June 2006

Ever wonder why the Amazon is loaded with different species while the Antarctic boasts just a few? Well, so did New Zealand biologists Shane Wright, Jeanette Keeling, and Len Gillman — and the answer they discovered would be no surprise to any sun-worshipping tourist: duh, it’s the weather! The team compared DNA from tropical plant species with the DNA from species in cooler climates and found that the tropical species’ DNA had evolved about twice as fast as that of their cold-weather cousins! The researchers chalked this difference up to the warmer weather of the tropics — which encourages lush growth, higher metabolic rates, and hence, higher rates of mutation. Those high mutation rates could boost the pace of speciation — resulting in a myriad of tropical species.

Where's the evolution?

The researchers on this study looked at a fundamental question in evolution: why are there so many different species — and specifically, why are they concentrated at the equator? But answering a question like this can be tricky. After all, the species that currently inhabit the tropics evolved thousands or millions of years ago — how can we learn about that deep history from the evidence available today? The research team based their approach on the evolutionary clues passed down from generation to generation in the genes of different species.

Biologists first noted the diversity spike in the tropics several hundred years ago. More recently, biologists have found that metabolic rate (the speed of chemical reactions occurring within the body) is directly related to temperature: the higher the temperature, the higher organisms’ metabolisms will be simply because biochemical reactions occur more quickly at higher temperatures. The New Zealand team focused on one main hypothesis that links those two observations: tropical species evolve more quickly than cold-weather species because higher temperatures lead to higher mutation rates. This is a reasonable hypothesis since warm-weather organisms likely have higher metabolic rates, and some substances involved in metabolic reactions can cause DNA damage, potentially leading to a mutation. Mutations, in turn, increase genetic variation, the raw material of evolution. So warm weather means higher metabolism, which means higher mutation rates — which may mean that warm-weather species evolve more quickly than cold-weather species.

To study their hypothesis, the New Zealand team compared the number of mutations accumulated by warm-weather plant species to the number of mutations accumulated by cool-weather plant species. But figuring out those numbers required some detective work. The researchers studied pairs of closely related, similar plant species where one species lives in the tropics and one lives in a more temperate climate. They sequenced the same stretch of DNA in all of these plant species and compared those to the sequence of a more distantly related plant, called the outgroup (see below).

All three species share a common ancestor with the ancestral sequence some time in the past, and since then, each lineage has picked up its own set of mutations that make its sequence unique. In the example below, the outgroup has accumulated three mutations that set it aside from the other species. The tropical and temperate species, on the other hand, both inherited the same two mutations from their immediate common ancestor — but the tropical species has four mutations that belong to it alone and the temperate species has only one mutation that belongs to it alone. Now if we compare the differences in sequence between the tropical species and the outgroup (yellow highlighting below), we see that they differ at nine spots (four of which occurred in the tropical species alone). However, the temperate species and the outgroup (green highlighting) differ at only six spots (one of which occurred in the temperate species alone).

By comparing each species to the outgroup, the researchers could figure out which species (the tropical or the temperate) accumulated more unique mutations. In this case, the tropical species (which differed from the outgroup at nine spots) accumulated more mutations than the temperate species (which differed from the outgroup at just six spots).

The New Zealand team did this comparison for 45 pairs of species and found that tropical species accumulated many more mutations than temperate species — about twice as many! That evidence strongly supported their hypothesis that higher temperatures cause higher mutation rates, and hence, faster evolution — at least in the plants they studied.

Of course, this work represents just one piece of the puzzle: many alternative hypotheses have been proposed to explain the tropics’ magnificent diversity, and more work is needed to figure out the most accurate explanations. However, if this general hypothesis (that higher temperatures and mutation rates lead to faster evolution in the tropics) holds true for other groups, it has some important implications. For example, if global warming continues, it could speed up the pace of evolution on Earth!