Which of the following best explains the mechanism that maintains reproductive isolation between goats and sheep?

V.B. Patterns of Genetic Divergence for Other Species Traits

Postzygotic reproductive isolation is only one trait that has generally diverged between species. In order to formulate a broader picture of the genetic patterns of divergence, it is necessary to compare the genetic architecture of this trait with what is seen for other traits that diverge between species (reviewed in Hollocher, 1998). The traits that have been looked at in some genetic detail include interspecific mate discrimination and interspecific differences in secondary sexual characteristics. Overall, the general pattern that emerges is that these traits, too, are governed by many genes of small effect. However, in contrast to what was found for the genetic basis of postzygotic reproductive isolation, epistasis does not play a dominant role in governing the evolution of these traits.

Comparison of patterns of genetic variation within species versus patterns of genetic variation between species for the same traits can be useful for gaining insights into the evolutionary mechanisms that may have played a role during species divergence (reviewed in Hollocher, 1998). If these within-species versus between-species comparisons reveal strong similarities, then generally it can be concluded that speciation proceeded through the same general action of evolutionary forces (in terms of type, direction, and strength) normally operating on these traits within species. In contrast, strikingly different patterns of within-species versus between-species comparisons could reveal the operation of a different set of evolutionary forces operating during divergence of species than what normally occurs within species.

Interestingly, within- and between-species patterns of genetic variation for mate discrimination and secondary sexual characteristics are very similar, indicating that divergence of these traits probably reflects the direct extension of the same evolutionary forces (most likely directional sexual selection in this case) that operate on these traits within species. In contrast, within-and between-species genetic patterns of sterility and inviability do not show similar patterns at all. Not only is the role of epistasis drastically different between the two comparisons, the relative frequency of genes that affect sterility versus inviability is completely reversed depending on whether within- or between-species patterns are considered. This disjunction between the genetic patterns observed within species versus those observed between species suggests that the evolution of postzygotic reproductive incompatibilities may result from the accumulated action of relatively rare evolutionary events happening over long periods of time. Such rare events may include periodic episodes of random genetic drift happening alone or in combination with natural and sexual selection working on these traits with varying intensity or changes in direction over the course of evolution.

Evidence of Natural Selection

Several investigators have shown that reproductive isolation has evolved as a by-product of adaptive divergence, a process called “ecological speciation.” For example, the evolution of mimicry appears to have played an important role in speciation in the butterfly genus Heliconius (Jiggins et al., 2001). The recently split sister species H. melpomene and H. cydno have diverged to mimic the color patterns of different model taxa (Fig. 3A), but strong assortative mating (preferential mating among individuals with similar phenotypes) based on the mimetic coloration results in substantial prezygotic isolation (Fig. 3B). Rare hybridization events produce individuals with poorly adapted intermediate phenotypes, demonstrating that divergent patterns of mimicry also result in some postzygotic isolation. Among plants, species in the genus Mimulus have evolved highly divergent floral morphologies that appear to be adaptations to different types of pollinators (either bees or hummingbirds) that in turn effectively reproductively isolate sympatric populations (Schemske and Bradshaw, 1999). Using controlled laboratory experiments, several studies have also generated strong assortative mating among replicate lines of houseflies and species of Drosophila by subjecting them to artificial divergent selection on behavioral, morphological, and physiological traits (e.g., Dodd, 1989). Control populations exposed to the same selective pressures show little behavioral isolation, indicating that divergent selection, rather than genetic drift, caused the incidental evolution of prezygotic isolation.

Sexual selection is also probably important in allopatric speciation given that divergence in sexually selected traits will necessarily diminish interbreeding (Ritchie, 2007). Many experiments have shown that among species where a female chooses males to mate with, females make choices based on sexually dimorphic traits in males, such as large body size, bright coloration, large antlers, and elongated tail feathers. Sexual selection is likely involved in the evolution of gametic (sperm and egg) incompatibilities and isolation in some marine organisms that release their gametes into the water column in mass spawnings, a scenario that results in intense sperm competition among males. Analysis of the underlying DNA sequences for some sperm and egg surface proteins indicates that the evolution of these proteins is driven by selection (Galindo et al., 2003). Rapid development of gametic incompatibility may explain how some populations evolve reproductive isolation during only brief periods of transient geographic isolation (Levitan and Ferrell, 2013).

Genome-wide patterns of genetic divergence between evolving species or ecotypes have provided numerous examples of the potential role of natural selection in speciation. Scanning thousands of loci across the genome allows identification of very strongly differentiated loci (“outliers”) between geographically isolated populations, a pattern often interpretted as evidence of natural selection (Luikart et al., 2003). However, as with any difference between recently-evolved species or ecotypes, it remains difficult to determine if outlier loci are a cause or consequence of reproductive isolation. Genome scans of allopatric ecotypes that have evolved more than once (repeated or parallel evolution) provide much greater power to identify the genetic loci responsible for adaptation and reproductive isolation between allopatric lineages. For example, in sticklebacks, whole-genome sequencing of several freshwater ecotypes, that each repeatedly evolved independently from a marine ancestor, reveals that many outlier loci are shared among freshwater ecotypes (Jones et al., 2012).

Evidence of selection

Several investigators have shown that reproductive isolation has evolved as a by-product of adaptive divergence, a process called ‘ecological speciation’. For example, the evolution of mimicry appears to have played an important role in speciation in the butterfly genus Heliconius. The recently split sister species H. melpomene and H. cydno have diverged to mimic the color patterns of different model taxa (Figure 3a), but strong assortative mating (preferential mating among individuals with similar phenotypes) based on the mimetic coloration results in substantial pre-zygotic isolation (Figure 3b). Rare hybridization events produce individuals with poorly adapted intermediate phenotypes, demonstrating that divergent patterns of mimicry also result in some post-zygotic isolation. Among plants, species in the genus Mimulus have evolved highly divergent floral morphologies that appear to be adaptations to different types of pollinators (either bees or hummingbirds) that in turn effectively reproductively isolate sympatric populations. Using controlled laboratory experiments, several studies have also generated strong assortative mating among replicate lines of houseflies and species of Drosophila by subjecting them to artificial divergent selection on behavioral, morphological, and physiological traits. Control populations show little behavioral isolation, indicating that divergent selection, rather than genetic drift, caused the incidental evolution of pre-zygotic isolation.

Sexual selection is also probably important in allopatric speciation given that divergence in sexually selected traits will necessarily diminish interbreeding. Many experiments have shown that among species where a female ‘chooses’ males to mate with, females make their choices based on sexually dimorphic traits in males, such as large body size, bright coloration, large antlers, and elongated tail feathers. Sexual selection is probably involved in the evolution of gametic incompatibilities and isolation in some marine organisms that release their gametes into the water column in mass spawnings, a scenario that results in intense sperm competition among males. Analysis of the underlying DNA sequences for some sperm surface proteins indicates that the evolution of these proteins is driven by selection. Rapid development of gametic incompatibility may explain how some populations evolve reproductive isolation during only brief periods of transient geographic isolation.

Insect Pollination in the Fossil Record and the Angiosperm Radiation

The role of plant–pollinator interactions in reproductive isolation has also led to the much grander hypothesis that insect pollination was a “key innovation” leading to the coradiation of flowering plants (Angiosperms) and anthophilous insects (the groups most involved in pollination), including certain bees and wasps (Hymenoptera), various families of flies (Diptera), and butterflies and moths (Lepidoptera).

One necessary prediction of the insect-pollination Angiosperm-radiation hypothesis is concurrent diversification of the angiosperms and anthophilous insects. Despite recent progress in our understanding of flower evolution (Specht and Bartlett, 2009; Endress, 2011), controversy still surrounds the dates of angiosperm diversification (Crepet, 2008). Nevertheless, recent analysis of the land plant record has produced a richer story that even more powerfully supports a tight evolutionary relationship between plants and pollinators (Labandeira and Sepkoski, 1993; Crepet, 2008). Indeed, insect pollination seems to have preceded the origin of angiosperms and insect diversity may have waxed, waned, and resurged with the fortunes of different plant groups.

Fossil plant and insect morphology suggests that many ancient gymnosperms, cycads, and early seed plants from the Middle Jurassic and Early Cretaceous were pollinated by insects with body parts specialized to obtain pollen, nectar, or other plant rewards (Labandeira, 2010). However, the global turnover from gymnosperm to angiosperm floras in the early Cretaceous apparently extirpated much of this diverse plant–pollinator biota and reduced insect diversity for 20 My until the subsequent diversification of angiosperm flowers and pollinators.

The earliest angiosperms tended to have bowl-shaped, open flowers accessible to a variety of small unspecialized insects (Friis et al., 1987). These would not have facilitated reproductive isolation. After another 45 My, as angiosperms began to dominate the flora, reproductive isolating mechanisms became evident. The fossil record of this time suggests pollinator specialization, including the earliest fossil evidence of bees and other long-proboscid insects, and floral traits such as tube-shaped flowers, which restrict floral rewards to specialized pollinators (Crepet, 2008). Molecular and fossil data also suggest the concomitant rise of new angiosperm groups characterized by rapid diversification of lineages with specialized pollinator associations (Lavin et al., 2005).

III.B.1.b. Insect Pollination and the Angiosperm Radiation

The role of plant–pollinator interactions in reproductive isolation has also led to the much grander hypothesis that insect pollination was a “key innovation” leading to the co-radiation of flowering plants (angiosperms) and anthophilous insects, which are those groups most involved in pollination, including certain bees and wasps (Hymenoptera), various families of flies (Diptera), and butterflies and moths (Lepidoptera). Fossil evidence suggests that the angiosperms diversified very rapidly, and many hypotheses have been advanced to explain this phenomenon. Like most hypotheses in the historical sciences, however, these have been very difficult to test. One necessary prediction of the insect-pollination angiosperm radiation hypothesis is concurrent diversification of the angiosperms and anthophilous insects.

Considerable controversy surrounds the dates of diversification of angiosperms and anthophilous insects. As early as the Carboniferous, seed ferns had large pollen that was probably too heavy for wind transport and may have been pollinated by paleodictyopteran insects found in the same formations. The first direct evidence associating insects with plant pollen appears in the Lower Permian. However, the radiation of the insect groups that today are most strongly associated with angiosperm pollination probably occurred in the late Middle to early Upper Cretaceous, the period most commonly thought to have witnessed the radiation of the flowering plants. While not proving the codiversification hypothesis, these estimates at least do not rule it out.

Distinguishing Similar Species

Most modern taxonomists subscribe to the biological species concept in which total reproductive isolation between organisms is taken as an indication of species status. Although the concept has considerable merit, several problems are associated with implementation. For example, most taxonomists work primarily with preserved museum specimens, and reproductive isolation cannot be tested in museum preserved material. Biological control workers, with laboratory and insectary facilities available, are better equipped than most museum taxonomists to carry on reproductive isolation studies.

The confirmation of reproductive isolation through hybridization studies in some cases has led to reevaluation of the comparative morphology of sibling species, which, in turn, has elicited minor but consistent anatomical differences. Examples of hybridization studies with closely related taxa pertinent to biological control include Muscidifurax (Kogan & Legner, 1970), Aphytis (Rao & DeBach, 1969a, 1969b, 1969c) and Trichogramma (Nagarkatti and Nagaraja, 1977). It is important to emphasize that the taxonomy of the cultures and species involved must be carefully researched before taxonomic decisions are made based on hybridization work.

The extent of reproductive isolation has been shown to vary among organisms. Hybridization experiments with natural enemies for use in biological control projects often yield living samples of closely related natural enemies from geographically and ecologically diverse localities. These can provide the raw material for the basic hybridization studies needed to clarify the taxonomic status of similar entomophagous forms.

Not all organisms reproduce sexually. In so-called uniparental organisms, the biological species concept cannot be used to test reproductive isolation because males do not exist or exist at very low percentages of the offspring and may not be functional. The phenomenon of female-only species is called thelytoky by workers in biological control. Unfortunately for biological control workers, thelytoky is common among natural enemies of agricultural pests and presents an obstacle to accurate identification. In the absence of tests for reproductive isolation, morphometric analysis may provide clues to identity of closely related or morphologically nearly identical forms.

Parlatoria pergandii Comstock (chaff scale) represents a problem on citrus in Texas. Aphytis hispanicus (Mercet) and A. comperei (DeBach & Rosen) are among the natural enemies found on chaff scale. Both species are thelytokous and similar (cryptic species). Key anatomical characters used to distinguish the species overlap. However, Woolley and Browning (1987) have used principal component analysis and canonical variate analysis to distinguish between the species. These and other statistical techniques may be used by museum taxonomists when electrophoretic analysis is not possible.

4. Other Morphological Differences Possibly Enhancing Spatial Segregation

Most of the mechanisms which maintain spatial segregation in sibling species are most likely related to reproductive isolation. As the structure of the habitats and their biological components are distinct in most of the cases, this spatial segregation would provide different ecological pressures and foster further divergent morphological adaptation in sibling species pairs. To test this notion, we used quantitative characters, average weights and wing lengths, to reveal potentially repetitive patterns in morphological differences between the sibling species, patterns indicative of constant trends in the divergent evolution of both (see Table I). Comparisons permit an assessment of the degree and trend of morphological differences and their possible relation to the spatial niches of each species and of whether general rules may be deduced from the results.

In pairs of species sharing the same primary forest habitat (genera Neocossyphus and Malachocynchla) and the same secondary vegetation (genera Sorathrura, Spermestes, and Nectarinia), three species pairs show no size differences and the other shows minimal differences. In species pairs inhabiting the primary forest with vertical segregation, similar size is observed in the sibling species of flycatchers (Dyaphorophyia, Myoparum, and Tchitrea). In the bulbuls, the canopy species Andropadus gracilis is smaller than the undergrowth species Andropadus ansorgei; but in Phyllastrephus it is the canopy species P. xavieri that is bigger and the undergrowth species P. icterinus that is smaller. In the primary forest sibling species showing horizontal spatial segregation (one species in the plateau, the other in riverine habitat), the cuckoos Cercococcyx both are similarly sized, in Alethe the riverine species poliocephala is bigger and the plateau species castanea is smaller, and in Geockichla the plateau species princei is bigger and the riverine species Cameronensis is smaller.

The category of sibling species pairs of species A, the mature forest inhabitant, and species B, restricted to modified habitats, is of special interest because such pairs probably have evolved in allopatry under different ecological regimes. The question then can be asked whether their different origins have resulted in divergent evolution in both size and weight. Our data rule out this possibility, as comparisons between such pairs show that within-pair differences are variously oriented among pairs. In most species pairs, these differences appear hardly significant (e.g., in Tauracus, Apaloderma, Psalidoprocne, Pogonolius, and Alcedo). In other cases, species A (forest species) is bigger (e.g., in Centropus), but species of Merops and Dicrurus show the opposite trend, with species A being the smaller. Further, our initial prediction that species evolving in dense vegetation might have shorter wings and lower weights than species evolving in relatively open habitat is not supported by the M'Passa data. Morphological differences in the sibling species appear primarily related to secondary sexual characters rather than to morphological adaptations to different habitats.

Genetic Structure of Livestock Populations

Livestock breeds represent a partitioning of the genetic diversity within each species. Through many generations of selection and relative reproductive isolation, each of these subpopulations has become a distinctive genetic entity possessing specific combinations of genes and associated phenotypic characteristics. The core value of these breeds lies in the reliable access they provide to these combinations of genetically defined characteristics. For example, ewes of the prolific Russian Romanov sheep breed are capable of producing three to six lambs at a time. Sheep breeders interested in improving prolificacy can thus rely on this breed as a source of useful genes.

The traditional (i.e., mid-twentieth century) genetic structure of most livestock species was thus characterized by many regionally isolated subpopulations. In addition, several distinct subpopulations could often be identified within each region, maintained in more or less pure form by breeders who favored animals of a specific type. These dual mechanisms for maintaining reproductive isolation fueled development of levels of observed genetic diversity that are generally far in excess of those observed in wild species.

It would be incorrect, however, to view the livestock breeds as fixed, immutable genetic entities. Migration, or the exchange of genetic material, among breeds is not uncommon. Even among breeds that maintain pedigree records, decisions to “open” the herd book occasionally occur to allow incorporation of desirable animals from outside the breed. Breeds wax and wane in popularity. Some breeds merge, others are absorbed by more popular breeds, and a few simply lose favor and gradually disappear. Breed evolution is thus an accepted part of livestock breeding. This traditional genetic structure of livestock breeds is remarkably similar to that proposed by Wright (1940) as optimal for evolution in natural populations, with relatively high levels of reproductive isolation, modest effective population sizes, diverse selection pressures, and levels of migration that provide for periodic infusions of genes without seriously compromising the genetic integrity of the subpopulations.

Livestock breeds in most cases possess very significant amounts of within-breed genetic diversity. Although breeds can rightly be viewed as mildly inbred lines, they do not approach the levels of genetic uniformity commonly found in plant breeding lines in which self-fertilization and clonal propagation can strongly limit, and in some cases completely eliminate, genetic diversity. Thus, essentially all livestock breeds retain significant evolutionary potential and can undergo significant genetic changes in response to selection, either natural or artificial. Livestock breeds retain the capacity to change, sometimes radically, in response to changes in breeders' preferences or market demands without losing fundamental characteristics of the breed.

Finally, within each of the domestic species, a very large number of individuals exist that do not exhibit or possess unambiguous breed ancestry. Worldwide, many domestic animals commonly display apparent breed affinity, manifested by the color or morphology of recognized local breeds, but lack either the pedigree documentation or the full array of phenotypic characteristics necessary to confirm breed identity. For example, more than 90% of the dairy cows in the United States would be visually classified as Holsteins, but at most one-third of these would be recorded in herd books as purebred animals. The proportion of animals recorded in herd books is much lower, rarely exceeding 10%, in cattle used for meat production, sheep, and goats.

Many other animals are obvious crossbreds, showing mixtures of specific breed characteristics that suggest a somewhat predictable breed ancestry. Also, large numbers of animals are often truly nondescript, exhibiting a collage of various breed characteristics that indicate a more diverse ancestry. The value of crossbred or nondescript animals as a genetic resource is hotly debated. Populations are often large and well adapted to prevailing environmental conditions. Although adapted local breeds are often viewed as “contaminated” by crossing with less well adapted, imported breeds, these populations also provide opportunity for creation of new genetic combinations and for selective elimination of undesirable breeds. Thus, these animals represent a potentially useful genetic resource but lack the predictability of the pure breeds for use in commercial livestock production.

The Genetics of Species Differences

A remarkable and repeated pattern in speciation is the tendency for interspecific hybrids to be either sterile or inviable (collectively known as postzygotic reproductive isolation). This phenomenon is so striking and common that it forms the central premise of the biological species concept (see Most Commonly Used Species Concepts) and has been the focus of intense genetic study over the past several decades. A central organizing principle for the study of the genetics of postzygotic reproductive isolation has been Haldane's rule. Haldane (1922) observed, “When in the F1 offspring of two different animal races one sex is absent, rare or sterile, that sex is the heterogametic sex.” As Haldane's rule is obeyed when males are the heterogametic sex as well as when females are the heterogametic sex, the genetic mechanisms that explain Haldane's rule are thought to be fundamental to speciation in all taxa.

Several explanations for Haldane's rule have been posited and an almost equal number have been rejected (reviewed in Hollocher, 1998; Coyne and Orr, 2004). Part of this controversy stems from the difficulties inherent in performing genetic analyses on such traits as hybrid sterility and hybrid inviability. Part also stems from the fact that most researchers naturally sought a single, universally applicable genetic mechanism to account for all cases conforming to Haldane's rule. The general consensus today is that Haldane's rule requires separate explanations depending on whether or not hybrid inviability or hybrid sterility is being considered.

For hybrid inviability, Haldane's rule results because genetic incompatibilities causing inviability that evolve between species tend to act recessively. Given this recessivity, genes causing hybrid inviability will be expressed in the heterogametic sex while remaining masked in the homogametic sex, thus generating Haldane's rule. For hybrid sterility, the explanation is more complicated and involves the joint action of several different processes. As is the case for hybrid inviability, Haldane's rule for hybrid sterility results because genetic incompatibilities causing sterility also tend to act recessively. In addition, however, when males are the heterogametic sex, the evolution of hybrid male sterility is accelerated, most likely due to the additional action of sexual selection driving the rapid divergence of male sexual traits (see Sexual Selection; Wu et al., 1996; Rice article in Howard and Berlocher, 1998; Coyne and Orr, 2004). Much detailed work on the characterization of the genetic basis of postzygotic reproductive isolation (namely male hybrid sterility) has revealed that many genes of small effect are involved and that epistasis is of primary importance for the expression of this trait (reviewed in Wu and Palopoli, 1994; see Wu and Hollocher article in Howard and Berlocher, 1998; Orr, 2001; Coyne and Orr, 2004).

Postzygotic reproductive isolation is only one trait that has generally diverged between species. To formulate a broader picture of the genetic patterns of divergence, it is necessary to compare the genetic architecture of this trait with what is seen for other traits that diverge between species (reviewed in Hollocher, 1998; Orr, 2001). The traits that have been looked at in some genetic detail include interspecific mate discrimination and interspecific differences in secondary sexual characteristics as well as more general morphological traits. Overall, the pattern that emerges is that these traits, too, are governed by many genes of small effect. However, in contrast to what was found for the genetic basis of postzygotic reproductive isolation, epistasis does not play a dominant role in governing the evolution of these species differences. Because both hybrid sterility and hybrid inviability are only manifested when gene interactions are rendered incompatible, the ubiquitous and rapid evolution of these traits reflect the highly integrated nature of gene networks operating in living organisms and the internal selection pressure these networks exert on physiological and developmental systems to constantly adjust to small changes in the genome that accumulate through the normal action of mutation, selection, and drift. Changes in morphological traits, however, are more tightly circumscribed developmentally. Although morphological traits also require integrated gene networks for their proper expression, genetic changes that evolve between species must still be able to operate well within that particular developmental system, whereas hybrid sterility and hybrid inviability signal the breakdown of gene interactions more globally.

B. Recognition Concept of Species

An important attack on the biological species concept came from H. E. H. Paterson in the early 1980s. His claims were twofold: first, that the Dobzhansky/Mayr term isolating mechanisms implied that reproductive isolation was adaptive, which Paterson felt was unlikely; second, that the true reality underlying species was prezygotic compatibility, consisting of mating signals and fertilization signals. According to Paterson (1985), this compatibility is strongly conserved by stabilizing selection, whereas isolating mechanisms such as hybrid sterility or inviability are nonadaptive and can be argued to be a result rather than a cause of species separateness. To Paterson, the true reality of species must be adaptive. He termed his idea of species the “recognition concept” versus Mayr's “isolation concept,” and its important characteristics “specific mate recognition systems” (SMRSs) instead of isolating mechanisms. Species were defined as “that most inclusive population of individual biparental organisms which share a common fertilization system” (Paterson, 1985).

The idea is generally recognized as a useful critique and has gained strong currency in some circles. However, it has been pointed out that SMRSs are more or less the inverse of prezygotic isolating mechanisms, and that the recognition concept therefore differs from the biological species concept mainly by focusing on the subset of isolating mechanisms occurring before fertilization. The interbreeding concept had always stressed a common gene pool and compatibility within a species, as well as isolation between species.