Emma Birch

Galápagos Marine Iguana Expert

On Daphne Major: Darwin’s Finches

Somewhere amidst the Galápagos Archipelago, on the sea surrounding an island known as Daphne Major, I stand on a boat watching the picturesque sunset which hangs in the sky. It casts a blazing orange across the cerulean water, just catching on the crests of each little wave. It alights the western-facing side of the volcanic island with warm hues of yellow and pink.

The straining light of the setting sun illuminates the outline of a single bird perched along the coastline of the island. The bird resembles a sparrow, a starling, a finch, even, though it is a species of entirely its own. Its distinguishing physical characteristic lies within its beak, strong and sharp, wide and yet coming to a point with steep gradation. Geospiza magnirostris. One of thirteen species of a subfamily of finches known as the Geospizinae — perhaps more recognizably, as Darwin’s finches.

The scientific study of G. magnirostris, or the large ground finch has been significant to developing our current understandings of evolutionary concepts such as genetic drift, gene flow, and genetic diversity. With this, the study of the finches on Daphne Major illustrates the role of chance and determinism in colonization. “Colonization and subsequent immigration”, as seen in the Galápagos large ground finch, may in fact be “the model pattern of founder events”[1]. The founding individuals on Daphne were a random sample of the greater population of finches from the surrounding islands[2]. The founding individuals represented a random selection of the various characteristics which a finch might have. As these finches and their offspring reproduced, the traits being expressed by individuals became a genetically nonrandom sample. The subsequent genetic variety (or perhaps lack of variety) can be defined by an evolutionary concept known as genetic drift.

Genetic drift refers to the idea that subpopulations of a species can be randomly selected through natural events to become genetically representative of the species. When these populations are founded by a relatively small number of individuals, genetic drift may cause a significant decrease in genetic diversity within the population. The founding individuals can only be so genetically diverse when the number of founding individuals colonizing the new population are limited. The subsequent generations of the species, after the initial event which caused genetic drift, will always reflect less genetic diversity than the parent population. In the context of a finch population, then, what might genetic drift imply in terms of genetic variability?

Whether you’ve read about it in a biology textbook, seen it on a lecture slide, or heard about it from your Introduction to Evolution and Diversity professor, you are likely to be at least somewhat familiar with the notion that Darwin’s finches are famous for their diversity in beak size and function. If the finch population were to experience genetic drift, though, wouldn’t genetic drift very likely limit the beak size of the individuals within the population to very certain, very specific dimensions? How, then, can the considerable variability between the beak size and function of the individuals of the large ground finch population be explained?

The answer, as it turns out, lies in the conclusions of a long-term study of the large ground finch population on Daphne Major. Over the course of several decades, now-famous scientists Peter and Rosemary Grant dedicated their lives to tracking, tagging, and tracing the life histories of the entire population of finches on Daphne[3]. In the early years of their study, they found that immigrant birds would migrate to Daphne just after breeding season, every year, only to leave again at the beginning of the next breeding season[4]. Why? During years of drought, Daphne was (for obvious reasons) not the ideal environment for parents to raise their offspring. It wasn’t until an exceptionally rainy year (1982-1983) that thirteen finches stayed on Daphne for the breeding season to reproduce, establishing the first large ground finch population on Daphne[5]. The finches have continued to do so ever since.

What does this imply as far as the growth of the finch population on Daphne? The finches on Daphne very much needed to inbreed to establish a stable population. Of course, there are ways in which the world naturally deters such behaviors, and not by issuing a misdemeanor or felony. Usually an inbreeding population will be affected by what’s called inbreeding depression, or the reduced ability to produce offspring because of inbreeding. Although the large ground finches did experience effects of inbreeding depression (such as asymmetry between the bones in their feet, for example), they were able to continue to reproduce with relatively high breeding success[6]. By 1992, the finch population on Daphne had grown to approximately twenty breeding individuals[7]. By 1993, it was twice that[8].

Individuals within this isolated population, through inbreeding, will undoubtedly produce offspring with reduced variability of traits. This loss of diversity, known as the “founder effect”, can be detrimental to the ability of a species to survive, adapt, and reproduce. When there is very little gene variation within a species, it becomes difficult for individuals to successfully reproduce healthy offspring, which are more likely to suffer the effects of inbreeding. Without genetic variation, a population cannot evolve in response to changing environmental variables and, as a result, may face an increased risk of extinction.

For example, if a population is exposed to a new disease, selection will act on genes for resistance to the disease if they exist in the population. If they do not exist, if the right genetic variation is not present, the population will not evolve and could be wiped out by the disease. As a species’ population size dwindles, it loses genetic variation. Even if the species recuperates, its level of genetic variation will not. Genetic variation will only slowly be restored through the accumulation of mutations over many generations. For this reason, endangered species with low genetic variation may risk extinction long after its population size has recovered. Evolutionary theory suggests that, for the long-term survival of a species, is not only important that the individual members of a species survive, but also a species' ability to evolve in the face of changing environmental variables — to maintain its genetic diversity.

Meticulous measurements of body dimensions and beak size over time, as collected by the Grants, clearly indicated the vastness of the genetic diversity amongst Darwin’s finches. Even so, it wasn’t until the development of genome sequencing technologies that scientists became aware of the true extent of the genetic diversity amongst the large ground finch population[9]. With the information revealed through genome sequencing, it became evident that the species G. magnirostris was actually made up of three genetically distinct groups, each group representing an individual species of finch which had coexisted on whichever island from which the Daphne population of G. magnirostris had originated[10].

Darwin’s finches had been interbreeding long before their colonization of Daphne Major[11]. Colonization and subsequent immigration events have occurred amongst the finches and across the Galápagos archipelago. Genetic drift, as experienced by the large ground finch population on Daphne, may have decreased the genetic diversity of the subsequent generations of finches. Simultaneously, however, gene flow between finch populations prior to the colonization of Daphne has allowed the finches to retain their extensive genetic diversity.

It is important to note that G. magnirostris, the large ground finch, is only able to continue to evolve, to survive and reproduce on Daphne Major because of this genetic diversity. When their survival is challenged, given their genetic variability, the finches are able to naturally select for the individuals within the population with a specific beak size and function which will improve their overall fitness. When seasonal, local weather events such as El Niño and La Niña occur, for example, the sudden onset of a drought may impact the availability of natural resources which sustain the finch population. The large ground finches will improve upon their foraging strategies and, therefore, reproductive success by naturally selecting for individuals with shorter, wider beaks, capable of breaking the larger seeds buried underneath the soil[12]. G. magnirostris, then, will adapt this beak size and function, and in doing so will improve its ability to successfully survive and reproduce.

Maybe one day, the effects of genetic drift will overcome the genetic diversity of the finch population and individuals will suffer the effects of inbreeding depression. Maybe one day, the effects of gene flow via interbreeding will become so conducive to such genetic diversity that the island of Daphne Major will no longer be able to sustain the finch populations. Maybe one day, the finches will go on to migrate to another island in the Galápagos archipelago. Maybe those individuals will found an entirely new population there. Maybe the study of those finches will continue to contribute to our understandings of evolutionary biology, just as their ancestors did.

Until one of those days comes, I take a moment to observe the species which populate Daphne, today. As the sky darkens behind me, I find myself squinting a little to make out a single bird perched just along the coastline of the island. Although standing on two asymmetrical feet, the finch stands strong. It leans forward in that peculiar way that birds sometimes do, just ever so slightly over the tips of its talons. It peers outwardly to the world around it. (I hope that it knows that I am doing the same right back.) Tufts of dark black feathers cascade from the top of the head down the body of the bird, which is otherwise a light tan color. The outlines of its wings glow a golden brown as the last strains of sunlight sifts through its feathers. The large ground finch resembles a sparrow, a starling, even, though it is a species of entirely its own. Geospiza magnirostris. Just as Darwin saw it, just as Peter and Rosemary saw it, just as I see it today.