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Reproductive isolation: Natural selection at work
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Reproductive isolation: Natural selection at work Brian Charlesworth and Deborah Charlesworth
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Although much is known about the genetic basis of reproductive isolation between species, little is understood about its underlying evolutionary causes. A study of two very closely related, but reproductively isolated, plant species has provided some valuable insights. Address: Institute of Cell, Animal and Population Biology, University of Edinburgh, Ashworth Laboratories, King’s Buildings, West Mains Road, Edinburgh EH9 3JT, UK. Current Biology 2000, 10:R68–R70 0960-9822/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved.
The crucial step in the separation of two populations of a sexually reproducing species into two new species — speciation — is the acquisition of reproductive isolation between them. Such isolation ensures that they can evolve independently of each other, and so end up with radically different genotypes and phenotypes. This causes the biological world to be divided up into the discrete taxonomic groups which we take for granted. Many different ways in which populations can fail to interbreed have been described, ranging from simple ecological separation of time or place of breeding, through behavioural barriers to interbreeding, to inviability or infertility of hybrids [1]. Reproductive isolation has long been thought to evolve mainly as a by-product of the divergence of populations that are prevented from interbreeding by external barriers such as spatial separation. There is clearly no selection to maintain compatibility of mating behaviour between individuals from geographically or ecologically separated populations, or to maintain harmonious interactions between genes that have acquired different alleles in different populations. Given enough evolutionary divergence between populations, complete reproductive isolation may therefore be expected to arise [1]. This is no more surprising in principle than the fact that electrical plugs of British design do not function in Continental European sockets. While this simple model is consistent with most of the facts relating to the genetics and ecology of reproductive isolation, it leaves several important questions unanswered. These include the amount of restriction on gene flow that will permit enough divergence for reproductive isolation to evolve, the time-scale needed for speciation to be completed, the genetic basis of reproductive isolating barriers, and the causes of the evolutionary divergence that drives reproductive isolation. More or less complete answers to the first three questions are
available. In many ways, the last question is the hardest to answer, as it is usually impossible to relate a particular example of reproductive isolation to the underlying evolutionary forces, even if we have considerable knowledge of its genetic basis. Pollinator preferences can lead to reproductive isolation between flowering plant species, and may provide good examples for studying the evolutionary origin of the features that lead to new species formation [2,3]. A recent study of a pair of closely related plants, Mimulus lewisii and M. cardinalis, has examined the adaptive evolution of their reproductive isolation [4]. Most species in the monkeyflower genus Mimulus are pollinated by bees, and their floral morphology can be interpreted as a set of adaptations to this mode of pollination, as in the case of M. lewisii (in which 100% of 233 visits observed in a natural population were by bees) [4]. In some cases, however, there has been a shift to pollination by hummingbirds, as in M. cardinalis (with more than 97% of 146 visits from hummingbirds). The change to pollination by hummingbirds in M. cardinalis is accompanied by a set of changes in the flowers (Figure 1, Table 1). These changes have almost certainly been driven by selection for more effective attraction of hummingbird pollinators. Similar differences between species with different pollinators are common in plant genera [5–7], and hummingbird pollinated species have repeatedly and independently evolved tubular flowers, often with orange or red colours different from those of their congeners [7–9]. In the case of the two Mimulus species, observations on pollinator behaviour in the wild suggest that there is little chance that a bee that Figure 1
Flower morphology of the two Mimulus species studied by Schemske and Bradshaw [4]. (Images courtesy of Douglas Schemske.)
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Table 1
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Figure 2
Floral characteristics of the two Mimulus species studied. Species
M. lewisii
M. cardinalis
Pollinators
Bee
Hummingbird
Flower size
Small
Large
Flower shape Wide, with ‘landing platform’
Narrow
Flower colour
Pink (low carotenoids)
Red (high anthocyanins)
Nectar
Moderate, high sugar
Abundant, low sugar
has visited M. lewisii will subsequently visit M. cardinalis, or that a hummingbird that has visited M. cardinalis will visit M. lewisii [4]. A cross between the two Mimulus species, made for the purpose of analysing the genetics of the traits distinguishing them [10], has now provided an unusual opportunity for testing the relation between phenotype and pollinator preference, and the effect of this on the degree of reproductive isolation between them [4]. The F2 hybrid plants show a wide range of flower colours and morphological differences. To test their effects on pollinators, arrays of plants were placed in the field at a locality where both plant species, and both kinds of pollinators, coexist. Because the species differ at a number of loci controlling several independent characters, many different character combinations are produced, so that the effects of individual floral characteristics can be separately analysed, using multiple regression. The last three characters in Table 1 were studied in detail, based on samples of two flowers per individual F2 plant, taken before they were transplanted into the field — that is, at a time when all plants were in a reasonably uniform environment, and before nectar was removed by pollinators. About 95% of bee visits to the transplanted plants of the parent species were by native bumblebees, and all hummingbird visits were by Anna’s hummingbirds, very similar to what is observed in natural populations of the two plant species. Based on more than 8,000 pollinator visits to 228 F2 plants, bee visitation rate was significantly negatively correlated, and hummingbird visitation significantly positively correlated, with flower anthocyanin content. These opposing effects show that anthocyanins contribute to reproductive isolation (Figure 2). Interestingly, however, other characters merely affected one kind of pollinator or the other. Increased carotenoid content caused a reduction in bee visits but did not increase visits by hummingbirds, while nectar volume per flower increased hummingbird visits without affecting bee visitation, and increased petal area correlated only with bee visitation. Bee pollination is
Effects of several flower character differences on daily visitation rates by bees and hummingbirds.
presumably the ancestral condition, and so it is interesting that, unlike most Mimulus, which are yellow-flowered, M. lewisii has pink flowers (perhaps making it easier to evolve the red colour of M. cardinalis). Given that adaptation to a new pollinator is unlikely to occur in a single step, the initial changes making flowers attractive to hummingbirds should thus be advantageous if hummingbirds are common, but only if bee visitation is not greatly decreased. This study provides a convincing case in which a set of changes driven by natural selection have led to the reproductive isolation of a pair of closely related populations (natural hybrids do not occur between the two Mimulus species, see [11]). Although the order of the evolutionary changes, and the nature of the change that started the process of isolation, are still unknown, the work has afforded an opportunity to dissect the genetic basis of the traits involved, by a quantitative-trait linkage (QTL) mapping study involving numerous molecular markers [10]. The carotenoid-content difference in the upper petals of these two species is largely due to a single major gene [11]. This difference alone significantly affects bee visits; in F2 plants, homozygosity for the M. cardinalis (recessive) allele for high carotenoid content reduced bee visits by over 75%. It is intriguing that increased carotenoid content was presumably disadvantageous until after the change to high anthocyanin (which attracts hummingbirds). Schemske and Bradshaw [4] even suggest that its advantage in M. cardinalis may have been
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to deter bees rather than to attract hummingbirds. It would thus be very interesting to know how the different flower colour characteristics interact in the effects on pollinators observed with the F2 plants. For several other traits, there is also evidence for major effects — defined as ≥25% of the species difference — associated with individual molecular markers [10]. For instance, a single molecular marker difference is associated with 41% of the parental difference in nectar volume [10]. Tests on the F2 population showed that this has a significant effect on visitation, this time by hummingbirds only [4]. Schemske and Bradshaw [4] provocatively suggest that their evidence for the involvement of major genes conflicts with neo-Darwinian orthodoxy that “adaptation and reproductive isolation are caused by a nearly infinite number of mutations of individually small effect”. The existence of such an orthodoxy is questionable, however. Fisher’s [12] often cited proposition, that small random mutational perturbations to a complex system are much more likely to be selectively advantageous than large ones, must be viewed in the context of his pioneering work on ecological genetics (with several collaborators), which utilized natural major gene polymorphisms to study natural selection in the wild [13]. Indeed, much classic work on the quantitative genetics of adaptive differentiation between local populations of the same species has been summed up as indicating contributions from “a few loci with major effects though probably many with minor ones” (page 365 in [14]). As Wright pointed out in his review [15] of Goldschmidt’s masterpiece of evolutionary heterodoxy [16] “The neoDarwinists have not been concerned primarily with ... the types of mutation involved in evolution. Their primary concern is the dynamics of the process, accepting all types of mutation actually observed”. The critical distinction is between the idea that complex differences between species arise through a single “macromutational” change affecting a whole set of traits simultaneously, as postulated by Goldschmidt [16] and by some advocates of evolution by punctuated equilibria [17], and the view that selection works on individually advantageous changes affecting separate traits [5]. The Mimulus case provides yet more support for the latter viewpoint. References 1. Dobzhansky T: Genetics and the Origin of Species. New York: Columbia University Press; 1937. 2. Grant V: Modes and origins of mechanical and ethological isolation in angiosperms. Proc Natl Acad Sci USA 1994, 91:3-10. 3. Waser NM: Pollination, angiosperm speciation, and the nature of species boundaries. Oikos 1998, 82:198-201. 4. Schemske DW, Bradshaw HD: Pollinator preferences and the evolution of floral traits in monkeyflowers (Mimulus). Proc Natl Acad Sci USA 1999, 96:11910-11914. 5. Darwin CR: The Origin of Species. London: John Murray; 1859.
6. Darwin CR: The Various Contrivances by which Orchids are Fertilised by Insects. London: John Murray; 1862. 7. Proctor MCF, Yeo PF: The Pollination of Flowers. London: Collins; 1973. 8. Grant V: Floral isolation between ornithophilous and sphingophilous species of Ipomopsis and Aquilegia. Proc Natl Acad Sci USA 1992, 89:11828-11831. 9. Richards AJ: Plant Breeding Systems (2nd edn). London: George Allen and Unwin; 1997. 10. Bradshaw HD, Otto SM, Frewen BE, McKay JK, Schemske DW: Quantitative trait loci affecting differences in floral morphology between two species of monkeyflower (Mimulus). Genetics 1998, 149:367-382. 11. Hiesey WM, Nobs MA, Björkman O: Experimental Studies on the Nature of Species. V. Biosystematics, Genetics and Physiological Ecology of the Euryanthe Section of Mimulus. Washington, D.C.: Carnegie Institute of Washington; 1971. 12. Fisher RA: The Genetical Theory of Natural Selection. Oxford: Clarendon Press; 1930. 13. Ford EB: Ecological Genetics (Fourth Edition). London: Chapman and Hall; 1975. 14. Wright S: Evolution and the Genetics of Populations. Vol.4. Variability Within and Between Populations. Chicago, Illinois: University of Chicago Press; 1978. 15. Wright S: The material basis of evolution: a review. Sci Monthly 1941, 53:165-170. 16. Goldschmidt RB: The Material Basis of Evolution. New Haven, Connecticut: Yale University Press; 1940. 17. Gould SJ: Is a new and general theory of evolution emerging? Paleobiology 1980, 6:119-130.
02/08/2000
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Dispatch
Reproductive isolation: Natural selection at work Brian Charlesworth and Deborah Charlesworth
t-
Although much is known about the genetic basis of reproductive isolation between species, little is understood about its underlying evolutionary causes. A study of two very closely related, but reproductively isolated, plant species has provided some valuable insights. Address: Institute of Cell, Animal and Population Biology, University of Edinburgh, Ashworth Laboratories, King’s Buildings, West Mains Road, Edinburgh EH9 3JT, UK. Current Biology 2000, 10:R68–R70 0960-9822/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved.
The crucial step in the separation of two populations of a sexually reproducing species into two new species — speciation — is the acquisition of reproductive isolation between them. Such isolation ensures that they can evolve independently of each other, and so end up with radically different genotypes and phenotypes. This causes the biological world to be divided up into the discrete taxonomic groups which we take for granted. Many different ways in which populations can fail to interbreed have been described, ranging from simple ecological separation of time or place of breeding, through behavioural barriers to interbreeding, to inviability or infertility of hybrids [1]. Reproductive isolation has long been thought to evolve mainly as a by-product of the divergence of populations that are prevented from interbreeding by external barriers such as spatial separation. There is clearly no selection to maintain compatibility of mating behaviour between individuals from geographically or ecologically separated populations, or to maintain harmonious interactions between genes that have acquired different alleles in different populations. Given enough evolutionary divergence between populations, complete reproductive isolation may therefore be expected to arise [1]. This is no more surprising in principle than the fact that electrical plugs of British design do not function in Continental European sockets. While this simple model is consistent with most of the facts relating to the genetics and ecology of reproductive isolation, it leaves several important questions unanswered. These include the amount of restriction on gene flow that will permit enough divergence for reproductive isolation to evolve, the time-scale needed for speciation to be completed, the genetic basis of reproductive isolating barriers, and the causes of the evolutionary divergence that drives reproductive isolation. More or less complete answers to the first three questions are
available. In many ways, the last question is the hardest to answer, as it is usually impossible to relate a particular example of reproductive isolation to the underlying evolutionary forces, even if we have considerable knowledge of its genetic basis. Pollinator preferences can lead to reproductive isolation between flowering plant species, and may provide good examples for studying the evolutionary origin of the features that lead to new species formation [2,3]. A recent study of a pair of closely related plants, Mimulus lewisii and M. cardinalis, has examined the adaptive evolution of their reproductive isolation [4]. Most species in the monkeyflower genus Mimulus are pollinated by bees, and their floral morphology can be interpreted as a set of adaptations to this mode of pollination, as in the case of M. lewisii (in which 100% of 233 visits observed in a natural population were by bees) [4]. In some cases, however, there has been a shift to pollination by hummingbirds, as in M. cardinalis (with more than 97% of 146 visits from hummingbirds). The change to pollination by hummingbirds in M. cardinalis is accompanied by a set of changes in the flowers (Figure 1, Table 1). These changes have almost certainly been driven by selection for more effective attraction of hummingbird pollinators. Similar differences between species with different pollinators are common in plant genera [5–7], and hummingbird pollinated species have repeatedly and independently evolved tubular flowers, often with orange or red colours different from those of their congeners [7–9]. In the case of the two Mimulus species, observations on pollinator behaviour in the wild suggest that there is little chance that a bee that Figure 1
Flower morphology of the two Mimulus species studied by Schemske and Bradshaw [4]. (Images courtesy of Douglas Schemske.)
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Table 1
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Figure 2
Floral characteristics of the two Mimulus species studied. Species
M. lewisii
M. cardinalis
Pollinators
Bee
Hummingbird
Flower size
Small
Large
Flower shape Wide, with ‘landing platform’
Narrow
Flower colour
Pink (low carotenoids)
Red (high anthocyanins)
Nectar
Moderate, high sugar
Abundant, low sugar
has visited M. lewisii will subsequently visit M. cardinalis, or that a hummingbird that has visited M. cardinalis will visit M. lewisii [4]. A cross between the two Mimulus species, made for the purpose of analysing the genetics of the traits distinguishing them [10], has now provided an unusual opportunity for testing the relation between phenotype and pollinator preference, and the effect of this on the degree of reproductive isolation between them [4]. The F2 hybrid plants show a wide range of flower colours and morphological differences. To test their effects on pollinators, arrays of plants were placed in the field at a locality where both plant species, and both kinds of pollinators, coexist. Because the species differ at a number of loci controlling several independent characters, many different character combinations are produced, so that the effects of individual floral characteristics can be separately analysed, using multiple regression. The last three characters in Table 1 were studied in detail, based on samples of two flowers per individual F2 plant, taken before they were transplanted into the field — that is, at a time when all plants were in a reasonably uniform environment, and before nectar was removed by pollinators. About 95% of bee visits to the transplanted plants of the parent species were by native bumblebees, and all hummingbird visits were by Anna’s hummingbirds, very similar to what is observed in natural populations of the two plant species. Based on more than 8,000 pollinator visits to 228 F2 plants, bee visitation rate was significantly negatively correlated, and hummingbird visitation significantly positively correlated, with flower anthocyanin content. These opposing effects show that anthocyanins contribute to reproductive isolation (Figure 2). Interestingly, however, other characters merely affected one kind of pollinator or the other. Increased carotenoid content caused a reduction in bee visits but did not increase visits by hummingbirds, while nectar volume per flower increased hummingbird visits without affecting bee visitation, and increased petal area correlated only with bee visitation. Bee pollination is
Effects of several flower character differences on daily visitation rates by bees and hummingbirds.
presumably the ancestral condition, and so it is interesting that, unlike most Mimulus, which are yellow-flowered, M. lewisii has pink flowers (perhaps making it easier to evolve the red colour of M. cardinalis). Given that adaptation to a new pollinator is unlikely to occur in a single step, the initial changes making flowers attractive to hummingbirds should thus be advantageous if hummingbirds are common, but only if bee visitation is not greatly decreased. This study provides a convincing case in which a set of changes driven by natural selection have led to the reproductive isolation of a pair of closely related populations (natural hybrids do not occur between the two Mimulus species, see [11]). Although the order of the evolutionary changes, and the nature of the change that started the process of isolation, are still unknown, the work has afforded an opportunity to dissect the genetic basis of the traits involved, by a quantitative-trait linkage (QTL) mapping study involving numerous molecular markers [10]. The carotenoid-content difference in the upper petals of these two species is largely due to a single major gene [11]. This difference alone significantly affects bee visits; in F2 plants, homozygosity for the M. cardinalis (recessive) allele for high carotenoid content reduced bee visits by over 75%. It is intriguing that increased carotenoid content was presumably disadvantageous until after the change to high anthocyanin (which attracts hummingbirds). Schemske and Bradshaw [4] even suggest that its advantage in M. cardinalis may have been
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to deter bees rather than to attract hummingbirds. It would thus be very interesting to know how the different flower colour characteristics interact in the effects on pollinators observed with the F2 plants. For several other traits, there is also evidence for major effects — defined as ≥25% of the species difference — associated with individual molecular markers [10]. For instance, a single molecular marker difference is associated with 41% of the parental difference in nectar volume [10]. Tests on the F2 population showed that this has a significant effect on visitation, this time by hummingbirds only [4]. Schemske and Bradshaw [4] provocatively suggest that their evidence for the involvement of major genes conflicts with neo-Darwinian orthodoxy that “adaptation and reproductive isolation are caused by a nearly infinite number of mutations of individually small effect”. The existence of such an orthodoxy is questionable, however. Fisher’s [12] often cited proposition, that small random mutational perturbations to a complex system are much more likely to be selectively advantageous than large ones, must be viewed in the context of his pioneering work on ecological genetics (with several collaborators), which utilized natural major gene polymorphisms to study natural selection in the wild [13]. Indeed, much classic work on the quantitative genetics of adaptive differentiation between local populations of the same species has been summed up as indicating contributions from “a few loci with major effects though probably many with minor ones” (page 365 in [14]). As Wright pointed out in his review [15] of Goldschmidt’s masterpiece of evolutionary heterodoxy [16] “The neoDarwinists have not been concerned primarily with ... the types of mutation involved in evolution. Their primary concern is the dynamics of the process, accepting all types of mutation actually observed”. The critical distinction is between the idea that complex differences between species arise through a single “macromutational” change affecting a whole set of traits simultaneously, as postulated by Goldschmidt [16] and by some advocates of evolution by punctuated equilibria [17], and the view that selection works on individually advantageous changes affecting separate traits [5]. The Mimulus case provides yet more support for the latter viewpoint. References 1. Dobzhansky T: Genetics and the Origin of Species. New York: Columbia University Press; 1937. 2. Grant V: Modes and origins of mechanical and ethological isolation in angiosperms. Proc Natl Acad Sci USA 1994, 91:3-10. 3. Waser NM: Pollination, angiosperm speciation, and the nature of species boundaries. Oikos 1998, 82:198-201. 4. Schemske DW, Bradshaw HD: Pollinator preferences and the evolution of floral traits in monkeyflowers (Mimulus). Proc Natl Acad Sci USA 1999, 96:11910-11914. 5. Darwin CR: The Origin of Species. London: John Murray; 1859.
6. Darwin CR: The Various Contrivances by which Orchids are Fertilised by Insects. London: John Murray; 1862. 7. Proctor MCF, Yeo PF: The Pollination of Flowers. London: Collins; 1973. 8. Grant V: Floral isolation between ornithophilous and sphingophilous species of Ipomopsis and Aquilegia. Proc Natl Acad Sci USA 1992, 89:11828-11831. 9. Richards AJ: Plant Breeding Systems (2nd edn). London: George Allen and Unwin; 1997. 10. Bradshaw HD, Otto SM, Frewen BE, McKay JK, Schemske DW: Quantitative trait loci affecting differences in floral morphology between two species of monkeyflower (Mimulus). Genetics 1998, 149:367-382. 11. Hiesey WM, Nobs MA, Björkman O: Experimental Studies on the Nature of Species. V. Biosystematics, Genetics and Physiological Ecology of the Euryanthe Section of Mimulus. Washington, D.C.: Carnegie Institute of Washington; 1971. 12. Fisher RA: The Genetical Theory of Natural Selection. Oxford: Clarendon Press; 1930. 13. Ford EB: Ecological Genetics (Fourth Edition). London: Chapman and Hall; 1975. 14. Wright S: Evolution and the Genetics of Populations. Vol.4. Variability Within and Between Populations. Chicago, Illinois: University of Chicago Press; 1978. 15. Wright S: The material basis of evolution: a review. Sci Monthly 1941, 53:165-170. 16. Goldschmidt RB: The Material Basis of Evolution. New Haven, Connecticut: Yale University Press; 1940. 17. Gould SJ: Is a new and general theory of evolution emerging? Paleobiology 1980, 6:119-130.