How many is a bunch?
Many years ago when I had first taken an interest in mating pool size and the survival of that pool there were questions of:
How many is too few?
How many is too many?
How many is best?
Initially I had been conditioned by the usual social forces to think that the bigger the gene pool the better. I thought one would not find the size was too many until it was in the tens of thousands and included enormous diversity of background as well. Over the years I have been beaten back by the evidence to believing that a few dozen is probably best and what is tolerable is something between one and two hundred in mammals. But I still try to keep an eye on any data that might help restrict the possibilities.
Two bits of data have now appeared. There was a population of Raso larks (The Rebounding of the Lark SCIENCE vol. 336 no. 6078 April 13, 2012 page 137 reviewing work by a team led by Michael Brooke of the University of Cambridge publishing in ANIMAL CONSERVATION) in the Cape Verde Islands has risen from 65 to 1490 over a period of eight years. That last number may not be the gene pool. Nature probably has given the larks a way to divide their gene pool. If it is the gene pool, I would predict a catastrophe. There is a question in my mind about just how much diversity there was among the 65. From what I can tell, the population had declined slowly to that level so that there was a limit on the diversity. So it seems that 65 is good number for good fertility provided the diversity within that population is acceptable.
So that would be a middling number. At the low end (SNV Bottleneck Dynamics, SCIENCE vol. 335 no. 6074 March 16, 2012 page 1281 reviewing work by Oliver and Piertney published in Mol. Biol. Evol. 29, 10.1093) a population of water voles on a Scottish island was deliberately reduced by introducing sheep, which reduced the population to just 5 voles, after which the sheep were removed. They measured the diversity of the survivors by examining something called microsatellite DNA, which seems not to have a great effect on the moles. They also measured their diversity of MHG, which means “major histocompatibility group genes (alleles, specifically), and insufficient MHG diversity is known to depress immune status. So 5 seems to be too small a number. There is no surprise there.
I have been following a blog called HBD Chick, referenced at the top of the home page, and have been much impressed by one commentator who calls himself “Snoopy,” although he capitalizes it differently. He has made remarks that seem relevant here. He says the quotes from a book Life in a Hutterite Colony saying that from 1880 to 1950 the community would deliberately subdivide when the population size reached 120 to 130 people, and in so doing achieved the highest population growth rate ever recorded for humans. He added some other examples.
On the MHG front, on another occasion, he reported that you can have too much MHG complexity as well as too little with an ideal range in between. I am mightily impressed with his knowledge. He says he got it from Wikipedia. He thought to look.
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