SCIENCE · INVESTIGATE AND EXPLAIN

Separate allele divergence from species formation

Track how exchange reduces a difference while examining the limits of a one-locus model.

Learning outcome: Explain symmetric gene flow and distinguish geographic separation, genetic difference and reproductive isolation.

Starting recap

Gene flow moves alleles between populations through successful reproduction. Reproductive isolation concerns barriers to gene exchange, not simply physical distance.

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Try

Equal-sized populations begin at A frequencies 90% and 10%. Exchanging 10% each way gives 82% and 18% after one generation.

Open the experiment

Predict

Predict the effect of no exchange and of exchanging 50% each way. Does a persistent allele-frequency gap prove two species?

Your prediction and reason

Experiment

Compare 0%, 10% and 50% exchange for one generation, then inspect ten generations. Track both frequencies and their mean. Capture one persistent and one reduced gap.

Track mixing between two populations

Worked teaching model · synthetic values · Two equally sized haploid populations begin at A frequencies 90% and 10%. Each generation exchanges the same fraction in both directions. No selection, mutation or drift occurs; only the frequencies of this one locus are tracked. Allele divergence alone is not a species test.

Fraction exchanged each generation: 10 % Generations: 1

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Frequency through generations

Solid line: population 1. Dashed line: population 2. A missing next-generation frequency is left undefined rather than drawn as zero.

Inspect the generation trace

Generation	Population 1 A (%)	Population 2 A (%)
0	90.000	10.000
1	82.000	18.000

Calculated values on the same fixed scale
Row	Quantity	Value (%)
1	Initial population 1 A frequency	90
2	Initial population 2 A frequency	10
3	Final population 1 A frequency	82
4	Final population 2 A frequency	18
5	Mean A frequency	50

Final between-population gap: 64.000 percentage points.

Each update mixes both populations from their preceding states. Equal sizes and symmetric exchange keep the mean at 50%. The gap is multiplied by (1 − 2m) each generation; this model tests mixing, not whether reproductive isolation has evolved.

Observation to attach to a capture

Captures preserve recipes and replay their calculated results. They are synthetic model evidence, not physical measurements.

Understand

Mix both populations from the same preceding state

At initial frequencies 0.9 and 0.1, exchanging a fraction m = 0.1 each way gives population 1 a new frequency of 0.9×0.9 + 0.1×0.1 = 0.82. Population 2 becomes 0.9×0.1 + 0.1×0.9 = 0.18. Both calculations use the old values.

The mean remains 0.5 because populations have equal size and exchange symmetrically. The gap shrinks from 0.8 to 0.64: multiplication by 1 − 2m = 0.8. After ten generations it is 0.8×0.8¹⁰, about 0.0859, or 8.59 percentage points.

At m = 0 the original gap persists. At m = 0.5 both populations immediately become 50% A. No allele is favored; this is mixing rather than selection.

A difference is not a species test

A persistent difference at one locus does not establish reproductive isolation. Populations within one species can differ in allele frequency, and different species can share alleles. Geographic separation may reduce gene flow, but its biological effect depends on dispersal and reproduction.

Reproductive barriers can act before fertilization or reduce the success of hybrid offspring. Their accumulation can contribute to speciation; a short frequency trace cannot demonstrate that process. Different species concepts also apply differently to asexual organisms and difficult boundary cases. Background: OpenStax, formation of species.

State what is missing

The browser omits selection, mutation, drift, mating preferences, hybrid fertility and ecological interactions. It predicts the effect of a chosen exchange fraction, not how that fraction arises.

Use two captures to explain the mixing mechanism. Then propose evidence about successful reproduction or barriers to gene exchange that would be needed for a species argument. Keep that claim separate from the one-locus arithmetic.

Apply

Public practice checks a specific claim. It does not establish independent mastery.

Give an example of evidence about reproductive isolation that the frequency model cannot supply. Explain why a barrier reducing gene flow can permit divergence without guaranteeing speciation.

Your explanation and limitations

Continue

Completion requires a saved prediction, at least one capture and a reflection. Presence checks do not grade your reasoning.

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Previous: Compare tree hypotheses with character evidence Next: Trace a negative-feedback response

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