Download Darwin`s Finches

Galapagos Islands (Ecuador)
~ 91° west at the equator
Daphne Major – a small (0.34 km2) island (area similar to a square ~0.4 mile on
each side)
Large-beaked G. fortis
G. magnirostris
Small-beaked G. fortis
Seeds of Tribulus cistoides (principal food of G. magnirostris)
are contained in a fruit consisting of five mericarps.
Mericarp: One segment of a fruit that breaks at maturity into units derived from the individual carpels.
1973 Beak size of G. fortis small on island (“small-beaked”). Lack says (no evidence) due to absence of small
ground finch (G. fulignosa).
1977 Drought. Small seeds scarce. G. fortis relies on large, hard seeds (Tribulus cistoides – Jamaican feverplant –
Zygophyllaceae – creosote bush family). Directional selection for increased bill size. (This and all future trends
documented by Peter & Rosemary Grant.)
1982 Breeding population of G. magnirostris (large ground finch) established. Three times faster at processing
Tribulus seeds. Physically exclude G. fortis from Tribulus feeding sites. Reduce density of Tribulus. But
density of G. magnirostris low - minimal effect on G. fortis.
2003 Drought (2003-2005). Density of G. magnirostris now high. Seeds scarce. Many birds of both species die
(starvation). G. fortis declines from ~235 to ~ 80. G. magnirostris decline from ~150 to 13. Strong competition
for few Tribulus seeds. Strong selection in favor of smaller billed G. fortis.
“(These results) support models of ecological community assembly that incorporate evolutionary effects of
interspecific competion, in contrast to null or neutral models.” Grant & Grant. 2006. Science 313:224-226.