Saturday, July 13, 2013

An Evolutionary Tour of the Galapagos (6): Can We See Evolution in the Beak of the Finch?

June 22
From Genovesa, we navigated overnight to North Seymour Island, where we hiked in the morning before ending our tour on the Cormorant and leaving for the Baltra airport.

From North Seymour, we could see the small island of Daphne Major, which is also easily seen when one flies into or out of the Baltra airport.  This bleak-looking island has become world famous as the main site for the research of Peter and Rosemary Grant in studying "Darwin's finches." 

Almost every high school and college textbook in general biology refers to this research as proof for Darwin's theory of evolution, because it seems that the Grants have actually observed evolution in action.  But does this really prove the evolution of new species by natural selection?

                                                Peter and Rosemary Grant

                                                         Daphne Major

                                         The 14 Species of Galapagos Finches

                         The Adaptive Radiation of Finches in Beak Morphology

In touring the Galapagos, I occasionally asked members of my tour group whether they believed we were seeing proof for Darwin's theory of evolution.  They were puzzled by my question, because it seemed obvious to them that yes, of course, life on the Galapagos demonstrates Darwinian evolution beyond any doubt.  One person who is a public school biology teacher responded: "Haven't you read The Beak of the Finch?"  Jonathan Weiner's book The Beak of the Finch: A Story of Evolution in Our Time is a popular survey of the Grants' studies of the finches, and it received the Pulitzer Prize for General Non-Fiction in 1995.  The powerful impact of this research is conveyed in the subtitle--"evolution in our time"--that is, the thought that we can actually see Darwinian evolution occurring among Darwin's finches, and therefore evolution is not just a speculative theory but an observable fact.

And yet a careful reading of Weiner's book suggests an ambiguity in the story of the finches' evolution.  He writes: "It is one thing to demonstrate, as the Grants have done, that natural selection leads to evolution.  It is another and much more complicated thing to demonstrate precisely how this evolution leads to new species; and despite the title of his greatest book, Darwin himself never spells out the details" (128).  Weiner indicates that while many readers will accept the evolution by natural selection of "slight individual differences," they will find it hard to see how this kind of evolution--"as an editor of beaks and bodies"--can create completely new species.

The fundamental ambiguity in the research of the Grants on the finches is that while it clearly shows microevolution within a species, it is not at all clear that it shows the macroevolution of a new species from an ancestral species.  Even some of the most enthusiastic supporters of Darwinian science see this problem in the finch research.  In Darwin in Galapagos, Thalia Grant and Greg Estes provide a favorable survey of the research conducted by the Grants, and yet they observe: "They have not shown speciation (no one has), but they have shown evolution" (151).  In other words, the Grants have shown evolutionary change within a species, but they have not shown macroevolutionary speciation--the evolution from one species to another.  (Thalia Grant has some intimate knowledge of this research, because the Grants are her parents, and she has worked with them in the Galapagos.)

Biologist Kathleen Donohue has edited an anthology of all the major writing on the evolution of the finches--Darwin's Finches: Readings in the Evolution of a Scientific Paradigm (University of Chicago Press, 2011).  Although she generally accepts the conclusions that the Grants have drawn from their research, she observes: "The tenuous link between adaptive microevolution and macroevolution remains one of the greatest challenges in the field of evolutionary biology" (353).

As I have indicated in some previous posts, creationists and intelligent design theorists can accept the finch research of the Grants as showing how evolution by natural selection can create microevolutionary change within a species--such as changes in the size and shape of finch beaks in response to changing ecological conditions.  But they can still argue that this does not support a Darwinian explanation for the origin of species.  For example, intelligent design proponent Jonathan Wells concedes that there is "good evidence that natural selection can produce limited modifications within existing species such as Darwin's finches"  (Icons of Evolution, 245).  But these "limited modifications within existing species" do not show any evolutionary emergence of new species.  If so, then one can accept the research of the Grants while still believing that the creation of species (or of "kinds")  requires a Creator or Intelligent Designer.

I am persuaded that the research of the Grants makes a plausible case for the adaptive radiation of Darwin's finches as a product of Darwinian macroevolution, particularly when one considers that the alternative theories--the theory of special creation and intelligent design theory--offer no testable hypotheses about exactly when, where, and how the Creator or Intelligent Designer brought new species into being.  But the plausibility of the Darwinian case falls short of absolute demonstration, because the Grants have not directly observed the evolution of new species, although they have directly observed evolutionary change within species. 

Thus the Grants are in the same situation as Darwin himself, who admitted in The Origin of Species that while he thought he had made a persuasive argument for the evolutionary origin of species, he had not demonstrated it, because there remained many "difficulties" for his theory (to which he devoted five of the fifteen chapters of his book).

As far as I know, the Grants have never explicitly responded to this challenge.  But they do implicitly recognize the problem in their book How and Why Species Multiply: The Radiation of Darwin's Finches (Princeton University Press, 2008), which is a general survey of all of their research.

The Grants began their study of Darwin's finches on Daphne Major in 1973, and they continued that study for 34 years.  They see themselves as extending the work of David Lack, who died in 1973 just as they were beginning their work.  In 1947, Lack published Darwin's Finches, based on his field research in the Galapagos and analysis of specimens in museums. 

Contrary to a popular legend, Darwin's theory of evolution did not come to him suddenly when he studied the finches during his time in the Galapagos, because it was only after he returned to England that he began to suspect that the different species of finches on the Galapagos might show an evolutionary adaptation by natural selection to the diverse environmental conditions of the islands.  Lack confirmed that this was the case by arguing that the Galapagos finches showed an adaptive radiation from a common mainland ancestor.  Individual finches that were better adapted to the diverse conditions of life in the different islands would be more likely to survive and reproduce, and thus some varieties of the birds would be favored by natural selection, and eventually these varieties could diverge so greatly as to become new species.  When different species competed with one another on the same island, this competition could create even greater divergence through character displacement, so that, for example, different species might evolve different shapes and sizes in their beaks as adaptations for feeding on different kinds of food. 

Lack showed how this illustrated the three principles of Darwin's theory--variation, inheritance, and selection.  There is variation in the morphology of finch beaks.  This variation is heritable, because the beaks of offspring tend to resemble the beaks of their parents.  And, finally, this heritable variation is subject to selection insofar as it confers some advantage or disadvantage in the struggle for life.  Lack's view of the evolution of the finches was then adopted by biology textbooks as the standard example of Darwinian evolution.

Although they disagree with Lack on some points, the Grants have supported his general account of finch evolution with their own meticulous research.  On Daphne Major, they captured each individual finch, measured its weight and beak morphology, and then banded it for identification.  They could then identify individual variations, compare offspring and parents to determine inheritance, and see the consequences for survival and reproduction in the changing circumstances of the island to determine natural selection. 

In 1977, the Grants saw the consequences of a severe drought on Daphne Major.  85% of the medium ground finches (Geospiza fortis) died.  Normally, these birds feed on small and soft seeds, but when the supply of these seeds was depleted by the drought, the birds were forced to try to feed on large and hard seeds, which can be cracked only by large birds with large beaks.  Consequently, the smaller birds with smaller beaks died at a high rate, the larger birds with larger beaks were more likely to survive, and by 1978, the average body size and average beak size of this population of finches had increased.  "Natural selection had occurred," the Grants concluded (53).  By extrapolation, they reasoned, if such droughts occurred at least once a decade for at least 200 years, this could produce a new species of finch.

But, then, in 1983, the heavy rainfall of an El Nino event promoted the growth of plants with small seeds, which favored the survival of smaller birds with smaller beaks.  As a result, the average body size and beak size of the birds declined.  "The direction of evolution had been reversed," the Grants concluded (55).  But if this evolution is fluctuating in opposite directions, it's hard to see how this could lead to the evolution of new species. 

In 2004, this story of evolution became even more complicated.  2004 was another year of severe drought, and consequently the numbers of medium ground finches declined even more than they did in the drought of 1977.  But in contrast to 1977, the evolutionary effect of the drought of 2004 was to decrease the average beak size of the medium ground finches after this drought.

To explain this change in selection pressures, the Grants argue that the finches in 2004 were showing "character displacement," in which closely related species living close together must diverge in their foraging behavior to avoid interspecific competition.  In 1982, a breeding population of large ground finches (Geospiza magnirostris) was established by two females and three males.  Large ground finches and medium ground finches compete for the same seeds.  By 2004, this competition had become intense, because the population of large ground finches had grown to 150, while the population of medium ground finches was 235.  The drought severely reduced the numbers of both species, and the medium ground finches with large beaks were more likely to die, because they were competing with the large ground finches for the same seeds (68-74).

So, once again, we see microevolutionary changes within a species, but we still don't see any macroevolutionary emergence of new species.

The research of the Grants also illustrates the unpredictable complexity of evolutionary biology that arises from historical contingency.  In 1978, the Grants could interpret the consequences of the drought of 1977 as an event of natural selection.  But this post-hoc explanation did not allow them to precisely predict the consequences of future droughts.  They could not predict the arrival of 5 large ground finches in 1982, which would lead to a growing population of large ground finches on Daphne Major, which would then so change the ecological circumstances that a drought in 2004 would have an evolutionary effect on the medium ground finches opposite to that of the drought of 1977. 

Evolutionary science is a historical science, and the Grants are historians of the finch populations on Daphne Major.  Like all historians, they can explain the past, but they cannot make precise predictions about the future, because of the irreducible complexity and unpredictable uncertainty of historically contingent events.

The evolutionary history of the finches that the Grants have observed on Daphne Major includes cultural history.  Reproductive isolation is a crucial element of their two-sentence definition of species: "Species comprise one or more populations whose members are capable of interbreeding with little or no fitness loss.  They are reproductively isolated from all other populations, either because there is no interbreeding, or because any interbreeding is rare and usually results in relatively unfit (or no) offspring being produced" (119).  This reproductive isolation can have cultural rather than genetic causes, and this is true for the finches.  Female finches choose their mates based on the songs sung by the males that identify them as members of the species.  Unlike beak morphology, the songs are transmitted not by genetic coding but by cultural learning:  males imprint on the songs they hear sung by their fathers or other males between day 10 and day 40 after hatching.  The songs as sung by mature males are shaped by sexual selection in that they must be attractive to females and effective in repelling male competitors.  There is individual variation in these songs, and in one case, the Grants observed that a single immigrant male large ground finch introduced a new song, and since he was a productive breeder, this song increased in frequency (87).  Here then individual variation can lead to changes in cultural history that determine the patterns of breeding that set the boundaries for species.  Thus, the evolutionary history of natural selection and sexual selection is not only genetic but also cultural.

Consequently, evolutionary science must include cultural evolution as well as genetic evolution.  And, indeed, gene-cultural coevolution has become one of the most vibrant areas of evolutionary theory today.  The importance of cultural evolution was recognized by Darwin, particularly in his Descent of Man, in which he explained the evolution of human morality as the joint product of social instincts, cultural learning, and individual judgments.

But even if human culture is open to evolutionary explanation, we might still wonder whether Darwinian evolutionary science can fully explain every feature of human nature.  Can it explain the human soul?  Or does the soul require a religious understanding that is beyond natural science?

Although David Lack thought evolutionary science could fully explain the finches of the Galapagos, he thought any full explanation of human beings required religious beliefs that transcended science.  In a book published in1957--Evolutionary Theory and Christian Belief--he argued that it was possible to be a theistic evolutionist, accepting both the scientific truth of evolution and the religious truth of Christianity.  To take such a position, we must see, on the one hand, that the story of Creation in the book of Genesis is "poetic imagery" that is not meant to be a scientific account of origins, and that God could have allowed Darwinian evolution to govern the history of life without any need for miraculous interventions to create new species.   But we must also see, on the other hand,  that "science has not accounted for morality, truth, beauty, individual responsibility or self-awareness" (36-37, 72, 77, 111-14).  "It seems inconceivable . . . that a soul could be evolved by natural selection," and so the human soul must be understood through religious belief (89).  Lack thus joined a long tradition of theistic evolutionists--from Asa Gray to C.S. Lewis to Francis Collins.

This question of whether evolutionary science can fully explain human life--including our moral, political, and religious experience--was one of the central questions for the Mont Pelerin Society conference on "Evolution, the Human Sciences, and Liberty" on the Galapagos island of San Cristobal.  In my next series of blog posts, I will give my account of the conference.

Some posts on related topics can be found here, here, here, here, here, and here.

After reading this blog post, Peter Grant sent me a copy of the following paper:  Peter Grant and Rosemary Grant, "The Secondary Contact Phase of Allopatric Speciation in Darwin's Finches," Proceedings of the National Academy of Science 106 (December 1, 2009): 20141-20148.

Here are a few excerpts from that paper, beginning with the abstract.

"Speciation, the process by which two species form from one, involves the development of reproductive isolation of two divergent lineages.  Here, we report the establishment and persistence of a reproductively isolated population of Darwin's finches on the small Galapagos island of Daphne Major in the secondary contact phase of speciation.  In 1981, an immigrant medium ground finch (Geospiza fortis) arrived on the island.  It was unusually large, especially in beak width, sang an unusual song, and carried some Geospiza scandens alleles.  We followed the fate of this individual and its descendants for seven generations over a period of 28 years.  In the fourth generation, after a severe drought, the lineage was reduced to a single brother and sister, who bred with each other.  From then on this lineage, inheriting unusual song, morphology, and a uniquely homozygous marker allele, was reproductively isolated, because their own descendants bred with each other and with no other member of the resident G. fortis population.  These observations agree with some expectations of an ecological theory of speciation in that a barrier to interbreeding arises as a correlated effect of adaptive divergence in morphology.  However, the important, culturally transmitted, song component of the barrier appears to have arisen by chance through an initial imperfect copying of local song by the immigrant.  The study reveals additional stochastic elements of speciation, in which divergence is initiated in allopatry; immigration to a new area of a single male hybrid and initial breeding with a rare hybrid female."

". . . it is too early to tell whether reproductive isolation is transitory or likely to be enduring.  The odds would seem to be against long-term persistence of the immigrant lineage as a reproductively isolated population.  First, numbers are small and stochastic fluctuations in population size may result in extinction.  Second, the new populations might run the risk of competitive exclusion from G. fortis  and/or G. magnirostris if the food environment changed.  Third, it might disappear through interbreeding with G. fortis and/or G. scandens, an example of reproductive absorption of one species by another, initiated perhaps by extra-pair mating or misimprinted song.  Fourth, it might suffer from inbreeding depression."

This appears to be close to observing actual speciation, although it also seems unlikely to persist.  Here the Grants are reaffirming what they say at the end of How and Why Species Multiply:  although the speciation of Darwin's finches depends on a combination of behavioral, ecological, geographical, and genetic factors, "behavior plays the key role in birds like Darwin's finches inasmuch as the origin of new species is the origin of barriers to interbreeding, and those barriers are created by behavior" (164-65).  Remarkably, these behavioral barriers to interbreeding are not genetic but cultural, because they are based on the cultural learning of song.  This does not come through in the textbook reports of the Grants' finch research.

1 comment:

Shawn Deny said...

But does this really prove the evolution of new species by natural selection?

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