Article Summary–Ecotones and Speciation

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Article Summary–Ecotones and Speciation

Smith, T.B., R.K. Wayne, D.J. Girman, and M.W. Bruford. (1997). A role for ecotones in generating rainforest biodiversity. Science 276: 1855-1857.

Tropical biodiversity is generally assumed to be sustained by speciation involving geographic isolation within the species-rich rainforests, hence most research and conservation efforts are focused on central rainforest habitats. Smith et al. suggest that the ecotones, the transition zones between grasslands and forest, may in fact play an essential role in generating new species. They propose that bird populations emigrating into the ecotone from the forest are subjected to strong natural selection, thereby generating novel traits that may form the basis for speciation. This study is unique in measuring both levels of gene flow and degrees of character divergence between the forest and ecotone habitats, combining ideas from molecular genetics, ecology, and biogeography.

The research focuses on morphological and genetic divergence between six ecotone and six rainforest populations of an African passerine bird, the little greenbul (Andropadus virens). Four out of five morphological characters differed between the ecotone and forest populations, whereas only one trait differed within the ecotone and two within the forest. The degree of morphological divergence between pairs of ecotone and forest populations decreases with increasing levels of gene flow (measured using both mitochondrial and nuclear DNA), whereas no correlation was observed between gene flow and divergence in forest-forest or ecotone-ecotone comparisons. If the divergence was simply related to genetic drift and geographic isolation, one would expect the same negative correlation in all population comparisons. This suggests that natural selection in the ecotone may be driving the divergence.

The degree of morphological divergence between ecotone and forest populations of A. virens is as great as the morphological divergence between two different bird species sharing the same habitat. Thus, while the authors have not demonstrated the reproductive isolation between ecotone and forest populations that marks a complete speciation event, they have shown that dramatic morphological divergence can occur through natural selection in the face of substantial gene flow. This clearly represents a first step in speciation. If potential new species often arise in the transition zone between habitats, conservationists will need to include these neglected regions in any overall plan for sustaining biological diversity.

1. One would generally expect to see a negative relationship between gene flow and morphological diversity between populations; what is an explanation for the lack of correlation between forest-forest and ecotone-ecotone populations?
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

2. The authors argue at one point that genetic evidence points to a recent rapid expansion of the overall A. virens population. Would a species with less rapid population expansion be expected to show as high a degree of divergence?
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

3. What ecological factors in the ecotone might be responsible for the apparently strong natural selection argued for in this paper? What evidence is provided that these factors play a role?
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

4. Deforestation creates more transition zones between forest and grassland. If such zones are the birthplaces of new species, does this mean that we do not need to worry about preserving tropical forests to sustain biological diversity?
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.




1.	One would generally expect to see a negative relationship between gene flow and morphological diversity between populations; what is an explanation for the lack of correlation between forest-forest and ecotone-ecotone populations? To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



2.	The authors argue at one point that genetic evidence points to a recent rapid expansion of the overall A. virens population. Would a species with less rapid population expansion be expected to show as high a degree of divergence? To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



3.	What ecological factors in the ecotone might be responsible for the apparently strong natural selection argued for in this paper? What evidence is provided that these factors play a role? To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



4.	Deforestation creates more transition zones between forest and grassland. If such zones are the birthplaces of new species, does this mean that we do not need to worry about preserving tropical forests to sustain biological diversity? To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.

Chapter 15: Literature: Speciation, Hybridization, and Ecotones

Article Summary–Ecotones and Speciation