Adaptive Radiation Constrained By Niche Availability

Understanding why there are so many species is an essential question in biology that continues to generate considerable curiosity and drive evolutionary research. Moreover, people seek to understand how species that are closely related can reside in very different niches and co-occur in areas. Darwin observed such an adaptive radiation with finches in the Galapagos, where the species of birds differ in the size and shape of their beaks, allowing them to specialize on particular food sources, thus reducing competition between the species. African rift lakes such as Lake Tanganyika and Lake Malawi also contain spectacular examples of adaptive radiations with cichlids. These fish not only have brilliant colorations, but they have diverged to specialize on particular food sources, such as fish eggs or the eyes of other fish.

While there are increasingly many examples of adaptive radiations that have occurred, we don’t necessarily know why they occur in the first place and to which clades. Is it when there is sufficient ecological opportunity, through pathways described by Simpson (1953) such as colonization, new resource availability, species extinctions, or novel resource utilization through trait evolution, or will radiations happen despite the lack of opportunities, as Losos summarizes in which a clade can supplant another clade through competition or it could radiate and form a new ecological opportunity that did not previously exist (Losos 2010)? Recently, Gómez and Buckling have explored the role of ecological niche availability in the experimental diversification of a strain of Pseudomonas flourescens bacteria. In soil microcosms, they test the diversification of this species in the presence and absence of a resident community and with or without a bacteriophage. They found that the bacteria experienced adaptive morphological diversification in the absence of a resident community, but not in the presence and that phages did not impact diversification. Selection was stronger in the absence of the community, as well. Moreover, the new morphotypes were capable of utilizing more variation in substrates, which would be consistent with greater radiation to utilize unoccupied niches, but remains a constraint when the niches are already filled.

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I think it is excellent that the authors consider how the community context that a species is establishing in may influence the evolutionary trajectory of a species or clade following that establishment. While they recognize the resident community is difficult to completely recreate in the lab, their paper is built upon a series of simple and elegant experimental designs with clear predictions and very interesting results that support field observations in other systems and can serve to help inform future studies exploring the context which adaptive radiations occur in. While “nothing in biology makes sense, except in the light of evolution” (Dobzhansky 1973) it is important to also consider that “nothing in evolutionary biology makes sense, except in the light of ecology” (Grant and Grant 2008).

Pedro Gómez and Angus Buckling. 2013. Real-time microbial adaptive diversification in soil. Ecology Letters 16: 650-655.

Kane Keller

About Kane Keller

Kane is a community ecologist who studies how mutualisms and intraspecific variation in the mutualist species can influence community and ecosystem functioning.
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2 Responses to Adaptive Radiation Constrained By Niche Availability

  1. Jen LauJen Lau says:

    Cool post, Kane! I also love this paper…Super elegant experiments on a big issue. The idea that biodiversity begets biodiversity is one explanation for the high speciation rates and huge species numbers in the tropics. The argument goes that increased diversity leads to more biotic interactions. These biotic interactions drive coevolution. And coevolution promotes adaptation, diversification, and ultimately speciation. A few experimental studies on bacteria and virus systems have supported this idea. For example, bacteria diversify more rapidly in the presence of a deadly virus (Paterson et al. 2010). But here, as you describe above, they find the opposite pattern. So which is it? Does diversity promote or inhibit evolution? As Gomez and Buckling mention in the close of their paper, it might really matter…This result is super relevant to a wide range of topics in both basic and applied science. While previous studies focusing on the presence/absence of a single strong interacting species (voracious predators or deadly parasites) suggest that species interactions drive diversification, these effects may be reduced in the more diverse communities we find in nature. And this could be important to us! What does this result mean for the evolution of our pests, pathogens, and weeds that dominate sites where biodiversity has been wiped out? As the authors argue, perhaps the evolution of important traits such as antibiotic resistance in pathogens or herbicide resistance in weeds also will be more rapid in habitats where natural microbial communities have been eradicated, like our “sterile hospitals” or our depauperate agricultural landscapes.

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