatoshi Nei, wherein he describes simulations of population crashes and subsequent rapid expansion. The upshot is: even absurdly unrealistic population crashes (down to N=2 individuals) do not eliminate most of the variation. I think in that extreme instance, the reduction was only from 15% down to 8% - so long as your 2 individuals are randomly selected, of course. This is pretty remarkable, and it suggests that the immediate problem will not be fitness loss from low genetic variation - you'll lose most rare, private variations (variation that only exists in you & your immediate relatives, e.g.), but most of this stuff is unimportant or even harmful. What WILL be retained is the bulk of frequent variation - stuff that is either beneficial or neutral, and so has not been eliminated by purifying selection. Of course, it takes on the order of 1/(mutation rate) generations (108) to recover your initial level of variation, but never mind that. We're merely concerned with survival, here.

The second problem is inbreeding depression. This is exactly what it sounds like: when you have babies with your parent or sibling, you're much more likely to encounter severe recessive phenotypes that drastically reduce fitness. Note that this is a DIFFERENT problem from low genetic variation - I am genetically not all too distant from anyone on earth, but inbreeding depression results mostly from the expression of rare, private variation that no one else has - except my relatives.

So, after a bottleneck, inbreeding depression will surely be a problem. This possibly results in 'purging', that is, the speedy elimination of deleterious variation via selection in a highly homozygous/inbred population. This puts pressure on the population - inbreeding load - fewer individuals are surviving and the general fitness of the population is lower. Populations may founder at this stage, although purging is believed to result in a rapid recovery of fitness.

At any rate, 50,000 individuals is, in genetic terms, a great many, and I'm pretty certain that inbreeding depression would not be a severe problem in such a population (especially if they're all free to mingle as the Galactica population would be).

Last night's conversation included: group selection in chickens and its eugenics implications, a recitation of some Hindu mythology (mostly the Avatars of Vishnu), some tales of the Buddha and other Zen masters, the story of Kalidas, a discussion of the Inquisition as viewed by the book Demon Lovers, and Hitler's vegetarianism. The evening culminated with us watching Triumph des Willens.*

* Which prompted the observation on my part, "Americans will never be able to take the German language seriously."


If you're looking to assemble a team of people to dispose of the undesirables, I'm available most days after my mid-morning nap. Also, I am pretty good at carrying torches and stuff. 

Posted by Mist 1

your household sounds like it could rent seats at the table for the evening.

human beings are remarkably genetically uniform 

among endogamous species? anyway what i meant to ask was, what's the distribution for genetic variety, and where are some other species scattered. who's widely varied? 

Posted by hibiscus

The distribution is actually surprisingly narrow, since there are few species that maintain large, continuously breeding populations. In fact much of this debate started because of the initial observation that the amount of genetic variation in real populations was more or less the same for all observed species (I'm not sure which were surveyed) regardless of their actual population size - reflecting the fact that most populations go through weird contusions of population structure. That said, there are obviously some exceptions and some variation in levels of genetic variation. The sea squirt, ciona intestinalis , is one such famous one. Sea squirts have a gigantic population and a short generation time, which allows them to produce tremendous amounts of genetic variation - I think the figure is 10%. That is, two random sea squirts will differ, in average, on one in every ten genome positions. Holy shit! By comparison, we're on the short end - 0.04%. Even chimps, which have a real population of 200,000 individiuals, have an effective population size more than twice ours - 25,000 or 35,000, which puts their level of variation at 0.1%-0.14%.  

Posted by saurabh

This will make it hard for, say, oceanic large fish populations to truly jump back even if we leave magically start leaving them alone, won't it?

Backstage of my cephalopod-obsessed joint, we were speculating on the lack of tentacled land creatures. The best we could think of was the Elephant. The immediate reaction was, "oh, that probably doesn't mean it won't work for --- reason". . .but the mere fact that phenome isn't around doesn't mean there is something innately wrong with it, right? It could just be that it never had a chance to flower bc of some random extinction event.

Posted by Saheli

For tentacled monsters - why don't monkeys or snakes qualify? They both have pretty adept, pliable appendages/bodies. 

Posted by saurabh

or fauna with very long tongues/lips, which is prolly a closer relative 

Posted by hibiscus

(in spirit, i mean -h.)

Brad, of the blog Sadly No, has an article about Galactica  in The American Prospect that is really good. 

Posted by hedgehog

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