Evolution. A quick Google search will tell you evolution is “the process by which different kinds of living organism are believed to have developed from earlier forms during the history of the earth”. That answers the what. But who is actually evolving, and when, and why?
Evolution is not about the individual, but about populations and their ability to survive and adapt to changing environmental situations. In order to survive, individuals of that population must reproduce, and in doing so pass on genetic information to their offspring. Evolution is constant, though one of the reasons it has taken so long to become a (more or less) accepted idea in society is because it is also a slow, hard-to-observe process. However, with the constantly evolving field of genetics there is now plenty of evidence to support the theory of evolution. I have recently come across a study of freshwater snails in New Zealand by Britt Koskella and Curtis Lively which provides convincing evidence that evolution, in particular co-evolution, is still occuring.
Co-evolution is a form of evolution, involving more than one species evolving together. Most often this particular evolutionary process is seen in the form of host-parasite interactions. Which is exactly what is going on here. The players in this evolutionary game of cat and mouse are tiny native freshwater snails, Potamopyrgus antipodarum, and even smaller parasitic sterilising trematodes or flukes from the Microphallus species.
The parasitic eggs are present in many of the lakes where the snails live. During feeding the snail may ingest an egg which is then able to infect and reproduce in the snails tissues, forming cysts. These cysts can replace the snails sexual organs, thereby reducing reproductive success. But it doesn’t end there. To complete the parasitic lifecyle the unsuspecting snail is eaten by waterfowl, the final host. The parasitic eggs will then go back into the water when the bird defacates, starting a new cycle.
Koskella and Lively seek “To determine whether parasite populations are able to adapt to infect common host genotypes, and whether this selection pressure on host populations is sufficient to significantly alter the frequencies of different host genotypes in the experimental population”. In plain English, the study wants to test whether evolution is demonstrated through adaptations in the parasite and the host. Because the study focuses on snail populations, this can be tested and demonstrated over a relatively quick period.
At a genetic level all snails are different. But genetics can also be used to determine which snails are the most similar. This is done through the method of genotyping. A genotype is a set of genes that are responsible for a particular trait, for example; eye colour. Check out the personal genetics education project website https://www.pged.org for more information. In this case genotyping was used to identify snails that belong to distinct clonal lineages. This has allowed researchers to determine if the genetic makeup of a snail population changed over time (i.e., whether it evolved), something which cannot be visually identified.
Snails were collected from Lake Alexandrina in the South Island and put in 12 different tanks. Some of these tanks were treated with parasites (recycled), and others were not (control). In order to reinfect the parasite treated snail tanks, snails were collected and analysed to determine infection status. Infected snails were then fed to mice (the final host), and the mice droppings used to reinfect the tanks with the parasite eggs. At generation zero (when the snails were collected from the lake) a subsample was taken for genotyping. Genotyping was also carried after three and six generations.
These graphs compare the results of the parasite treated snail tanks with the control snail tanks that had no parasites at the end of generation six. The various symbols represent different genotypes that were found, where “C” stands for genotypes that were found at a high frequency in the population at generation zero (the initial frequency), and “R” indicates genotypes that were initially rare in the population. (Red circle, C1; Purple square, R1; Yellow diamond, R2; Green triangle, R3; Orange inverted triangle, R4). Each individual symbol represents the results from one tank (population).
Based on this we can see that in the recycled population C1 was present at a frequency of 0.32 at generation zero. Over the course of the experiment this frequency decreased by at least 0.1 in most populations. In contrast the R1 genotype was not detected in the population at generation zero (initial frequency). But by generation six it had increased in frequency in all recycled populations except one.
The slope of the line in the recycled population is much steeper than that of the control, which is almost horizontal. This suggests that there has been a greater change in the frequency of genotypes in the population of experimental snails compared to the control. Indeed statistical analysis concluded there was significant negative frequency-dependent selection ocurring in the parasite infected population and that there was no significant selection process ocurring in the control tanks. This means that the most common genotypes were being selected against in the recycled populations.
This study shows that the most common snail genotype (C1) was being targeted by the parasites as the change in frequency of C1 in most recycled populations was negative. This then allowed other rarer genotypes (such as R1) to become more common in the population, as seen by the mostly positive change in frequency for initially rare genotypes in the recycled population. Though the experiment did not continue past the sixth generation we can speculate that the parasites would eventually begin to target the new common genotype (R1), this would then allow another genotype to become more prevalent in the population. Thereby continuing the cycle.
But this doesn’t just happen in a controlled lab environment. The researchers went back to Lake Alexandrina towards the end of the experiment and collected another set of snails from the same site for genotyping. Genotype C1 was significantly less common in the newly collected field snails compared to the control experiment snails, but there was no significant difference in the frequency of C1 between the parasite infected experimental snails and the freshly collected field snails. This suggests that the field population may be evolving in a similar way (C1 becoming less common) to the lab populations, but at a slower rate.
For further details and more information on the other experiments performed, check out the paper; Evidence for negative frequency-dependent selection during experimental coevolution of a freshwater snail and a sterilizing trematode, by Britt Koskella and Curtis Lively, 2009.