Natural selection — a process with no purpose and no direction makes a direction

Nobody changed, yet the whole has moved

If people choosing for a few hundred years changed pigeons this much, how much more could nature do, choosing for millions of years with life and death at stake? Run Darwin's “how much more” here for yourself.

The first pair of sliders — you need both

Chance alone will not do it, and neither will selection alone

There are 260 finches, and their beaks differ a little in size. The island is full of hard seeds, so the bigger beaks leave slightly more young. There are two sliders: how varied they are and how hard the choosing presses. Try setting either one to zero.

With selection at zero the mean only wobbles and goes nowhere. With variation at zero, however hard the choosing, nothing changes. A direction appears only when chance (variation) and selection are multiplied together.

The second board — same conditions, different rules

How did the giraffe's neck get long?

On the left is the idea that a neck stretched during life is passed to the young (Lamarck); on the right, the idea that those born with longer necks leave more young (Darwin). The mean lengthens in both, but the shape differs.

If acquired traits were inherited

If it is natural selection

On the left the whole herd moves as one block; on the right it moves while its spread narrows, because the shorter ones leave fewer young and vanish. When one result can be explained in two ways, this difference in shape is what showed which was right.

What this tool shows, and what it does not
  • The model — it looks at a single trait and holds the population at 260. The larger the trait, the higher the chance of leaving offspring; with selection at zero that chance is the same for all. The aim is to show why the shape comes out as it does, not to reproduce figures from real organisms.
  • What is simplified — in reality one trait rarely corresponds to one gene, the environment keeps changing, and sexual selection, genetic drift and gene flow all act together. Here only variation and selection are left in.
  • The giraffe story — whether the neck lengthened because of high leaves is still argued over (another explanation points to fights between males). It is used here to show the difference in shape between the two models, not to give the answer about giraffe necks.

Having a direction is not the same as having a purpose. The population moves one way, but it was not heading there. If the environment changes, it goes the other way. Evolution acquires direction, but it has no goal.

No individual changed itself. Yet as generations piled up, the whole population moved. If something intricate was made with no designer and no purpose — what is intricacy evidence of?

The words in this tool

These are the names for what you handled here. The half Darwin did not know — why variation arises and how traits are passed on — was filled in only long after his death.

Natural selection Mutation Traits, dominant and recessive Gene Inheritance of acquired traits The Weismann barrier The modern synthesis
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