Evolution of Species Without Natural Boundaries
Genetic Change by Mutation and Acquisition from Other Species
Jul 28, 2009
Genetic changes can arise in different ways – they can be the result of mutations or come from the transfer of genetic material between species.
Evolution of Species Without Natural Boundaries
Enormous numbers of different animal species live on this planet (estimates have varied between 3 and 30 million), and all this diversity is difficult to explain if new species can only evolve when an original population is effectively split in two by an emerging physical boundary (like the formation of a mountain range, or an island splitting in two).
Charles Darwin’s theory of evolution by natural selection explains how new species evolve when an ancestral population gets split in two by a physical boundary. The two (newly separated) populations gradually acquire adaptations to their environments, and eventually end up sufficiently different from one-another to be considered as related species. The two populations can no longer interbreed successfully.
Recent work, using computer simulations, suggests that there might not always need to be a physical barrier – a widespread species might accumulate genetic differences that vary from place to place. Animals living on the seabed, for example, could acquire different genetic mutations in different areas, and eventually become sister species without the need of any significant natural boundary between them. Les Kaufman ("Global patterns of speciation and diversity" Nature 460, 16 July 2009) has shown how diverse species can arise from the arrangement of organisms across an area, without any influence from geographical barriers. Over generations, the genetic distance between organisms in different regions increases, and organisms spontaneously form groups that can no longer mate. The number of species then increases rapidly until it reaches a relatively steady state.
Evolution of Species Without Clear Family Trees
Horizontal (or lateral) gene transfer is well known among the bacteria, but it has recently been shown that genes can find their way into animals as well. For example the adzuki bean beetle (Callosobruchus chinensis) has acquired genetic material from its gut bacteria (Wolbachia). This fact plays havoc with the idea of ‘family trees’ – if genes can move from one species to another then it is not simply a question of individuals passing on genes that they have inherited from their ancestors, they might be passing on genes originated elsewhere. The clear ‘lines of descent’ that Darwin envisaged are probably more like ‘webs of descent’, with any modern animal tracing its genetic history back through many chance events.
These two bits of new research show that Darwin’s original theory needs to be modified, but that does not mean that it has been discredited in any way.
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