40 Years of Evolution: Darwin's Finches on Daphne Major Island

Many of us are fascinated by the biological world around us. We marvel at the diversity of color, pattern, form, shape, size, ferocity, tameness, speed, and ingenious things that animals and plants do to find food and mates and avoid being eaten. Some of us have peered into microscopes that have opened up a new and wonderfully diverse world. Others have had the same thrilling experience in diving off coral reefs and being dazzled by the variety of fish. Yet others have been simultaneously bewildered and stimulated by the overwhelming diversity of a tropical rain forest. All this is so...

On first entering the Galápagos Archipelago, a naturalist is struck by the distribution of closely related species of finches. Populations of the same species differ on average in body and beak size from island to island, whereas on any one island sympatric species differ discretely from each other in these characteristics. How did the morphological differences between species come about? Two previous investigators confronted this question. Lack (1947) laid stress on three factors—natural selection, diversification on separate islands, and competition between species for food when two closely related lineages came together on secondary contact (ch. 1)—whereas Bowman (1961)...

For evolution to occur there must be heritable variation, so one of the first questions that arise from both the small size offortison Daphne and the evolutionary trajectory in figure 1.6 is how heritable are size traits? We addressed this question by using the methods of quantitative genetics to estimate heritable variation from degree of family resemblance. Many nests were found (box 3.1) and mapped (appendix 3.1), their owners (banded or not banded) were identified, nestlings were banded when they had reached a posthatching age of 8 days, and, sometime after they had fledged, they were captured in...

G. fortisis highly variable in size, and the variation is strongly heritable; therefore it has the potential to evolve when the environment changes. This chapter describes what we learned about selection and evolution by following the population through contrasting regimes of wet and dry conditions. Figure 4.1 provides a general framework for the specific observations we describe. It shows how fitness of individuals varies according to the interaction between genotypes, phenotypes, and both social and general aspects of the environment. The center point, indeed the central focus of our field study of evolution, is phenotypic variation. We work back...

There is more to fitness than surviving dry seasons (fig. 4.1). The genetic fitness of an individual is its ability to surviveandreproduce (Charlesworth 1980, De Jong 1994). Whether its potential is realized or not is determined by its behavior, physiology, and morphology, as well as by chance. At certain critical times in dry seasons, survival is dependent on beak and body size (chapter 4). It is possible that breeding success is also dependent on beak size and body size in wet seasons. Some birds may be better than others at gaining a territory and a mate, and converting...

Darwin answered the dilemma above in two ways. First, he suggested that deep channels and rapid currents would be strong barriers to the dispersal of sedentary organisms, such as snails and centipedes, although not for mobile animals like birds. Second, he pointed out that potential colonists arriving at an island would have to compete with the local inhabitants. Darwin’s second answer remained in the realm of conjecture until an unexpected colonization event occurred in the extraordinary El Niño year of 1982–83. The Large Ground FinchGeospiza magnirostris(fig. P.1) established a breeding population on Daphne. We were exceptionally lucky...

Colonization of Daphne bymagnirostrishad a profound effect on the community of finches, after a lag of 22 years.G. magnirostrisapparently caused an evolutionary change infortismorphology as a result of competing with it for food (character displacement; Grant and Grant 2006, Pfennig and Pfennig 2010, 2012). An evolutionary effect of competition in the present is the complement of character release, evolution offortismorphology in the absence of competitors in the past (chapter 2).G. magnirostrisis a potential competitor with the two resident species, especiallyfortis, because their diets overlap (fig. 6.11). The potential for...

Our study on Daphne began as a small part of an archipelagowide attempt to understand the evolutionary diversification of the finches. Since the strength of the evolutionary response to natural selection is governed by the amount of genetic variation (chapter 3), morphological averages should not be considered in isolation from the variances. Average beak size differs between species as well as between populations of the same species (fig. B.1.2), and so do phenotypic variances (Lack 1947). For example, in chapter 3 we showed thatfortisvary more thanscandensphenotypically and genetically. These contrasts raise the question of why some...

Many bird species, approximately one–tenth of roughly ten thousand species, are known to hybridize (Panov 1989, Grant and Grant 1992b, McCarthy 2006). Hybridization and subsequent introgression through backcrossing can be a powerful source of significant genetic variation because it can generate new combinations of genes in either or both parental populations, allowing a rapid evolutionary response to new or changing environments. In the Galápagos Archipelago closely related species occur in sympatry in various combinations (Grant 1986). This provides many opportunities for occasional hybridization with introgression to contribute to the high variation observed in morphological traits. As we mentioned in...

The main implication of high hybrid fitness and continuing hybridization described in the previous two chapters is that interbreeding species should converge toward an intermediate morphology, and, if unchecked, this convergence would result in complete fusion of the previously reproductively isolated populations (Dobzhansky 1941, Clarke et al. 1998, Grant et al. 2004, Taylor et al. 2006, Grant and Grant 2008b, 2010b, Seehausen et al. 2008, Behm et al. 2010, Webb et al. 2011). It would then represent speciation in reverse, or despeciation of sister species such asfuliginosaandfortis(Grant et al. 2004). A better metaphor for the situation...

Long-term field studies help to bridge the gap between two very different temporal scales of evolution, the micro and the macro. Fossils reveal temporal patterns in size, shape, and complexity of organisms as diverse as trilobites, oysters, and dinosaurs over millions of years. The patterns include long-term linear trends with or without reversal (Darwin 1859, Simpson 1953), long-term stasis punctuated by rare, abrupt, and substantial change (Eldredge and Gould 1972, Eldredge et al. 2005) and bursts of diversification following major tectonic activity (Williams and Duda 2008), concentrated periods of extinction (Bapst et al. 2012), or entry into new environments (Hou...

Chapter 1 introduced the main problem that has motivated research on Daphne: the problem of explaining how one species of Darwin’s finch became many by repeated speciation through morphological and ecological diversification. Speciation occurs when one species splits into two populations or sets of populations (fig 1.2) that do not interbreed or interbreed rarely with little genetic consequence (Grant and Grant 2008a). We observe the products, species, and the challenge we face is to reconstruct the process by which they were formed. One purpose of this chapter is to use information from the study on Daphne to reconstruct and interpret...

Over the past 20 years hybridization has been seen increasingly as an important factor in speciation (reviewed in Schwenk et al. 2008, Abbott et al. 2013). An exchange of genes between related species or even genera was once thought to be a phenomenon largely restricted to plants, but is now known to be widespread and to occur from bacteria (Jain et al. 1999, Ochman et al. 2000) and corals (Veron 1995, Willis et al. 2007) to arthropods (Mallet 2005), birds (Grant and Grant 1992b, McCarthy 2006), and primates (Arnold and Meyer 2006, Schwenk et al. 2008). Even in our own...

Our goal has been to gain insights into the past by studying the present for long enough to capture significant change (Grant and Grant 1996c, 2011b), to use the information to extrapolate to the broader context of the Darwin’s finch radiation (Grant and Grant 2008a), and to look into the future and ask such questions as what will the Daphne finch community be like at the end of the century? Predictions are hazardous (appendix 10.1), as the cautionary quotations above imply; nonetheless 40 years of experience are a better basis for extrapolation than, say, 4 years.

The present is a...

Figure 15.1 summarizes the four profound morphological changes that took place in the Daphne finch community over 40 years. Two changes were additions, brought about by colonization (magnirostris) and speciation (Big Bird lineage). Two changes were morphological transformations offortisandscandensthrough selection and introgressive hybridization.

From start to finish three themes dominated the study: interspecific competition for resources, enhanced variation in continuously varying traits like beak size, and the role these two play in species formation. Three themes emerged as the study progressed: natural selection as an observable, repeatable, and interpretable phenomenon with evolutionary consequences; introgressive hybridization; and...

We studied finches on Daphne: only one island! Are the evolutionary dynamics we uncovered just an interesting case history applicable only to Galápagos, or are they indicative of ubiquitous processes that are normally too subtle to be revealed in most field studies? In other words how representative are they of other island communities, in Galápagos, or elsewhere, or of mainland birds, or of animals in general? Can Darwin’s finches on Daphne be considered a model system of evolution in a fluctuating environment? Obtaining answers to questions of this sort require comparative studies of other species in other places (e.g., Price...

Long-term processes require long-term study. A few such field studies have contributed disproportionately to our knowledge of how natural populations fluctuate in numbers and evolve (e.g., Anderson et al. 1991, Weider et al. 1997, Majerus 1998, Clegg et al. 2002, Beaugrand and Reid 2003, Packer et al. 2005, Baltensweiler et al. 2008, Ozgul et al. 2009, Armitage 2010, Clutton-Brock and Sheldon 2010, Husby et al. 2010, Qvarnström et al. 2010, van der Pol et al. 2010, Hanski 2011, Alberts and Altmann 2012, Lebige et al. 2012, Rijssel and Witte 2013). In the experience of many field biologists the longer the...