Conservation science, like all biological fields, is underpinned by evolutionary theory. Overharvesting, habitat degradation and destruction share a number of features which are key components and processes in natural evolutionary biology. Species which are overharvested or which live in disturbed environments often show a reduction in population size, a reduction in genetic diversity and reduced gene flow between smaller populations. These are known to have a number of effects. A reduced population size means that mutations are more likely to be lost or fixed by chance rather than selection, which can impede adaptive responses to selection pressure. Reduced genetic diversity can do the same, and increased inbreeding can lead to the accumulation of mutations which negatively affect the health and survival of individuals in a population, putting them at further risk of extinction.
Isolation of populations can have other effects over and beyond gene flow. Whilst in some circumstances it may increase the rate of local adaptations, it can also lead to an increase in inbreeding. It can also mean populations becoming so isolated that random fluctuations in population size cannot be stabilised by immigrants so the populations is at greater risk of dying out.
Invasive species face similar problems when they first colonise a new habitat. Their population size will usually be small, probably with reduced genetic diversity and little or no gene flow with other populations. Because of this, immediately after a colonisation event population numbers of the invasive species remain small. But if they are able to persist and adapt to the local environment they may then rapidly increase in population size.
So, to combat invasive species we should tackle them early on in their colonisation event before they get past the problems of having a small population. To conserve species which face over harvesting and habitat destruction we need to help them overcome these same problems. Our understanding of evolution can help us to do this.
For example, studies of micro-arthropods in artificial moss ‘islands’, and experiments on butterflies, have shown that habitat ‘corridors’ can be used to inter-connect patches of undisturbed regions which would otherwise be isolated from each other. This has been shown to promote gene flow and increase the effective population size and immigration/emigration rates. This practice has the potential, if widely implemented, to reduce local extinction rates.
A common strategy to conserve species threatened with extinction is to create a captive or wild ‘refuge’ population to use as a kind of backup in case wild populations go extinct. These refuge populations also suffer from reduced population sizes, and may evolve away from optimal fitness for their wild habitat. To prevent this, policies which allow some gene flow between wild and refuge populations could be implemented.
It may also be possible to use these refuge populations to create a population which is better adapted to the changing conditions in the wild than are wild populations. By artificial selection or by exposing them to less extreme changes over a number of generations, when released, these individuals may be better able to survive in the wild.
Finally when boosting dwindling wild populations with individuals either from refuge or other wild populations we also need to consider what effects this will have. Perhaps the dwindling population has some local adaptation, or has purged most of its deleterious mutations! Introducing foreign individuals could break up local adaptation or spread new deleterious mutations amongst a small population so leading to inbreeding depression.