Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Biological species concept: a species is a group that can interbreed in nature and produce viable, fertile offspring. It applies to sexually reproducing organisms, not to asexual ones or fossils.
- A new species forms when populations become reproductively isolated: no gene flow, so mutation, selection and drift change each gene pool separately.
- Allopatric speciation needs geographic isolation. Sympatric speciation happens in the same area, for example by polyploidy (plants) or a host shift (apple maggot fly, hawthorn to apple).
- Gradualism: slow, steady change over very long times. Punctuated equilibrium: long stasis, then rapid change.
- Divergent evolution: related populations adapt to different habitats and spread apart; fastest in adaptive radiation when new habitats open. Convergent evolution: similar pressures give similar traits in unrelated lineages (Caribbean Anolis ecomorphs).
Key relationships
| Barrier | When it acts | Example |
|---|---|---|
| Habitat | pre-zygotic | live in different parts of the same area |
| Temporal | pre-zygotic | breed in different seasons or times of day |
| Behavioural | pre-zygotic | courtship songs or displays not recognised |
| Mechanical | pre-zygotic | reproductive parts do not fit |
| Gametic | pre-zygotic (after mating) | sperm cannot fuse with egg |
| Reduced hybrid viability | post-zygotic | hybrids die young |
| Reduced hybrid fertility | post-zygotic | hybrids sterile (mule: 32 + 31 = 63 chromosomes) |
| Hybrid breakdown | post-zygotic | hybrids fertile, their offspring weak or sterile |
| Polyploidy check | Working |
|---|---|
| Tetraploid × diploid | gametes 2 sets + 1 set = 3 sets (triploid), usually sterile |
| Tetraploid × tetraploid | 2 sets + 2 sets = 4 sets, fertile |
Assumptions behind the conclusions
- A species boundary is judged by gene flow in nature. Hybrids made only in a laboratory do not show that two groups are one species.
- Isolation can be partial. Speciation is a process, so some populations are “in between”.
Mistakes to avoid
- Saying two groups are one species just because they can mate or make hybrids.
- Calling gametic isolation post-zygotic (it acts before the zygote forms).
- Saying sympatric speciation needs a barrier.
- Treating “rapid” in punctuated equilibrium as overnight.
- Saying convergent species share a recent common ancestor.
- Saying individuals, rather than populations, become new species.
Quick self-check
- Two moth species fly on the same nights but one is attracted only to its own pheromone. Barrier? (Behavioural, pre-zygotic.)
- A diploid plant has 2n = 20. How many chromosomes in a triploid hybrid with a tetraploid of the same species? (10 + 20 = 30.)
- Why are radiations fast on new islands? (Many empty niches and few competitors, so divergent selection acts in many directions at once.)
Next: practice questions.