Darwinian natural selection The logic of The Origin

Darwinian natural selection The logic of The Origin of Species by Means of Natural Selection

Darwin’s four postulates 1. Individuals within populations are variable variation 2. Some of these variations are passed on to offspring heritability from the Illustrated London News (1851), in Secord (1981)

Darwin’s four postulates 3. In every generation, more offspring are produced than can survive. Some individuals survive and reproduce better than others. • Most populations are stable in number over years, despite great reproductive potential (adopted from Malthus)

Darwin’s four postulates 4. The individuals that survive and reproduce the most are those with favorable variations. They are naturally selected. 1. Survival and reproduction are not random 2. They are linked to the variation among individuals

Darwinian natural selection Selection on beak shape in Galápagos (“Darwin’s”) finches

There are 15 species of “Darwin’s” finches, derived from the grassquit (genus. Tiaris) of mainland Central and South America This complex of species is closely related but shows remarkable variation in beak size and shape

This great variety of bill characteristics allows use of a broad spectrum of diets Cactospiza pallida

15 species are on the Galápagos islands off Ecuador, with one species on Isla Cocos off Costa Rica

* The ground finches: a phylogenetic group primarily of fruit and seed eaters The medium ground finch Geospiza fortis cracks seeds at the base of its thick bill

Peter and Rosemary Grant and colleagues have tagged ~100% of the G. fortis population on Daphne Major (~1200 birds average), and studied them for > 30 years

Testing postulate 1 Variation

1976 Data Figure 3. 6 Variation in beak depth in G. fortis. The finches obey Darwin’s 1 st postulate: the population on Daphne Major is highly variable for beak shape (e. g. depth) and beak size.

Testing postulate 2 Heritability

Figure 3. 7 Heritability of beak depth in G. fortis. Peter Boag has shown that variation is passed to offspring (Darwin’s Postulate 2). Tagging allows them to follow parent/offspring relationships. Offspring beak depth resembles that of their parents.

Testing postulate 3 Do only a fraction of offspring survive to reproduce?

The Galápagos have wet (Jan May) and dry seasons (June Dec) Extreme drought occurred during the 1977 wet season (1/5 th average rainfall)

The drought led to a severe reduction in the number of seeds available to G. fortis.

During the drought, the usual variety of plants did not seed. Tribulus cistoides is an annual plant whose seeds were the majority available during the 1977 drought

This led to larger and harder seeds available on average to ground finches. Only large birds with deep, narrow beaks can crack open and feed on these harder seeds.

Only 16% of the ground finch population survived the drought to reproduce in later years. Darwin’s postulate 3 held true—more offspring were produced than survived.

Testing postulate 4 Did natural selection occur?

Figure 3. 9 Beak depth before and after the drought. Was survival nonrandom with respect to beak shape (Darwin’s postulate 4)? Yes—the average survivor of the drought had a deeper beak than pre 1977.

This proves that the 1977 drought produced natural selection for deeper beaks But did the population of G. fortis evolve?

Evolution is a change, over generations, in the genetic composition of a population. Evolution is a response to natural selection—did this occur? To find out, Grant and Grant measured beaks of the offspring of the survivors. Offspring beaks had become deeper, proving that genetic change over generations (evolution) had occurred.

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