What correlation between environmental conditions and beak size did the Grants find when they analyzed the ground finches on Daphne Major Island?

V. SUMMARY

Previous studies of Darwin's finch communities on several Galápagos islands over a period of just under a decade established the importance of food supply in determining the number of species breeding on an island, which particular species they were, ecological differences between them, and their abundances. In this chapter we describe the main ecological results obtained from extending a medium-term study lasting less than 10 years (<2 generations) on the island of Daphne Major to a long-term study of 19 years (>3 generations).

Our medium-term study happened to include an extremely rare environmental perturbation, an El Niño event of exceptional severity and duration. Even so, our knowledge of rainfall patterns and their effects upon finches underwent a large change in the following decade. Estimates of mean annual rainfall fell and estimates of variance increased. We encountered sequences of dry or wet years that had not occurred in the medium term.

Similarly, numerical relationships of the two main species changed profoundly over the long term. Cumulative mean and cumulative standard deviation of G. fortis numbers on a log scale increased, whereas cumulative mean numbers of G. scandens decreased and the cumulative standard deviation stabilized. Changes in finch numbers were brought about by changes in their respective food supplies. A change from a preponderance of large seeds to a preponderance of small seeds, witnessed at the end of the medium-term study, persisted to the end. The abundance of Opuntia cactus also changed. In the face of these changes the diet of G. scandens remained basically unaltered, and as a result these finches were inflexibly at the mercy of a food supply that changed in composition and abundance. Geospiza fortis tracked the changing food supply to a greater extent and experienced higher survival.

The community of finches changed in one other respect. Geospiza magnirostris, a repeated immigrant to the island before the El Niño event of 1982–1983, became a member of the breeding community in December 1982. Numbers of breeders increased exponentially over the next decade as a result of local recruitment supplemented by additional immigration. No species went extinct.

These results show how consumers cope with erratically fluctuating food resources. The community of finches and the component species can only loosely be described as equilibrial, given the large and somewhat irregular fluctuations in annual rainfall upon which primary and secondary production depends, and the imperfect ability of consumer populations to track resources at all times. Dynamics on the small island of Daphne, characterized by biotic interactions periodically superseded in importance by environmental pertur-bations, could be representative of community dynamics over a broad geographical range of similar terrestrial environments affected directly or indirectly by El Niño events.

We use these results and interpretations to speculate about the ecology of finch communities over the last 15,000 years when Daphne became an island. Climatic conditions changed across millenia, and extinctions on Daphne probably occurred during relatively dry periods. This dynamic view of the past, combined with modern observations of frequent dispersal to the island and one documented colonization event, implies that the Daphne community of finches has been “assembled” many times in the island's history; there have been many opportunities for finch species to combine to form the community. It also suggests that the community has a core of species, and other species are sometimes added or subtracted according to environmental circumstances. Thus the biological consequences of extreme climatic conditions experienced during our long-term study provide us with a view of community dynamics in the historical and pre-historical past.

Lack of an Evolutionary Response

The study of Darwin's finches is remarkable also, because the phenotypic responses to natural selection could be accurately predicted from estimated heritabilities, selection gradients, and genetic correlations among the measured traits (Grant and Grant, 1995, 2002). Overall, this is the exception rather than the rule in quantitative genetic studies of natural selection in the wild. Predictions of this quality are usually restricted to studies under controlled laboratory conditions (Lynch and Walsh, 1998). Particularly, there are numerous examples from unmanipulated natural populations in which substantial heritabilities and selection gradients failed to produce a change in the trait mean (Merilä et al., 2001). Careful dissection of some of these cases has illustrated how the interplay between organismal features and environmental variables affects the efficacy of natural selection.

A contrast between apparent selection and observed phenotypic change was discovered in an analysis of red deer (Cervus elaphus) antler size based on a 30-year study on the Scottish island of Rum. By applying the animal model, Kruuk et al. (2002) showed that antler size was substantially heritable (h2=0.33) and also significantly correlated with lifetime breeding success (β=0.445 phenotypic SD). From these figures, antler size was predicted to increase over the course of the study, but instead it showed a decrease that was driven by increasing population density. After correction for the latter effect, there was no evidence for a genetic correlation between antler size and fitness and no indication of any evolutionary change in this trait. Instead, the data strongly suggested that the phenotypic correlation between antler size and fitness came about via a third, unmeasured, condition-dependent trait. Here, as in the hypothetical example of egg-laying dates in birds (see Potential Biases in Estimated Selection Strengths), environmental covariances between fitness and a given trait can generate the spurious inference of natural selection in action (see also Kruuk et al., 2003; Morrissey et al., 2010).

This example also illustrates how contrasting trends of phenotypic versus genetic change can obscure ongoing evolutionary processes. Classic examples are phenotypic clines of increasing body size and development time with altitude that mask trends in genotypic values in the opposite direction. For example, mountain populations of green frogs (Rana clamitans) face a much shorter larval growing season than their conspecifics in the lowlands. This forces them to overwinter once or even twice before metamorphosis. The resulting intense selection for rapid development is revealed under constant laboratory conditions, where tadpoles from upland populations develop significantly faster than those from the lowlands and especially so at low temperatures (Berven et al., 1979). Such counter-gradient selection (for a review see Conover and Schultz, 1995) can result in phenotypic homogeneity, when environmental and genetic influences balance each other. Similarly, phenotypic stasis within a given population over time might be caused by an ongoing evolutionary response to directional selection that balances an environmental trend in the opposite direction.

Temporal variation in heritability contributed to an unexpectedly low selection response in an intensely studied population of Soay sheep (Ovis aries) on one of the St. Kilda islands (Wilson et al., 2006). Birth weight in these sheep is both heritable and strongly positively associated with the survival of lambs during their first summer. Nevertheless, it showed no increase over the 20-year study period. Overall, variation in density and climatic conditions caused substantial annual variation in the total first-summer survival. This variation affected both the heritability and strength of selection on birthweight: strong selection and low heritability coincided in low survival years, whereas weak selection and increased heritability were observed in good years. The resulting negative correlation between the two independent variables in the breeder's equation reduced the maximum possible response to selection.

There is an interesting twist in this story. Total heritability in birthweight has two components: one based on the additive genetic variance of the trait in the offspring and a second one due to genetic maternal effects. Maternal effects comprise all means by which a mother's phenotype influences the phenotype of her offspring over and above the additive effects of the transmitted genes. Maternal effects used to be thought of mainly as components of environmental variance, as in the case of larger offspring from females that are in good condition. However, there can also be genetic maternal effects due to heritable phenotypic variation among females that contributes to the variation in some measure of offspring phenotype. Any component of maternal investment and parental care might in principle display such effects, and they are increasingly recognized as an important variance component in natural populations (for a review see Räsänen and Kruuk, 2007). In the case of the Soay sheep, temporal variation in total heritability of birthweight could indeed be attributed to maternal genetic effects. Under good environmental conditions, there was relatively more heritable variation in the mothers’ ability to promote the in utero growth of their offspring.

Taken together, the examples in this section illustrate that point estimates of heritabilities and selection strengths provide only a rough, first approximation of the scope for microevolutionary change in natural populations. Temporal variation in either of these variables can considerably alter the course of ongoing adaptive phenotypic change, and insufficient temporal resolution in a given dataset may miss some or all of the dynamics. Furthermore, a purely phenotypic correlation between a trait and fitness gives the erroneous impression that the trait plays a causative role in fitness variation. Finally, spatial or temporal trends in environmental effects can override or balance genetic change in the opposite direction. An additional impediment to adaptive phenotypic change is considered in the next section.

Galapagos Finches

There are currently 14 recognized species of Darwin's finches in six genera, which have evolved from a common ancestor (Figure 6; Lack, 1947; Grant, 1986). Of these, 13 live in the Galapagos Islands. Based on morphological, behavioral, and ecological data, they have been divided into three lineages: First, the ground finches, Geospiza (6 species), which are found in more arid areas of the archipelago and feed on seeds on the ground. Three of the species which are considered the most “finch-like” differ primarily in body and beak size and are known as the large (G. magnirostris), medium (G. fortis), and small (G. fuliginosa) ground finches. The 3 other species of ground finches have longer beaks. Two feed on cactus flowers and pulp as well as seeds and are known as the large (G. conirostris) and small (G. scandens) cactus ground finches. Finally, the sharp-beaked ground finch (G. difficilis) supplements its diet with the eggs and blood of other birds and reptile ticks. Second, the tree finches, which are found mostly in trees and shrubs, are divided into 3 genera: Cactospiza [the woodpecker finch (C. pallida) and the mangrove finch (C. heliobates)], Camarhynchus [the large tree finch (C. psittacula), the medium tree finch (C. pauper), and the small tree finch (C. parvulus)], and Platyspiza (the vegetarian finch, P. crassirostris). All except P. crassirostris are insect eaters. Finally, the warbler-like finches, which are small with slender beaks, are in 2 genera: The warbler finch (Certhidea olivacea) catches insects like a warbler, and the Cocos finch (Pinaroloxias inornata) is the only Darwin finch that lives outside the Galápagos Archipelago. It appears to have colonized Cocos Island from the Galapagos.

Darwin's finches share common features of nest architecture, egg pattern, and courtship displays. They differ in song, morphology, and plumage. Based on morphology, allozyme, and DNA sequence data, the warbler finch C. olivacea appears to be closest to the ancestral form. However, recent molecular data indicate that the Cocos finch P. inornata is closest to the tree finches of the Galapagos (Sato et al., 1999; Petren et al., 1999). Also, the vegetarian finch appears to be ancestral to both the tree and ground finches rather than being a member of only the tree finch group.

Grant (1986) proposed a three-step process to speciation to explain the adaptive radiation of the group: (i) The ancestral species arrives in the archipelago; (ii) the species spreads to other islands and as a result of this, there will probably be some selection and hence differentiation in size because the islands are different; and (iii) members of the original and derived populations encounter each other, and as a result of competition for food and selection against intermediates character displacement causes rapid divergence in feeding structures between the species when they come together. Therefore, the radiation appears to be based on (i) the isolation of the archipelago, which provided ecological opportunity; (ii) considerable distances between the different islands, which has led to infrequent interisland exchange; and (iii) different environments on the different islands, which have selected for different feeding niches both within and between islands.

A Hypothesis

The ability to alter body architecture enhances allocation plasticity, optimizing each age group's allocation patterns.

Body architecture often constrains resource allocation to life history and foraging traits. For example, beak size in Darwin's finches affects the cost of eating (or the possibility of eating) seeds of particular size (Grant et al., 1976). Body cavity size constrains the amount of reproduction in many animals (see Calder, 1984, for a thorough review). “Furriness,” or the depth of modified scales on the adult thorax, affects temperature regulation in Colias butterflies, with consequent effects on available flight time during which reproduction and foraging occur (Kingsolver, 1983a,b; Jacobs and Watt, 1994).

Changes in body architecture are a form of allocation plasticity. Such changes should result in shifts to new combinations of life history and foraging patterns, as resource allocation changes under the new architectural constraints. Architectural changes are predicted to allow an organism to optimize structure, function, and metabolism for expected future demands for allocation to particular traits. Such changes in response to expected demand should obviate future trade-offs among life history traits that would occur without changes in body architecture.

Architectural changes may also allow the partitioning of gathering of specific resources into different parts of the life cycle. Absence of mouth-parts in some adult moths, for example, relegates all feeding to the larval stage. Changes in architecture thus may be in response to future expected allocation demands, but they also will themselves affect resource demand or intake.

III.A. Evidence for Natural Selection

If Darwin generally lacked evidence for natural selection in nature, modern evolutionary biology has supplied an abundance of such evidence (Endler, 1986), including classic studies of industrial melanism and recent studies of drought selection in Darwin's finches (Grant, 1986). However, there are many other examples of natural selection in the wild, dating back to W. R. F. Weldon's study of carapace width in estuarine crabs in the 1890s. Indeed, natural selection is such an obvious feature of the living world that it is now considered in discussions of such practical medical problems as the prescription of antibiotics and the treatment of the human immunodeficiency virus (Freeman and Herron, 1998). Thus, the general principle that there is a process of adaptation involving natural selection is not in any reasonable doubt.

The evidential problems instead concern the importance of the process in any particular instance. The idea of an adaptive process shaping the course of evolution is very attractive because it can be used to support the interpretation of evolutionary change in terms of natural selection. However, as discussed previously, the demonstration that such a process is occurring is usually very difficult. Also, the possibility that other evolutionary processes are involved—processes that do not involve adaptation by natural selection for the character of interest—cannot be dismissed out of hand. This renders most casual post hoc invocations of natural selection essentially dubious. Whatever the specific features of natural selection, casually invoking it as an explanation for all features of life is no longer reputable behavior in evolutionary biology.

This means that, although there are some specific studies that provide excellent evidence for adaptation by natural selection, in most cases scientists are not in a position to interpret an evolutionary process as being driven by natural selection. It may be allowed as a possibility, but further study is usually required before a particular evolutionary change can be considered as being brought about by natural selection, even when such an interpretation seems intuitively natural.

Evidence for Natural Selection

If Darwin generally lacked evidence for natural selection in nature, modern evolutionary biology has supplied an abundance of such evidence (e.g., Endler, 1986). There are the classic studies of industrial melanism and there are the recent studies of drought selection in Darwin's finches (Grant, 1986). But there are many other examples of natural selection in the wild, dating back to Weldon's study of carapace width in estuarine crabs during the 1890s. Indeed, natural selection is such an obvious feature of the living world that it is now considered in discussions of such practical medical problems as the prescription of antibiotics and the treatment of the human immunodeficiency virus. Thus the general principle that there is a process of adaptation involving natural selection is not in any reasonable doubt.

The evidential problems instead concern the importance of the process in any particular instance. The idea of an adaptive process shaping the course of evolution is very attractive, because it can be used to support the interpretation of evolutionary change in terms of natural selection. But as discussed already, the demonstration that such a process is occurring is usually very difficult. And the possibility that other evolutionary processes are involved, processes that do not involve adaptation by natural selection for the character of interest, cannot be dismissed out of hand. This renders most casual post hoc invocations of natural selection essentially dubious. Whatever the specific features of natural selection, casually invoking it as an explanation for all features of life is no longer reputable behavior in evolutionary biology.

This means that, while there are some specific studies that provide excellent evidence for adaptation by natural selection, in most cases scientists are not in a position to interpret an evolutionary process as being driven by natural selection. It may be allowed as a possibility, but further study is usually required before a particular evolutionary change can be taken as brought about by natural selection, even when such an interpretation seems intuitively natural.

Ecology, Mechanics, and Vocal Diversity

If physical limitations on vocal production mechanisms do indeed influence the evolution of birdsong, what happens when some components of the vocal apparatus are under strong selection for some behavior other than song (Nowicki et al. 1992)? Bird beaks provide an especially interesting case, given the essential role they play in song production, and their frequent specialization for feeding, as illustrated by Darwin's finches of the Galápagos Islands. These birds represent a premier example of adaptive radiation, in which a single ancestor has diversified into multiple descendent species inhabiting a variety of ecological niches (Schluter 2000). Adaptive radiation in Darwin's finches has centered around the diversification of feeding niches, and includes birds that eat seeds, insects, flowers, fruits, pollen, nectar, leaves, cactus pods, and even blood (Lack 1947; Grant 1999). This diversification is matched by an equally impressive array of specializations in beak form and function. The ground finches, for instance, have evolved heavy beaks analogous to a linesman's pliers, suitable for crushing hard seeds, whereas the warbler finches have light, slender beaks analogous to needlenose pliers, more efficient for probing and manipulating insect larvae (Bowman 1963; Fig. 11.6, left).

The evolutionary link between food type and beak evolution in Darwin's finches is firmly established. Peter and Rosemary Grant and colleagues (Gibbs & Grant 1987; Grant 1999; Grant & Grant 2002b) have demonstrated that even short-term fluctuations in food availability, driven largely by seasonal variations in weather patterns, lead to evolutionary changes in beak morphology via natural selection. During a severe drought, for instance, beak size in the medium ground finch increased over a single generation, presumably because of the advantage held by larger-beaked birds in husking large and hard seeds (Boag & Grant 1981). Beak evolution may also be driven by competition among species for common resources. Evidence for competition comes from patterns of morphological variation that suggest a history of character displacement between sympatric species (Lack 1947; Schluter et al. 1985). Directional selection on descendent species, together with broad-scale morphological responses to shifts in the ecological environment, appear sufficient to explain species differences in beak form and function in this group of birds (Grant 1999; Grant & Grant 2002b).

The diversity of beaks among Darwin's finches reflects not only the diversity in feeding adaptations but also, in light of what we now know about beaks and song production, differentiation in vocal production capacities. To the extent that beak characteristics shape the acoustic properties of song, then species diversity in beak morphology should be matched by corresponding diversity in vocal behavior. Early discussions of song diversity in Darwin's finches gave no support to this view, because it was believed that the songs of different species were mostly indistinguishable (Orr 1945; Lack 1947; Grant 1999). With the advent of sonographic analysis, however, differences among species in song structure became apparent (Fig. 11.6, right; CD1 #90), in spite of the statistical overlap among species for song features (Ratcliffe 1981; Bowman 1983). The birds themselves seem highly skilled, although not unerringly so, in using song for species recognition (Ratcliffe & Grant 1985; Grant & Grant 1996b).

How might the kind of variation in beak morphology expressed by Darwin's finches translate into vocal variation? As a starting point, it seems likely that species with an ability to apply large forces with their beaks, for example, those with beaks adapted to crush hard seeds, would face particularly strong constraints on the speed at which they could change the configuration of their vocal tract during sound production (Podos 2001). There is an intrinsic tradeoff in vertebrate motor systems between force and speed (Vogel 1988), which suggests that a beak and associated musculature adapted for strength would necessarily be compromised in their speed of movement. Furthermore, the intrinsic speed of muscle declines with increasing muscle size, because greater numbers of contractile units require more time to activate (Vogel 1988). Species thus ‘encumbered’ by large beaks with comparatively massive jaw musculature are expected to face comparatively severe restrictions on the types of songs they could sing. By comparison, birds with smaller beaks should suffer fewer motor constraints on beak dynamics and thus should be free to produce songs at higher performance levels.

To test this prediction, the songs and morphology of individually marked birds from eight Darwin's finch species were examined on Santa Cruz, one of the central Galápagos Islands (Podos 2001). Measurements taken included three dimensions of beak size and three dimensions of body size. Banded birds were then recorded and their songs analyzed, specifically to measure the two variables we have linked previously to vocal performance capacities – trill rate, and frequency bandwidth (Podos 1997; Fig. 11.2A). The vocal performance of a song was defined by its deviation from a performance boundary determined as the upper bound regression of the relationship between trill rate and bandwidth (Fig. 11.2B).

Consistent with the vocal tract constraint hypothesis, vocal performance of Darwin's finch songs correlated with measures of beak morphology, with lower performance songs being associated with larger beaks (Podos 2001). This correlation was significant not only across different species, for which variation in beak morphology is expected to be pronounced, but also among individuals in a single population of the medium ground finch (Podos 2001; Fig. 11.7). This finding does not of course mean that variation in vocal performance caused by differences in beak morphology and musculature is the sole driver of vocal diversification in these birds. Other causes of vocal evolution not directly related to performance, such as copying inaccuracies during song learning, might better explain variation in other vocal features such as note structure or song syntax (Ratcliffe 1981; Grant & Grant 2002b), and may also explain variation in trill rate and frequency bandwidth in some instances. For example, the two forms of the warbler finch, recently recognized as distinct species based on genetic analyses (Petren et al. 1999), differ in vocal performance and in one beak measure (beak length); here, vocal performance diverges in a direction opposite to that predicted by the vocal constraint hypothesis (Grant & Grant 2002a). Nevertheless, some proportion of the variation in the songs of Darwin's finches, particularly across species and also within some species, appears to be accounted for by differences in beak morphology (Podos 2001).

Relationships between beak morphology and song in other groups of birds appear to be more varied. In reed buntings, several song parameters, including the number and diversity of syllable types, correlate positively with a beak measure (beak depth) that is highly variable across this species’ geographic range (Matessi et al. 2000). Further, the degree to which reed bunting populations are distinct in song structure corresponds to levels of morphological divergence, but not to geographical divergence. These patterns imply a functional role for the beak in song divergence (Matessi et al. 2000), although a mechanical link between syllable diversity and beak morphology has not been established. Among the Neotropical woodcreepers (Dendrocolaptidae), beak length correlates negatively with peak acoustic frequencies (Palacios & Tubaro 2000). This finding is consistent with expectations drawn from our understanding of vocal mechanics; birds with larger beaks should be able to filter harmonic overtones from source sounds with comparatively low fundamental frequencies.

A contrasting example comes from the work of Slabbekoorn & Smith (2000), who analyzed the songs of large- and small-billed forms, or ‘morphs’ of the black-bellied seed-cracker in Africa. Large-billed morphs are specialized to eat comparatively hard seeds, and thus are expected to face comparatively severe constraints on vocal performance. However, no differences were detected between morphs of this finch in a wide range of acoustic frequency and timing parameters (Slabbekoorn & Smith 2000). One possible explanation for this lack of difference is that the song of this species, described by Chapin (1954) as a short, pleasant warble, might not be particularly challenging to produce; thus differences in potential vocal proficiency among morphs may exist but are not expressed. For example, only a subset of songs contained trills, in which birds need to quickly repeat the same set of sounds. Another possible explanation for a lack of difference among morphs is that the pressure of sexual selection is weak and thus that birds do not necessarily approach potential performance limits in their realized vocal output (Slabbekoorn & Smith 2002b).

Recognition of the dual role of the beak in both feeding and singing provides new insights into mechanisms of speciation. Darwin's finches are a classic system for studying how ancestral species split into multiple descendent species, and consensus has been reached on a number of points (Swarth 1934; Lack 1947; Bowman 1961; Grant 1999). First, speciation in Darwin's finches appears to have been primarily if not exclusively allopatric (Grant 1999), which is not surprising given the many opportunities for geographic isolation on the Galápagos archipelago. Second, pre-mating isolation mechanisms such as song are thought to play a particularly strong role in speciation, as is illustrated by the occasional production of successful hybrids among various Darwin's finch species (Grant & Grant 1997b, 1998). This argues against the existence of genetic or physiological post-mating isolation mechanisms. Third, trait differences among populations and species are thought to be driven by rapid and precise adaptation to divergent local ecologies (Grant 1999). Adaptation is manifest largely as evolutionary changes in beak morphology, and is driven by natural selection for feeding ecology, (Gibbs & Grant 1987) and by selection against interspecific competition (Schluter et al. 1985). On the basis of these consensus views, Grant (1999) and collaborators have argued that ecological adaptation in Darwin's finches is the primary driving force in their diversification, that evolutionary changes in mating signals occur as incidental correlates of ecological adaptation, and that the diversification of mating signals, including song, plays an important role in speciation, by providing cues about species identity (Grant & Grant 2002b).

Niche Picking

Sulloway has argued that children adopt different roles or niches within the family. Some of this are the result of genetic differences, some are connected to sex differences, and some are due to birth order. Specialization of roles within the family, like specialization of species in the wild (the most famous example being Darwin's finches), reduces levels of competition. Specialization also makes children more unique. If your older brother is an awesome football player, a younger brother may be better served by a different niche, attempting to excel in a different sport or a totally different area such as academics or arts. Eldest siblings also often occupy the role of surrogate parent with its sense of responsibility and adherence to rules. For later born children, there is no advantage to trying to duplicate that role; they have to find their own niche and their openness to experience with less adherence to rules and authority facilitates this.

High mobility—Movers

Mobility, as a species trait, is complex and varies markedly among taxa. The major driver of mobility seems to be related to resource acquisition over space and time. For conservation purposes, it may be useful to focus on traits within clades. This could reveal biological trends, such as with Darwin's finches, but could also be used more broadly to determine species at high risk of extinction, such as migratory wetland birds. It is illogical to determine global biodiversity trends by extrapolating from one clade, especially where representatives vary markedly in mobility. However, the comparison of mobility capabilities within one clade can be used effectively in quantifying threatening processes, such as effects of fragmented landscapes on butterflies or freshwater fish. Mobility is an important trait for conservation as it can make the difference between escaping or being consumed by a threatening process such as fire, changing environmental conditions or invasive species (Mason et al., 2018a,b). The mobility of taxa may also act as a predictive trait for the assessment of conservation status over large areas, such as with birds.

Speciation

Speciation is the process by which new independently evolving lineages (species) arise from a single ancestral lineage. In general, efficient dispersal between populations (patches) increases the rate of gene flow between them and hence tends to reduce genetic differentiation and to restrain speciation. In contrast, inefficient dispersal between existing populations leads to geographic isolation and tends to increase genetic differentiation and to promote speciation. This type of speciation, called allopatric speciation, is considered to be the most frequent one. Jump range expansion occurs with very low probability (see The Biogeographic Perspective) and is thus unlikely to be followed by additional dispersal; this isolation thereby causes rare dispersal events (in this context also called founder events) to be relevant to allopatric speciation. The adaptive radiation of Darwin's finches in the Galápagos Islands is one of the many examples of allopatric speciation that has resulted from dispersal events. However, as discussed in the Introduction, a dispersal barrier may divide a previously continuous distribution. Hence, allopatric speciation may also result from a vicariant event.

A second way in which dispersal affects speciation requires the opposite conditions. The breakdown of a dispersal barrier, and the following biotic interchange (see Removal of Barriers), brings organisms to dissimilar environments and thus opens new chances for diversification. The colonization of North American mammals during the Great American Interchange (see Removal of Barriers) was followed by extensive diversification in South America. In the same manner dispersal events cause speciation. They affect the genetic structure within the species range, sometimes to the point that the populations are evolving independently, and can be considered as distinct species. Theoretical models and molecular studies have shown clear effects of LDD on the spatial structure of the genetic variation within a species during the postglacial expansion of European and North American biota (see Holocene Postglacial Spread). As predicted by models, jump range expansion well ahead of the “normal” advancing wave generates large patches of low genetic variation, due to the so-called founder effects; consequently, genetic diversity decreases from south to north.