Species and reproductive isolating mechanisms
What a species is
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The biological species concept defines a species as a group of organisms that can interbreed in nature to produce fertile offspring.
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Two requirements are doing work here, and both are examinable:
- The offspring must be fertile. A horse and a donkey produce a mule, which is sterile, so they remain separate species.
- They must interbreed in nature. Two populations that will only interbreed in captivity are still separate species.
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Speciation is therefore the evolution of reproductive isolation. Everything in this standard is about how populations stop being able to interbreed.
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The concept has real limits, worth knowing for Excellence:
- It cannot be applied to organisms that reproduce asexually.
- It cannot be applied to fossils, since interbreeding cannot be tested.
- Some species hybridise occasionally in nature yet remain distinct, so the boundary is not always sharp.
Reproductive isolating mechanisms
- A reproductive isolating mechanism is any feature that prevents successful interbreeding between populations.
- They fall into two groups according to when they act, and using this framing makes answers much clearer:
- Prezygotic mechanisms act before a zygote forms — mating or fertilisation never happens.
- Postzygotic mechanisms act after a zygote forms — the offspring is inviable or infertile.
Prezygotic mechanisms
| Mechanism | How it prevents interbreeding | Example |
|---|---|---|
| Geographical | Populations are physically separated, so individuals never meet | A mountain range, ocean, or river |
| Ecological | Populations occupy different habitats in the same area, so they rarely encounter each other | One form in the canopy, another on the forest floor |
| Temporal | Populations breed at different times — different seasons, or different times of day | One plant flowers in spring, another in autumn |
| Behavioural | Courtship signals differ, so individuals do not recognise each other as mates | Different songs, displays or pheromones |
| Structural | Reproductive structures are physically incompatible | Flower shape suits a different pollinator; genitalia do not fit |
- Geographical isolation is different from the rest, and this catches people out. It is external to the organisms — remove the barrier and the populations can interbreed again immediately. The other mechanisms are properties of the organisms themselves and persist wherever they meet.
- This is why geographical isolation starts speciation but does not complete it. Speciation is only complete when an intrinsic mechanism has evolved.
Postzygotic mechanisms
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Hybrid inviability — the zygote forms but the embryo fails to develop, because the two sets of chromosomes or gene products are incompatible.
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Hybrid sterility — the hybrid is healthy but cannot reproduce. Usually the two parental chromosome sets cannot pair at meiosis, so functional gametes are not produced. The mule is the standard example.
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Hybrid breakdown — the first generation is fertile, but the second generation is weak or sterile.
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Postzygotic mechanisms are costly, because the parents invest in gametes and offspring that produce no descendants at all.
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That cost has an important consequence: selection favours prezygotic mechanisms. Any heritable feature that stops an individual wasting gametes on a hopeless mating is advantageous, so once postzygotic isolation exists, prezygotic isolation tends to be strengthened.
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This strengthening is called reinforcement, and it explains why closely related species that overlap often differ more in courtship than those that never meet.
Selective advantage
- Isolating mechanisms are not "for" making new species — that would be group selection. Each is advantageous to the individual:
- Behavioural and structural mechanisms mean an individual does not waste gametes, energy and a breeding season on a mating that would produce inviable or sterile offspring. An individual that mates only with its own kind leaves more surviving, fertile descendants.
- Temporal differences often arise because each population is timed to its own local conditions — flowering when its pollinators are active, or when its seed can ripen. Isolation is a by-product of that local adaptation, not its purpose.
- Speciation is therefore usually an outcome of individuals being selected to reproduce successfully, not a goal being pursued.
Worked Example
Worked Example
Two populations of a native alpine plant grow on adjacent mountains. Researchers record:
- Population X flowers in November–December; population Y flowers in January–February.
- X is pollinated mainly by a native bee; Y mainly by a native fly. X has a deep tubular flower; Y has a shallow open flower.
- Hand-pollinating X with Y pollen produces seed, but only 11% germinates, compared with 84% for within-population crosses.
- The few hybrid plants that grow to maturity produce almost no viable pollen.
Identify the isolating mechanisms present, classify them, and explain whether these are separate species.
Answer:
Identifying and classifying the mechanisms. Four distinct mechanisms are operating, and they fall into both groups.
Prezygotic — no zygote would form in nature:
- Temporal isolation. X flowers November–December and Y flowers January–February, so their flowering periods do not overlap. Pollen from one is not available when the other is receptive, so cross-pollination cannot occur regardless of anything else.
- Structural isolation. X has a deep tubular flower and Y a shallow open one. The structures suit different pollinators and are physically incompatible, so pollen is not transferred between them even where they grow together.
- Ecological isolation (via pollinators). X is pollinated by a bee and Y by a fly. Each pollinator visits its own flower type, so pollen is carried within populations rather than between them.
Postzygotic — a zygote forms but the offspring fails:
- Hybrid inviability. Hand-crossed seed germinates at only 11% against 84% for within-population crosses, so most hybrid embryos fail to develop. The two genomes are largely incompatible.
- Hybrid sterility. The few hybrids reaching maturity produce almost no viable pollen, so even surviving hybrids cannot reproduce. This is typically because the two parental chromosome sets cannot pair properly at meiosis.
Are these separate species?
Applying the biological species concept — organisms that can interbreed in nature to produce fertile offspring — they are separate species, and the evidence is strong on both counts:
- They do not interbreed in nature. Non-overlapping flowering times alone would prevent it, and the structural and pollinator differences would prevent it even if the timing overlapped. Interbreeding occurred here only because researchers hand-pollinated them, which is exactly what the phrase "in nature" excludes.
- The offspring are not fertile. Even the hybrids that survive produce almost no viable pollen, so they cannot reproduce.
Either finding alone would be sufficient; together they make the conclusion secure.
Why the redundancy matters. Note that isolation here is multi-layered. Speciation is complete well beyond the minimum, and this is typical: once populations diverge, differences accumulate across many traits rather than stopping at the first one that achieves isolation.
There is also a likely causal sequence worth stating. The postzygotic incompatibility means any cross-pollination would waste an entire season's reproductive investment on seed that mostly fails. Selection therefore favours any heritable feature that prevents such crosses — so the prezygotic differences in flowering time and flower structure would have been strengthened by selection once the genomes became incompatible. This is reinforcement, and it explains why the prezygotic barriers are so thorough.