View source for Evolutionary Biology and Natural Selection

== 3. Speciation: The Origin of New Species ==

Evolutionary changes within a single lineage are known as microevolution. When these changes accumulate to the point where a lineage splits into two distinct species, it is called speciation (macroevolution). For speciation to occur, gene flow between populations must be interrupted.

[[File:Speciation_Diagram.jpg|thumb|right|Comparison of Sympatric vs. Allopatric Speciation.]]

* '''Allopatric Speciation:''' The most common form of speciation. It occurs when a population is divided by a physical, geographic barrier (e.g., a mountain range, a canyon, or a river). Isolated from one another, the two populations evolve independently through different selective pressures and genetic drift until they can no longer interbreed.
* '''Sympatric Speciation:''' Occurs without a geographic barrier. Populations diverge into distinct species while living in the same area. This is often driven by sexual selection, polyploidy (common in plants), or exploiting a new, highly specific micro-niche within the same environment.

Once populations are separated, '''reproductive isolating mechanisms''' (such as different mating times, incompatible anatomy, or sterile hybrid offspring like the mule) ensure they remain distinct species even if they come back into contact.