Speciation - introduction
Speciation - introduction
Evidences for "macroevolution", What is macroevolution, Speciation (evolution101), more, hybrids
List of 26 species concepts, Jerry Coyne on speciation, Gibbon - Siamang hybrid, Speciation examples
Speciation is one of evolution's most misunderstood concepts. Here's my attempt to shed light on it.
To begin with we need a definition of species. There are many, but the most useful is Ernst Mayr's "biological definition of species". Simplified, a species is a group of organisms which for practical purposes doesn't interbreed with other groups. We can exclude trivial barriers such as geography.
Common misconception - physical differences are a good indicator of genetic variability.
Physical appearances can be misleading. Sibling species are physically indistinguishable, but do not interbreed. Dog breeds are physically distinctive, but all of a single species. In fact there is very little genetic variation in dogs (less than among humans!) The physical differences result from selection.
Do outward differences correlate with differences at organ or cellular levels?
No! As the scale gets smaller organisms are more and more similar. For example, primate organs resemble each other far more than do physical appearances of the whole organisms. At cellular and molecular levels organisms are yet more similar. In fact all cells use the same basic molecular machinery. Cell biologists and biochemists see great similarity where anatomists may see little. Common ancestry is most obvious at microscopic levels.
How do organisms speciate?
There are many mechanisms. Some of the simplest are in plants where chromosome doubling may create a new species which can no longer interbreed with it's parent. In some cases two related species may form a hybrid that contains the full set of chromosomes from each parent. The hybrid in these cases is self fertile, but will not hybridize with either parent. Wheat is a classical example of a hybrid species.
Factors which contribute to speciation:
In animals speciation is more complex. Separate populations of a single species undergo different selection pressures and will become physically different. Nucleotide sequences of DNA drift over time (~0.5% per million years). Eventually separate populations will differ sufficiently to become incompatible (millions of years). Chromosome structures may change over shorter time periods also producing incompatibility. Two species may become adapted to disparate ecosystems. If hybrids are poorly are poorly adapted to either ecosystem, then, where the species overlap, mechanisms to prevent interbreeding will be selected (disruptive selection). Recently genes leading to speciation of Drosophila melanogaster and simulans have been identified
Behavioral changes (influence mating)
Chromosome rearrangements (prevent proper chromosome pairing, alter gene regulation)
Drift (time dependent DNA sequence changes)
Timing - Animals may be active at different times of day. Plants may be fertile at different times of the year.
Incompatible gene combinations
Physical separation (prevents interbreeding)