Speciation and Phylogeny
Explain reproductive isolation, speciation, cladograms, derived traits, and how phylogenetic trees represent evolutionary relationships.
A lesson of Biology, in Evolution and Diversity of Life, about 10 minutes.
By the end of this lesson, you will be able to define species and speciation, explain allopatric and sympatric speciation, describe reproductive isolation mechanisms, and read phylogenetic trees.
What is a Species?
Biological species concept: a group of organisms that can interbreed in nature and produce fertile offspring; reproductively isolated from other species.
Limitations: doesn't apply to asexual organisms, fossils, or populations that are geographically separated.
Alternative concepts: morphological (appearance), phylogenetic (evolutionary history), ecological (niche).
Speciation
Speciation: the evolutionary process by which one species splits into two or more distinct species.
Requires that populations become reproductively isolated — unable to exchange alleles.
Allopatric Speciation (Geographic Isolation)
Most common mode. A physical barrier (mountain range, river, ocean, land mass separation) divides a population → the separated populations evolve independently under different selection pressures → eventually accumulate enough differences that they can no longer interbreed even if brought back together.
Example: Darwin's finches — isolated on different Galápagos islands, each evolved different beak shapes suited to island-specific food.
Sympatric Speciation (Same Geographic Area)
Speciation without geographic isolation. Less common in animals; more common in plants.
Polyploidy (most common mechanism in plants): chromosomal duplication → new species that can no longer interbreed with parents.
Example: most crop plants (wheat, cotton, tobacco) are polyploids.
Reproductive Isolation Mechanisms
Pre-zygotic (before fertilization):
- Habitat isolation: use different habitats in the same area
- Behavioral isolation: different courtship signals or rituals
- Temporal isolation: mate at different times
- Mechanical isolation: physically can't mate
- Gametic isolation: sperm can't fertilize eggs
Post-zygotic (after fertilization):
- Hybrid inviability: hybrid embryo doesn't survive
- Hybrid sterility: hybrid can't reproduce (e.g., mule = horse × donkey)
- Hybrid breakdown: first-generation hybrids fertile, but later generations have reduced fitness
Phylogenetic Trees
A phylogenetic tree (cladogram) shows evolutionary relationships — which organisms share common ancestors and when lineages diverged.