Environmental Engineering Reference
In-Depth Information
Processes Shaping Biodiversity and Sustainability in Ecotones
Ecotones and Evolutionary Processes
Ecotones are “natural laboratories” for studying a range of evolutionary processes,
such as the process by which new species form, also termed speciation. This process
is of major interest to evolutionary biologists who define three major types of
speciation: allopatric, parapatric, and sympatric. These models are based on the
degree of geographical subdivision between populations that lead to the formation
of new species. Allopatric speciation happens in geographical isolation, and has
been for many years considered the major form of speciation. Parapatric speciation
occurs in adjacent populations with gene flow among them, often along clines.
Sympatric speciation occurs when populations are geographically congruent, and
are found in the same area. The study of ecotones has led to a better understanding
of the potential importance of parapatric and sympatric speciation as mechanisms
for speciation. Ecotones have been proposed by some authors to be centers of
evolutionary novelty that maintain evolutionary process, and as regions where
parapatric (or sympatric) speciation processes may take place [
33
]. As such,
ecotones and areas of environmental transition have been suggested as natural
laboratories where evolutionary processes and barriers to gene flow can be
examined [
33
].
A review by Moritz et al. [
27
] summarized the major models of evolutionary
processes that promote diversification of rain forest faunas. They include the
divergence-with-gene-flow model, which suggests that adaptive divergence caused
by selection forces occurs across environmental gradients, leading to speciation
even in the face of gene flow across ecotones. This means that speciation does not
require isolation in cases where selection is strong enough to separate populations.
This process is expected to occur especially where very different environments
meet in the ecotone, for example, at the border between a dry and wet rain forest.
This may mean that even when gene flow continues, strong selection pressures can
lead to divergence.
Support for the divergence-with-gene-flow model comes from recent research
examining divergence using molecular genetic, phenotypic, and experimental
approaches [
27
,
33
,
34
,
37
]. Smith et al. [
37
], studying the little greenbul
(
Andropadus virens
), a passeriform bird in the rain forest-savannah ecotone
region of Cameroon, found especially high morphological divergence in the
ecotone. The authors proposed that when the ecotone is large enough, natural
selection processes could be strong enough to generate morphological differences
similar to those seen in reproductively isolated species even when high rates of
gene flow occur. Their data support the divergence-with-gene-flow model of
speciation [
38
], leading them to propose that ecotones may be integral to the
production and maintenance of high biodiversity in tropical rain forests. Quantifi-
cation of morphological and geographic distances in olive sunbird (
Nectarinia
olivacea
) populations in West African forests and ecotones revealed similar
divergence patterns [
38
].
