When One Becomes Two

One population, split in two, four different amounts of contact

Two populations start identical at forty loci. Each is pushed by its own local selection, each drifts, and a fraction of each moves into the other every generation. That fraction is the only thing that differs between the four runs. Pick one and watch what it decides.

Migration each way—

Individuals moving per generation—

Genetic distance after 300 generations—

Generations to reach 0.25—

The four runs side by side

Migration each way	Individuals per generation	Distance at the end	Generations to 0.25

Why gene flow is the thing that matters

What a species is, and where the definition runs out

Notice what the definition is made of. It does not mention how similar two organisms look, how much DNA they share, or whether a taxonomist has given them separate names. It mentions one thing: whether genetic information passes between them. That is why this whole topic is one idea with several names.

Seven barriers, each acting on what the last one let through

Not one of these is complete on its own. Switch them on one at a time and watch the total. The arithmetic is multiplication, not addition, and that is the whole reason partial barriers are enough.

Barriers switched on—

Strongest one on its own—

Pairings still getting through—

Total isolation—

All seven, in the order they act

Barrier	Type	Blocks	Removes here	Isolation so far

Before the zygote and after it

The result that is not obvious

This is worth holding onto, because it is a general fact about anything that acts in a sequence, not a fact about orchids. The last filter in a line can be the finest one in the building and still catch almost nothing, because everything was caught upstream. Strength and effect are different quantities.

And the correction to it, which matters more

The panel above is easy to over-read into “so the early barrier is the important one”. That is false, and the arithmetic says exactly how false. Here is the same set of seven barriers put in four different orders.

Barriers in this order	First barrier	It removes	Total isolation

What each barrier is actually worth

A different question, with a different answer: if one barrier stopped acting altogether, how much isolation would be lost?

If this barrier stopped acting	It blocks	It removes, in place	Isolation falls to	Cost

Two questions that sound like one question

So both statements are true at once, and neither is the other. How much does this barrier remove? depends on where it sits, because it can only act on what reached it. How much would we lose without it? depends only on how strong it is, because the total is a product and every factor in a product counts the same wherever you write it. The first question is about the barrier's visible workload. The second is about its value. An exam question asking which barrier is “most important” is ambiguous between them; a question asking which removes the most, or which the population could least afford to lose, is not.

Two tempos, one endpoint

Gradualism and punctuated equilibrium are claims about the shape of change over time, not about how much of it there is. Both lineages below start in the same place and finish in the same place. Only the middle distinguishes them — which is exactly the part of the fossil record that is usually missing.

Total change, gradualism—

Total change, punctuated—

Time spent in stasis, punctuated—

Fastest single interval, ratio—

Where the gene flow stops

	Allopatric	Sympatric

Apart, and together again

	Closely related?	Becoming	Because

When speciation is fast