Experimental evidence that competition promotes divergence in adaptive radiation

Generalist sticklebacks were placed in experimental ponds, half of them alongside a specialist competitor that feeds on open-water plankton and half without. Growth of individual generalists was measured against their body form, so that the fitness consequence of resembling the competitor could be estimated directly.

Studied in

Gasterosteus aculeatus

Sample size

experimental pond populations with and without a competitor

With the competitor present, generalists resembling it grew more slowly, while those differing from it grew better. Without the competitor, that relationship disappeared.

• Experimental ponds over one growing season, not a long-term evolutionary trajectory.
• Growth is a component of fitness rather than the whole of it, and reproduction was not measured.

• Measuring what it costs to resemble your competitor

  Similar species often differ more where they overlap. Does competition actually push them apart, or does that pattern have another explanation?

• Character displacement

  Put a competitor in a pond and the fish that resemble it grow worse; take it away and that penalty vanishes. On Daphne Major, a competitor arrived, drought struck, and beak size shifted within a generation.