The Princeton Guide to Evolution

VI.3

Geography, Range Evolution, and Speciation

Albert Phillimore

OUTLINE

1. Geographic patterns of species and speciation

2. The geography of speciation

3. Island patterns and their implications

4. Speciation and area

5. Geographic and geological triggers of speciation

6. Challenges and prospects

The distribution of biodiversity is very uneven across the earth, a testament to geographic variation in the net contribution made by speciation, shifting geographic ranges, and extinction. One of the most obvious diversity patterns on the planet is the tendency for species richness to be greatest close to the equator and to decline toward the poles. However, identifying the mechanisms whereby speciation, geographic range dynamics, and extinction contribute to this global latitudinal diversity gradient continues to confound ecologists and evolutionary biologists alike. Geography features prominently in speciation research—so much so, that until recently the geographic distributions of species undergoing speciation formed the basis for classifying speciation into three main modes: allopatric, parapatric, and sympatric. Although population-genetic-based classifications of speciation are now preferred, the geographic distributions of populations reveals something about the potential for gene flow, which in turn affects the ease with which reproductive isolation and speciation can arise. Unfortunately, because species ranges can expand, contract, and move substantially through time, little can usually be inferred about the geography of speciation from species’ present distributions. Island-dwelling species may be the exception; their isolation serves to limit postspeciation range movements. This has made studying the geography of speciation on islands especially informative. Islands offer a further attraction to evolutionary biologists, in that teasing apart the mechanisms whereby immigration, extinction, and speciation contribute to their diversity patterns has proven more tractable than for continental patterns.

GLOSSARY

Allopatric Speciation. Geographically separated populations among which reproductive isolation accumulates in the absence of gene flow.

Parapatric Speciation. Speciation of populations in the face of gene flow at a level that is greater than zero (allopatric speciation) and less than complete mixing (sympatric speciation). Populations may have abutting or geographically separated distributions.

Peripatric Speciation. A mode of allopatric speciation arising when some individuals disperse outside the current range and beyond a preexisting barrier to establish a new population in a remote location. After establishment, the rarity of long-distance dispersal means that little or no gene flow occurs.

Species Richness. The total number of species in a clade or inhabiting a specified area.

Sympatric Speciation. Under a population genetic definition, a mode of speciation that results when two reproductively isolated species arise from within a randomly mating ancestral population.

Sister Species. Two species that are each other’s closest extant relative, typically identified using a molecular phylogeny.

Vicariant Speciation. A mode of allopatric speciation that involves a physical barrier, such as an ocean channel or mountain range, that subdivides a range and prevents gene flow between the two resulting populations.

1. GEOGRAPHIC PATTERNS OF SPECIES AND SPECIATION

The number of species found in different parts of the planet varies enormously. To visualize this variation, imagine dividing the surface of the earth into equal-sized grid cells, say 100 km by 100 km, by drawing invisible lines across its surface and then counting the species found within each cell. For breeding birds, species richness would vary from as few as zero to one species in Greenland and Antarctica to about 950 species in the Andean foothills of Ecuador. Similarly, mammalian richness would range from zero to 250 species, with the maximum found in the Great Rift Valley of the Democratic Republic of Congo. Estimates of vascular plant diversity are subject to greater uncertainty, but geographic disparity in their richness is even more pronounced, exceeding 6000 species in a single grid cell in the rain forests of Central America.

While some spatial variation in diversity is idiosyncratic to each group of organisms, there are pronounced general trends. For example, terrestrial species richness tends to be high in the foothills of mountain ranges and low on islands, whereas marine species richness tends to be highest in continental shelf and coastal regions. But the global species-richness pattern that receives the most attention is the tendency for species richness to decline from the equator (low latitude) to the poles (high latitudes), a pattern known as the latitudinal diversity gradient. If we reconsider the grid cells where birds, mammals, and plants are most diverse, each of these lies near the equator. In fact, the latitudinal diversity gradient holds across most cosmopolitan groups of terrestrial and marine organisms and is observable in the fossil record.

Identification of the processes that give rise to diversity gradients in general, and the latitudinal gradient in particular, has been the goal of innumerable studies. Yet, perhaps surprisingly, a definitive explanation is still lacking. We know that a location can gain species only via immigration and speciation (see chapter VI.2) and lose species via local extinction; therefore, all richness patterns must arise via differential contributions of these three processes over time. More than 100 hypotheses have been put forward to explain the latitudinal gradient, but most of these can be assigned to one of three classes. These classes of explanation identify the major difference between tropical and extratropical regions as one of (1) ecological limit, or, in other words, the overall carrying capacity; (2) diversification rate; and (3) the amount of time that has been available for diversity to accumulate.

We will consider how these explanations might apply to the latitudinal diversity gradient, and the evidence for each. However, we will later see that variations on these explanations may be applied to other instances where diversity varies spatially—the depauperate nature of islands, for example.

Ecological explanations for the latitudinal gradient often boil down to the following chain of reasoning: because the tropics receive the most energy, plants there are more productive, supporting more species in more niches at successively higher trophic levels. Proponents of this hypothesis point to the positive relationship between energy (often measured as potential evapotranspiration) and diversity that is often apparent on a continental to global scale. However, we do not yet know whether the standing diversity at different latitudes has already reached or is even close to its local carrying capacity. In certain locations, such as some Northern Hemisphere temperate forests, there is good reason to believe that diversity has not reached its potential. The fossil record suggests that prior to the last glacial maxima, the European temperate and East Asian forests were comparable in diversity, but now the former possess about a tenth of the species diversity of the latter. It is also possible that carrying capacities are to some extent elastic and that as diversity increases, key innovations provide the opportunity for the accumulation of greater diversity.

Until recently, ecological explanations for the latitudinal diversity gradient received much more attention than evolutionary ones, though this situation is changing. The diversification rate hypothesis states that the net contribution of speciation minus extinction decreases toward the poles. A variety of mechanisms leading to faster speciation in the tropics have been proposed (the “cradle” hypothesis). For example, at high latitudes, selection may be driven by abiotic conditions that vary more seasonally than spatially. In comparison, the seasons in tropical areas are less pronounced, a consequence of which may be that geographic variations in biotic conditions come to the fore as drivers of geographic selection and speciation. Perhaps, higher temperatures in the tropics accelerate the mutation rates of ectotherms and plants. Alternatively, extinction may have been slower in the tropics (the “museum” hypothesis) because either tropical areas are more climatically stable or population sizes are larger. The empirical evidence for increased diversification rates in the tropics is currently quite mixed. Phylogenetic and taxonomic studies on representatives of birds, butterflies, marine bivalves, and flowering plants all found that diversification was fastest among low-latitude clades. However, support for a very different model comes from a comparison of the ages of sister species of birds and mammals at different latitudes conducted by Jason Weir and Dolph Schluter. They found that while both speciation and extinction rates were faster at high latitudes, the diversification rate (the speciation rate minus the extinction rate) was consistent across latitudes. It now seems likely that diversification rates themselves may change through time, perhaps owing to a negative feedback of clade-wide ecological limits on the rate of speciation, implying that the ecological limit and diversification rate hypotheses are both at play and interact.

The third major type of explanation, of which Alfred Russel Wallace was a proponent, posits that the time available for diversity to accumulate is key. A mere 20,000 years ago, ice sheets covered North America as far south as New York, as well as much of northern Europe. Pollen recovered from sediment cores reveals the huge effects of the climate and ice sheets on the geographic ranges of temperate and boreal tree species. The distributions of North American oak and spruce trees are now about 1000 km farther north than they were at the last glacial maximum, and the fossil record for North American mammals tells a similar story of postglacial range expansions and shifts. Therefore, we can surmise that postglacial dispersal and range shifts must have played a major role in forming the communities that now inhabit high-latitude terrestrial areas.

During glacial periods, conditions also changed at lower latitudes: many biomes contracted in total area, and arid areas expanded. Species-rich wet tropical biomes appear to have been relatively climatically stable and, at various times in the last 50 million years, have extended much farther north. It is suggested therefore that a combination of greater area and more time have provided the wet tropical biomes with a greater opportunity for diversification. Support for this explanation comes from phylogenetic studies revealing that clades inhabiting higher latitudes are often nested within (i.e., younger than) those inhabiting lower latitudes.

Each of the processes described by these three classes likely contributes to the latitudinal diversity gradient, though they may differ in their importance. Unfortunately, as we have only one earth and no truly independent replication, even if we accumulate additional data and develop more sophisticated statistical models, a full explanation for this fascinating pattern may continue to elude us.

2. THE GEOGRAPHY OF SPECIATION

While speciation is one of the key contributors to geographic diversity patterns, the process of speciation is itself influenced by geography. In fact, the geography of speciation has been the subject of debate for more than a century. Arguments have revolved around both the feasibility and importance of three main geographic modes of speciation: allopatric, parapatric, and sympatric. Often in the past, these modes have been defined on the basis of geography alone, but here we adopt definitions based on gene flow and used by population geneticists (see the glossary).

Gene flow is useful in defining modes of speciation because, for sexual organisms, it plays a key role in determining the ease with which speciation occurs. In the absence of gene flow, any process that leads to genetic divergence, even genetic drift, will eventually result in allopatric speciation. By contrast, the presence of gene flow impedes divergence by homogenizing genetic variation across populations. The splitting of lineages to form two separate species often requires divergence in two or more traits (e.g., one trait that is subject to divergence under selection and another trait used to choose a mate). However, when there is gene flow between two diverging populations, genetic recombination (the process whereby DNA is exchanged between the two strands of a chromosome during meiosis) can cause the genes that underlie these traits to become disassociated (see chapter VI.9).

Theory has shown that speciation in the face of gene flow is easiest when disruptive (or divergent) selection and assortative mating are both present. Selection is described as disruptive when the fitness of individuals with extreme phenotypes is greater than that of individuals with intermediate phenotypes, and divergent if different phenotypes are favored in different locations. Assortative mating is the nonrandom mating of individuals; that is, members preferentially mate with individuals of similar phenotype. It is relatively easy to envisage how both conditions may arise in a parapatric geographic scenario. For example, consider a species that inhabits two adjacent habitats. There is likely to be divergent selection for adaptation to each habitat, and spatial separation of populations will have the effect of making individuals occupying each habitat more likely to mate with individuals from the same rather than the different habitat. In comparison, sympatric speciation is easiest where a single trait is subject to disruptive selection and assortative mating (or where the genes underlying the traits are situated in proximity on a chromosome), thereby preventing recombination or making it less frequent. The beak of the Geospiza genus of Darwin’s finches could be just such a trait: the size and shape of the finch’s beak is subject to selection for handling seeds that vary in size and hardness, and the dimensions of the bill also affect the song of male birds, on which basis females choose mates. It is therefore possible that reproductive isolation may emerge as a by-product of natural selection as populations adapt to different resources. However, there is no compelling evidence to suggest that any completed speciation in Darwin’s finches has actually been sympatric from start to finish.

Allopatric speciation has been classified into two geographic subtypes, vicariant and peripatric. Vicariant speciation is inferred if sister species have nonoverlapping geographic ranges, and the age of the split between them, as revealed by molecular phylogenetics, is consistent with the emergence of a known barrier. For instance, when the Panamanian land bridge joined North and South America about 3 million years ago, gene flow between many coastal marine animal populations inhabiting the Caribbean and Pacific sides ceased. A review of 115 comparisons of nucleotide divergence of populations of echinoderms, crustaceans, mollusks, and fish sampled from both the Caribbean and Pacific coasts found that 30 percent had divergence times consistent with the final closure of the isthmus, but the majority of the remainder (63.5 percent of cases) diverged substantially earlier.

The second type of allopatric speciation, peripatric speciation, arises when individuals colonize a new locality via long-distance dispersal and become reproductively isolated from the source population. Drawing on examples of endemic species occupying remote islands, biologists have long been aware that peripatric speciation sometimes occurs, though its frequency of occurrence on continents and in the oceans remains to be established. Ernst Mayr, famous for introducing Modern Synthesis ideas on evolution to the study of systematics and biogeography, was interested in the processes responsible for the peripatric speciation of island populations. In a work published in 1954, Mayr observed that there were islands off the coast of New Guinea that differed little in environment from one another or from New Guinea, and yet many of these islands had their own endemic kingfisher. He argued that the similarity of the islands’ environments meant that divergent natural selection could not account for this speciation. Therefore, to explain such endemism he proposed the founder effect speciation model. According to this model, a small number of individuals, carrying a fraction of the parent population’s genetic diversity, colonize a novel area. Thereafter, genetic drift in a massively reduced population causes many alleles to be rapidly fixed or lost. Mayr argued that this process could dramatically alter the genetic background on which selection operates, thereby allowing rapid speciation even in the absence of divergent selection. For some time the founder effect speciation model and variants thereof were considered by many to be an important mode of speciation, especially on islands. However, they have received a great deal of criticism on theoretical grounds. One of the key objections is that even the superficially similar environments on the islands that Mayr discussed actually differ sufficiently in their abiotic and biotic environments for divergent natural selection to operate.

Given that allopatric speciation and sympatric speciation are two extremes of a continuum and that parapatric speciation covers everything in between, it appears likely that parapatric speciation is common. However, demonstrating that two species arose in parapatry has proven especially difficult. We often observe sister species with adjacent and abutting geographic ranges, sometimes with each species adapted to different habitats, exactly as expected under the parapatric model. But on the basis of this evidence alone, we cannot reject the possibility that such distribution patterns arose through allopatric speciation followed by range expansion and finally competition or hybridization along the abutting ranges (i.e., allo-parapatric speciation). One of the best examples of parapatric speciation in action comes from a grass species (Anthoxanthum oderatum) that has evolved tolerance to high levels of heavy metals in soils brought to the surface by miners. Accompanying the adaptation, some (though not complete) reproductive isolation through divergent flowering times has arisen between the population tolerant to heavy metals and nearby populations on normal soil.

Because theory tells us that the conditions under which allopatric speciation can proceed are much more permissive than those required for sympatric speciation, the most persuasive empirical evidence for sympatric speciation usually comes from sister species for which an allopatric phase can be ruled out or was unlikely. There now exist a handful of cases that meet this criterion. A famous example concerns two Amphilophus cichlid sister species confined to a single volcanic crater lake in Nicaragua. While the two species are genetically quite similar, implying that they diverged very recently (sometime in the last 10,000 years), they have divergent morphology, occupy different niches in the water column, and are reproductively isolated. Whether the ancestral population mated randomly, as required under our definition of sympatric speciation, we will perhaps never know. In the crater-lake cichlids, reproductive isolation seems likely to have accumulated as a by-product of disruptive natural selection and in the face of ongoing gene flow, but few cases of putative sympatric speciation fit this description. More often, it appears that there is a rapid shift to nonrandom mating from within an initially panmictic population, followed by a rapid decline in gene flow. For instance, in speciation via host shift, mating of the diverging forms is often assortative with respect to host type. In polyploid speciation (see chapter VI.9), an important mode of speciation in plants, reproductive isolation between polyploid and diploid individuals can be substantial or even complete instantaneously.

If we accept that allopatric, parapatric, and sympatric speciation is each possible in theory and sometimes occurs in nature, how much does each mode contribute to global diversity? As long ago as the turn of the twentieth century, David Starr Jordan and Moritz Wagner observed that most closely related species had nonoverlapping geographic distributions, which has long been held as evidence that allopatric and/or parapatric speciation predominate. More recently, Daniel Bolnick and Benjamin Fitzpatrick (2007) reported that 224 of 309 (72.2 percent) sister species included in recent studies had completely nonoverlapping geographic ranges. A problem with sister species comparisons is that we know from the fossil and pollen records that species geographic ranges sometimes shift substantially over the course of just a few thousand years, meaning that current distributions may not be informative regarding distributions during speciation. One solution to this problem is to consider the geographic distributions of the youngest sister species, for which postspeciation range movements should have been less extensive. Studies that combine information on phylogeny and geographic range reveal that the youngest sister species of many groups—including birds, mammals, and the South African cape flora—often have completely nonoverlapping ranges. In comparison, studies of some groups of herbivorous insects, most famously Rhagoletis pomonella (apple maggot fly), reveal substantial range overlap among young sister species or races, consistent with the important role of sympatric divergence in these groups but also explicable in terms of allopatric or parapatric speciation followed by range expansion. Interestingly, molecular analyses of apple maggot fly host races in North America by Jeffrey Feder and colleagues have shown that the phylogeographic history of the genes that underlie the sympatric host shift—from hawthorn to apple trees, which fruit earlier—is much more complex than had earlier been supposed. While the host shift occurred in sympatry, selection for divergence in the timing of emergence acted on genetic variation that arose much earlier in a more southerly allopatric population.

The contributions of different modes of geographic speciation will almost certainly vary among groups depending on their biology. In fact, some of the polarized opinions in the geography-of-speciation debate reflect the worldview of researchers as shaped by their own study taxa. For instance, ornithologists have been especially vocal proponents of allopatric speciation, which is not surprising given that most sister species of birds have nonoverlapping geographic distributions, whereas researchers working on herbivorous insects point to the frequency of host shifts and have been among the strongest advocates of sympatric models.

3. ISLAND PATTERNS AND THEIR IMPLICATIONS

As discussed earlier, the possibility that species’ geographic ranges move postspeciation makes it difficult to infer the geography of speciation from the contemporary distributions of closely related species. For species that are restricted to islands, however, it may be reasonable to assume that their geographic distributions have been more static through time, with expansions and shifts limited by physical constraints on dispersal. As a consequence, studies of island taxa have yielded important insights into the geography of speciation, a point to which we will return shortly.

Island systems (note that lakes and mountaintops can be viewed as island systems) also provide the replication required to conduct robust tests of evolutionary and ecological hypotheses; for this reason, islands are sometimes referred to as “natural laboratories.” Indeed, identifying the contributions of immigration, speciation, and extinction to the species richness of islands has proven much more tractable than teasing apart the mechanisms whereby they contribute to the latitudinal diversity gradients in continental and marine systems. Islands tend to have fewer species than continental regions of the same size. The community on an island close to a mainland source of colonists is liable to be composed of a sample of species that are present in the source area(s), implying that immigration is the main species input. Robert H. MacArthur and Edward O. Wilson’s (1967) Theory of Island Biogeography described how the species richness of island communities might represent a dynamic equilibrium between immigration and extinction. They suggested that as the species richness of an island increases, immigration rates decline and extinction rates increase, with equilibrium species richness located where the lines cross.

On more isolated islands, we often find endemic species (i.e., species that are unique to an island). In fact, as island isolation increases, avian species richness declines, but the number of endemic bird species actually increases. Note that isolation is relative to an organism’s dispersal ability, such that birds may frequently reach an island that is remote for other taxa such as amphibians. Two types of island endemic species, anagenetic and cladogenetic, are recognized on the basis of the geographic distribution of their relatives. An island endemic species is termed anagenetic if its closest relatives are not confined to the island, and cladogenetic if its closest relatives are endemic to the same island.

Anagenetic species are encountered most frequently on islands of intermediate isolation, suggesting that anagenetic speciation may be subject to a “Goldilocks effect.” On islands that are situated too close to a mainland source of immigrants, gene flow between immigrants and residents impedes speciation, whereas on islands that are too far from the mainland, too few individuals arrive to initiate speciation.

On the most isolated islands, cladogenetic speciation within islands may assume importance as the greatest source of island diversity. Two stunning examples are the radiations of ancestral cichlids into hundreds of species within each of the African Great Lakes—Victoria, Tanganyika, and Malawi—and within-island speciation of Anolis lizards to occupy distinct niches in the Greater Antilles (see chapter VI.10 for a fuller discussion of adaptive radiations).

When cladogenetic speciation creates two or more forms on a single island, we are often interested in determining whether speciation has been sympatric, parapatric, or allopatric. As well as offering evolutionary biologists the aforementioned advantage of making postspeciation range shifts less likely, isolated islands are expected to offer conditions that are conducive to sympatric speciation, such as unoccupied niches and increased intraspecific competition, which may promote disruptive selection. For these reasons, a team of researchers studied the geography of speciation of plants on the minute and remote Lord Howe Island (it has an area of less than 16 km², is located 600 km to the east of Australia, and is remote from any other island). Using molecular phylogenies, Alex Papadopulos and colleagues identified 11 cases in which the closest relative of a plant species endemic to Lord Howe Island was also endemic to the island (i.e., cladogenetic species). Although these cases are almost certainly sympatric under a biogeographic definition, we do not know whether they would satisfy our population genetic–based definition. In fact, in-depth study of one of these cladogenetic sister species, the Howea palms, revealed that, while speciation likely occurred in the face of strong gene flow, it was most likely parapatric under a population genetic definition. From this research into Lord Howe’s flora we might infer that within-island speciation in the face of gene flow is not as rare as has been suspected for some groups. However, as we will discuss, research into the relationship between area and speciation on islands reveals a somewhat different story about the relative frequency of different geographic modes of speciation.

4. SPECIATION AND AREA

The species-area relationship, a positive correlation between geographic area and species richness, is one of the most ubiquitous patterns in ecology. Across small geographic units this pattern is believed to arise solely from ecological processes, with communities assembled through immigration and local extinction. But across larger geographic units, speciation within the geographic unit makes an important contribution.

All else being equal, an increase in geographic area is expected to facilitate speciation. For instance, as a species’ geographic range size increases, so does the probability that a randomly placed knifelike barrier, such as a mountain range or river, will bisect it and give rise to vicariant speciation. A larger area should also present more opportunities for long-distance dispersal and colonization of new areas, thereby promoting peripatric speciation. Moreover, given that geographic area tends to correlate with habitat diversity, we expect to find that habitat-driven parapatric speciation will generally be more common in larger geographic areas.

Empirical evidence for the relationship between geographic area and speciation relies heavily on insights from island diversity patterns. Barring one as-yet-unproven case of buntings on remote islands in the Tristan da Cunha archipelago, there is no evidence that birds undergo within-island/cladogenetic speciation on any island smaller than Jamaica (11,189 km²). In comparison, within-island speciation of anoles, a speciose radiation of New World lizards, starts to make a substantial contribution to diversity on Caribbean islands larger than 3000 km². The observation that for both birds and anoles within-island speciation appears only on islands that are larger than a certain size implies that speciation is likely to be allopatric or parapatric and that sympatric speciation is unlikely to play a major role in either group.

Yael Kisel and Tim Barraclough (2010) explored the relationship between island area and the probability of observing cladogenetic species across a broad set of taxa, including birds, mammals, snails, and angiosperms, and found that the probability of within-island speciation generally increases with area. Moreover, when they compared the minimum-sized island on which within-island speciation occurs among groups, they found a positive correlation between island size and dispersal ability. In other words, snails are able to speciate clad-ogenetically on much smaller islands than are birds. This finding implies that the dominant geographic modes of speciation on islands are those that proceed more easily when dispersal (and therefore gene flow) is reduced or absent—namely, parapatric and allopatric speciation—rather than sympatric speciation.

It is becoming clear that the influence of available geographic area on speciation is more complex than outlined thus far. In an important recent paper, Daniel Rabosky and Richard Glor (2010) found that island area determines equilibrium species richness for anole lizards in the Greater Antilles, but not solely via immigration and extinction, as MacArthur and Wilson suggested. Rather, Rabosky and Glor reported that the speciation rate on each island started high and then declined through time, as would be expected if the diversity of anole species on an island induces a negative feedback on the speciation rate. They found that on the two smallest islands (Puerto Rico and Jamaica), speciation rates had started high in the past and rapidly declined to zero. In contrast, on the largest island, Cuba, the decline in speciation rate was much slower, and contemporary speciation rates are still above zero, meaning that the Anolis diversity on this island may still be increasing. This result represents some of the most compelling evidence that the processes responsible for diversity patterns involve an interaction between ecological limits (which are, in part, a property of the area available) and diversification rates.

5. GEOGRAPHIC AND GEOLOGICAL TRIGGERS OF SPECIATION

If geographic area places a limit on diversity, via either a negative feedback of diversity on speciation rates or a positive feedback on extinction rates, a corollary is that colonization of a geographic region where there are few competitors should present increased opportunities for diversification. Sometimes, an organism’s superior dispersal ability affords it the opportunity to colonize a new region before its competitors. Indeed, the success of anoles in dispersing to Caribbean islands and radiating there may be a case in point.

Alternatively, geological events may present taxa with new opportunities for colonization and diversification. For example, until the Panamanian land bridge joined North and South America, only stronger dispersers, such as some birds and plants, would cross between the two continents, meaning that many of the lineages making up the two biotas evolved in isolation from one another. The mammalian fossil record reveals that following formation of the land bridge about 3 million years ago, poorer dispersers were able to cross it in either direction for the first time. This event is called the Great American Interchange, and while it precipitated some extinction, it also produced some evolutionary winners. For instance, some lineages, such as the rodent subfamily Sigmodontinae from South America, diversified into many species on the new continent.

6. CHALLENGES AND PROSPECTS

After more than a century of research into the geography of speciation, some general principles appear to hold. For example, if we adopt the population genetic definition, sympatric speciation is unusual. On the basis of contemporary distributions it also seems likely that in many groups gene flow is low, though perhaps not absent, during speciation. But as yet, we do not know for sure what a plot of the amount of gene flow on the x-axis and the frequency of speciation on the y-axis would look like. Nor do we know whether this distribution changes at different stages of speciation. For instance, perhaps speciation often begins in the absence of gene flow but is completed in the face of some gene flow. Recent work on speciation has seen a shift in focus away from classifying cases into broad geographic modes and toward characterizing and quantifying the processes involved. Thus, new questions have arisen, for example, How important are biotic versus abiotic drivers of ecological speciation?

Our understanding of how speciation contributes to broad-scale geographic patterns is also in a state of flux. An explosion in the availability of molecular phylogenies for many groups is yielding fascinating insights into the role of speciation, extinction, and time to species diversity patterns. However, findings such as those of Rabosky and Glor for islands in the Greater Antilles highlight complex interactions among geography, ecology, and diversification. Thus, two questions arise: First, do the same general principles that apply to island diversity also apply to broad-scale continental and marine diversity patterns? Second, if diversity limits are an emergent property of a feedback of diversity on diversification, how elastic are these limits? In light of the increasing availability of molecular and paleontological information, there are good prospects for obtaining answers to question such as these in the coming years.