Minimum species population to ensure adequate genetic diversity?
February 16, 2010 8:44 AM   Subscribe

If a species is nearing extinction, what is the minimum population needed to give it a reasonable chance of survival, in terms of genetic diversity?

Assume for the sake of argument that whatever threat or change in the environment that has already reduced the species' population is reversed, so that the species has a chance to re-propagate itself. Also assume we're talking about a complex animal, not a paramecium.

I'm guessing that below some threshold, negative traits like hemophilia would tend to get amplified in succeeding generations, and hurt the chances of the species as a whole from flourishing. Obviously population isn't strictly equivalent to genetic diversity—if all remaining members of a population are members of the same family, the genetic diversity is lower. I also realize this probably isn't a precise number.
posted by adamrice to Science & Nature (14 answers total) 7 users marked this as a favorite
In addition to minimum population and genetic diversity, you may also need to consider fertility rates and average death rates. Also add in geographic proximity of members to facilitate breeding.

It may be too abstract a query to satisfactorily answer without narrowing down a few more parameters.
posted by Like the Reef at 8:49 AM on February 16, 2010

I did a bit of Googling and found a 1991 paper from the National Oceanic and Atmospheric that examines the population question specifically. It mentions a "50/500" rule:

The "50/500" rule of thumb initially advanced by Franklin (1980) and Soule (1980) comes the closest of any to attaining "magic number" status (Wilcox 1986). This rule prescribes a short-term effective population size (Ne) of 50 to prevent an unacceptable rate of inbreeding, and a long-term Ne of 500 to maintain overall genetic variability. The Ne=50 prescription (termed "the basic rule" by Soule 1980) corresponds to an inbreeding rate of 1% per generation, approximately half the maximum rate tolerated by domestic animal breeders (Franklin 1980). The Ne=500 prescription is an attempt to balance the rate of gain in genetic variation due to mutation with the rate of loss due to drift, and is based on a genetic study of bristles in Drosophila (Franklin 1980).

I've also heard 150 thrown out as the lowest viable population.
posted by lunalaguna at 8:53 AM on February 16, 2010

I think the commonly used term for this is minimum viable population. Here's a link to a paper on grizzly bears in Yellowstone that may detail some of what you're looking for.
posted by pappy at 8:55 AM on February 16, 2010

You can shoot numbers around all you want, but it's really important to nail down specifics. Octopuses lay hundreds of thousands of eggs at a time. Thousands of those are killed and thousands are eaten, but at worst the octopus has still given thousands of offspring. A whale gives birth to a single calf.

In writing this, though, it occurred to me that one could argue that animals who lay eggs in such great numbers might never run the risk of extinction unless environmental factors instantly killed all the offspring.
posted by InsanePenguin at 9:17 AM on February 16, 2010

Effective population size (Ne)=500 is the rule of thumb for conservation biologists.

HOWEVER... It's very important to understand that Effective population size and census population size aren't always the same. Variables such as breeding cycles, sex ratio, mating strategy, life span and historic population size all go in to calculating the effective population size. You can have a census size (number of actual individuals of the species) in the thousands and not have an effective population size > 500.

Any species that gets down to very low numbers, for example 20, will have an effective population size of 20 for generations, even if the number of individuals bounces back to over 500. Yellowstone Grizzlies are a great example of this.
posted by Maude_the_destroyer at 9:20 AM on February 16, 2010 [3 favorites]

Cheetahs may be the test case of this. Genetic diversity among cheetahs is amazingly small. For instance, they're all genetically close enough so that skin transplants between unrelated animals take without rejection. And whereas all tigers have different patterns of stripes, and all leopards have different patterns of spots, all cheetahs have the same pattern of spots.

The assumption is that they suffered a nearly complete population collapse at some time in the past, possibly to as little as a single breeding pair, and then rebounded from it.

They have some serious health issues, especially related to breeding, and their poor genetic variability leaves them highly vulnerable to environmental changes, so they may well die out some time in future. (Ultimately, extinction awaits us all.) But for the moment they are doing OK.

A different test case would be examples of feral domestic animals who are well established now, specifically dingos and mustangs. In both cases the basis for the modern population is a small number of animals who escaped from domestication. They both seem to be doing well, though.

That 50/500 rule is a guideline, not a law.
posted by Chocolate Pickle at 9:33 AM on February 16, 2010 [1 favorite]

You may be interested in a couple previous questions (both specifically about humans).
posted by ssg at 9:39 AM on February 16, 2010

Following up on what Chocolate Pickle said, having a small range of genetic variability is not automatically a kiss of death for a species, and over time it will be come less important as variation increases in subsequent generations. The main issue facing species is lack of resistance to disease (the entire genealogy might be particularly susceptible to a particular strain of staph bacteria for example) and an increased likelihood of genetic disorders. I have read that at some point around 70,000 years ago our species suffered a tremendous collapse and there may have been only a few thousand (or hundred!) homo sapiens alive at one point. Over time we have recovered, but for the most part we are still fairly homogeneous genetically, and most people can find a common ancestor with almost any stranger within the past millennium.
posted by BobbyDigital at 10:13 AM on February 16, 2010

Another part of the equation to consider is whether your animals will repopulate by themselves (choosing their own partners/s) or will they be partnered according to the most efficient way of maintaing genetic variability.
posted by dstopps at 10:55 AM on February 16, 2010

BobbyDigital, that is a great point....human effective population is estimated to be around 30,000. (Can't remember the citation of the paper that calculated that) We are not a very genetically-diverse species, thanks to that bottleneck.
posted by Maude_the_destroyer at 10:58 AM on February 16, 2010

There are species of Taraxacum (dandelions) which have somehow-or-other lost the ability to reproduce sexually. They produce flowers every year, but there are no pistils or stamens. The seeds are produced from the genes of the parent plant. Effectively, they reproduce by cloning. Which means that genetic diversity is zero.

But any home owner can tell you that dandelions are not endangered.
posted by Chocolate Pickle at 11:19 AM on February 16, 2010

I don't have time to get into more research, but species survival plans are used by zoos to maintain genetic diversity among their collections, especially as for many species no genetically novel organisms have been added in many years.
posted by fermezporte at 11:57 AM on February 16, 2010

In writing this, though, it occurred to me that one could argue that animals who lay eggs in such great numbers might never run the risk of extinction unless environmental factors instantly killed all the offspring.
That argument would suggest that there should be no record of endangered or extinct frogs, which is clearly not the case.

If a species has a stable population, and its individuals lay ten thousand eggs over their lifetime, that suggests that only two eggs per ten thousand survive to breed successfully. I'd interpret this oppositely from how you did: the survival rate is so low that relatively small changes in the environment could drive it to zero.
posted by fantabulous timewaster at 9:39 AM on February 17, 2010 [1 favorite]

In the mid-1980s, the black-footed ferret was completely extinct in the wild and at one point was down to 18 captive individuals. The worry was that the genetic viability was far too compromised by inbreeding to support a viable wild population. Apparently that hasn't happened. I had heard some years ago that the conclusion that the science was leading researchers to was that when a population has a big genetic bottleneck, then the species responds by increasing the mutation rate of genes to try to avoid catastrophic inbreeding. While this is its own obvious risk, in the case of the BFF, it hasn't stopped their recovery, insofar as there are now some 1500 ferrets in around ten independent wild colonies.

I did find this article that has a discussion on the subject. The conclusion states that some species are quite a bit more susceptible to inbreeding depression, but that the ferret looks like it isn't one of them, and that the biggest threats to their continued recovery remain the things that took them to the brink of extinction-- habitat loss and the plague.

By the way, they are also very cute.
posted by norm at 2:09 PM on February 17, 2010

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