Newsgroups: rec.birds
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!sci.ccny.cuny.edu!christ
From: christ@sci.ccny.cuny.edu (Chris Thompson)
Subject: Re: Importing parrots for genetic variability
Message-ID: <1991Jun15.154957.3243@sci.ccny.cuny.edu>
Organization: City College of New York - Science Computing Facility
References: <1991Jun12.131020.22423@zoo.toronto.edu>
Date: Sat, 15 Jun 1991 15:49:57 GMT

	An Australian theoretical geneticist (Warren Ewens) has spent some 
time talking about the problem of genetic variation, and minimum viable
population sizes.  ("Minimum viable population" is the current hot catch-
phrase in conservation biology).  Anyway, the problem here is that when you
want to calculate a minimum viable pop. size, you DON'T use the actual
population size.  You have to include the effective population size.  I don't
recall the formula offhand, but it has to do with sex ratios, and the number
of animals that actually breed, and survival rates.  Now, this can obviously
all be manipulated with captive populations, but still, the effective
population size is always much smaller than the actual population size.
	Also, there has always been this mystique associated with the number
500.  This has been bandied about as a standard minimum viable population
size---below 500, most species will not maintain enough genetic variation
to make it for more than about 50 years.  Warren Ewens again (this was the
subject of a seminar he gave at Columbia University 1.5 years ago) 
thouroughly debunked this notion, showing that the number is almost always
much greater than 500.  His position was that the number 500 was based on a
series of "heroic assumptions", which werem't valid.  So I don't think I'd
spend a lot of time trying to catch wild birds to maintain genetic
variation in captivity.  I think I'd rather see it remain in wild 
populations.

Chris Thompson
Biology Department, CCNY

-- 
"Never count a human dead until you've seen the body.  And even
then you can make a mistake".
			-Lady Fenring