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PubMed1994

Modes and origins of mechanical and ethological isolation in angiosperms.

Mechanical and ethological isolation between species is widespread in angiosperms with specialized animal-pollinated flowers, being recorded in 29 species groups belonging to 27 genera and 16 families. Mechanical isolation occurs in two forms. (i) The common type, designated the Salvia type, operates when two or more species of flowers are adapted for different groups of pollinators with different body sizes and shapes. (ii) In the Pedicularis type two flower species have the same species of pollinator but pick up pollen from different parts of the pollinator's body. Four forms of ethological isolation are recognized. (i) In the Aquilegia type, which is widespread, ethological isolation is a side effect of mechanical isolation. (ii) The flower-constancy type, as the name suggests, is based on flower-constant foraging behavior. (iii) In the Ophrys type, floral scents attract male bees or wasps and play a role in their mating behavior; different species of flowers, often orchids, have different scents and attract different sets of hymenopteran species. (iv) The monotropy type occurs in plants pollinated by hymenopterans with species-specific or group-specific flower preferences for nutritive purposes (monotropic and oligotropic bees and fig wasps). Three modes of origin of floral isolation are confirmed by evidence: (i) mechanical and ethological isolation arising as a by-product of allopatric speciation, (ii) ethological isolation developing by selection for reproductive isolation per se, and (iii) mechanical isolation arising as a by-product of character displacement. Mode of origin i accounts for the Salvia and Aquilegia types of isolation in nine known species groups and for the Ophrys type in one group. Mode of origin ii accounts for the flower-constancy type of ethological isolation in two species groups. Mode of origin iii explains mechanical isolation in two groups. Sympatric origin of floral isolation by hybrid speciation and by flower constancy has been proposed, but these modes are undocumented and improbable.

Journal Article↗

PubMed1975

Haptoglobin-ABO interaction: a possible explanation for the excess of Hp 1 among offspring of ABO incompatible matings.

The Hp1 frequency among ABO phenotypes varies in the Hutterite population as follows: O less than A less than B less than AB. Within group O, the Hp1 frequency is significantly lower than the Hp1 frequency among the other groups combined. The Hp1 frequencies among ABO genotypes, known by means of family pedigrees, vary as follows: OO less than AO less than BO less than AB less than AA less than BB. This holds for both main subjects of this isolate, although they have been reproductively isolated since World War I. The higher Hp1 frequency among type A, B, and AB individuals explains the observation of the higher Hp1 frequencies found among H-leut offspring who are incompatible with their mothers (mainly AO offspring of OO mothers) compared to offspring from the same matings who are compatible with their mothers.

ABO Blood-Group System↗

PubMed2026

A haploid wild yeast resource for exploring the natural ecology of Saccharomyces cerevisiae.

Saccharomyces cerevisiae occurs predominantly in the diploid state in nature, limiting genetic analyses of wild populations. Here, we establish a haploid collection from 32 Taiwanese S. cerevisiae isolates through targeted HO disruption. This resource spans predomesticated Asian wild lineages and enables the investigation of reproductive isolation and ecological trait variation. Although all pairwise hybridizations formed zygotes, many yielded reduced spore viability, revealing strong postzygotic barriers. Genome analyses associated reduced hybrid fertility with lineage-specific structural variation, including elevated levels of intra-chromosomal inversions in H413-8/TW1 and inter-chromosomal rearrangements in PD35A/CHN-V, rather than sequence divergence alone. Phenotyping revealed ecological differentiation, with TW1 favoring cooler growth and a natural hybrid exhibiting heterosis with expanded thermotolerance. Most wild strains grew poorly on maltose, whereas anthropogenic strains displayed enhanced utilization linked to MAL + regulatory alleles and maltose-specific transporters. Together, this haploid collection links structural variation and metabolic divergence to ecological and reproductive differentiation in wild S. cerevisiae.

Saccharomyces cerevisiae↗

PubMed2002

A test of ecologically dependent postmating isolation between sympatric sticklebacks.

Ecological speciation occurs when reproductive isolation evolves ultimately as a result of divergent natural selection between populations inhabiting different environments or exploiting alternative resources. I tested a prediction of the ecological model concerning the fitness of hybrids between two young, sympatric species of threespine sticklebacks (Benthics and Limnetics). The two species are ecologically and morphologically divergent: the Benthic is adapted to feeding on invertebrates in the littoral zone of the lake whereas the Limnetic is adapted to feeding on zooplankton in the open water. The growth rate of two types of hybrids, the Benthic backcross and the Limnetic backcross, as well as both parent species, was evaluated in enclosures in both parental habitats in the lake. The use of backcrosses is ideal because a comparison of their growth rates in the two habitats estimates an ecologically dependent component of their fitness while controlling for any intrinsic genetic incompatibilities that may exist between the Benthic and Limnetic genomes. The backcross results revealed a striking pattern of ecological dependence: in the littoral zone, Benthic backcrosses grew at approximately twice the rate of Limnetic backcrosses, while in the open water, Limnetic backcrosses grew at approximately twice the rate of Benthic backcrosses. Such a reversal of relative fitness of the two cross-types in the two environments provides strong evidence that divergent natural selection has played a central role in the evolution of postmating isolation between Benthics and Limnetics. Although the rank order of growth rates of all cross-types in the littoral zone was Benthic > Benthic backcross > Limnetic backcross > Limnetic, neither backcross differed significantly from the parent from which it was mainly derived. Implications of this result are discussed in terms of ecological speciation and possible introgressive hybridization between the species. Results in the open water were less clear and were not fully consistent with the ecological model of speciation, mainly as a result of the low growth rate of Limnetics. However, analysis of the diet of the fish in the open water suggests that these enclosures may not have been fully successful at replicating the food regimes characteristic of this habitat.

Animals↗

PubMed1999

Ecological parallelism and cryptic species in the genus Ophiothrix derived from mitochondrial DNA sequences.

We addressed the long-standing problem of species assignment of two nominal species of the genus Ophiothrix (Echinodermata, Ophiuroidea) by phylogenetic analysis of a segment of the mitochondrial 16S rDNA. Our phylogeny identified two distinct mitochondrial lineages that do not correspond to the present species assignments. Individuals of the endemic Mediterranean species O. quinquemaculata were clustered with individuals of O. fragilis in both mitochondrial lineages. We thus suggest that these taxa are not biological species but ecotypes. Differences between the two ecotypes in morphological and physiological characteristics may be explained by adaptation to environmental conditions at different water depths. Despite the observed ecomorphological variability within each of the two major mitochondrial lineages, the large genetic distance (9.0-12.0%) between them does suggest the existence of two distinct biological species. Their reproductive isolation could result from differences in reproductive strategy rather than by ecological and/or morphological differentiation.

Animals↗

PubMed2000

The likelihood of homoploid hybrid speciation.

New species may be formed through hybridization and without an increase in ploidy. The challenge is for hybrid derivatives to escape the homogenizing effects of gene flow from parental species. The mechanisms hypothesized to underlie this process were modelled using a computer simulation. The model is of recombinational speciation, in which chromosomal rearrangements between parental species result in poor fertility of F1 hybrids, but through recombination, novel homozygous types are formed that have restored fertility. In simulations, stable populations bearing the recombinant karyotypes originated frequently and were maintained when the fertility of F1 hybrids was high. However, this high rate of origination was offset by low genetic isolation, and lower F1 hybrid fertility increased the evolutionary independence of derived populations. In addition, simulations showed that ecological and spatial isolation were required to achieve substantial reproductive isolation of incipient species. In the model, the opportunity for ecological isolation arose as a result of adaptation to extreme habitats not occupied by parental species, and any form of spatial isolation (e.g. founder events) contributed to genetic isolation. Our results confirmed the importance of the combination of factors that had been emphasized in verbal models and illustrate the trade-off between the frequency at which hybrid species arise and the genetic integrity of incipient species.

Animals↗

PubMed2006

Linkage of butterfly mate preference and wing color preference cue at the genomic location of wingless.

Sexual isolation is a critical form of reproductive isolation in the early stages of animal speciation, yet little is known about the genetic basis of divergent mate preferences and preference cues in young species. Heliconius butterflies, well known for their diversity of wing color patterns, mate assortatively as a result of divergence in male preference for wing patterns. Here we show that the specific cue used by Heliconius cydno and Heliconius pachinus males to recognize conspecific females is the color of patches on the wings. In addition, male mate preference segregates with forewing color in hybrids, indicating a genetic association between the loci responsible for preference and preference cue. Quantitative trait locus mapping places a preference locus coincident with the locus that determines forewing color, which itself is perfectly linked to the wing patterning candidate gene, wingless. Furthermore, yellow-colored males of the polymorphic race H. cydno alithea prefer to court yellow females, indicating that wing color and color preference are controlled by loci that are located in an inversion or are pleiotropic effects of a single locus. Tight genetic associations between preference and preference cue, although rare, make divergence and speciation particularly likely because the effects of natural and sexual selection on one trait are transferred to the other, leading to the coordinated evolution of mate recognition. This effect of linkage on divergence is especially important in Heliconius because differentiation of wing color patterns in the genus has been driven and maintained by natural selection for Müllerian mimicry.

Animals↗

PubMed1989

Genetics of sexual isolation between two sibling species, Drosophila simulans and Drosophila mauritiana.

Drosophila simulans and Drosophila mauritiana are sibling species that show substantial sexual isolation in one of their two reciprocal hybridizations. Genetic analysis reveals that in females this isolation is caused by at least one recessive gene on each autosome, while the X chromosome has little or no effect. Our results, combined with those of previous studies, show that in Drosophila the genetics of sexual isolation differs from that of postzygotic reproductive isolation, which invariably involves large effects of the X chromosome.

Animals↗

PubMed2002

Patterns of divergence in the effects of mating on female reproductive performance in flour beetles.

Sexual selection can lead to rapid divergence in reproductive characters. Recent studies have indicated that postmating events, such as sperm precedence, may play a key role in speciation. Here, we stress that other components of postmating sexual selection may be involved in the evolution of reproductive isolation. One of these is the reproductive investment made by females after mating (i.e., differential allocation). We performed an experiment designed to assess genetic divergence in the effects of mating on female reproductive performance in flour beetles, Tribolium castaneum. Females were mated to males of three different wild-type genotypes at two different frequencies, in all possible reciprocal combinations. Male genotype affected all aspects of female reproduction, through its effects on female longevity, total offspring production, reproductive rate, mating rate, and fertility. Moreover, male and female genotype interacted in their effects on offspring production and reproductive rate. We use the pattern of these interactions to discuss the evolutionary process of divergence and suggest that the pattern is most consistent with that expected if divergence was driven by sexually antagonistic coevolution. In particular, the fact that females exhibited a relatively weak response to males with which they were coevolved suggests that females have evolved resistance to male gonadotropic signals/stimuli.

Animals↗

PubMed1992

An empirical test of the meiotic drive models of hybrid sterility: sex-ratio data from hybrids between Drosophila simulans and Drosophila sechellia.

Recently, there has been much discussion regarding the hypothesis that divergence of meiotic drive systems in isolated populations can generate the patterns of reproductive isolation observed in animal hybridizations. One prediction from this hypothesis is that the sex ratio of hybrids with heterospecific sex chromosomes should greatly deviate from the Mendelian expectation of 50% female. From sex-ratio data in our Drosophila hybridization studies, we find no such deviation: the sex ratio of offspring of males with introgressed heterospecific Y chromosomes with various autosomal backgrounds does not differ from that of the pure species. We also discuss other aspects of the current meiotic drive models.

Animals↗

PubMed1992

Floral isolation between ornithophilous and sphingophilous species of Ipomopsis and Aquilegia.

The Ipomopsis aggregata group (Polemoniaceae) and Aquilegia formosa-Aquilegia caerulea group (Ranunculaceae) in western North America contain species with ornithophilous flowers and related species with sphingophilous flowers. The ornithophilous and sphingophilous species are sympatric over large areas and remain distinct in some sites where they grow close together. Floral isolation-a combination of mechanical and ethological isolation-plays a significant role in the reproductive isolation. The primary pollinators of the ornithophilous taxa are the common western American species of hummingbirds, and the primary pollinators of the sphingophilous taxa are western hawkmoths. The ornithophilous and sphingophilous flowers are adapted to their respective primary pollinators. A corollary of these specializations is that the differences between the two types of floral mechanisms significantly reduce interspecific pollination. The floral isolation is incomplete. However, it acts not alone but in conjunction with ecological and seasonal isolation, which are also incomplete. The combination of these three incomplete external isolating mechanisms is sufficient to maintain the distinctness of the ornithophilous and sphingophilous species in some areas of sympatric coexistence.

Journal Article↗

PubMed2006

Timing and tempo of primate speciation.

Published molecular clocks for primates are used to estimate typical divergence times for phylogroups (1.6 Ma), species (3.3 Ma), sister species (2.7 Ma), genera (8.9 Ma) and sister genera (8.6 Ma). Significant median differences exist between major groups (infraorders and superfamilies) for various divergence times. These data are employed to estimate typical maximum duration of speciation. Typical primate values (1.1 Ma) suggest this process to be faster than is characteristic of many vertebrates. However, after considering divergence times for hybridizing congeneric and confamilial primates, this value is likely only to estimate the commencement of prezygotic isolating mechanisms, rather than the completion of reproductive isolation. Thus, speciation typically takes around 1.0 Ma to more than 4.0 Ma to occur, depending on whether prezygotic or post-zygotic isolating mechanisms are emphasized. Typical primate genus age is around 5.3 Ma, but we note differences among major groups. In light of these estimates, the classification of humans and chimpanzees is reconsidered using a molecular yardstick approach. Three taxonomic frameworks may flow from molecular analyses, all of them having major implications for understanding the evolution of humans and chimpanzees.

Animals↗

PubMed2005

Assortative mating in sympatric host races of the European corn borer.

Although a growing body of work supports the plausibility of sympatric speciation in animals, the practical difficulties of directly quantifying reproductive isolation between diverging taxa remain an obstacle to analyzing this process. We used a combination of genetic and biogeochemical markers to produce a direct field estimate of assortative mating in phytophagous insect populations. We show that individuals of the same insect species, the European corn borer Ostrinia nubilalis, that develop on different host plants can display almost absolute reproductive isolation-the proportion of assortative mating was >95%-even in the absence of temporal or spatial isolation.

Animals↗

PubMed1984

[Sensitivity of Anopheles gambiae s.l. to insecticides in the Selingue dam area].

In the Selingue hydroelectric dam area the An. gambiae complex members are susceptible to DDT, resistant to dieldrin (the adults mainly) and also susceptible to temephos, chlorpyrifos, fenthion, fenitrothion and malathion. The LC50 of the larvae for fenthion is 0.00325 ppm. Due to the abundance of An. gambiae s.s. in this area (nearly 98%), it is probable that these data refer mainly to this species. Recent cytogenetic studies have shown that this species is composed of three chromosomal types (Bamako, Savanna and Mopti) with partially reproductive isolation. In the case of resistance of members of a species complex to an insecticide, it will be of great interest to know how this resistance is distributed within the complex. This can be particularly important for An. gambiae s.s. and An. arabiensis, and also for the chromosomal forms Bamako and Mopti of An. gambiae s.s. which are reproductively well isolated.

Animals↗

PubMed2025

Strong but diffuse genetic divergence underlies differentiation in an incipient species of marine stickleback.

Understanding how lineages proceed along the "speciation continuum" and how species boundaries are maintained over time remain central questions in evolutionary biology. Populations early in the speciation process can give us detailed insight into the reproductive barriers that first initiate speciation. In this study, we explore the nature of genomic divergence between two sympatric marine stickleback ecotypes from Atlantic Canada, "whites" and "commons". Males of each ecotype exhibit distinct nuptial colorations, nesting habits, and parental care strategies. Using population genomic analyses of SNPs and copy number variants (CNVs; deletions and duplications) we show that whites and commons consistently form distinct populations. We uncover genomic differentiation in the white ecotype characteristic of an incipient species, showing extremely low genome-wide differentiation (FST) and very recent divergence (~1 kya). Demographic analysis detected very low levels of ongoing gene flow between populations. Our results and prior genomic studies suggest that reproductive isolation is being maintained between ecotypes despite recent evidence that hybridization in nature does occur. Contrary to other systems, we found many small, but dispersed regions of high differentiation throughout the genome rather than explicitly within chromosomal inversions or the sex chromosomes. On chromosomes VII and XVI, we identified CNVs overlapping genes enriched for olfaction, which may play a role in differences in reproductive strategies between ecotypes. Ultimately, our results demonstrate that genome-wide rather than localized differences can underlie the early stages of divergence, and that this pattern is corroborated by both SNPs and CNVs.

Copy Number Variation↗

PubMed2025

Extensive Introgression Failed to Erode Species Boundaries Among Multiple Sympatric Closely Related Species of Roscoea.

How species boundaries are maintained among sympatric closely related species experiencing gene flow is a puzzling question in evolutionary biology. Although introgression is commonly documented, the dynamics and gene function of introgression have rarely been explored to probe why frequent introgression does not necessarily destroy species boundaries in sympatry. In this study, we employ whole-genome resequencing data to examine introgression among five closely related species of Roscoea that coexist in a 'sky island' with seventeen distinct morphological traits. Our findings reveal that introgression has led to the phylogenomic discordance between nuclear and chloroplast genomes among these morphologically distinct species. Additionally, introgression is predominantly asymmetrical in both intensity and gene function, particularly concerning recombination. Notably, the majority of gene functions associated with introgressed loci are unrelated to reproductive processes. Our results suggest that closely related species with incomplete allele assortment can coexist despite theoretical predictions, highlighting the semipermeable nature of species boundaries as reproductive isolation develops. This provides a critical conceptual framework for understanding the interplay between introgression and species persistence. Our finding offers insights into how related sympatric species boundaries can be maintained in the face of frequent asymmetrical gene introgression.

Sympatry↗

PubMed2000

Intraspecific variation in sexual isolation in the jewel wasp Nasonia.

Divergence in mate recognition systems can lead to reproductive isolation. In this study, we investigate patterns of intraspecific variation that contribute to premating isolation within and between two haplodiploid species, Nasonia vitripennis and N. longicornis. In a broad-scale survey of 17 North American isofemale lines encompassing the two species, we report strong asymmetric sexual isolation between species and a dramatic level of intraspecific variation for mate discrimination between species. A general lack of incipient speciation was found, with the exception of low levels of interpopulational sexual isolation within N. vitripennis. Regression analysis shows that the degree of intraspecific variation for within-species mating frequency is not associated with the degree for between-species mating frequency. Reinforcement or reproductive character displacement may be involved in some of the variation in interspecies premating isolation.

Animals↗

PubMed2006

Phenotypic differentiation and pre-mating isolation between allopatric populations of Girardinichthys multiradiatus.

Sexual selection may lead to reproductive isolation between populations through divergence in female mate choice, and population differentiation driven by female mate choice is expected to produce pre- but not post-mating isolation. We tested these hypotheses by looking at whether allopatric populations of the Amarillo (Girardinichthys multiradiatus), a sexually dimorphic viviparous fish with effective female choice, (i) have undergone phenotypical differentiation that may be attributed to divergence in female mate choice, and (ii) are already separated by pre- and/or post-mating reproductive barriers. We found substantial divergence in morphological traits which are the target of female mate choice, and in male courtship behaviour. Strong female preferences for homogametic males indicate substantial and symmetric pre-copulatory isolation, but the few successful heterogametic crosses produced in confinement yielded litters of the same size as those produced in homogametic matings, suggesting that post-copulatory isolation between populations is non-existent or weak. It appears that the studied populations have undergone incipient speciation with a pattern that is consistent with speciation driven by sexual selection, yet further work should assess whether divergence in female preferences has promoted male phenotypic differentiation or whether variation in male attributes has driven divergence in female preferences.

Animals↗