Since the first reports of the origin of a new species of finch on the island of Daphne Major in the Galapagos archipelago appeared, there has been a flood of questions about just what exactly it was that Peter and Rosemary Grant observed, and how their observations relate to the larger question of macroevolution. As many evolutionary biologists (including me) anticipated, creationists and intelligent design ("ID") supporters have moved the goalposts, arguing that they have always accepted that speciation occurs, but that it does not necessarily mean anything for macroevolution, especially if one defines "macroevolution" as the origin of higher taxa (i.e. taxonomic categories above the level of species). So, what did the Grants observe, and how are their observations related to the larger question of the origin of higher taxa (i.e. macroevolution)?
The answer is that this long-term research project has provided direct evidence for the initial stages of macroevolution in the field. To be precise, what is at issue in the research reported by the Grants is what is known as “secondary contact”. This is what happens after a sub-population has become reproductively isolated from the population from which it was derived. According to Theodosius Dobzhansky and Ernst Mayr (two of the founders of the “modern evolutionary synthesis”), speciation is the result of genetic isolation resulting from geographic isolation: the members of two geographically separated populations of organisms no longer interbreed, and therefore genetic differences between the two populations accumulate over time.
This process, commonly known as allopatric speciation, can be considered to consist of six discrete, successive stages:
1) Vicariance: A subpopulation (in this case, a couple of finches) becomes geographically isolated (on Isla Daphne Major) from its former panmictic conspecifics (i.e. the species Geospiza fortis on Isla Santa Cruz, a neighboring island);
2) Divergence: The genomes of the members of the vicariant subpopulation diverge from the genomes of the members of the panmictic source population as the result of various genetic mechanisms (for a list of such mechanisms, click here);
3) Reproductive Isolation: The reproductive anatomy, physiology, and behavior of the members of the vicariant subpopulation diverge from the reproductive anatomy, physiology, and behavior of the members of the original source population, resulting in reproductive isolation and (eventually...at least sometimes) reproductive incompatibility;
4) Secondary Contact: Successful hybridization between members of the diverging sub-population and the original source population decreases in frequency as the result of pre-zygotic and post-zygotic isolating mechanisms (for more, click here);
5) Reinforcement: Hybrids continue to decrease in frequency as non-hybrids increase in frequency as the result of microevolutionary mechanisms (i.e. mutation,natural selection, gene flow, genetic drift, and inbreeding depression), resulting in reinforcement of reproductive isolation and species boundaries; and
6) Maintenance: Species incompatibility is continuously reinforced via pre-zygotic and post-zygotic isolating mechanisms, resulting in continued genotypic and phenotypic divergence.
This is why Alfred Russell Wallace entitled his paper (which he mailed to Darwin in April 1858), “On the Tendency for Varieties to Depart Indefinitely from the Original Type”.
Note that none of these stages is absolutely defined; rather, they integrade in what Darwin characterized as an “insensible series”. Also note that stages 4 through 6 can be condensed into one stage (i.e. “reinforcement”), in which reproductive incompatibility increases steadily over time. Finally, some evolutionary biologists (most notably C. H. Waddington, Mary Jane West-Eberhard, Eva Jablonka and Marion Lamb) have proposed that stages 2 and 3 probably happen in reverse order (a process known as genetic assimilation).
This is the theoretical model; what actual empirical studies have shown is that diverging phylogenetic lines frequently become reintegrated, separating and then re-integrating more than once. Sometimes they become sufficiently reinforced that they remain separate and diverge continuously, and sometimes they “collapse” back into a single, panmictic “species”.
The importance of all of this to the theory of macroevolution is that divergence is divergence: phylogenetic divergence via reproductive isolation is macroevolution. Speciation is simply the first stage in the origin of all higher taxa.
Therefore, what is ultimately at issue between evolutionary biologists and creationists (including most ID supporters) is not speciation per se nor the mechanisms by which it occurs or is reinforced, but rather whether there are “natural” limits to the degree of divergence that can take place as a result of the mechanisms that comprise the “engines of variation”.
Despite much posturing on both sides, this is not a question that can be answered via pure theoretical (i.e. mathematical) speculation. However compelling a theoretical model may appear, it must be tested empirically to see if it conforms to the evidence from nature. This is what evolutionary biologists do all the time, and what ID theorists seem either unable or unwilling to do. Until this situation changes (if it ever does), no reputable empirical scientist anywhere will ever take ID seriously.
************************************************
As always, comments, criticisms, and suggestions are warmly welcomed!
--Allen