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Posts Tagged ‘drosophila

When adaptation doesn’t happen

“Evolutionary biology has been enriched by considering not only how adaptation happens, but also why it often does not happen,
or at least does not happen as we might naively expect.”

Douglas Futuyma (2010)

In 2005, a group of scientists from La Trobe University in Australia investigated how species will adapt to global warming by studying a species of rainforest fly, Drosophila birchii (later published in Science).  Increased temperatures may lead to drier conditions in rainforests, so the authors wanted to see how quickly this fly could adapt and develop resistance to desiccation.

As in many directed evolution experiments, they took the flies to a lab and exposed them to dry conditions, with most of the flies unable to survive the dryness.  The survivors were then bred to one another.  Thus generation after generation, only the most desiccation resistant flies would survive, resulting in a population that could survive dry conditions potentially induced by global warming.  Right?

Well, that was the idea at least.  At first, their population of flies did show a bit of increased resistance.  However, in the proceeding 28 generations of selection, there was no increased resistance to desiccation, despite that previous papers had found increased resistance in 3 other Drosophila species.  Drosophila birchii was unable to adapt to these conditions.  What went wrong?

The assumptions of natural selection

Natural selection itself is based on three assumptions in a population.  The first is that there will be variation in traits, such as multiple colors of eyes or hair.  The second is that these traits be heritable through the generations, that children will inherit the traits of their parents.  The third is that these variable traits have differential fitness, or that some versions of a trait might help you survive better than another.  Thus certain trait variants will help its carrier organism survive better, passing that trait to its offspring which will in turn bear this trait.

In order for populations to adapt by natural selection, these three requirements must be fulfilled.  When a biologist sees any population, he or she typically assumes that these they are met, and I can’t really blame them.  All cellular life we know of on this planet has a hereditary mechanism, the gene, which has differential fitness depending on the variation, thus meeting requirements two and three.

But what about requirement one, genetic variation in a population?  It has frequently been assumed to also exist in all populations.  In his 2010 review, Douglas Futuyma quotes the famous geneticist Richard Lewontin, who concluded in his 1974 book that “genetic variation relevant to all aspects of the organism’s development and physiology exists in natural populations,” for “[t]here appears to be no character—morphogenetic, behavioral, physiological or cytological—that cannot be selected in Drosophila.”

The idea here is that at any point in a healthy population, there will be many variants of genes in the population, which are replenished infrequently by new mutations.  Thus, when selective pressures come around, there is a full stock of variants to differentially survive and lead to new adaptations.  This is why small populations and inbreeding are considered such problems: the small gene pool means fewer gene variants and an increased inability to adapt to new conditions.

But why should this have to be true?  Is a lab-bred stock of Drosophila really enough to show that there is variation in natural populations, when the lab has a completely different set of selective pressures?

Selection bias in evolutionary studies

When the researchers studying the rainforest Drosophila birchii analyzed the genetic diversity of their collected populations, they found very low variation in the desiccation gene group compared to other genes in the population such as wing size.  This lack of variation prevented the flies from adapting to new conditions – though the researchers weren’t sure why.  Perhaps these dessication genes had (relatively) recently been under selective pressure, and had not had time to reconstitute the genetic diversity in the population.

Unfortunately, there are very few studies of this kind so it’s hard to draw any conclusions.  After all, people studying evolution usually want to study EVOLUTION IN ACTION and not evolution when it doesn’t happen.  I suspect that past experiments that have found low genetic diversity have been tossed simply because it seems less interesting evolutionarily – when maybe it is in fact more interesting.

This would be a form of selection bias: the choice of study populations or species unconsciously (or consciously) swayed by the desired outcome of EVOLUTION IN ACTION.  In his 1991 Croonian Lecture at the at the Royal Society, Anthony Bradshaw gave a compelling example of this sort of selection bias in evolutionary studies (published here).

In Prescot, Merseyside, a copper refinery opened up next to a meadow that had many species of grasses and wildflowers.  Over time, the soil was contaminated by copper, killing off many of the plants.  However, after 70 years, there were 5 species that were still able to thrive in these meadows!  They had adapted and developed a resistance to copper.

Even now I’m thinking to myself, “oh! cool!  How did they adapt that way?  How did that mechanism work?  How quickly did the resistance gene spread?” etc.  But what I’m forgetting is that, while five species did adapt, twenty-one species failed to adapt.  If there really was massive diversity at all genes in each population, you would think that at least one would confer some benefit to survive the copper.  But this did not happen: twenty-one species went locally extinct.

Just as interesting a question as “How did these 5 adapt?” is “Why did these 21 fail to adapt?”  But it’s a question that’s only begun to be reconsidered.  Death and extinction are far more powerful forces in shaping the whole of biodiversity on our planet than successful adaptation, but these evolutionary failures that occur all around us are little studied.  Especially as we anticipate global climate change causing unknown impacts on species worldwide, we should be studying non-evolution to get a better sense of what natural genetic variation actually looks like and what we may be facing in the future.

Literature Cited

Bradshaw, A. (1991). The Croonian Lecture, 1991: Genostasis and the Limits to Evolution Philosophical Transactions of the Royal Society B: Biological Sciences, 333 (1267), 289-305 DOI: 10.1098/rstb.1991.0079

Futuyma, D. (2010). EVOLUTIONARY CONSTRAINT AND ECOLOGICAL CONSEQUENCES Evolution, 64 (7), 1865-1884 DOI: 10.1111/j.1558-5646.2010.00960.x

Hoffmann, A. (2003). Low Potential for Climatic Stress Adaptation in a Rainforest Drosophila Species Science, 301 (5629), 100-102 DOI: 10.1126/science.1084296

Written by Hanner

January 5, 2011 at 10:58 am