01Biological species concept
A species is a group that can interbreed and produce fertile offspring in nature. Speciation happens when populations become reproductively isolated.
Allopatric speciation: a geographic barrier splits populations (Grand Canyon squirrels). Sympatric speciation: no barrier — via polyploidy (common in plants), habitat or behavioral differences.
02Reproductive barriers
Prezygotic: habitat, temporal (breeding at different times), behavioral (mating songs), mechanical, gametic isolation.
Postzygotic: reduced hybrid viability, hybrid sterility (mule), hybrid breakdown.
Tempo: gradualism (slow steady change) vs. punctuated equilibrium (long stasis, rapid bursts). Adaptive radiation: rapid diversification into new niches.
03Notebook box
Prezygotic = before fertilization. Postzygotic = after.
Worked example: Two frog species breed in the same pond but in different months. Barrier? → Temporal isolation (prezygotic).