What is Evolution and Speciation?
Evolution is the change in heritable traits of populations across generations, driven by natural selection, mutation, genetic drift and gene flow. Speciation is the evolutionary process by which one population splits into two or more populations that can no longer interbreed.
Speciation happens when populations of the same species become reproductively isolated — usually after a period of separation — and accumulate enough genetic differences that they can no longer produce fertile offspring together.
- 1↓Ancestral populationOne interbreeding population shares a common gene pool.
- 2↓Geographic isolationA barrier — river, mountain range, or ocean — splits the population in two.
- 3↓Independent evolutionMutation, natural selection and genetic drift act differently on each isolated group.
- 4↓Reproductive isolationGenetic and behavioral differences build up until the groups can no longer interbreed.
- 5New speciesEven if reunited, the two populations no longer produce fertile offspring together.
Step-by-step worked examples
Darwin's finches on the Galápagos Islands descended from one mainland ancestor. How did so many finch species form?
A few finches colonized different islands Each island population was geographically isolated from the others Natural selection favored different beak shapes for different foods (seeds, insects, cactus) Over generations, isolated populations became too different to interbreed → separate species
Ensatina salamanders form a 'ring species' around California's Central Valley, where neighboring populations interbreed but the two ends of the ring cannot. Is this speciation?
Populations spread gradually around a geographic barrier Each neighboring population can still interbreed with its immediate neighbor Genetic differences accumulate gradually along the ring Where the ring closes, the two end populations are different enough to be reproductively isolated → effectively separate species
A new plant species forms instantly when a hybrid's chromosome number doubles (polyploidy), even though both parent species still live in the same field. What type of speciation is this?
No geographic barrier separates the parent populations Chromosome doubling instantly makes the hybrid unable to breed with either parent This is sympatric speciation — it occurs without geographic isolation Common in plants (e.g., modern wheat, cotton)
Flashcards
Quick quiz
Q1.Which factor is the classic trigger for allopatric speciation?
Q2.Polyploidy leading to instant reproductive isolation without geographic separation is an example of:
Q3.Two species produce offspring that survive but are sterile (like mules). This is an example of:
Q4.What ultimately defines two populations as separate species under the biological species concept?
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Common mistakes
Speciation always requires physical separation. — Correct: Sympatric speciation can occur without any geographic barrier, e.g. through polyploidy.
Evolution and speciation are the same thing. — Correct: Evolution is ongoing change in a population; speciation is one possible outcome — a split into new species.
Speciation happens in a single generation. — Correct: Most speciation (except polyploidy) takes many generations of accumulated genetic change.
Once separated, populations always become new species. — Correct: Isolated populations only become new species if enough reproductive isolation accumulates; some remain interfertile indefinitely.
FAQ
What is speciation in biology?
Speciation is the evolutionary process by which populations diverge until they become distinct species that can no longer interbreed successfully.
What is the difference between allopatric and sympatric speciation?
Allopatric speciation requires geographic separation; sympatric speciation occurs within the same area, often through mechanisms like polyploidy.
What causes evolution?
Evolution is driven by natural selection, mutation, genetic drift, and gene flow acting on heritable variation within populations.
What are examples of speciation?
Darwin's finches (allopatric), Ensatina ring species (gradual allopatric), and polyploid plants like modern wheat (sympatric) are classic examples.