Speciation. 26. Key Concepts. Speciation occurs when populations of the same species become genetically isolated by lack of gene flow and then diverge from each other due to natural selection, genetic drift, or mutation.
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Speciation occurs when populations of the same species become genetically isolated by lack of gene flow and then diverge from each other due to natural selection, genetic drift, or mutation.
Populations can be recognized as distinct species if they are reproductively isolated from each other, if they have distinct morphological characteristics, or if they form independent branches on a phylogenetic tree.
Populations can become genetically isolated from each other if they occupy different geographic areas, if they use different habitats or resources within the same area, or if one population is polyploid and cannot breed with the other.
When populations that have diverged come back into contact, they may fuse, continue to diverge, stay partially differentiated, or have offspring that form a new species.
The biological species concept considers populations to be evolutionarily independent if they are reproductively isolated from each other, i.e., they do not interbreed or they fail to produce viable, fertile offspring.
Therefore, no gene flow occurs between these populations.
The biological species concept has disadvantages:
The criterion of reproductive isolation cannot be evaluated in fossils or in species that reproduce asexually.
It can only be applied to populations that overlap geographically.
Subspecies are populations that live in discrete geographic areas and have their own identifying traits but are not distinct enough to be considered a separate species.
Several subspecies of dusky seaside sparrow live along the Atlantic and Gulf Coasts and are physically isolated from one another; scientists believed that there was little or no gene flow between populations.
Based on the biological species and the morphospecies concepts, these subspecies were considered to be separate species.
Scientists launched a conservation program for one subspecies thought to be nearing extinction, the dusky seaside sparrow.
However, phylogenetic analysis of gene sequences from different seaside sparrow populations showed that only two distinct monophyletic groups of seaside sparrows exist.
The dusky sparrow was shown to be genetically indistinguishable from the other Atlantic Coast sparrows and thus did not need to be individually preserved to preserve the genetic diversity of the species.
If two populations have diverged extensively and are distinct genetically, it is reasonable to expect that their hybrid offspring will have lower fitness than their parents. The logic here is that if populations are well-adapted to different habitats, then hybrid offspring will not be well-adapted to either habitat.
When postzygotic isolation occurs, there is strong natural selection against interbreeding.
Selection for traits that isolate populations reproductively is called reinforcement.