Broken barriers

[ This entry was written by Erika Crispo; I am just posting it.  I added the images and captions, so if they are screwed up somehow, that's not Erika's fault.  BCH ]

Humans are directly affecting wildlife through various activities, leading to declines in the numbers of individuals, populations, and species. It is less clear, however, how these activities are indirectly affecting wildlife through impacts on the genetic constitution of populations and species. In a recent review paper, we identify two general ways in which human activities can influence genetic exchange.

Humans may be reducing post-zygotic reproductive isolation in the Medium Ground Finch of the Galápagos Islands by augmenting the food supply.  Photo by Putney Mark.

First, lineages that were previously isolated by geographic barriers can be put into contact, thus allowing genetic exchange should these lineages be able to interbreed. This might occur if (1) humans alter landscapes such that increased dispersal is possible, (2) anthropogenic climate change allows increased dispersal into areas that were previously uninhabitable, or (3) humans introduce individuals directly (intentionally or unintentionally) by physically moving them.

A second way in which humans can influence genetic exchange is by disrupting reproductive barriers, such that lineages that were previously reproductively isolated in sympatry can now interbreed. This might occur if (1) the sensory (e.g. visual, chemical, auditory) environment is altered so that individuals are no longer able to recognize and chose suitable mates, or (2) the adaptive landscape is altered so that selection against hybrid individuals no longer occurs.

Habitat degradation due to human activity in Lake Victoria (East Africa) has led to biodiversity loss through hybridization between the endemic haplochromine cichlids there, a process called reverse speciation.  Photo by Haplochromis.

Whether genetic exchange has positive or negative effects on fitness and population viability is context-specific. For example, negative effects of genetic exchange should occur if populations are locally adapted and gene flow results in the incorporation of maladaptive alleles into the populations. On the other hand, positive effects of genetic exchange would occur for small, isolated populations by increasing genetic variability and thus evolutionary potential. An additional consideration is that increased gene flow and decreased reproductive isolation could lead to genomic extinction, i.e. extinction of the unique combinations of genetic material that are characteristic of a particular species or lineage. For conservation and management purposes, we argue that it is important to take a gene-centric approach, by appraising not only the direction and amount of genetic exchange, but which genes are being exchanged and their relevance to the sustainability and uniqueness of the populations in question.

For further information and empirical examples, please see our manuscript:

Crispo, E.*, Moore, J.-S.*, Lee-Yaw, J.A., Gray, S.M., and Haller, B.C. (2011) Broken barriers: human-induced changes to gene flow and introgression in animals. BioEssays, 33, 508-518.
*Both are first authors