AUTHOR: Daniel Cressey
SOURCE: Nature.com News
COMMENTARY: Allen MacNeill
As I have noted in several recent blogposts, Charles Darwin's Origin of Species
was published 150 years ago this month
. One of Darwin's crucial examples of descent with modification in the Origin
was the evolutionary diversification of a group of finches now usually referred to as "Darwin's finches"
. In the Origin
Darwin did not speculate as to how long this evolutionary diversification took place, except to suggest that it would require "the passage of long ages". However, in a private letter to one of his correspondents, Darwin suggested that it would take at least fifty years to see unambiguous effects of natural selection. A recent publication in the Proceedings of the National Academy of Sciences
lends empirical support to Darwin's suggestion. Peter and Rosemary Grant
have been studying the finches of Daphne Major
, a small island in the Galapagos archipelago
since 1973. In PNAS
, they have proposed that a population of finches on Daphne Major may be on the verge of becoming a new species of finch. Here's how their proposal was reported at Nature.com News
"It was in 1981, that the Grants spotted an unusually heavy medium ground-finch (Geospiza fortis). At 29.7 grams, the male was more than 5 grams heavier than any they had seen on Daphne Major before. Genetic analysis showed that it probably came from the neighbouring island of Santa Cruz.
The Grants numbered the bird 5110 and followed it and all its known descendants over seven generations. Many of its descendants stuck out from the other G. fortis on Daphne Major: they had unusually shaped beaks and their songs differed from those of the other finches.
In the fourth generation, a severe drought hit the island and 5110's descendants were reduced to one male and one female — a brother and sister. From then on the immigrant lineage isolated itself, breeding with no other G. fortis on the island....
"No study of this sort has been done before, and it shows one way in which speciation can get started," say the Grants from Japan, where they are receiving the Kyoto Prize for basic science for their life work."
Several further points from the Grants' report are significant. Many evolutionary biologists (including Tijs Goldschmidt
) have speculated that sexual selection
may be a significant cause of evolutionary diversification, including the origin of new species as the result of female choice. It appears that sexual selection has played an important part in the differentiation of the incipient species of finch on Daphne Major:
"The fact that 5110's descendants haven't mixed could be because they differ from the natives. The Grants note that the descendants have a differently shaped beak from those native to Daphne Major. As finch beaks are vital in identifying potential mates, this could serve to keep them reproductively isolated.
5110's offspring also have the avian equivalent of a strange accent. These finches learn their songs from their father, and the Grants suggest that 5110 sang the songs from his birth home of Santa Cruz then modified his come-hither ballad by roughly copying the Daphne Major birds'. This imperfect copying, they suggest, has over time acted as a barrier to interbreeding.
Also, the Grants' research has illustrated an important point concerning Darwinian speciation. As Darwin pointed out in the Origin
, the distinction between varieties, subspecies, and species are "entirely arbitrary", and ultimately depend on reproductive isolation:
"The Grant's aren't yet ready to call 5110's lineage a new species, a term fraught with difficulty for evolutionary biologists. "There is no non-arbitrary answer to the question of how many generations should elapse before we declare the reproductively isolated lineage to be a new species," they say. "For the present it is functioning as a [separate] species because its members are breeding only with each other."
The Grants think there is only a small chance that 5110's descendants will remain isolated long enough to speciate. If they do, the new species will have to be named: "When discussing these birds we call them 'big birds'," the Grants say. "That could be translated into Latin."
According to the biological species concept
, since 5110's descendants are not interbreeding with the other finch species on Daphne Major, they should already be considered to be a separate species. If at some point in the future they do interbreed with the other species of finches, this would not violate their status as a distinct species, any more than the hybridization between blue-winged warblers (Vermivora pinus) and golden-winged warblers (Vermivora chrysoptera)
indicates that these two species are not "genuine" biological species.
Finally, the observation that this incipient species of finch resulted from the consanquineous mating of a pair of full siblings lends support to my proposal that speciation can be facilitated by first degree inbreeding
. Here's what I wrote about this proposal on Thanksgiving in 2006:
"Wouldn't [the genetic rearrangements usually accompany reproductive isolation] have to occur within at least two members- one male, one female- of the same population in order for it to have any chance of getting passed on? And therefore, wouldn't this make such genetic rearrangements difficult, if not impossible to pass on?"
To which I answered:
No. All that would need to happen to make this possible would be for two first-degree relatives carrying the genetic rearrangement to mate and have offspring. First degree relatives (i.e. parents and offspring or full siblings) can easily have the same chromosomal mutation (i.e. a fusion, fission, translocation, or inversion), as they would inherit it from a single parent. If they were to mate with each other (a not uncommon event among non-humans...and even among some humans), they would be able to produce fertile offspring carrying the same chromosomal mutation.
Yes, it is true that first degree mating carries with it the possibility of reinforcement of recessive lethal alleles. However, as many geneticists and evolutionary biologists have repeatedly pointed out, this is actually beneficial to the population within which such reinforcement happens, as the alleles are "purged" from the population as a result.
In other words, mating between first degree genetic relatives within a small, isolated population would have the effect of both removing deleterious alleles from the population and allowing chromosomal mutations to spread throughout the population, especially if such mutations were at all beneficial (although they would diffuse almost as well if they were selectively neutral, as would probably be the case given that no change in overall genetic information would have occurred).
Furthermore, the hypothesis that I have presented above squares very well with the currently prevailing theory of speciation: that of peripatric speciation
, as first proposed by Ernst Mayr
. According to Mayr's theory, speciation occurs most often in small, isolated populations on the periphery of large, panmictic populations. There is abundant natual history evidence that this is the case, especially in animals.
However, to my knowledge no one has yet proposed a mechanism explaining how peripatric speciation would come to be associated with the kinds of chromosomal changes that are commonly associated with reproductive isolation and speciation. My hypothesis – that first-degree inbreeding facilitates chromosomal speciation – is an attempt to reconcile those two observations.
In a large, panmictic population, selection would tend to eliminate individuals who mate with first-degree relatives as a result of decreased viability due to inbreeding depression and the increased frequency of expression of homozygous lethal alleles.
However, in very small, isolated populations individuals who occasionally mate with first degree relatives (i.e. "facultative first degree inbreeders") could easily have a selective advantage of individuals who avoid mating with first degree relatives (i.e. "obligate outbreeders").
Males in particular would tend to loose less as the result of mating with first degree relatives, as their parental investment in offspring is lower (i.e. they can waste gametes and even zygotes by mating with their first degree relatives, without significantly decreasing their reproductive success).
However, even females can cut their losses by mating with first degree relatives if the likely alternative is failure to mate at all due to unavailability of non-relatives. This would especially be the case in small, isolated populations, which are exactly the kind of populations in which speciation is most likely to occur.
The effects described above would be facilitated by increased genomic homogeneity, such as would result from genetic bottlenecks and founder effects. This is because close inbreeding intensifies genomic homogeneity and decreases genetic variation, especially in isolated populations with decreased gene flow from other populations.
This hypothesis – that first degree inbreeding facilitates chromosomal speciation – immediately suggests a series of predictions, all of which are empirically testable:
• The frequency of mating between first degree relatives should be inversely correlated with effective breeding population size. That is, the smaller the effective breeding population, the greater the frequency of mating between first degree relatives (i.e. “first degree inbreeding”).
• The increased frequency of “first degree inbreeding” in such populations should be more pronounced in males. That is, males should be more likely to attempt mating with first degree relatives, especially in small, isolated populations.
• The frequency of “chromolocal mutations” (that is, chromosomal fission/fusion/inversion/translocation mutations) should also be inversely correlated with effective breeding population size. That is, the smaller the effective breeding population, the greater the frequency of viable “chromolocal mutations.”
• Peripatric speciation events should be correlated with small population size, chromolocal mutations, and first degree inbreeding.
• Speciation resulting from chromolocal mutations should be much less common in large, panmictic populations.
• First degree inbreeding should also be much less common in large, panmictic populations.
• The success rate of artificial (i.e. facilitated/forced) first degree mating should be directly correlated with the degree of inbreeding. That is, the more inbred a population, the more successful artificial first degree inbreeding should be.
• Paleogenomic analysis should find close correlations between genetic bottlenecks, founder events, and peripatric speciation events and the frequency of chromolocal mutations and genetic homogeneity (resulting from first degree inbreeding).
• Relatively large changes in phenotype resulting from chromolocal effects should be more common in small, isolated populations.
• Speciation should be easier (and therefore more frequent) among asexually reproducing eukaryotes, such as plants and parthenogenic animals (among whom aneuploidy is largely irrelevant).
Let me stress two things about the foregoing:
• What I am suggesting is, at this stage, merely a hypothesis, but one that generates a series of immediately testable predictions.
• The hypothesis is, of course, based on the idea that incest (i.e. first degree inbreeding) is the most likely explanation for the diffusion of chromolocal mutations throughout small, isolated populations of animals. Let me stress as strongly as possible that I am NOT advocating incest, I am simply pointing out that first degree inbreeding would facilitate the kind of chromolocal mutations that are often correlated with species differences in animals. The same is also true for plants, of course, but in plants we don't call it "incest," we call it "self-pollination."
At the time that I proposed this hypothesis in November, 2006, I was a little perplexed at why no one has yet proposed this mechanism, given the fact that it is already used as the explanation for speciation in plants via polyploidy
. The only explanation that seems reasonable to me is that most evolutionary biologists assume that animals will always avoid mating with first-degree relatives as a result of the increased frequency of inbreeding depression and expression of homozygous lethal alleles that result from it.
However, the Grants' observation of incipient speciation among the finches of Daphne Major, which was apparently facilitated by first-degree inbreeding lends support to my hypothesis.
And that's a reason for me to give thanks next week — Happy Thanksgiving, one and all!
As always, comments, criticisms, and suggestions are warmly welcomed!
Labels: Daphne Major, Darwin's finches, Galapagos, Peter and Rosemary Grant