How did the medium ground finch population on Daphne Major change following the 1977 drought?

Author’s note: Are Darwin’s finches “a particularly compelling example of speciation” as well as “evolution in action”? In a series of posts, I offer some notes on the question of whether macroevolution is happening on the Galápagos Islands. Please find the full series here.

It may perhaps sound a bit confusing and contradictory when we read (Lamichhaney et al. in Nature 2015, referred to in my last post) that, “After a year of drought the finches with slightly larger beaks survived earlier than those with smaller beaks,” and also: “When the weather is dry, bigger-beaked birds do better. When the rain returns, smaller-beaked birds return to previous levels,” etc., and that during the time of the drought: “the researchers found that the large-beak HMGA2 variant was more common in birds that starved to death, while the small-beak variant was more common in birds that survived” (emphasis added). 

Solving the Riddle

The solution to the riddle?

First phase (for the Wikipedia article, see here):

During the rainy season of 1977 only 24 millimeters of rain fell. Two of the main finch species were hit exceptionally hard and many of them died. The lack of rain caused major food sources to become scarce, causing the need to find alternative food sources. The smaller, softer seeds ran out, leaving only the larger, tougher seeds. The finch species with smaller beaks struggled to find alternate seeds to eat. The following two years suggested that natural selection could happen very rapidly. Because the smaller finch species could not eat the large seeds, they died off. Finches with larger beaks were able to eat the seeds and reproduce. The population in the years following the drought in 1977 had “measurably larger” beaks than had the previous birds.

Second phase (for the Wikipedia article that is the source for this and subsequent quotations, see here): 

Over the course of 1982–1983, El Niño brought a steady eight months of rain. In a normal rainy season Daphne Major usually gets two months of rain. The excessive rain brought a turnover in the types of vegetation growing on the island. The seeds shifted from large, hard to crack seeds to many different types of small, softer seeds. This gave birds with smaller beaks an advantage when another drought hit the following year. Small-beaked finch could eat all of the small seeds faster than the larger beaked birds could get to them.

Third phase:

In 2003, a drought similar in severity to the 1977 drought occurred on the island.

Events Repeated?

Now you would expect that the events that happened in 1977 would be repeated, namely: 

Because the smaller finch species could not eat the large seeds, they died off. Finches with larger beaks were able to eat the seeds and reproduce. The population in the years following the drought in 1977 had “measurably larger” beaks than had the previous birds.

But far from it. Instead, the following was observed:

Following the drought, the medium ground finch population had a decline in average beak size, in contrast to the increase in size found following the 1977 drought.

Note the Subjunctive

Why? The authors offer the following selectionist explanation, which is nevertheless uncertain. Note the repeated use of the subjunctive: 

…in the time between the droughts (beginning in late 1982), the large ground finch (Geospiza magnirostris) had established a breeding population on the island. This species has diet overlap with the medium ground finch (G. fortis), so they are potential competitors. The 2003 drought and resulting decrease in food supply may have increased these species’ competition with each other, particularly for the larger seeds in the medium ground finches’ diet. This was hypothesized to be due to the presence of the large ground finch; the smaller-beaked individuals of the medium ground finch may have been able to survive better due to a lack of competition over large seeds with the large ground finch.

So, the situation concerning natural selection is not as simple as usually presented in the textbooks or in the later deleted comment of the National Academy of Sciences, which I discussed in the first two posts in this series. Rather, there are obviously important open questions yet to be solved, including the possible (not even mentioned above) effects of hybridization between Geospiza magnirostris and G. fortis. 

Nevertheless, even if the competition hypothesis between these two species were true, it would change the “Sisyphean evolution of Darwin’s finches” only with regard to its length and the number of its steps to fulfill the Sisyphean cycle. 

The more extensive but misleading claim of Peter and Rosemary Grant is that “selection oscillates in a direction.” Even if this doubtful assertion were true, it would ultimately be irrelevant for the origin of primary species and higher systematic categories. A sieve (natural selection), after all, cannot create new grains.

Next, “Island Evolution of Species: Typogenesis, Typostasis or Typolysis?”

Editor’s note: This article was updated on November 30, 2020.

How did the medium ground finch population change after the drought?

How did the population of medium ground finches on the island of Daphne Major change as the result of environmental change?

What happened to the finches at Daphne Major?

What event occurred on Daphne Major in 1977 and reduced the medium ground finch population by 85 %?