Biology · Species, Speciation & Macroevolution
Reproductive isolation and barriers to gene flow
A barrier removes 60% of possible reproductive routes. A second removes 75% of the routes still available. Adding 60% and 75% gives an impossible-looking 135%, because the percentages refer to different sets. Follow the surviving routes to see how several barriers can combine.
Evolutionary Processes & Adaptation supplies allele frequencies, selection, drift, gene flow and the distinction between observation and explanation. Heredity, Variation & Evolution introduces branching ancestry. Meiosis & Patterns of Inheritance helps with the chromosome-set model; Development, Differentiation & Stem Cells helps with constraints and historical context. The examples require proportions, weighted averages and reading simple trees; each rule is explained where it is used. Species definitions for sexual organisms are introduced with their limits rather than assumed to classify every form of life.
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Locate barriers before and after zygote formation, and combine conditional barrier effects using the correct denominators.
Barriers limit successful inherited exchange
Reproductive isolation comprises factors that reduce successful reproduction between groups. Prezygotic barriers act before a zygote forms: populations may use different habitats or breeding times, respond to different mating signals, have incompatible reproductive interactions, or fail at gamete recognition and fertilisation.
Postzygotic barriers act after fertilisation. Hybrid offspring may fail to develop or survive, have reduced fertility, or produce later descendants with lower success. A living hybrid is therefore different from a fertile hybrid, and a fertile hybrid is different from demonstrated transmission into a later population.
Some barriers depend on surroundings, such as where organisms meet or whether a hybrid can use a habitat. Others can be expressed even in a common test environment. Field and laboratory comparisons can therefore answer complementary questions. Bringing organisms together artificially may bypass a natural encounter barrier without removing later incompatibilities.
Isolation can be partial, directional and different across regions of a genome. A population's total exchange is affected by encounter, reproduction, inheritance and subsequent selection. Count visitors, mating attempts, offspring and later inherited contribution as different quantities.
Multiply conditional survival through a route
Our model begins with 1,000 baseline cross-lineage reproductive opportunities. A shared breeding period and encounter permits 40%, leaving 400. Compatible mating or pollen delivery permits 25% of those, leaving 100. Fertilisation succeeds for 80% of those, producing 80 zygotes. Finally, 25% produce viable fertile hybrid offspring, leaving 20.
The surviving fraction is 0.40×0.25×0.80×0.25 = 0.02. The fraction removed somewhere along the route is 1−0.02 = 0.98, or 98%. These probabilities are conditional on reaching their stages; multiplying them does not require treating the four stages as independent events with unconditional probabilities.
The removed counts are 600, 300, 20 and 60. They add to 980 because each removed route is counted at its first stopping point. As fractions of the original 1,000, their contributions are 60%, 30%, 2% and 6%. Those percentages share a denominator and can be added; the stage-specific removal percentages cannot.
Follow the remaining reproductive routes
Read the diagram in words
Start with 1,000. Encounter passes 400 and first removes 600; mating/pollen delivery passes 100 and removes 300; fertilisation passes 80 and removes 20; survival and fertility passes 20 and removes 60. Conditional passing fractions are 0.40, 0.25, 0.80 and 0.25. Their product is 0.02. The four first-stopped counts sum to 980; they have one common original denominator.
| Stage | Barrier stage | Routes entering this stage | Conditional fraction passing | Routes passing this stage | First stopped here |
|---|---|---|---|---|---|
| Opportunity to encounter in a shared breeding period | Before a zygote forms | 1000 | 0.4 | 400 | 600 |
| Compatible mating or pollen delivery | Before a zygote forms | 400 | 0.25 | 100 | 300 |
| Fertilisation succeeds | Before a zygote forms | 100 | 0.8 | 80 | 20 |
| Hybrid survives and is fertile | After a zygote forms | 80 | 0.25 | 20 | 60 |
Initial opportunities: 1,000; 80 zygotes; 20 viable fertile hybrid offspring; Overall passage: 2%; removal: 98%
One assigned cross direction in an invented life cycle. Start with 1,000 baseline cross-lineage reproductive opportunities. Each probability applies conditional on reaching that stage. Losses are exclusive because a route removed at one stage does not reach the next. Endpoints are viable fertile hybrid offspring; later backcross transmission is not measured.
Investigate a barrier rather than merely naming it
A monkeyflower study examined several components of isolation between Mimulus lewisii and M. cardinalis, including geographic distributions, pollinator behaviour, pollen competition and hybrid performance. It found barriers at several stages, with much of the estimated isolation acting before hybrid formation. The model pipeline illustrates the accounting principle, not that study's measured values.
To test a proposed barrier, compare suitable within-group and between-group outcomes, specify direction and conditions, and ask what the intervention changed. A low hybrid count alone does not tell us whether different habitats, mate choice, fertilisation or hybrid survival produced it.
After groups come into contact, selection can favour traits reducing costly hybrid matings; this strengthening is called reinforcement. It is a possible process, not an inevitable response to low hybrid fitness. Continuing exchange, changing barriers or fusion can also occur. Evidence about heritable mating differences and their reproductive consequences is needed to explain the outcome.
Worked example
Remove a barrier without restarting the account halfway
Suppose fertilisation now succeeds for every route that reaches it, while the other assigned conditional fractions stay the same. Then 1,000×0.40×0.25×1×0.25 = 25 viable fertile hybrid offspring. The removed fraction becomes 97.5%, rather than 98%.
The improvement affects the 100 routes reaching fertilisation, not all 1,000 starting opportunities directly. Earlier barriers still remove most routes. This comparison holds the other conditional fractions fixed; a real intervention could also change them.
Check your understanding
Why is the final passing fraction 2% rather than 40%+25%+80%+25%?
Show reasoning
Each fraction describes passage conditional on reaching its own stage. Follow successive remaining routes by multiplication: 0.40×0.25×0.80×0.25=0.02. Adding fractions with different denominators does not calculate total passage.
If the final hybrid-survival-and-fertility fraction rises from 0.25 to 0.50, how many model outputs remain?
Show reasoning
Eighty zygotes reach the final stage, so 80×0.50=40 viable fertile hybrid offspring. They are 4% of the initial 1,000; the other conditional fractions are held fixed.
Does observing a hybrid that survives establish that it transmitted inherited material into a parental population?
Show reasoning
No. It also needs a successful reproductive route, such as fertile backcrossing and descendants contributing later. Survival, fertility and realised gene flow are separate observations.
What to carry forward
Locate each barrier and preserve its denominator; partial barriers can combine into strong isolation without making every route impossible.
References
Resources supporting this lesson’s explanations and examples.
- Components of reproductive isolation between the monkeyflowers Mimulus lewisii and M. cardinalis (Phrymaceae)Justin Ramsey, H. D. Bradshaw Jr. and Douglas W. SchemskeResearch abstract
- Biology 2e — formation of species, reconnection and ratesMary Ann Clark, Matthew Douglas and Jung ChoiOpen textbook — selected sections18.2 Formation of New Species18.3 Reconnection and Speciation Rates
- How common is homoploid hybrid speciation?Molly Schumer, Gil G. Rosenthal and Peter AndolfattoResearch review — selected article text