The process by which a new species arises is called speciation. When geographic isolation accumulates, populations become genetically differentiated, and reproductive isolation is eventually established, they are recognized as separate species.
Basic Knowledge
Species definition (biological species concept, Mayr): A group of populations whose members can actually or potentially interbreed and are reproductively isolated from other groups.
Mechanisms of speciation: Allopatric speciation = populations are divided by geographic isolation → independent evolution → reproductive isolation is established → separate species. Sympatric speciation = speciation without geographic isolation (through polyploidy, a change in diet, and so on).
Types of reproductive isolation: Prezygotic isolation (behavioral, ecological, and geographic) and postzygotic isolation (hybrid sterility = a mule is unable to reproduce; hybrid breakdown).
Adaptive radiation: Diversification into many species by adapting from a common ancestor to diverse environmental niches. Examples: Galápagos finches, Australian marsupials, and the Cambrian explosion.
Convergent evolution: Organisms from unrelated lineages evolve similar traits in response to similar environmental pressures. Examples: sharks, ichthyosaurs, and fish (streamlined bodies); bats and flying squirrels (gliding).
Describe the difference between allopatric and sympatric speciation, and give one specific example of each.
Answer: Allopatric speciation: populations are divided by geographic isolation, and reproductive isolation is established through independent evolution. Example: Galápagos finches (populations isolated on different islands diverged into separate species). Sympatric speciation: divergence occurs without geographic isolation. Example: polyploidy in plants (individuals with doubled genomes become reproductively isolated from the original species).
- Species definition = reproductively isolated populations (biological species concept)
- Allopatric speciation = geographic isolation + independent evolution + reproductive isolation
- Sympatric speciation = no geographic isolation, such as polyploidy
- Postzygotic isolation = hybrid sterility (mules cannot reproduce) and hybrid breakdown
- Adaptive radiation = diversification from a common ancestor into diverse niches
- Convergent evolution = different lineages evolve similar traits in the same environment
- Polyploidy = common in plants; reproductive isolation is established immediately
Points to Note
① Homologous organs are evidence of adaptive radiation (divergence from a common ancestor). Analogous organs are products of convergent evolution. ② A mule is a hybrid of a horse and a donkey and is unable to reproduce, making it a typical example of postzygotic isolation. ③ Polyploidy is common in plants and is also found in many crops, such as wheat.
Practice
- State the definition of a species according to the biological species concept.
- Explain the difference between adaptive radiation and convergent evolution using specific examples.
- Explain why polyploidy in plants can lead to sympatric speciation.