A few quick definitions, I find that people don’t understand evolution because they don’t know the nuances of the definition. First evolution is change over time. No direction, no getting better, no superiority complex, just change. This is important because random mutations occur in populations and even if they aren’t beneficial they are still considered evolution (we call these random mutations genetic drift). Natural selection is a pressure that drives evolution in a direction (this is the heart of what Darwin and Wallace described in their research). When a habitat changes (gets dryer for example) an organism has to change (evolve) to survive the drought (a plant might have leaves with less evaporation). Any individual in the population that already has a random mutation that has less evaporation will be selected for, making that individual more successful and better able to pass its genes on to the next generation. So now you know that change (evolution) can happen by a pressure (natural selection) that makes an organism better able to survive and reproduce (the real currency in biology is reproduction because this is how genes persist) in a changing landscape.
Ok the lizards research, what did they discover? This study creatively used an occurring invasion by a novel lizard (Anolis sagrei) on a set of small spoil islands in Florida to test the impacts on a native lizard (Anolis carolinensis). These two species of Anolis eat the same thing, this creates a novel competition that the native lizards were never previously exposed to, a new selective pressure. This forced the native lizards to perch higher in the tree canopies. However, the features of these trees change as you go higher in the canopy, the branches are thinner and the bark is smoother. This makes it hard for the native lizards to hold on. The researchers found that over 10 years the native lizards changed their feet, they had larger toe pads and more lamellae (little scales on their feet that allow them to cling onto smooth surfaces better). This is incredible, in only 10 years these lizards evolved to take advantage of unoccupied space.
Ok into the nuts and bolts. So there are a few important considerations to understand when talking about natural selection. First, the absolute time is not really important, selection happens over generations, it is effectively selection of the fittest over many generations (this is because selection happens on heritable traits). So the researchers estimate that it took approximately 20 generations for these lizards to evolve these adaptive feet. This would only be a few weeks for some bacteria, or 500 years for humans (human generation time is somewhere between 20 and 30 years so for this estimation I’ve used 25 years). While this study is an impressive illustration of how fast evolution can work, the results are so striking because these researchers chose an animal with relatively short generation times.
There are some questions that I have that directly contribute to the speed of change in this system, but they aren’t mentioned in the study. What is the variation in toe size and lamellae number in a natural population? This gets at the very essence of how selection works. Natural variation is the raw material of selection. If there were a wide variety of toe sizes in the lizards before the introduction of the invader, there would be a lot of raw material for natural selection to act on. However, if there was very low frequency of larger toes in the native lizards, you could imagine that it would take longer for these traits to develop. Also what is the selective pressure of competition? This is a very difficult thing to measure, how “powerful” is competition in driving change over time. In many ways this study shows that competition is a strong selective pressure. But we don’t know the relative effect in relation to other factors, novel diet, changing environment, etc. These questions don’t discount the importance of this published work, they just highlight important nuances that might have changed the rate of change in these lizards.
One of the reasons I really like this paper is the researchers’ approach to testing WHY the toes changed size. Instead of assuming that invasive lizards drive changing behaviors in the native lizards the researchers tested other potential explanations. First the authors tested whether toe size and lamellae number were variable during the life of a lizard but not necessarily heritable traits. To do this they took eggs from parents exposed to invaders and not exposed and raised newborn lizards in the same environment, called a common garden. Lizards from parents exposed to the invaders always had larger toes and more lamellae regardless of the environment they were raised in, showing a heritable difference among the lizards.
Next the researchers tested whether lizards with larger toes had migrated to the islands. Using genetic techniques that measure the number of mutations in gene sequences they found that each island contained lizards that were more related to each other than to lizards on any other island. The researchers were able to measure 121,973 mutations in these lizards. This is an amazing amount of information, previously science relied on a handful of mutations to identify population level differences, but recent technology has allowed sequencing of many more mutations. This created a very strong test for lizard relatedness within and between islands. The researchers did find a few lizards that had migrated from different islands but overall there was a higher level of relatedness within an island and the islands were very distinct from each other. This is a very exciting result and this data could also be used to identify which genes influence toe size and lamellae number, I expect there will be some more interesting discoveries that come from this data.
Using multiple experiments this study is a powerful example of rapid evolution in response to an invasive species. This is an exciting study because it is actively occurring on a time scale that we can observe and in a real world situation. Many tests of evolution have been conducted in the laboratory but it is difficult to apply laboratory results to complex natural habitats. The invasion of lizards to the spoil islands in Florida offer a great natural experiment to study the mechanisms and speed of lizard evolution.
Here is the full citation for this paper:
Stuart, Y, Campbell, T., Hohenlohe, P., Reynolds, R., Revell, L., Losos, J. (2014) Rapid evolution of a native species following invasion by a congener. Science 346: 463-466.