Hybridization is an adaptive strategy for species survival

Large ground finch. Linda Hall Library

We introduced Darwin’s finches to our readers in our previous post.  We told you about the research of Rosemary and Peter Grant on the Galápagos Islands that documented the rapid adaptation of the finches to radical changes in their food sources resulting from extreme weather events.  In this post we will continue the story by telling you about another of the amazing discoveries of the scientists studying the finches over a period of nearly 30 years.

Natural selection resulted in the survival of finches with body sizes and shapes that were best suited to the availability and type of food.  Sexual selection enhanced those physical characteristics during periods in which females had more choice because they were greatly outnumbered by males.  In addition to these adaptations, the birds increased their cross-breeding with other species and the resulting hybrids actually had a survival and breeding advantage over their species “pure” parents.*

In the first five years of the research study, there was little evidence of different finch species interbreeding, known as hybridizing.  On those rare occasions when species interbred, the resulting generation was not as successful as their parents, with respect to finding a mate and raising another generation.

Such lack of success of hybrids is considered the norm in nature.  In fact, many hybrids are sterile, incapable of reproducing.  Think of the sure-footed but sterile mule—the offspring of a horse and a donkey—as the classic example of a hybrid.

After the severe drought of 1977 and the flood of 1983, the Grants began to notice an increasing number of cross-breeding birds.  It seemed that the resulting hybrids were having more breeding success than the pre-drought hybrids and the data confirmed their observation.

This counter-intuitive conclusion required some careful consideration and the conclusion is a valuable lesson in our rapidly changing environment.  The environment on the islands was radically transformed by the severe drought and subsequent flood.  The cactus was overwhelmed by a vine that smothered it.  The plants with big, hard seeds were attacked by a fungus that decimated the population.  The small seeded plants thrived and became the dominant food source.

The rapidly changing environment was causing more rapid evolution and the genetic variability of hybrids was giving them an advantage.  If the environment is changing rapidly in unpredictable ways, the birds could increase the odds of finding a winning strategy by increasing the variability of their genes, sometimes resulting in novel traits.

We cannot and should not, however, anthropomorphize the birds by imputing motives to the selection of a mate of another species.  The starving cactus finch probably observes that a male of another species—a seed-eating ground finch, for example—appears to be more fit than a male of her own species.  She is not thinking of the odds of increasing genetic variability.  Natural selection operates without the conscious effort of species.

The implications of hybridization

We are experiencing a period of rapid change because of the anthropogenic (caused by humans) impacts on the environment, most notably climate change, but surely many other impacts which we don’t necessarily understand.  These would seem the ideal conditions for the hybridization of species which speeds up evolution by increasing genetic variability. 

Unfortunately, one of many strategies of the native plant movement and nativism in the animal kingdom is to prevent hybridization because it is perceived as a threat to native plants and animals.  We have reported to our readers some examples of such attempts to prevent hybridization and there are many more in the literature:

The variety of California poppy being eradicated from the Presidio in San Francisco.

Are efforts to prevent hybridization depriving plant and animal species of opportunities to adapt to the rapidly changing environment?  We don’t know the answer to that question, but we find it a provocative line of inquiry.

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*This information is drawn from:   Jonathan Weiner, The Beak of the Finch, Vintage Books, 1994

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.

Galapagos Islands, satellite photo. Daphne Major is too small to be visible.

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

Large ground finch (Geospiza magnirostris). Linda Hall Library

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