Speciation and Macroevolution

1. What is a species?

The Biological Species Concept (Ernst Mayr) defines a species as a group of populations that can interbreed and are reproductively isolated from other such groups. Reproductive isolation — not appearance — is the key test. The concept has limits: it does not apply to asexual organisms, cannot be tested on fossils, and is ambiguous for separated populations or species that hybridize (such as wolves and coyotes). Alternative concepts (morphological, ecological, phylogenetic) fill these gaps.

2. How new species form

Reproductive isolation is built from reproductive barriers. Prezygotic barriers prevent mating or fertilization: habitat isolation (different places), temporal isolation (different breeding times), behavioral isolation (different courtship signals), mechanical isolation (parts do not fit), and gametic isolation (sperm and egg do not fuse). Postzygotic barriers act after fertilization: reduced hybrid viability (hybrids die young), reduced hybrid fertility (hybrids are sterile, like mules), and hybrid breakdown (first-generation hybrids are fine, but later generations fail).

Speciation happens in two main geographic modes. In allopatric speciation, a physical barrier (a river, mountain, or sea) separates a population; the groups evolve independently until they can no longer interbreed. In sympatric speciation, a new species arises in the same place, often through polyploidy — gaining an extra set of chromosomes in a single generation, common in plants (bread wheat is a polyploid formed from two hybridizations). Polyploids are instantly reproductively isolated from their parents.

3. The tempo of change and macroevolution

Microevolution is change within populations; macroevolution is the origin of new species and larger groups — the branching pattern of life's history, including adaptive radiation (one lineage diversifying rapidly into many forms) and extinction. Two models describe the pace of change: gradualism holds that species change slowly and steadily, while punctuated equilibrium holds that species stay mostly unchanged for long periods, with rapid change concentrated in brief speciation events. Both agree on the mechanisms (mutation, drift, selection, gene flow); they differ on tempo, and both patterns occur in nature.

How it works

Steps of allopatric speciation:

1. A geographic barrier (a river, mountain range, or rising sea) splits one interbreeding population into two.
2. Gene flow between the groups drops to near zero.
3. Each group accumulates different mutations and faces different selective pressures; genetic drift also acts, especially in small groups.
4. Over many generations, the groups diverge — different alleles become common, different adaptations arise.
5. If separation lasts long enough, reproductive barriers evolve as a byproduct of divergence.
6. If the barrier later disappears, the groups meet again but can no longer interbreed freely — they are now distinct species.

Common confusions

Memory aids

"Allo = Apart, Sym = Same." Allopatric speciation needs a geographic barrier that keeps populations apart; sympatric speciation happens in the same place. And "Pre-zygotic prevents the meeting; post-zygotic produces poor offspring."

Quick review

Topic Recap

• A species is a reproductively isolated group of interbreeding populations; isolation, not appearance, is the test.
• Prezygotic barriers block mating or fertilization; postzygotic barriers reduce hybrid fitness after fertilization.
• Allopatric speciation requires a geographic barrier; sympatric speciation (often via polyploidy) does not.
• Polyploidy creates instant reproductive isolation and is common in plants.
• Gradualism and punctuated equilibrium describe the tempo of change; both occur in nature and agree on mechanisms.
• Microevolution scales up to macroevolution through accumulated reproductive isolation.

Knowledge Check

1. A donkey and a horse produce a sterile mule. Which category of reproductive barrier does this represent, and why?
2. Two frog populations live in the same pond, but one breeds in early spring and the other in late summer. What type of barrier is this, and is it prezygotic or postzygotic?
3. How does a geographic barrier lead to allopatric speciation?
4. Why is a newly formed tetraploid (4n) plant instantly reproductively isolated from its diploid (2n) parents?
5. Do gradualism and punctuated equilibrium disagree about the mechanisms of evolution or about its tempo? Explain.

Answers and Rationales

1. Answer: Reduced hybrid fertility, a postzygotic barrier. Why: Fertilization succeeds and the hybrid survives, but it cannot reproduce, so gene flow between horse and donkey is blocked.
2. Answer: Temporal isolation, a prezygotic barrier. Why: The populations breed at different times, so they never mate — the barrier acts before fertilization.
3. Answer: The barrier stops gene flow, so each population accumulates different mutations and experiences different selection and drift until reproductive barriers evolve. Why: Without gene flow to keep them similar, the populations diverge independently over many generations.
4. Answer: Crossing a 4n plant with a 2n parent produces 3n offspring that are typically sterile, so no genes flow between them. Why: An odd number of chromosome sets disrupts meiosis — the new polyploid is isolated in a single generation.
5. Answer: They disagree about tempo — the pace and pattern of change — not about the mechanisms (mutation, drift, selection, gene flow). Why: Both models accept the same processes; they differ on whether change is slow and steady or concentrated in bursts around speciation events.