Jul 15 2008

Tasmanian Devil Evolution

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Growing up on Bugs Bunny this was my image of a tasmanian devil. Only later in life did I come to know the real animal, through books, documentaries, and now the internet. The tasmanian devil is the largest marsupial carnivore in the world (after the extinction of the tasmanian tiger).

For the past decade the tasmanian devil population has been decimated by an unusual disease – an contagious cancer. Devils commonly bite each other in the face as part of their social interaction, especially during mating. A facial cancer that can be spread through biting has reduced the devil population by about 90%. The cancer is fatal because it eventually prevents the devil from feeding. This has prompted breeding programs in captivity to rescue the species. It is believed that the cancer is able to spread in this fashion partly because the devil is so in-bred, therefore genetic diversity is low and devil’s immune systems do not reject what amounts to a tissue graft.

There are several distinct populations of tasmanian devils in Tasmania, all but one are affected by the cancer. There are two populations held in captivity as insurance. Biologists are also working on a vaccine for the cancer. The clock is ticking, however, because it is feared that the red fox, an invasive species, may take over the tasmanian devil’s habitat while the populations are low.

But, evolution may succeed first without the need for human intervention.

Devils typically reach sexual maturity at two year and then have multiple litters over the next few years, reaching old age by 5-6. But the cancer is decimating the older breeding populations. Rare precocious females reached sexual maturity at one year of age. But now the cancer is producing a new selective pressure on affected populations. A new study soon to be published in PNAS looked at tasmanian devil populations and found that the number of females reaching maturing and breeding at one year of age is increasing.

From the abstract:

Devils have shown their capacity to respond to this disease-induced increased adult mortality with a 16-fold increase in the proportion of individuals exhibiting precocious sexual maturity. These patterns are documented in five populations where there are data from before and after disease arrival and subsequent population impacts. To our knowledge, this is the first known case of infectious disease leading to increased early reproduction in a mammal.

Very interesting. The full article is not yet available. One question I had was how much of the increase in proportion of precocious females was due to the death of older devils vs an increase in the absolute numbers of young-breeding females. The notion that the devil is adapting or evolving in reaction to this new environmental stress implies the latter.

There is some interesting discussion of the study on this site.  Two questions are raised Рthe first is whether or not a change in reproductive age is genetic (and therefore subject to evolutionary pressures) or merely a response to the environment. This is an interesting question. If the devils already have hormonal or other biological mechanisms by which females reach maturity faster, perhaps due to lack of presence of many other females, then this change would not necessarily represent any change in gene frequency but rather just a hormonal response to the environment. As an analogy Рmodern human populations grew taller with better nutrition Рthis is not an evolutionary change but rather an environmental one.

The second question¬† – assuming that breeding age is determined solely or mostly by genetics, does such a change actually represent evolution?¬† Some objected to use of the world “speciation” to such a change, and I agree that this term is no appropriate in this case, even assuming a purely genetic change. That implies a permanent separation of populations. However, I disagree with those who argue that this is not evolution.

At its most fundamental level, evolution is about changes in gene frequency within a population. It does not have to involve a new mutation. The kind of change being documented in wild devils is exactly the stuff of evolution. It also demonstrates that populations frequently contain occasional or rare members with variations that can, by chance, prove to be adaptive to new situations. The raw material for possible evolutionary change is inherent in the diversity of a population. This is exactly why genetic diversity is so important to the long term survival of a species – and why we should be concerned about the loss of species diversity.

I certainly hope the tasmanian devil survives this challenge – with human help and with a little evolutionary luck. There is concern that if the red fox is allowed to take over former devil habitat they will wreak a devastating further extinction on Tasmanian wildlife. But also, Taz will always have a soft-spot in my heart – ever since I first saw him on Bugs Bunny.

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