Introduction

Evolutionary relatedness among various groups of organisms helps to explain community interactions in an ecosystem. There are three mechanisms that are often invoked to understand the structure of diversity within ecological communities; neutrality, competitive exclusion and abiotic filtering. In their study, Swenson et al., 2007 studied evidence for phylogenetic conservatism and spatial scale relatedness in tropical rainforest by analyzing smaller spatial scales across multiple different tropical forest communities, and eventually provided first analysis for combined influence of size and spatial scales on phylogenetic relatedness across multiple different tropical forest communities. According to Swenson et al., measures of phylogenetic dispersion of co-occurring species seem to provide reliable insights into the relative influence of biotic, abiotic and stochastic processes in structuring ecological functions. Hence, the tropical plant community structures and diversity and at which scales their processes occur is better understood. For instance, five traits (seed size, wood density, specific leaf area and leaf nitrogen and phosphorus content) in tropical trees generally considered to be important in determining tree life histories showed significant phylogenetic conservatism.

Discussion

Phylogenetic niche conservatism results when closely related species are more ecologically similar than expected suggesting that some process is constraining divergence among closely related species (Losos, 2008). In this process species evolve traits that are adapted to survival in a desired niche. One of the hypotheses used to explain incidence of high species richness in the tropics is the geographical area hypothesis which argues that the tropics are the largest biomes and that more area allows species to have more ranges and hence more populations. It is generally accepted that larger areas experience lower extinction rates and that larger areas are more likely to undergo allopatric speciation, which would increase rate of speciation. Larger areas therefore will result in phylogenetic divergence if this hypothesis is followed. This is because larger areas offer opportunity to species to occupy other environments other than competing for resources in a smaller environment.

Ecologists are however increasingly investigating the phylogenetic structure of communities to examine whether co-occurring species are more or less closely related to each other relative to species in a regional source pool (Webb et al., in press). Species that are phylgenetically closer exert stiffer competitive pressure on each other. In evolution, different species often develop functional traits to occupy a desired environment. This is referred to as trait convergence. It is a way of reducing competition or occupying a more resourceful environment. A specific niche will therefore have species that possess similar traits that enable them to survive in it. This also means that species that remain in the historical niche will maintain or develop traits that will enable them survive in that niche. Because of trait convergence, different unrelated species will occupy a niche or environment but ecological divergence of closely related species may yield similar results.

Many scientists have studied phylogenetic overdispersion in co-occurring species (i.e. co-occurring species being less phylogenetically related than expected by chance) and have attributed this phenomenon to competitive exclusion of closely related sympatric species, with the assumption that closely related species are ecologically similar (Losos, 2008). Another explanation could be that closely related species are ecologically divergent and that environmental filtering (in which only ecologically divergent species can exist in a site (Webb et al., 2002) is the reason distantly related, but ecologically similar species will occupy the same community (Kraft et al., 2007).

Phylogenetic conservatism

Phylogenetic niche conservatism can be viewed as a situation where organisms fail to adapt to conditions outside their ancestral niche. In a normal evolutionary trend, increased genetic diversity makes a population more adaptable, increasing the chance of niche expansion into previously unfavorable habitats. Genetic diversity might however also begin the process that results eventually into niche conservatism where the organism is highly specialized. Whereas it is common in matters of ecological theory to assume that species are genetically and phenotypically homogenous, evolutionary biologists assume that variation arises at every step along the way from reproduction through development. Swenson et al. in their study found that phylogenetic niche conservatism is likely widespread, indicating closely related species are more functionally similar.

Conclusion

Swenson et al. state that relative role of phylogenetic relatedness of species within different size classes and cohorts in structuring tropical tree communities have remained completely unexplored. Study of ancestry especially by identifying fossils (through morphological studies) is a big gamble. Morphological studies are known to be quite unreliable and unrelated species that occupy similar environments can resemble one another. Losos, 2008 (Ecology letters) observes that interpretation of patterns of phylogenetic composition of communities is not possible without actual ecological data for the constituent species. And this casts a cloud over phylogenetic studies, as Prof. Chuck Cannon, (AFEC-X lecture, 2009) observed “evolution is not optimal.”

References

Advance Field Ecology and Conservation Course (AFEC_X), 30^th July 2009, Lecture on Phylogenetics by Prof. Chuck Cannon.

Loso, Jonathan B., (2008) Phylogenetic niche conservation, phylogenetic signal and the relationship between phylogenetic relatedness and ecological similarity among species. Ecology letters. 11:995-1007

Swenson, N. G., Enquist, B. J., Thompson J. and Zimmerman, J. K. (2007). The in fluence of spatial and size scale on phylkogenetic relatedness in tropical forest communities, Ecology, 88, 1770-1780.

Webb, C. O., Ackerly, D. D., McPeek, M. A. and Donoghue, M. J. (2002). Phylogenetics and community ecology. Ann. Rev. Eco. Syst, 33, 475-505.