Interspecific Hybridization and Introgression in Animal Evolution

I was at a conference in Venice a few weeks ago on "Evolution in the Age of Genomics." The most interesting presentation at the meeting was by Peter and Rosemary Grant, Princeton biologists who have been studying Darwin's finches in the Galapagos for the past three-plus decades. This work is all the more important because these birds, especially their beaks, have been the poster children of Darwinian evolution for a century and a half.

While most population biology is highly theoretical and conjectural, the Grants have been following what has actually been going on in the wild. Theirs is an exciting scientific and human story, including raising and educating their daughters in a tent while making field observations.

What the Grants emphasized, among many fascinating observations, was the major role hybridization and introgression between distinct "species" played in producing genetic variability in the wild populations. (Introgression means the introduction of part of the genome from a distinct species.)

Whenever there was high inherited variability in a particular population, examination of the DNA indicated that it arose from introgression from a different species. The Grants also described the formation of what would be classed as a new finch species resulting from the full hybridization of two distinct species.

In the discussion session following their joint presentation, someone asked why more attention had not been paid to these inter-species genome transfer events. Peter Grant answered, "Ernst Mayr". What Peter meant was the influence of Mayr's theoretical dictum that recently separated species did not interbreed.

Since Mayr was one of the neo-Darwinian giants of the Modern Synthesis, his speculations were taken as accepted fact. The answer prompted someone in the audience to comment, "Great biologists can only impede progress, not stimulate it."

While interspecific hybridization is now widely accepted in plant evolutionary biology, neo-Darwinian theorists like Jerry Coyne continue to minimize its importance in animals: "Polyploidy is a rapid form of evolution and speciation, one that is fairly common in plants, but very rare in animals. (The reason for its rarity in animals isn't understood, but we discuss the theories in the book I wrote with Allen Orr, Speciation."

Examples of introgression and interspecific hybridization in many different animals are accumulating. Documentation of these processes is aided, as in the Grants' studies, by the application of forensic DNA methods to determine the origins of various genome components. Using the same kind of "microsatellite" markers as in criminal investigations, field biologists can use small tissue samples from wild organisms to pinpoint the sources of DNA regions in their genomes.

A recent paper in Nature, "Butterfly genome reveals promiscuous exchange of mimicry adaptations among species" by The Heliconius Genome Consortium describes the role that interspecific DNA transfers play in the evolution of mimetic wing patterns in butterflies. Similar cases have recently been documented in rodents, newts, and flatfishes. It is likely that interspecific hybridization is far more common in animals than commonly believed.

The reason I was particularly interested in the Grants' observations was that they exemplified an overlooked aspect of population behavior that is relevant to natural genetic engineering. Introgression is a form of horizontal DNA transfer, and interspecific hybridization is one of the most important triggers of large-scale genome restructuring by natural genetic engineering. We are beginning to understand the molecular basis of this triggering because interspecific hybridization is also a destabilizing event for the epigenetic controls that regulate natural genetic engineering functions.

While neo-Darwinian theories focus on a "bourgeois" model of mating within well-defined population boundaries, as articulated by major figures like Ernst Mayr, the evidence increasingly tells us that evolutionary novelty comes from the more adventurous and improper ("bohemian") matings that are excluded in many conventional accounts. It is amusing (even if dispiriting) to think that maybe the anti-empirical biases in neo-Darwinian thought and discourse have as much to do with Victorian prudishness as with Victorian philosophical battles.