AP Syllabus focus:
‘Speciation occurs when populations become reproductively isolated and no longer exchange genetic information.’
Natural selection can push populations in different directions, but new species form only when gene flow between populations is reduced enough that they evolve independently. Reproductive isolation is the key turning point from variation to speciation.
Allopatric speciation is illustrated as a stepwise process in which a geographic barrier reduces gene flow between populations, allowing divergence to accumulate. Over time, reproductive isolation evolves so that the populations remain distinct even if they later come into contact. This reinforces the idea that speciation depends on sustained genetic separation, not just visible trait differences. Source
Core idea: reproductive isolation drives speciation
Reproductive isolation and gene flow
Reproductive isolation: A condition in which two populations can no longer effectively exchange genetic information through reproduction, reducing or eliminating gene flow between them.
When populations are reproductively isolated, alleles are not shared (or are shared too rarely to matter). This allows differences to accumulate in each population’s gene pool, eventually producing distinct evolutionary lineages.
What “no longer exchange genetic information” means
“Exchange genetic information” refers to successful reproduction that moves alleles between populations. Gene flow can stop in practice even if occasional mating attempts occur, because:
mating does not happen (individuals do not recognise each other as mates)
fertilisation fails
offspring are inviable or infertile
offspring have such low fitness that their alleles rarely persist
In AP Biology terms, speciation is supported when populations show strong barriers to gene flow, not merely physical separation or visible trait differences.
How reproductive isolation produces new species
Independent evolution after gene flow is reduced
Once gene flow is limited, populations can diverge through:
natural selection acting differently in each population
genetic drift changing allele frequencies by chance (especially in small populations)
new mutations arising and spreading separately
Even if environments are similar, drift and mutation alone can create genetic differences; selection often accelerates divergence when environments differ.
Accumulation of isolating differences (a gradual process)
Speciation typically occurs as a sequence of changes, rather than a single event:
Early stage: partial isolation; populations still share some alleles
Intermediate stage: reduced hybrid success or reduced mating; divergence increases
Late stage: isolation becomes strong enough that gene pools remain distinct even if contact occurs
A key idea is that reproductive isolation can evolve as a by-product of genetic divergence. Traits under selection in one context can incidentally change mating signals, reproductive timing, or gamete compatibility, further reducing gene flow.
Genetic incompatibilities and reproductive systems
As populations diverge, combinations of alleles that worked within each population may not work well together in hybrids.
The Dobzhansky–Muller model explains how hybrid problems can evolve without either population being “maladapted” on its own. Each isolated population fixes different genetic changes, but when combined in a hybrid, certain allele combinations interact incompatibly, reducing viability or fertility. This provides a genetic basis for postzygotic reproductive isolation that can intensify as divergence increases. Source
This can lower hybrid survival or fertility, which strengthens reproductive isolation because:
fewer alleles cross population boundaries
each population’s adaptations are preserved
selection may favour traits that prevent wasted reproductive effort
This relationship is circular: reduced gene flow promotes divergence, and divergence often further reduces gene flow.
Recognising speciation in populations (what to look for)
Evidence consistent with new species formation
In line with the syllabus statement (“reproductively isolated” and “no longer exchange genetic information”), strong indicators include:
consistent failure to produce viable, fertile offspring between populations
markedly reduced reproductive success of crosses compared with within-population mating
persistent genetic differentiation suggesting ongoing lack of gene flow
A practical emphasis is reproductive outcome: if allele exchange is effectively absent, populations can be considered separate evolutionary units, even if they appear similar.
Limits and common complications
Reproductive isolation is not always absolute
Some closely related species occasionally hybridise; the critical question is whether hybrids allow meaningful gene flow. If hybrids are rare or have low fitness, reproductive isolation can still be strong enough for speciation to be maintained.
Time and chance matter
How quickly isolation evolves depends on:
strength of selection against interbreeding
population sizes (drift effects)
genetic architecture of isolating traits (many genes vs few)
opportunities for mating between populations
FAQ
They assess whether alleles move between populations at a rate that prevents divergence. Evidence can include genomic clustering, low hybrid frequency, and reduced hybrid fertility/fitness over multiple generations.
Yes. Random mutation and genetic drift can produce divergence, and some changes (e.g., mating cues) can reduce interbreeding even in similar environments.
Reinforcement is selection that increases reproductive isolation because hybrids have lower fitness. It is most likely when partially isolated populations come into contact and hybridisation is costly.
Hybrids may have reduced mating success, behavioural mismatch, or ecological disadvantages that limit survival to reproduction, so their alleles rarely persist in either parent population.
Approaches include controlled crosses to measure fertilisation and offspring fertility, mate-choice assays to quantify mating preferences, and genomic analyses to estimate historical and current gene flow (e.g., admixture proportions).
Practice Questions
Define reproductive isolation and explain how it contributes to the formation of new species. (1–3 marks)
Correct definition: reduced/eliminated gene flow via unsuccessful reproduction between populations (1)
Explains that lack of gene flow allows independent evolution/allele frequency divergence (1)
Links divergence to speciation/new species formation (1)
Two populations of the same organism show increasing genetic differences over time. Describe how reproductive isolation can develop and lead to new species, referring to gene flow and reproductive success. (4–6 marks)
States that reproductive isolation reduces gene flow between populations (1)
Explains that reduced gene flow allows independent changes in allele frequencies (mutation, selection, drift) (1–2)
Describes that differences can reduce mating success, fertilisation success, or hybrid viability/fertility, lowering allele exchange (1–2)
Concludes that sufficiently reduced gene flow results in distinct gene pools consistent with speciation/new species (1)
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