Hybridization of G. fortis with scandens and fuliginosa in Relation to Speciation and Darwinian Concepts

Laura Steele

Darwin defined evolution with the simple phrase "descent with modification" (2). This three word description encompasses a much more complex meaning. The concept of evolution can be split into two main subsections; microevolution and macroevolution. The study of microevolution focuses on the changes in heritable traits or gene frequencies within a population or species. Macroevolution, alternatively, analyzes changes in the numbers or types of species in the world around us. Typically, this study concentrates on evolutionary events that occur at a large scale and over a relatively long period of time. While it would appear that these two types of studies are exceedingly different, some Neo Darwinists proposed that "macroevolution didn't need a separate explanation from microevolution; it was simply microevolution left running for a few million years" (4).

One of the most recurrent questions related to evolution and creation examines the origin of adaptation. As posed by Weiner in The Beak of the Finch, "How did blind creation make so many new kinds of tools? How do evolutionary inventions and innovations... get started, if their [Darwin's finches] raw material is merely random individual variations?" (3). The commonly accepted answer to this question is speciation, the process by which new species arise. To counter the numbers of new species, there must be another process to lower the numbers of others. This happens through extinction, which is defined as the loss, or disappearance, of a species. Both speciation and extinction are types of macroevolution (2).

The studies of Darwin's finches on the Galapagos Islands are one of the most well documented examples of microevolution. Peter and Rosemary Grant have spent over 20 years of their lives following these finches and comparing their observations from year to year, species to species, and island to island. One of the features they found most relevant to the topic of speciation dealt with the breeding habits and behavioral patterns of the birds. An example of a well studied finch was the medium ground finch, G. fortis. A defining feature of this species was its medium sized beak, designed to help it crack open different types of seeds. The average length and size of its beak and body fluctuated, depending on the different pressures coming from the environment, food supply, and competition (3).

On the island Daphne Major, a number of Darwin's 13 finches lived along side each other, held apart by invisible barriers created from their distinctive behavior. One barrier discovered was the males "advertising song," serving to prevent species from interbreeding through individual recognition of their correct song. Each male finch, no matter the species, sings their unique song from a singing post, which is typically the highest point of the particular territory. A key observation made by the Grant's was that while males of the same species will never have overlapping territories, males of opposite species may indeed share parts of the same land (3).

After a drastic season of rainfall, brought upon by the hurricane' El Nino, the Grants witnessed some unusual breeding patterns among finches on Daphne Major. Individuals from the species G. fortis interbred with members from both the scandens and fuliginosa species and were able to produce viable hybrid offspring. The fortis-scanden hybrids exhibited a combination of the medium beaked fortis and the small beaked cactus finch. In contrast, the fortis-fuliginosa hybrids were a combination between the medium beaked cactus finch and the small beaked cactus finch. These two hybridizations expanded the gene pool of the G. fortis species, though the extent of the expansion was not known without genetic analysis. The hybrid offspring from these pairings proved to
have the advantage over the offspring from individual fortis, scandens, or fuliginosa pairs. These offspring were more likely to survive and more likely to breed, ultimately passing on their new or slightly different characteristics to the future generations of finches on the island (3).

The hybridization between the species proved puzzling at first for the Grants, but once they put all the pieces together, they reached some possible, conclusions. Many changes occurred on Daphne Major during the wrath of the El Nino. Food supply, including both seeds and caterpillars, skyrocketed. In turn, according to Peter Gibbs, "the birds went crazy. . . [breeding] like hell," producing way over the ultimate carrying capacity of the island (3). As the numbers rose, territories continually got smaller, overlapping more and more. This intersecting led some fledgling male finches to learn the wrong song, the song of a neighboring species. It became evident to the Grants that "this [was] one way the finches [got] their lines crossed and [made] hybrids." The next question that must be asked was whether or not these hybrids will prove to produce an entirely new species (3).

To quote Charles Darwin, "the smallest grain in the balance, in the long run, must tell on which death shall fall, and which shall survive" (3). The "smallest grain in the balance" may be any difference that exists between members of a species that can better one's chance of survival over another. This concept has been shown to hold true for all organisms, whether they are considered a unique species or part of a hybrid group. It would appear that these hybrid individuals of G. fortis finches could ultimately lead to a new species, depending on the genetic variation. Stephen Jay Gould, a renown evolutionist, once noted "that a big change in an organism can sometimes arise from a small change in its genes" (3). It must be understood that although outward appearance might look very similar amongst organisms, one could be genetically better adapted for survival in the current landscape.

Darwin also makes two compelling statements in Chapter 2 of his book entitled On, the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life related directly to speciation. In the first statement to be examined, Darwin states "I attribute the passage of a variety, from a state in which it differs very slightly from its parents to one in which it differs more, to the action of natural selection in accumulating differences in structure in certain definite directions. Hence I believe a well-marked variety may be justly called an incipient species" (1). The above discussions of the two hybrid species seem to be examples that support Darwin's statement. They could be called "incipient species" as they are not yet defined as their own species but in the process of transgressing into one.

The second statement explains further that "these facts are of plain signification of the view that species are only strongly marked and permanent varieties …where the manufactory of species has been active, we ought to generally to find the manufactory still in action, more especially as we have every reason to believe the process of manufacturing species to be a slow one" (1). This statement appears to point directly to the studies being explored on the Galapagos Islands. It is apparent that the manufactory of species has actively occurred there over the years and can be thought to still be in action through the observation of the hybrid species.

After examining the hybridization of the G. fortis finches, as presented by Weiner, it can be concluded that this process could most likely lead to speciation. This cannot be said definitely, as it would take a more detailed analysis and explanation of the actual changes between the original species and hybrid offspring. Certain characteristics, specifically genetic differences between the original finches and the hybrids, would play an important part in the discerning of an entirely new species. Although in his books he fails to "document the origin of a single species", many of Darwin's mechanisms can be seen in the events on the Galapagos Islands and the changes amongst the finches (3). One bold comment made by Weiner pushes the Grant's study of the hybrid finches to an entire new level. If these hybrids really do lead to new species, then "that mystery of mysteries, the origin of species", has been solved (3).


References


1. Darwin, C. 1859. On the Origin of Species by Means of Natural Selection, or the
Preservation of Favoured Races in the Struggle for Life
.

2. Kardong, K. 2005. Introduction to Biological Evolution. McGraw Hill.

3. Weiner, J. 1995. The Beak of the Finch. Vintage Books, NY.

4. Zimmer, C. 1998. At The Water's Edge. The Free Press, NY.

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