Darwin’s Finches: An opportunity to observe evolution in action

The finches on the Galápagos Islands are called Darwin’s finches because of the important role they played in the development of his theory of natural selection and evolution of species.

Charles Darwin spent five weeks on the Galápagos Islands in 1835, near the end of a five year expedition.  Although he noticed the similarity of the birds on the different islands, he didn’t realize they were all related to one common ancestor until he returned home.  Fortunately, he collected many specimens of the birds to bring home for study.  It wasn’t until those specimens were examined by an ornithologist that he learned they were 13 species of finches, distinguished primarily by variations in the size of the bird and its beak size and shape.

Unfortunately, he hadn’t recorded which islands the specimens were from, so the implications of their differences were somewhat of a mystery.  He lamented in Voyage of the Beagle, “It is the fate of every voyager, when he has just discovered what object in any place is most particularly worthy of his attention, to be hurried from it.”

But Darwin was no dummy, so despite lacking the data necessary to prove his point, he speculated in his memoir, “…in the thirteen species of ground-finches, a nearly perfect gradation may be traced from a beak extraordinarily thick, to one so fine, that it may be compared to that of a warbler.  I very much suspect that certain members of the series are confined to different islands…”

Such development of new species from a common ancestor in response to varying environmental conditions is called adaptive radiation.  Species also diverge from one another to reduce competition by specializing in a particular food forage type or technique.  Nearly 200 years later, science has proven Darwin’s hunch, but just as he had no way of knowing how long this process of speciation took, modern science still cannot answer that question.

Darwin’s finches continue to change in response to changing conditions

Rosemary and Peter Grant have studied the finches on two Galápagos Islands (Daphne Major & Genovesa) for about thirty years.  Nearly every year they visited the finches, weighing and measuring every appendage of the birds, especially their beaks.  They banded the birds so they could follow their breeding success. They also measured their food:  how much food but more importantly how accessible the food is to the birds such as the difficulty of opening seeds.

The availability and type of food is what determines the shape and size of the birds’ beaks.  In a year in which there is plenty of rain, there is usually plenty of food which is relatively easy for the birds to eat.  When it doesn’t rain, the birds are reduced to the difficult task of trying to crack open a large, hard seed pod.  That’s when a big bird with a big beak has an advantage.   

Extreme weather is therefore a “selection event,” a time when not every bird is equipped to survive.  And the birds that survive are best equipped for those extreme conditions.  When the conditions improve, the bird that survived the hard time is not necessarily best equipped for the good times.

These are the principles of natural selection, but they were largely theoretical until the Grants spent many years watching the birds and how they survived such selection events.  They had the good fortune to witness two such events in the first twelve years of their study.

The drought

In the fifth year of the Grants’ study, 1977, there was a severe drought.  After one short storm in early January, there was no more rain for the remainder of the year.  In January, there were 1,300 finches on the island they studied that year.  At the end of the year, there were less than 300 finches left on the island.

The Grants measured and weighed the birds that survived the drought.  Then they returned to their lab at Princeton University to study their data:

• Not a single finch was born and survived on the island in 1977
• The surviving birds were 5-6% larger than the dead birds
• The average beak size of the birds that survived was 11.07 mm long and 9.96 mm deep.  The average beak size of the birds that did not survive was 10.68 mm long and 9.42 mm deep.  These critical differences were too small to see with the naked eye, but became evident when the measurements were analyzed by computer.  This makes a strong case for scientific measurement verses anecdotal observation, which passes for “evidence” amongst native plant advocates.
• Few female birds survived the drought, presumably because male birds are larger than females.

In the years following that drought, sexual selection played an important role in maintaining the population of larger birds with larger beaks.  Because the female birds were scarce, they could be very selective in their mates.  Who did they choose?  Of course, they chose the males with the traits that allowed the birds to survive the drought year.  When the ratio of males to females is more even, sexual selection plays a less important role in natural selection in monogamous species such as the finches.

The flood

Here on the West Coast, we are familiar with the weather phenomenon of El Niño, the nickname given to a heavy rain year resulting from an unusually warm ocean current.  In 1983, we experienced the strongest El Niño on record, as did the Galápagos Islands. 

In 1983, the Grants witnessed the reversal of the results of the 1977 drought:  “Natural selection had swung around against the birds from the other side.  Big birds with big beaks were dying.  Small birds with small beaks were flourishing.  Selection has flipped.” *

Lessons learned

Darwin’s finches give us reason for optimism about the future.  Nature can and will respond to changes in the environment.  Natural selection is not just an historical process that stopped when The Origin of Species was written nearly 200 years ago.  Natural selection is operating at all times, whether we notice it or not. 

However, the loss of nearly 80% of the birds on a Galápagos Island during a severe drought is not cause for celebration.  Although the species survived, hundreds of individual birds did not.  So, we are quick to add that our confidence in the adaptive abilities of nature is not an argument for abusing the environment.

Climate change has caused extreme weather events which are undoubtedly selection events for many species of plants and animals.  Unless we take action to reduce greenhouse gas emissions we can predict more of such events.  Destroying millions of trees solely because they are not native is irresponsible given the contribution their destruction makes to the greenhouse gases causing climate change.

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*Jonathan Weiner, The Beak of the Finch, Vintage Books, 1994