ECOLOGY, SEXUAL SELECTION, HYBRIDISATION AS SPECIATION MECHANISMS

What? Who said we geographic isolation alone?

The article below has some insights on mechanisms involved in speciation. The high biodiversity of cichlid fishes in great Lake of Malawi is one of the many examples of organisms that have appreciated this mechanisms and produced the unspoken wonders of the world.

Just get a dive in rocky habitats of Cape Maclear, Nkhata Bay, Likoma and Monkey Bay just to mention a few localities and you will appreciate what we are trying to communicate here.

Do not say I did not tell you.
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SEXUAL SELECTION AND SPECIATION
The following points are made by K. Kraaijeveld and A. Pomiankowski (Current Biology 2004 14:R419):1) When Charles Darwin (1809-1882) [1] proposed his theory of sexual selection he was concerned mainly with explaining the widespread occurrence of exaggerated sexual ornaments and courtship displays, as these traits could not easily be explained by natural selection. He also noted that taxonomic groups with more pronounced sexual ornaments tended to have more species. This suggests that sexual selection may elevate the rate at which populations diversify and give rise to new species. A new study [2] of female mate preferences in five populations of an East African cichlid species strongly supports the connection between sexual selection and speciation.2) With the surge of interest in sexual selection over the past few decades, the question of whether it can lead to speciation has also enjoyed renewed attention. A plethora of theoretical models have investigated the connection, and generally concluded that sexual selection can promote speciation (3). The main evolutionary mechanism proposed invokes the rapid coevolution of female mate preferences and male courtship traits, leading to reproductive isolation between groups of individuals. However, empirical evidence in support of the idea is scarce.3) An indirect way this idea has been tested involves looking across broad taxonomic groups for a link between the strength of sexual selection and species number. So far, the evidence from these studies has been conflicting. In birds for example, taxa with greater sexual differences in plumage color -- an indicator of sexual selection -- have higher species numbers compared to sister taxa subject to weaker sexual selection [4,5]. However, surveys in other groups (butterflies, mammals, and spiders) have failed to find such an association, and the positive result in birds has not been replicated in a recent reanalysis. It seems premature to conclude from this that speciation is independent of sexual selection. One reason for the lack of a strong linkage is that sexual selection may promote extinction as well as speciation, if it leads to the evolution of traits maladaptive to male and female survival. Another is that sexual selection can even retard speciation under certain conditions. So in the long term, species numbers may only loosely be connected to sexual selection.4) A more direct way of investigating the connection between sexual selection and speciation is to examine its action in closely related populations. Knight and Turner [2] attempted such a test using populations of the cichlid fish Pseudotropheus zebra from Lake Malawi. The cichlid fishes of the East African lakes, in particular Lake Victoria and Lake Malawi, are renowned for rampant speciation over a very brief period of time -- more than 1000 species have been generated in less than a million years. Some of this diversity is due to ecological specialization, facilitated by the "key innovation" of the cichlid pharyngeal jaw. But many closely related species show practically no differences except in male color, suggesting that sexual selection may be an important additional mechanism of speciation.
References (abridged):1. Darwin, C.R. (1871). The Descent of Man and Selection in Relation to Sex. (London: John Murray)2. Knight, M.E. and Turner, G.F. (2004). Laboratory mating trials indicate incipient speciation by sexual selection among populations of the cichlid fish Pseudotropheus zebra from Lake Malawi. Proc. R. Soc. Lond. B 271, 675-6803. Turelli, M., Barton, N.H., and Coyne, J.A. (2001). Theory and speciation. Trends Ecol. Evol. 16, 330-3434. Barraclough, T.G., Harvey, P.H., and Nee, S. (1995). Sexual selection and taxonomic diversity in passerine birds. Proc. R. Soc. Lond. B 259, 211-2155. Owens, I.P.F., Bennett, P.M., and Harvey, P.H. (1999). Species richness among birds: body size, life history, sexual selection or ecology?. Proc. R. Soc. Lond. B 266, 933-939

ON HABITATS AND ECOLOGICAL SPECIATION.
The following points are made by R. Ogden and R.S. Thorpe (Proc. Nat. Acad. Sci. 2002 99: 13612):1) Understanding speciation processes in rainforests is key to predicting changes in species number and planning conservation strategy (1). Ecological speciation due to divergent natural selection has emerged as an alternative theory to speciation in geographic isolation. Recent studies in support of an ecological gradient model of speciation in rainforests have shown morphological differences between habitats but have not tested for a reduction in gene flow (2,3) or have not reported such a reduction where it has been tested (3,4). Morphological variation along ecological gradients may indicate diversification, but speciation is not an inevitable consequence of population differentiation (5), and molecular evidence of reduced gene flow is needed to strengthen support for the theory of ecological speciation.2) The authors report a study in which molecular markers were used to examine the effects of allopatric divergence and habitat on levels of gene flow in the Caribbean lizard Anolis roquet. Three study transects were constructed to compare variation in microsatellite allele frequencies and morphology across phylogenetic and habitat boundaries in northern Martinique. Results showed reductions in gene flow to be concordant with divergent selection for habitat type. No evidence could be found for divergence in allopatry influencing current gene flow. Morphological data match these findings, with multivariate analysis showing correlation with habitat type but no grouping by phylogenetic lineage. The results support the ecological speciation model of evolutionary divergence, indicating the importance of habitats in biodiversity generation.References (abridged):1. Moritz, C. , Patton, J. L. , Schneider, C. J. & Smith, T. B. (2000) Annu. Rev. Ecol. Syst. 31, 533-5632. Schneider, C. J. , Smith, T. B. , Larison, B. & Moritz, C. (1999) Proc. Natl. Acad. Sci. USA 96, 13869-138733. Smith, T. B. , Schneider, C. J. & Holder, K. (2001) Genetica 112, 383-3984. Smith, T. B. , Wayne, R. K. , Girman, D. J. & Bruford, M. W. (1997) Science 276, 1855-18575. Magurran, A. E. (1998) Philos. Trans. R. Soc. London B 353, 275-286Proc. Nat. Acad. Sci.


HYBRIDS AND SPECIATION
The following points are made by Richard J. Abbott (Science 2003 301:1189):1) Why sex evolved and is maintained in most living organisms remains a key question in evolutionary biology (1). What is indisputable, however, is that sexual reproduction generates new gene combinations, some of which may render the organism better adapted to new environments. The range of different genotypes among offspring increases with the level of genetic divergence between parents. Therefore, matings between different species (that is, interspecific hybridization could potentially generate a vast range of different offspring genotypes, provided that the resulting hybrid zygotes develop and exhibit some fertility. For example, Rieseberg et al (2) describe how hybridization between two sunflower species generated offspring genotypes that are adapted to habitats very different from those occupied by the parents. This resulted in three diploid hybrid sunflower species that are ecologically isolated from each other and their progenitors. The Rieseberg et al findings provide proof that interspecific hybridization can be adaptive.2) In many plant groups, hybridization between different species is prevented by prezygotic barriers. Such barriers may arise when species have their own specific pollinator, occupy a habitat different from other species, or are spatially separate from other species. Under these conditions, postzygotic barriers manifested in the form of embryo abortion or low hybrid viability may be absent or weak. However, prezygotic barriers can be "leaky", especially when habitats are disturbed in some way, so hybrids are sometimes produced that are often sterile or exhibit reduced fertility. Such problems of low fertility can be overcome, either by chromosome doubling (allopolyploidy) or recombination (3), to produce a stable fertile hybrid that is reproductively isolated from its parents by a strong postzygotic barrier (4,5), and which is therefore regarded as a new species.3) Although postzygotic barriers are effective mechanisms of reproductive isolation, they present major obstacles to the establishment of a new hybrid species in the wild. Hybrids are born into populations comprising one or both parent species and will initially be represented as a minority component. Consequently, most matings by a fertile hybrid will be with a parent rather than another hybrid, and will result in the production of no offspring or sterile offspring. The hybrid therefore suffers from what is termed a "minority type disadvantage" (4). It can escape from this predicament by evolving a prezygotic barrier that prevents it from mating with its parents. This can be achieved through uniparental reproduction (asexual reproduction or selfing), by flowering earlier or later, by attracting a different pollinator, by occupying a different habitat (ecological isolation), or through spatial isolation due to geographical separation after dispersal (3). Ecological or spatial isolation will also enable a hybrid to avoid any adverse effects of interspecific competition with a parent.References (abridged):1. S. A. West et al., J. Evol. Biol. 12, 1003 (1999)2. L. H. Rieseberg et al. Science 301, 1211 (2003)3. V. Grant, Plant Speciation (Columbia Univ. Press, New York, ed. 2, 1981) 4. D. A. Levin, The Role of Chromosomal Change in Plant Evolution (Oxford Univ. Press, Oxford, 2002)5. J. F. Gutierrez-Marcos et al., Philos. Trans. R. Soc. London Ser. B 358, 1105 (2003)