Biology subjects

Montgomery, S. H.

Publications and source records attributed to Montgomery, S. H..

At least 37 records · Page 2Linked to original sources

bioRxiv2024

Genomics of Neotropical biodiversity indicators: two butterfly radiations with rampant chromosomal rearrangements and hybridisation

A major question in evolutionary biology is what drives the diversification of lineages. Rapid, recent radiations are ideal systems for addressing how new species arise because they still show key morphological and ecological adaptations associated with speciation. While most studied recent radiations have evolved in an insular environment, less research has been carried out on continental radiations with complex species interactions. Melinaea and Mechanitis butterflies (Nymphalidae: Ithomiini) have rapidly radiated in the Neotropics. They are classical models for Amazonian biogeography and colour pattern mimicry and have been proposed as biodiversity indicators. We generated reference genomes for five species of each genus, and whole-genome resequencing data of most species and subspecies covering a wide geographic range to assess phylogeographic relationships, patterns of hybridisation and chromosomal rearrangements. Our data help resolve the classification of these taxonomically challenging butterflies and reveal very high diversification rates. We find rampant evidence of historical hybridisation and putative hybrid species in both radiations, which may have facilitated their rapid diversification. Moreover, dozens of chromosomal fusions and fissions were identified between congeneric species, and even some within species. We conclude that interactions between geography, hybridisation and chromosomal rearrangements have contributed to these two rapid radiations in the highly diverse Neotropical region. We suggest that rapid radiations may be spurred by repeated periods of geographic isolation during Pleistocene climate oscillations, combined with lineage-specific rapid accumulation of incompatibilities during allopatric phases, followed by secondary contact with some gene exchange. Significance StatementUnderstanding factors contributing to rapid speciation is a key aim of evolutionary biology. Here we focus on two rapid radiations of Neotropical butterflies. Our genomic data with broad taxonomic and geographic coverage reveal rampant hybridisation and chromosomal rearrangements, each likely contributing to the high diversification rates. Our study highlights the use of genomic data to resolve taxonomically challenging species groups and elucidate drivers of diversification in rapid radiations. We show that for biodiversity hotspots with recent radiations, barcoding is insufficient to characterise species richness due to gene flow and recent speciation. The taxonomic implications of both introgression and karyotype diversity for species delimitation are important to consider during monitoring and management of biodiversity in these vulnerable habitats.

evolutionary biology↗

bioRxiv2024

Repeated evolution of reduced visual investment at the onset of ecological speciation in high-altitude Heliconius butterflies.

Colonisation of new habitats is typically followed by divergent selection acting on traits that are immediately important for fitness in the new habitat. For example, shifting sensory environments are often associated with variation in sensory traits critical for navigation and foraging. However, the extent to which the initial response to novel sensory conditions is mediated by phenotypic plasticity, and its contribution to early species divergence remains unclear. We took advantage of repeated cases of speciation in Heliconius butterflies with independent allopatric distributions in the west of the Colombian and Ecuadorian Andes. Using volumetric brain measurements, we analysed patterns of investment in sensory processing in brain components across different localities and habitats. We find that a higher-altitude species, H. chestertonii, differs in levels of investment in visual and olfactory brain centres compared to its lower altitude relative H. erato venus, mainly attributable to heritable variation as inferred from comparisons between wild and common-garden reared individuals. We compared these shifts with those reported for another high-altitude species, H. himera, and its parapatric lowland counterpart, H. erato cyrbia, and demonstrate parallel reductions in the size of specific optic lobe neuropils. Conversely, for the antennal lobe, we detected disparate trait shifts in H. himera and H. chestertonii in respect to their lowland erato neighbours. Overall, our findings add weight to the adaptive potential for neuroanatomical divergence related to sensory processing during early species formation. Lay summaryRepeated associations between trait variation and environmental shifts may indicate adaptation to local sources of natural selection. For instance, in fish, the presence of certain morphological traits in specific ecological conditions across independent populations is well documented, suggesting equivalent phenotypic responses to shared sources of natural selection. We compared independent cases of ecological divergence in Heliconius butterflies distributed along altitude gradients from sea level to mid mountain in the west of the Colombian and Ecuadorian Andes. Shifts in altitude involve repeated, abrupt transitions from wet, large-leaved, warm forests to higher dry, open, cold scrubs. We tested hypotheses about the role of these ecological shifts in driving adaptive evolution in neuroanatomical traits during early speciation. We showed that in Heliconius, independent changes in forest-type have been accompanied by heritable parallel patterns of divergence in sensory investment in visual processing in the brain. We propose these differences likely facilitate species divergence in the face of ongoing geneflow.

evolutionary biology↗

bioRxiv2024

Mosaic evolution of a learning and memory circuit in Heliconiini butterflies

A critical function of central neural circuits is to integrate sensory and internal information to cause a behavioural output. Evolution modifies such circuits to generate adaptive change in sensory detection and behaviour, but it remains unclear how selection does so in the context of existing functional and developmental constraints. Here, we explore this question by analysing the evolutionary dynamics of insect mushroom body circuits. Mushroom bodies are constructed from a conserved wiring logic, mainly consisting of Kenyon cells, dopaminergic neurons and mushroom body output neurons. Kenyon cells carry sensory identity signals, which are modified in strength by dopaminergic neurons and carried forward into other brain areas by mushroom body output neurons. Despite the conserved makeup of this circuit, there is huge diversity in mushroom body size and shape across insects. However, an empirical framework of how evolution modifies the function and architecture of this circuit is largely lacking. To address this, we leverage the recent radiation of a Neotropical tribe of butterflies, the Heliconiini (Nymphalidae), which show extensive variation in mushroom body size over comparatively short phylogenetic timescales, linked to specific changes in foraging ecology, life history and cognition. To understand the mechanism by which such an extensive increase in size is accommodated through changes in lobe circuit architecture, we first combined immunostainings of structural markers, neurotransmitters and neural injections to generate, to our knowledge, the most detailed description of a Papilionoidea butterfly mushroom body lobe. We then provide a comparative, quantitative dataset which shows that some Kenyon cell populations expanded with a higher rate than others in Heliconius, providing an anatomical parallel to specific shifts in behaviour. Finally, we identified an increase in GABA-ergic feedback neurons essential for non-elemental learning and sparse coding, but conservation in dopaminergic neuron number. Taken together, our results demonstrate mosaic evolution of functionally related neural systems and cell types and identify that evolutionary malleability in an architecturally conserved parallel circuit guides adaptation in cognitive ability.

evolutionary biology↗

bioRxiv2024

Sensory weighting reflects changing patterns of visual investment during ecological divergence in Heliconius butterflies.

Integrating information across sensory modalities enables animals to orchestrate a wide range of complex behaviours. The relative importance placed on one sensory modality over another reflects the reliability of cues in a particular environment and corresponding differences in neural investment. As populations diverge across environmental gradients, the reliability of sensory cues may shift, favouring divergence in neural investment and the weight given to different sensory modalities. During their divergence across closed-forest and forest-edge habitats, closely related butterflies Heliconius cydno and H. melpomene evolved distinct brain morphologies, with the former investing more in vision. Quantitative genetic analyses suggest selection drove these changes, but their behavioural effects remain uncertain. We hypothesised that divergent neural investment may alter sensory weighting. We trained individuals in an associative learning experiment using multimodal colour and odour cues. When positively rewarded stimuli were presented in conflict pairing positively trained colour with negatively trained odour, and vice-versa, H. cydno favoured visual cues more strongly than H. melpomene. Hence, differences in sensory weighting may evolve early during divergence and are predicted by patterns of neural investment. These findings, alongside other examples, imply that differences in sensory weighting stem from divergent investment as adaptations to local sensory environments.

evolutionary biology↗

bioRxiv2024

Selfish herd effects in aggregated caterpillars and their interaction with warning signals

Larval Lepidoptera gain survival advantages by aggregating, especially when combined with aposematic warning signals, yet reductions in predation risk may not be experienced equally across all group members. Hamiltons selfish herd theory predicts that larvae which surround themselves with their group mates should be at lower risk of predation, and those on the periphery of aggregations experience the greatest risk, yet this has rarely been tested. Here, we expose aggregations of artificial caterpillar targets to predation from free-flying, wild birds to test for marginal predation when all prey are equally accessible, and for an interaction between warning colouration and marginal predation. We find that targets nearer the centre of the aggregation survived better than peripheral targets and nearby targets isolated from the group. However, there was no difference in survival between peripheral and isolated targets. We also find that grouped targets survived better than isolated targets when both are aposematic, but not when they are non-signalling. Our data suggest that avian predators preferentially target peripheral larvae from aggregations, and that prey warning signals enhance predator avoidance of groups.

ecology↗

bioRxiv2023

Adaptive divergence in the eyes of Heliconius butterflies likely contributes to pre- and post mating isolation

When populations experience different sensory conditions, natural selection may favor whole sensory system divergence, from the peripheral structures to the brain. We characterized the outer eye morphology of sympatric Heliconius species from different forest types, and their first-generation reciprocal hybrids to test for adaptive visual system divergence and hybrid disruption. In Panama, Heliconius cydno occurs in closed forests, whereas Heliconius melpomene resides in more open areas. Previous work has shown that, among wild individuals, H. cydno has larger eyes than H. melpomene, and there are heritable, habitat-associated differences in the visual brain structures that exceed neutral divergence expectations. Notably, hybrids have intermediate neural phenotypes, suggesting disruption. To test for similar effects in the visual periphery, we reared both species and their hybrids in common garden conditions. We confirm that H. cydno has larger eyes and provide new evidence that this is driven by selection. Hybrid eye morphology is more H. melpomene-like despite body size being intermediate, contrasting with neural trait intermediacy. Thus, eye morphology differences between H. cydno and H. melpomene are consistent with adaptive divergence, and when combined with previous neuroanatomy data, suggest hybrid visual system disruption due to mismatched patterns of intermediacy and dominance in the visual pathway.

evolutionary biology↗

bioRxiv2023

Enhanced long-term memory and increased mushroom body plasticity in Heliconius butterflies

As highly labile structures on both individual and evolutionary time-scales, the mushroom bodies, a key site of learning and memory in insects, are an excellent model for investigating the evolution of cognitive variation. We explored the behavioural consequences of mushroom body expansion in Heliconius butterflies, which possess greatly expanded mushroom bodies relative to their closest outgroups. We conducted long-term visual memory assays across three Heliconius and three other Heliconiini species using trained food-colour associations. We confirm robust differences between clades, with Heliconius exhibiting greater fidelity to the trained colour after 8 days without reinforcement compared to other Heliconiini, with further evidence of stable preferences at 13 days. We extended this analysis to consider the plastic response of the mushroom body calyces over this time period, measuring the volume of the mushroom body calyx, and the number of neurons and synapses it contains. We find substantial post-eclosion expansion and synaptic pruning in calyx of Heliconius erato, but not in Dryas iulia. In Heliconius erato, visual associative learning experience specifically is associated with a greater retention of calyceal synapses. At an individual level, fidelity to the trained colour in Heliconius erato was also positively correlated with synapse number. These results point to an enhanced visual long-term memory across Heliconius, facilitated not only by phylogenetic expansion of the mushroom body, but also changes in its developmental response to learning experience. The co-evolution of mushroom body expansion, plasticity and specific behaviours provides an important case study in the evolution of cognition. Significance StatementHow are cognitive differences between species supported by evolutionary changes in the brain? We investigated this question using Heliconius butterflies which have expanded mushroom bodies, a region of the insect brain involved in learning and memory. We show that Heliconius have more stable visual long-term memories and exhibit more substantial age- and experience-related plasticity than a closely related genus with smaller mushroom bodies. Recall accuracy was also predicted by synapse number in Heliconius erato, but not Dryas iulia, suggesting functional importance. These results suggest that increases in the size of specific brain regions and changes in their plastic response to experience may co-evolve to shape the evolution of cognition.

animal behavior and cognition↗

bioRxiv2023

Parallel shifts in flight-height associated with altitude across incipient Heliconius species

Vertical gradients in microclimate, resource availability and interspecific interactions are thought to underly stratification patterns in tropical insect communities. However, only a few studies have explored the adaptive significance of vertical space use during early population divergence. We analysed flight-height variation across speciation events in Heliconius butterflies representing parallel instances of divergence between low and high-altitude populations. We measured flight-height in wild H. erato venus and H. chestertonii, lowland and mountain specialists respectively, and found that H. chestertonii consistently flies at a lower height. We compared these data with previously published results for H. e. cyrbia and H. himera, the latter of which flies lower and, like H. chestertonii, recently colonised high-altitude, dry forests. We show that these repeated trends largely result from shared patterns of selection across equivalent environments, producing parallel trait-shifts in H. himera and H. chestertonii. Although our results imply a signature of local adaptation, we did not find an association between resource distribution and flight-height in H. e. venus and H. chestertonii. We discuss how this pattern may be explained by variation in forest structure and microclimate. Overall, our findings underscore the importance of behavioural adjustments during early divergence mediated by altitude-shifts.

evolutionary biology↗

bioRxiv2023

Patterns of host plant use do not explain mushroom body expansion in Heliconiini butterflies

The selective pressures leading to the elaboration of downstream, integrative processing centres, such as the mammalian neocortex or insect mushroom bodies, are often unclear. In Heliconius butterflies, the mushroom bodies are three to four times larger than their Heliconiini, and the largest known in Lepidoptera. Heliconiini lay almost exclusively on Passiflora, which exhibit a remarkable diversity of leaf shape, and it has been suggested that the mushroom body expansion of Heliconius may have been driven by the cognitive demands of recognising and learning the leaf shapes of local host plants. We test this hypothesis using two complementary methods: i) phylogenetic comparative analyses to test whether variation in mushroom body size is associated with the morphological diversity of host plants exploited across the Heliconiini; and ii) shape learning experiments using six Heliconiini species. We found that variation in the range of leaf morphologies used by Heliconiini was not associated with mushroom body volume. Similarly, we find interspecific differences in shape learning ability, but Heliconius are not overall better shape learners than other Heliconiini. Together these results suggest that the visual recognition and learning of host plants was not a main factor driving the diversity of mushroom body size in this tribe.

animal behavior and cognition↗

bioRxiv2023

Reversal learning of visual cues in Heliconiini butterflies

The mushroom bodies, an integrative region of the insect brain involved in learning and memory, have undergone volumetric increase in several independent lineages includes bees and ants, cockroaches and some beetles. However, the selective pressures driving these expansion events are not fully understood. One promising system for investigating this question is the Neotropical butterfly genus Heliconius, which exhibits markedly enlarged mushroom bodies compared with other members of the Heliconiini tribe. Notably, this neural elaboration co-occurs with the evolution of trapline foraging behaviour and an improved capacity for learning complex visual cues and long-term memory. Here, we further investigate the behavioural consequences of this brain expansion by testing reversal learning ability, a commonly used measure of cognition and behavioural flexibility in both vertebrates and invertebrates, across three Heliconius and three closely-related Heliconiini species. We trained butterflies to associate a food reward with either purple or yellow flowers, before training them with the reversed associations, and then reversing the cues again. All six successfully learned the reversed cues, and, contrary to our expectations, we found no evidence that Heliconius performed better than the other Heliconiini species. These results are surprising, given previous evidence linking the mushroom bodies to reversal learning in other insects, and the enhanced performance of Heliconius in other cognitive tests. This serves as a reminder that the functional consequences of brain expansion can be multifaceted, and do not necessarily result in an overall increase in general cognitive ability, but rather enhanced performance in specific, ecologically-relevant tasks.

animal behavior and cognition↗

bioRxiv2022

Rapid expansion and visual specialization of learning and memory centers in Heliconiini butterflies

How do neural systems evolve to support new behaviors? Changes in the abundance and diversity of neural cell types, and their connectivity, shape brain composition and provide the substrate for behavioral variation. We describe a striking example of neural elaboration in an ecologically diverse tribe of Heliconiini butterflies. By building extensive new datasets of neural traits across the tribe, we identify major bursts in the size and cellular composition of the mushroom bodies, central brain structures essential for learning and memory. These expansion events are associated with increased innervation form visual centers and coincide with enhanced performance in multiple cognitive assays. This suite of neural and cognitive changes is likely tied to the emergence of derived foraging behaviors, facilitated by localized specialization of neural networks. One-Sentence SummaryMajor shifts in brain composition and behavior in butterflies with unique foraging and dietary behaviors.

evolutionary biology↗

bioRxiv2022

Evolutionary dynamics of genome size and content during the adaptive radiation of Heliconiini butterflies

Heliconius butterflies, a speciose genus of Mullerian mimics, represent a classic example of an adaptive radiation that includes a range of derived dietary, life history, physiological and neural traits. However, key lineages within the genus, and across the broader Heliconiini tribe, lack genomic resources, limiting our understanding of how adaptive and neutral processes shaped genome evolution during their radiation. We have generated highly contiguous genome assemblies for nine new Heliconiini, 29 additional reference-assembled genomes, and improve 10 existing assemblies. Altogether, we provide a major new dataset of annotated genomes for a total of 63 species, including 58 species within the Heliconiini tribe. We use this extensive dataset to generate a robust and dated heliconiine phylogeny, describe major patterns of introgression, explore the evolution of genome architecture, and the genomic basis of key innovations in this enigmatic group, including an assessment of the evolution of putative regulatory regions at the Heliconius stem. Our work illustrates how the increased resolution provided by such dense genomic sampling improves our power to generate and test gene-phenotype hypotheses, and precisely characterize how genomes evolve.

evolutionary biology↗

bioRxiv2022

Warning coloration, body size and the evolution of gregarious behavior in butterfly larvae

Many species gain anti-predator benefits by combining gregarious behavior with warning coloration, yet there is debate over which trait evolves first, and which is the secondary adaptive enhancement. Body size can also influence how predators receive aposematic signals, and potentially constrain the evolution of gregarious behavior. To our knowledge, the causative links between the evolution of gregariousness, aposematism and larger body sizes have not been fully resolved. Here, using the most recently resolved butterfly phylogeny and an extensive new dataset of larval traits, we reveal the evolutionary interactions between important traits linked to larval gregariousness. We show that larval gregariousness has arisen many times across the butterflies, and aposematism is a likely prerequisite for gregariousness to evolve. We also find that body size may be an important factor for determining the coloration of solitary, but not gregarious larvae. Additionally, by exposing artificial larvae to wild avian predation, we show that undefended, cryptic larvae are heavily predated when aggregated but benefit from solitariness, whereas the reverse is true for aposematic prey. Our data reinforce the importance of aposematism for gregarious larval survival, whilst identifying new questions about the roles of body size and toxicity in the evolution of grouping behavior.

evolutionary biology↗

bioRxiv2022

A butterfly pan-genome reveals a large amount of structural variation underlies the evolution of chromatin accessibility

Despite insertions and deletions being the most common structural variants (SVs) found across genomes, not much is known about how much these SVs vary within populations and between closely related species, nor their significance in evolution. To address these questions, we characterized the evolution of indel SVs using genome assemblies of three closely related Heliconius butterfly species. Over the relatively short evolutionary timescales investigated, up to 18.0% of the genome was composed of indels between two haplotypes of an individual H. charithonia butterfly and up to 62.7% included lineage-specific SVs between the genomes of the most distant species (11 Mya). Lineage-specific sequences were mostly characterized as transposable elements (TEs) inserted at random throughout the genome and their overall distribution was similarly affected by linked selection as single nucleotide substitutions. Using chromatin accessibility profiles (i.e., ATAC-seq) of head tissue in caterpillars to identify sequences with potential cis-regulatory function, we found that out of the 31,066 identified differences in chromatin accessibility between species, 30.4% were within lineage-specific SVs and 9.4% were characterized as TE insertions. These TE insertions were localized closer to gene transcription start sites than expected at random and were enriched for several transcription factor binding site candidates with known function in neuron development in Drosophila. We also identified 24 TE insertions with head-specific chromatin accessibility. Our results show high rates of structural genome evolution that were previously overlooked in comparative genomic studies and suggest a high potential for structural variation to serve as raw material for adaptive evolution.

genomics↗

bioRxiv2021

Repeated genetic adaptation to high altitude in two tropical butterflies

Repeated evolution can provide insight into the mechanisms that facilitate adaptation to novel or changing environments. Here we study adaptation to high altitude in two divergent tropical butterflies, H. erato and H. melpomene, which have repeatedly and independently adapted to high elevations on either side of the Andean mountains. We sequenced 518 whole genomes from elevational transects and found many regions under selection at high altitude, with repeated genetic differentiation across multiple replicates, including allopatric comparisons. In contrast, there is little molecular parallelism between H. erato and H. melpomene. With a further 85 whole genomes of five close relatives, we find that a large proportion divergent regions have arisen from standing variation and putative adaptive introgression from high-altitude specialist species. Taken together our study supports a key role of standing genetic variation and gene flow from pre-adapted species in promoting parallel genetic local adaptation to the environment.

evolutionary biology↗

bioRxiv2021

The effects of chromosome fusions on genetic diversity and evolutionary turnover of functional loci consistently depends on chromosome size

Major changes in chromosome number and structure are linked to a series of evolutionary phenomena, including intrinsic barriers to gene flow or suppression of recombination due to chromosomal rearrangements. However, chromosome rearrangements can also affect the fundamental dynamics of molecular evolution within populations by changing relationships between linked loci and altering rates of recombination. Here, we build chromosome-level assembly Eueides isabella and, together with the chromosome-level assembly of Dryas iulia, examine the evolutionary consequences of multiple chromosome fusions in Heliconius butterflies. These assemblies pinpoint fusion points on 10 of the 21 autosomal chromosomes and reveal striking differences in the characteristics of fused and unfused chromosomes. The ten smallest autosomes in D. iulia and E. isabella, which have each fused to a longer chromosome in Heliconius, have higher repeat and GC content, and longer introns than predicted by their chromosome length. Following fusion, these characteristics change to become more in line with chromosome length. The fusions also led to reduced diversity, which likely reflects increased background selection and selection against introgression between diverging populations, following a reduction in per-base recombination rate. We further show that chromosome size and fusion impact turnover rates of functional loci at a macroevolutionary scale. Together these results provide further evidence that chromosome fusion in Heliconius likely had dramatic effects on population level processes shaping rates of neutral and adaptive divergence. These effects may have impacted patterns of diversification in Heliconius, a classic example of an adaptive radiation.

evolutionary biology↗

bioRxiv2020

Re-emergence and diversification of a specialised antennal lobe morphology in ithomiine butterflies

How an organisms sensory system functions is central to how it navigates its environment and meets the behavioural challenges associated with survival and reproduction. Comparing sensory systems across species can reveal how facets of behaviour and ecology promote adaptive shifts in the relative importance of certain environmental cues. The insect olfactory system is prominent model for investigating how ecological factors impact sensory reception and processing. Notably work in Lepidoptera led to the discovery of vastly expanded structures, termed a macroglomerular complex (MGC), within the primary olfactory processing centre. These structures typically process pheromonal cues and provide a classic example of how variation in size can influence the functional processing of sensory cues. Though prevalent across moths, the MGC was lost during the early evolution of butterflies, consistent with evidence that courtship initiation in butterflies is primarily reliant upon visual cues, rather than long distance olfactory signals like pheromones. However, a MGC has recently been reported to be present in a species of ithomiine, Godryis zavaleta, suggesting this once lost neural adaptation has re-emerged in this clade. Here, we show that MGCs, or MGC-like morphologies, are indeed widely distributed across the ithomiine tribe, and vary in both structure and the prevalence of sexual dimorphism. Based on patterns of variation across species with different chemical ecologies, we suggest that this structure is involved in the processing of both plant and pheromonal cues, of interlinked chemical constitution, and has evolved in conjunction with the increased importance and diversification of plant derived chemicals cues in ithomiines.

evolutionary biology↗

bioRxiv2020

Neural divergence and hybrid disruption between ecologically isolated Heliconius butterflies

The importance of behavioural evolution during speciation is well established, but we know little about how this is manifest in sensory and neural systems. Although a handful of studies have linked specific neural changes to divergence in host or mate preferences associated with speciation, how brains respond to broad environmental transitions, and whether this contributes to reproductive isolation, remains unknown. Here, we examine divergence in brain morphology and neural gene expression between closely related, but ecologically distinct, Heliconius butterflies. Despite on-going gene flow, sympatric species pairs within the melpomene-cydno complex are consistently separated across a gradient of open to closed forest and decreasing light intensity. By generating quantitative neuroanatomical data for 107 butterflies, we show that H. melpomene and H. cydno have substantial shifts in brain morphology across their geographic range, with divergent structures clustered in the visual system. These neuroanatomical differences are mirrored by extensive divergence in neural gene expression. Differences in both morphology and gene expression are heritable, exceed expected rates of neutral divergence, and result in intermediate traits in first generation hybrid offspring. This likely disrupts neural system function, leading to a mismatch between the environment and the behavioral response of hybrids. Our results suggest that disruptive selection on both neural function and external morphology result in coincident barriers to gene flow, thereby facilitating speciation.

evolutionary biology↗