Conservation efforts usually aim to bring in more individuals to increase the population size of endangered species. This is done with a view of promoting gene flow. Such a move is based on the understanding that bringing in (migrating) more members of the endangered group will lead to a greater import of genetic variation and thereby a greater variation in the focal population.
Experiments done by researchers from the Indian Institute of Science Education and Research (IISER), Pune, indicate that in contrast to the simple plan outlined here, other factors, namely, the nature of the environment and the genetic constitution of the immigrants can play a role in deciding whether the evolutionary outcomes of immigration are beneficial. The study has been accepted for publication in Journal of Evolutionary Biology.
Role of environment
In the experiments, the researchers subjected populations of Escherichia coli that were adapting to complex and fluctuating environments to migration of colonies from outside. These colonies consisted of varying proportion of clonal ancestral immigrants. That is to say, the immigrants consisted of a mix of clonal members and those carrying genetic variation. In parallel, the researchers studied the effects of varying the ratio of clonal members to those carrying genetic variation.
One of the major studies done in this field, published in 2007 in the journal Proceedings of the Royal Society B: Biological Sciences, showed that when bacterial cells were subjected to high level of stress — so high that it was lethal — then increasing the levels of migration increased the chances of survival in the stress environment. This implied that migration promotes adaptation.
However, the present study looks at more general conditions and yields a more complex result. They show that the results of the 2007 study do not hold true when the bacteria is subjected to non-lethal but stressful environment.
“When the stress environment does not lead to complete mortality, the effects of migration on adaptation is reversed,” says Sutirth Dey from IISER, Pune, in whose lab the experiment was conducted, in an email to The Hindu. This means that increasing the proportion of immigrants actually decreases the adaptation.
Genetics crucial
S. Selveshwari, from IISER Pune and an author of the paper, adds, “The genetic makeup of the immigrant individuals is another crucial factor in determining the effects of migration.”
She explains that in contrast to clonal immigrants (those that do not have much of a genetic variation amongst themselves), immigrants carrying greater amount of genetic variation can counter the negative effects of migration in such environments.
A most likely explanation for the contrast in evolutionary outcomes between the two experiments may be the following, according to Prof. Dey.
The effect of migration is not just a property of the evolving individuals but results from an interaction between the nature of their environment and the genetics of the immigrating individuals. Unlike in the 2007 study where the bacterial cells were subjected to lethal concentrations of antibiotics, the present study subjected bacterial cells to fluctuating environments that are stressful but not lethal stresses.
“We also subject our bacterial cells to broad-spectrum stresses such as oxidative stress, osmotic stress and pH stress, whereas [in the other work] antibiotic stresses were much narrower,” says Prof. Dey.
