Hybridization among Darwin's finches

This is an old paper by Peter and Rosemary Grant, from 2002:

Unpredictable Evolution in a 30-Year Study of Darwin's Finches
Peter R. Grant and B. Rosemary Grant
Evolution can be predicted in the short term from a knowledge of selection and inheritance. However, in the long term evolution is unpredictable because environments, which determine the directions and magnitudes of selection coefficients, fluctuate unpredictably. These two features of evolution, the predictable and unpredictable, are demonstrated in a study of two populations of Darwin's finches on the Galápagos island of Daphne Major. From 1972 to 2001, Geospiza fortis (medium ground finch) and Geospiza scandens (cactus finch) changed several times in body size and two beak traits. Natural selection occurred frequently in both species and varied from unidirectional to oscillating, episodic to gradual. Hybridization occurred repeatedly though rarely, resulting in elevated phenotypic variances in G. scandens and a change in beak shape. The phenotypic states of both species at the end of the 30-year study could not have been predicted at the beginning. Continuous, long-term studies are needed to detect and interpret rare but important events and nonuniform evolutionary change.

It seems like the purpose of the paper is to demonstrate the fluctuating evolution of the finches in response to environmental change. But I was drawn to it by the demonstration of biased gene flow from one species to the other:

The proportionally greater gene flow from G. fortis to G. scandens than vice versa has an ecological explanation. Adult sex ratios of G. scandens became male biased after 1983 (Fig. 4C) as a result of heavy mortality of the socially subordinate females. High mortality was caused by the decline of their principal dry-season food, Opuntia cactus seeds and flowers; rampantly growing vines smothered the bushes (16). G. fortis, more dependent on small seeds of several other plant species, retained a sex ratio close to 1:1 (Fig. 4C). Thus, when breeding resumed in 1987 after 2 years of drought, competition among females for mates was greater in G. fortis than in G. scandens. All 23 G. scandens females paired with G. scandens males, but two of 115 G. fortis females paired interspecifically. All their F1 offspring later bred with G. scandens (43) because choice of mates is largely determined by a sexual imprinting-like process on paternal song (42).

They conclude that this hybridization had an introgressive effect, acting in the same direction as selection for beak shape during the early 1990's, but continuing throughout the decade while there was little evidence for selection. There was little or no fitness loss to interspecific hybrids.

The case is distinctive because the effects of introgression on the characters of beak size and shape are consistently large enough to be measured each generation. This despite the fact that the proportion of F1 hybrids and first-generation backcrosses in the G. scandens population was never greater than 20 percent.

References:

Grant PR, Grant BR. 2002. Unpredictable evolution in a 30-year study of Darwin's finches. Science 296:707-711. Full text (subscription)