Speciation In Allopatry Research Articles

Evolution of choosiness dictates whether search costs of mate choice enhance speciation by sexual selection

The role of sexual selection in speciation is implicated in both empirical case studies and larger comparative works. However, sexual selection faces two major problems in driving speciation. First, because females with novel preferences search for their initially rare males, search costs are expected to curtail initial sexual divergence. Second, if these populations come back into sympatry, sexual divergence may be erased due to hybridization. A major goal is to understand which conditions increase the likelihood of overcoming these problems. Here we generated a diploid population genetic model of how female search costs and evolution of female 'choosiness' (i.e. preference strength) interact to drive speciation in allopatry and secondary contact. We studied the model using numerical simulations in the context of two different male traits, ecologically 'arbitrary' versus 'magic' traits. First, in allopatry, without female search costs only minor and fluctuating sexual isolation evolved. In contrast, with female search costs, sexual isolation was highly curtailed with arbitrary male traits but was greatly facilitated with magic traits. However, because search costs selected for reduced choosiness, sexual isolation with magic traits was eventually eroded, the rate determined by the genetic architecture of choosiness. These factors also played a key role in secondary contact; with evolvable choosiness and female search costs, pure sexual selection models collapsed upon secondary contact. However, when we added selection against hybrids (i.e. reinforcement) to this model, we found that speciation could be maintained under a wide range of conditions with arbitrary male traits, but not with magic male traits. This surprisingly suggests that arbitrary male traits are in some cases more likely to aid speciation than magic male traits. We discuss these findings and relate them to empirical literature on female choosiness within species and in hybrids.

Biogeographic Drivers of Evolutionary Radiations

Some lineages radiate spectacularly when colonizing a region, but others do not. Large radiations are often attributed to species’ adaptation into niches, or to other drivers, such as biogeography including dispersal ability and spatial structure of the landscape. Here we aim to disentangle the factors determining radiation size, by modeling simplified scenarios without the complexity of explicit niches. We build a spatially structured neutral model free from niches and incorporating a form of protracted speciation that accounts for gene flow between populations. We find that a wide range of radiation sizes are possible in this model depending on the combination of geographic isolation and species’ dispersal ability. At extremely low rates of dispersal between patches, each patch maintains its own endemic species. Intermediate dispersal rates foster larger radiations as they allow occasional movement between patches whilst sufficiently restricting gene flow to support further speciation in allopatry. As dispersal rates increase further, a critical point is reached at which demographically identical lineages may vary greatly in radiation size due to rare and stochastic dispersal events. At the critical point in dispersal frequency, some lineages remain a single species for a comparatively long time, whilst others with identical characteristics produce the largest radiations of all via a new mechanism for rapid radiation that we term a ‘radiation cascade’. Given a single species covering many patches connected with gene flow, a radiation cascade is triggered when stochastic dispersal is unusually low for a period, leading to an initial speciation event. This speciation means there are fewer individuals per species and thus further reduced gene flow between conspecifics. Reduced gene flow in turn makes it easier for further speciation to occur. During a radiation cascade, dispersal of individuals between patches continues at the same rate as before, but due to the increasing diversity it primarily introduces novel species that will later speciate, rather than adding to gene flow of existing species. Once a radiation cascade begins, it continues rapidly until it is arrested by a new equilibrium between speciation and extinction. We speculate that such radiation cascades may occur more generally and are not only present in neutral models. This process may help to explain rapid radiation, and the extreme radiation sizes of certain lineages with dispersing ancestors. Whilst niches no doubt play a role in community assembly, our findings lead us to question whether diversification and adaptation into niches is sometimes an effect of speciation and rapid radiation, rather than its cause.

Geographic and Ecological Dimensions of Host Plant-Associated Genetic Differentiation and Speciation in the Rhagoletis cingulata (Diptera: Tephritidae) Sibling Species Group.

Ascertaining the causes of adaptive radiation is central to understanding how new species arise and come to vary with their resources. The ecological theory posits adaptive radiation via divergent natural selection associated with novel resource use; an alternative suggests character displacement following speciation in allopatry and then secondary contact of reproductively isolated but ecologically similar species. Discriminating between hypotheses, therefore, requires the establishment of a key role for ecological diversification in initiating speciation versus a secondary role in facilitating co-existence. Here, we characterize patterns of genetic variation and postzygotic reproductive isolation for tephritid fruit flies in the Rhagoletis cingulata sibling species group to assess the significance of ecology, geography, and non-adaptive processes for their divergence. Our results support the ecological theory: no evidence for intrinsic postzygotic reproductive isolation was found between two populations of allopatric species, while nuclear-encoded microsatellites implied strong ecologically based reproductive isolation among sympatric species infesting different host plants. Analysis of mitochondrial DNA suggested, however, that cytoplasmic-related reproductive isolation may also exist between two geographically isolated populations within R cingulata. Thus, ecology associated with sympatric host shifts and cytoplasmic effects possibly associated with an endosymbiont may be the key initial drivers of the radiation of the R. cingulata group.

Contrasting evolutionary histories in Neotropical birds: Divergence across an environmental barrier in South America.

Avian diversity in the Neotropics has been traditionally attributed to the effect of vicariant forces promoting speciation in allopatry. Recent studies have shown that phylogeographical patterns shared among codistributed species cannot be explained by a single vicariant event, as species responses to a common barrier depend on the biological attributes of each taxon. The open vegetation corridor (OVC) isolates Amazonia and the Andean forests from the Atlantic Forest, creating a notorious pattern of avian taxa that are disjunctly codistributed in these forests. Here, we studied and compared the evolutionary histories of Ramphotrigon megacephalum and Pipraeidea melanonota, two passerines with allopatric populations east and west of the OVC that represent different subspecies. These species differ in their biological attributes: R.megacephalum is a sedentary, forest specialist mostly confined to bamboo understorey, whereas P.melanonota is a seasonal migrant and generalist species that ranges in a variety of closed and semi-open environments. We performed genetic and genomic analyses, complemented with the study of coloration and behavioural differentiation, to assess population divergence across the OVC. We found that the evolutionary histories of both R.megacephalum and P.melanonota have been shaped by this environmental barrier. However, these species responded in different and asynchronous manners to the establishment of the OVC and to past connections between the currently isolated South American forests, which can be mostly explained by their distinct ecologies and dispersal abilities. Our results support the fact that the biological attributes of species can make their evolutionary histories idiosyncratic.

Do Holarctic ant species exist? Trans‐Beringian dispersal and homoplasy in the Formicidae

AbstractAimContinents harbour unique faunas, and only a small percentage of species naturally inhabit more than a single continent. This pattern is most evident in the insects, a morphologically small and extremely diverse group. Nevertheless, 12 species of ants have traditionally been recognized as native to both North America and Eurasia, the Holarctic region. Since intercontinental dispersal is presumably rare in ants, allopatric speciation in the absence of gene flow can be expected over evolutionary time. Here, we reassess the existence of Holarctic ant species and reconstruct their biogeographical history.LocationThe Holarctic.TaxonAll known ant species with purportedly Holarctic distributions.MethodsWe reconstructed the phylogenetic relationships, biogeographical history and reassessed the taxonomic status of all known ants with Holarctic distributions using genetic data based on one mitochondrial and three nuclear genes and an ancestral area reconstruction of 310 specimens and 73 species (the 12 Holarctic species plus outgroup taxa).ResultsContrary to the currently accepted hypothesis, only three ant species have Holarctic native ranges, while six taxa separate into distinct Palearctic and Nearctic species. Four species are shown to be recent introductions from Europe to North America by human activity, one of which was thought to be native. Genetic diversity is considerably higher within the North American than within European species as currently defined.Main conclusionsThe Formicidae have repeatedly dispersed through Beringia, during and after land bridge formation, and in both directions between the Palearctic and Nearctic regions. However, only three cold‐tolerant species crossed the Bering Strait in relatively recent time. Our results highlight the potential existence of many unknown Nearctic ant taxa. Reliance on an evolutionarily labile morphological character, erect hairs, seems to have obscured species delimitation in these ant taxa. Based on our investigation, the typical time for speciation in allopatry for ants is 2–5 Ma.

Male-driven reproductive and agonistic character displacement in darters and its implications for speciation in allopatry.

Selection against hybridization can cause mating traits to diverge between species in sympatry via reproductive character displacement (RCD). Additionally, selection against interspecific fighting can cause aggressive traits to diverge between sympatric species via agonistic character displacement (ACD). By directly affecting conspecific recognition traits, RCD and ACD between species can also incidentally cause divergence in mating and fighting traits among populations within a species [termed cascade RCD (CRCD) and cascade ACD]. Here, we demonstrate patterns consistent with male-driven RCD and ACD in 2 groups of darters (orangethroat darter clade Ceasia and rainbow darter Etheostoma caeruleum). In both groups, males that occur in sympatry (between Ceasia and E. caeruleum) have higher levels of preference for mating and fighting with conspecifics over heterospecifics than do males from allopatry. This is consistent with RCD and ACD. We also found patterns consistent with CRCD and cascade ACD among species of Ceasia. Ceasia males that are sympatric to E. caeruleum (but allopatric to one another) also have heightened preferences for mating and fighting with conspecific versus heterospecific Ceasia. In contrast, Ceasia males that are allopatric to E. caeruleum readily mate and fight with heterospecific Ceasia. We suggest that RCD and ACD between Ceasia and E. caeruleum has incidentally led to divergence in mating and fighting traits among Ceasia species. This study is unique in that male preferences evolve via both RCD (male preference for conspecific females) and ACD (male preference to fight conspecific males) which leads to subsequent divergence among allopatric lineages.

Speciation: Where Are We Now? An Introduction to a Special Issue on Speciation

Although modern evolutionary biology started with a seminal volume whose title identified the origin of species as the central theme of the new theory (Darwin 1859), the topic of speciation received relatively little attention for several decades following the publication of the origin of species. Darwin and his contemporaries devoted much more attention to explain how changes occurred within species rather than how species originated. In fact, a search of the published scientific literature using web of science reveals that between 1864 and 1939 only 21 journal articles had the word speciation in their title (Fig. 1). As already summarized by Coyne and Orr (2004) interest in the origin of species greatly increased during the development of the modern synthesis, when mendelian genetics was reconciled with biogeography and natural selection by people like Dobzhansky (1937) and Mayr (1942). Mayr was the first to focus on the importance of species, introducing the biological species concept (BSC), which has dominated speciation research for the past seven decades, and championed the idea of speciation in allopatry. Dobzhansky’s work pointed out the importance of understanding how changes in allele frequencies could produce genetically distinct groups and the importance of reproductive isolating mechanisms. Between the 1940s and the 1970s, while the biogeography of speciation remained an active area of research for systematists and naturalists, the mechanisms of origin of barriers to reproductive isolation received relatively little attention by geneticists. During that time period most workers in fact remained more interested in demonstrating the strength and influence of natural selection in the wild, and in explaining how genetic variation accumulates and persists within species (e.g., Lewontin 1974). Starting in the 1980s however, the availability of new empirical tools such as molecular genetics, and theoretical and methodological approaches, such as phylogenetic and comparative methods, led to resurgence in interest in the origin of species. Speciation research, once predominantly the domain of systematists, paleontologists, and some geneticists, started to attract the interests of workers in other fields of biology, such as ecologists, ethologists, genome biologists, and developmental biologists. This caused a shift from largely pattern-oriented studies of speciation, in which often the description of patterns was accompanied by the suggestion of some (often untestable) hypotheses regarding what might have caused the events, to more process-oriented studies, in which attempts were made to directly test and explore the process. This resurgence of interest in speciation has been one of the main developments in evolutionary biology during the past 25 years, and has led to a new phase of speciation studies (Coyne and Orr 2004). During this new phase many of the major conclusions about speciation reached since 1859 have been re-examined. The debate over species concepts, once mostly the domain of systematists and philosophers, was reinvigorated through the active involvement of students of different branches of biology (e.g., F. Santini (&) Department of Ecology and Evolutionary Biology, University of California at Los Angeles, 610 Charles Young Dr. South, Los Angeles, CA 90095, USA e-mail: santini@ucla.edu

Genetic differentiation among populations of a Hispaniolan trunk anole that exhibit geographical variation in dewlap colour

Long neglected by classic island biogeographical theory, speciation within and among islands is increasingly recognized as a major contributor to insular diversity. Although the factors responsible for island speciation remain poorly understood, this process appears critically dependent on geographical variation and speciation in allopatry or parapatry. Here, we investigate geographical variation and speciation in a complex of Hispaniolan trunk anoles (Anolis distichus), where populations with strikingly distinct dewlap colours and patterns correspond with deeply divergent mtDNA structure. Using a multilocus, population-level analysis, we investigate whether these phenotypically and mitochondrially distinct populations exhibit the type of nuclear differentiation expected among species or incipient species. Along a transect that extends across a recently recessed marine barrier, our results are consistent with the persistence of an abrupt phenotypic and mitochondrial transition following secondary contact, in spite of little or no evidence for a reduction in nuclear gene flow. Along a second transect extending across a steep environmental gradient, our phenotypic and microsatellite data suggest a sharp genetic break with little or no admixture, whereas mtDNA recovers a signature of extensive unidirectional introgression. Together, these results are consistent with previous studies of Lesser Antillean anoles, suggesting that allopatric divergence alone is insufficient for speciation, whereas reduced gene flow and partial reproductive isolation may accumulate in the presence of ecological gradients.

Phylogeny of the Yponomeuta species (Lepidoptera, Yponomeutidae) and the history of their host plant associations

This study presents the results of cladistic analyses of the morphology of 1) the subfamilies of the Yponomeutidae, 2) the genera of the Yponomeutinae, and 3) the species of Yponomeuta Latreille, 1796. The sequential steps in the evolution of the host plant associations, the presumed key factor in the processes of speciation within Yponomeuta, as extrapolated from the cladograms are discussed. The hypothesis that the present-day host plant associations evolved from an ancestral relation with Celastraceae through speciation in allopatry mostly on Euonymus is supported by the underlying study. The biogeographical patterns suggest speciation through dispersion from Australia, the Oriental region, Africa to western Europe.

Divergence times and origin of neotropical sheath-tailed bats (Tribe Diclidurini) in South America

Times of divergence and origin of sheath-tailed bats (family Emballonuridae) in the New World were approximated with a relaxed molecular clock approach using Bayesian analysis of introns from the three nuclear genetic transmission systems in mammals (autosomal, X and Y sex chromosomes). An upper constraint of 30 mya for the oldest known Neotropical emballonurid fossil and a lower constraint of 13 mya for the only pre-Pleistocene fossil of an extant genus were used as calibration points. Differentiation began in the Late Oligocene with the appearance of two subtribes as independently corroborated by each gene. Following an explosive model of evolution, the genera diversified relatively suddenly in the Early Miocene with seven of the eight genera radiating within 1.4 myr and most intrageneric speciation occurring before the Pliocene. Optimization of ancestral areas onto the phylogeny suggests that the ancestor of New World emballonurid bats has its origin in Africa and this is the third report of placental mammals colonizing South America by trans-Atlantic dispersal and subsequent speciation in allopatry.

Editorial and Retrospective 2007

Editorial and Retrospective 2007

Mitochondrial phylogeny of African wood mice, genus Hylomyscus (Rodentia, Muridae): Implications for their taxonomy and biogeography

This paper investigates the usefulness of two mitochondrial genes (16S rRNA and cytochrome b) to solve taxonomical difficulties within the genus Hylomyscus and to infer its evolutionary history. Both genes proved to be suitable molecular markers for diagnosis of Hylomyscus species. Nevertheless the resolving powers of these two genes differ, and with both markers (either analyzed singly or in combination), some nodes remain unresolved. This is probably related to the fact that the species emerged during a rapid diversification event that occurred 2–6 Myr ago (4–5 Myr ago for most divergence events). Our molecular data support the recognition of an “ aeta” group, while the “ alleni” and “ parvus” groups are not fully supported. Based on tree topology and genetic divergence, two taxa generally recognized as subspecies should be elevated at the species level ( H. simus and H. cf kaimosae). H. stella populations exhibit ancient haplotype segregation that may represent currently unrecognized allopatric species. The existence of cryptic species within H. parvus is questioned. Finally, three potentially new species may occur in West Central Africa. The Congo and Oubangui Rivers, as well as the Volta and Niger Rivers and/or the Dahomey gap could have formed effective barriers to Hylomyscus species dispersal, favoring their speciation in allopatry. The pronounced shifts in African climate during the late Pliocene and Miocene, which resulted in major changes in the distribution and composition of the vegetation, could have promoted speciation within the genus (refuge theory). Future reports should focus on the geographic distribution of Hylomyscus species in order to get a better understanding of the evolutionary history of the genus.

EVOLUTIONARY RADIATION OF "STONE PLANTS" IN THE GENUS ARGYRODERMA (AIZOACEAE): UNRAVELING THE EFFECTS OF LANDSCAPE, HABITAT, AND FLOWERING TIME

Recent phylogenetic evidence suggests that the extraordinary diversity of the Cape Floristic Kingdom in South Africa may be the result of widespread evolutionary radiation. Our understanding of the role of adaptive versus neutral processes in these radiations remains largely speculative. In this study we investigated factors involved in the diversification of Argyroderma, a genus within the most spectacular of the Cape radiations, that of the Ruschioid subfamily of the Aizoaceae. We used amplified fragment length polymorphisms and a suite of morphological traits to elucidate patterns of differentiation within and between species of Argyroderma across the range of the genus. We then used a matrix correlation approach to assess the influence of landscape structure, edaphic gradients, and flowering phenology on phenotypic and neutral genetic divergence in the system. We found evidence for strong spatial genetic isolation at all taxonomic levels. In addition, genetic differentiation occurs along a temporal axis, between sympatric species with divergent flowering times. Morphological differentiation, which previous studies suggest is adaptive, occurs along a habitat axis, between populations occupying different edaphic microenvironments. Morphological differentiation is in turn significantly associated with flowering time shifts. Thus we propose that diversification within Argyroderma has occurred through a process of adaptive speciation in allopatry. Spatially isolated populations diverge phenotypically in response to divergent habitat selection, which in turn leads to the evolution of reproductive isolation through divergence of flowering phenologies, perhaps as a correlated response to morphological divergence. Evidence suggests that diversification of the group has proceeded in two phases: the first involving divergence of allopatric taxa on varied microhabitats within a novel habitat type (the quartz gravel plains), and the second involving range expansion of an early flowering phenotype on the most extreme edaphic habitat and subsequent incomplete differentiation of allopatric populations of the early flowering group. These results point to adaptive speciation in allopatry as a likely model for the spectacular diversification of the ice-plant family in the dissected landscapes of the southern African winter rainfall deserts.

EVOLUTIONARY RADIATION OF “STONE PLANTS” IN THE GENUS ARGYRODERMA (AIZOACEAE): UNRAVELING THE EFFECTS OF LANDSCAPE, HABITAT, AND FLOWERING TIME

Recent phylogenetic evidence suggests that the extraordinary diversity of the Cape Floristic Kingdom in South Africa may be the result of widespread evolutionary radiation. Our understanding of the role of adaptive versus neutral processes in these radiations remains largely speculative. In this study we investigated factors involved in the diversification of Argyroderma, a genus within the most spectacular of the Cape radiations, that of the Ruschioid subfamily of the Aizoaceae. We used amplified fragment length polymorphisms and a suite of morphological traits to elucidate patterns of differentiation within and between species of Argyroderma across the range of the genus. We then used a matrix correlation approach to assess the influence of landscape structure, edaphic gradients, and flowering phenology on phenotypic and neutral genetic divergence in the system. We found evidence for strong spatial genetic isolation at all taxonomic levels. In addition, genetic differentiation occurs along a temporal axis, between sympatric species with divergent flowering times. Morphological differentiation, which previous studies suggest is adaptive, occurs along a habitat axis, between populations occupying different edaphic microenvironments. Morphological differentiation is in turn significantly associated with flowering time shifts. Thus we propose that diversification within Argyroderma has occurred through a process of adaptive speciation in allopatry. Spatially isolated populations diverge phenotypically in response to divergent habitat selection, which in turn leads to the evolution of reproductive isolation through divergence of flowering phenologies, perhaps as a correlated response to morphological divergence. Evidence suggests that diversification of the group has proceeded in two phases: the first involving divergence of allopatric taxa on varied microhabitats within a novel habitat type (the quartz gravel plains), and the second involving range expansion of an early flowering phenotype on the most extreme edaphic habitat and subsequent incomplete differentiation of allopatric populations of the early flowering group. These results point to adaptive speciation in allopatry as a likely model for the spectacular diversification of the ice-plant family in the dissected landscapes of the southern African winter rainfall deserts.

Habitat selection and ecological speciation in Galápagos warbler finches ( Certhidea olivacea and Certhidea fusca )

We investigated phylogeographic divergence among populations of Galápagos warble finches. Their broad distribution, lack of phenotypic differentiation and low levels of genetic divergence make warbler finches an appropriate model to study speciation in allopatry. A positive relationship between genetic and geographical distance is expected for island taxa. Warbler finches actually showed a negative isolation by distance relationship, causing us to reject the hypothesis of distance-limited dispersal. An alternative hypothesis, that dispersal is limited by habitat similarity, was supported. We found a positive correlation between genetic distances and differences in maximum elevation among islands, which is an indicator of ecological similarity. MtDNA sequence variation revealed monophyletic support for two distinct species. Certhidea olivacea have recently dispersed among larger central islands, while some Certhidea fusca have recently dispersed to small islands at opposite ends of the archipelago. We conclude that females have chosen to breed on islands with habitats similar to their natal environment. Habitat selection is implicated as an important component of speciation of warbler finches, which is the earliest known divergence of the adaptive radiation of Darwin's finches. These results suggest that small populations can harbour cryptic but biologically meaningful variation that may affect longer term evolutionary processes.

Sexual selection and peripatric speciation: the Kaneshiro model revisited

Abstract The Kaneshiro model proposes a role for sexual selection in peripatric speciation. During population bottlenecks, derived males lose attractive traits and become discriminated against by ancestral females, whereas derived females are selected to be less choosy. This permits novel mate choice cues to evolve in derived populations. In a quantitative analysis of laboratory experiments, we show that bottlenecked males have indeed become less attractive, but females have not lost their ancestral preferences. Contrary to the model, mating asymmetries were not limited to bottlenecks, but regularly occurred between derived and ancestral populations. The simplest explanation for the observed mating asymmetries is loss of genetic variation and inbreeding in the derived populations. The Kaneshiro process is unlikely to isolate small daughter populations from their ancestor but with slight modifications it may become a strong candidate for speciation in allopatry manifested as isolation between daughter populations in secondary contact.

Raciation and speciation in house mice from the Alps: the role of chromosomes.

There are at least 24 different karyotypic races of house mouse in the central Alps, each characterized by a different complement of ancestral acrocentric and derived metacentric chromosomes; altogether 55 different metacentric chromosomes have been described from the region. We argue that this chromosome variation largely arose in situ. If these races were to make contact, in most cases they would produce F1 hybrids with substantial infertility (sometimes complete sterility), due to nondisjunction and germ cell death associated with the formation of long-chain and/or ring configurations at meiosis. We present fertility estimates to confirm this for two particular hybrid types, one of which demonstrates male-limited sterility (in accordance with Haldane's Rule). As well as a model for speciation in allopatry, the Alpine mouse populations are of interest with regards speciation in parapatry: we discuss a possible reinforcement event. Raciation of house mice appears to have happened on numerous occasions within the central Alps. To investigate one possible source of new karyotypic races, we use a two-dimensional stepping stone model to examine the generation of recombinant races within chromosomal hybrid zones. Using field-derived ecological data and laboratory-derived fertility estimates, we show that hybrid karyotypic races can be generated at a reasonable frequency in simulations. Our model complements others developed for flowering plants that also emphasize the potential of chromosomal hybrid zones in generating new stable karyotypic forms.

Evolutionary divergence between sympatric species of southern African Hakes, Merluccius capensis and M. paradoxus. I. Electrophoretic analysis of proteins

We estimated the amount of genetic divergence between two morphologically similar species of southern African hake, Merluccius capensis and M. paradoxus, with the electrophoretic analysis of proteins encoded by 31 loci. Nei's genetic distance between these taxa was 0·583 (±0·160) and is typical of evolutionary divergence between well differentiated congeneric species. We found no evidence of hybrid individuals. The mean heterozygosity over 13 samples of M. capensis was 0·055 and over 10 samples of M. paradoxus was 0·067. The present sympatric distributions of these fish are most likely the result of secondary contact after speciation in allopatry or the result of repeated dispersals of ancestral populations of other Atlantic Ocean hakes to southern Africa, rather than the result of sympatric speciation. There were significant excesses of rare alleles in both species as compared with that expected for neutral alleles in species at drift-mutation equilibrium. Average heterozygosities, however, were not appreciably reduced in comparison with other marine fish. Using genetic distance and the assumptions of the molecular clock, we estimate that the lineages leading to these species diverged from one another between 7 and 13 million years ago.

Darwin's finches: Population variation and sympatric speciation

The classical model of the adaptive radiation of Darwin's finches is one of repeated speciation in allopatry. Evidence presented here suggests that sympatric specification may have contributed to the radiation. On Isla Genovesa Geospiza conirostris displays several features that are consistent with a model of sympatric speciation. Males are polymorphic in song type. Those singing song A have significantly longer bills than those singing song B. The two groups of males forage in different ways that are functionally associated with the bill differences, particularly in the nonbreeding season when food is probably limiting. Territories of mated song A and song B males alternate in space, whereas territories of unmated males do not. This suggests that females can discriminate between males on the basis of song and position, and the pattern is consistent with a hypothesis of assortative mating within song groups. The population is therefore polymorphic; the morphs occupy different niches in which they may be separately regulated and they could be on the way to achieving full reproductive isolation through assortive mating. It is suggested that the population may oscillate between fission and fusion tendencies due to a changing selection regime in this variable and unpredictable environment. There is no evidence that one of the morphs originated allopatrically and then immigrated to Genovesa. The possibility of sympatric speciation being partly responsible for the adaptive radiation, dismissed more than 30 years ago, should be reinstated.