Saturday, February 18, 2017

Thinking About Galapagos (3): The Evolution of Iguanas, Finches, and Humans by Hybridization

 
Marine Iguana
 


                                                                   Land Iguana

 
Hybrid Iguana from Breeding of a Male Marine Iguana and Female Land Iguana
 
 
Video of Marine Iguana Feeding Underwater
 
 
South Plaza Island, Looking Towards North Plaza Island
 


                                                       A Video on the Hybrid Iguana


On Day 3 (January 30), I began the day before sunrise, sitting on the balcony of our stateroom, reading and writing in my journal.  As the sun came up, I saw South Plaza Island, a small island (measuring one tenth of a square mile), near the even smaller island of North Plaza, which are off the east coast of Santa Cruz island.  From my reading, I knew that one of the unique features of South Plaza is that it is the only place where hybrid iguanas have been found. 

Both marine iguanas and land iguanas are unique to Galapagos.  The marine iguana is the lizard in the world that swims and forages for food underwater.  DNA analysis has shown that hybrid iguanas have a marine iguana father and a land iguana mother.  This is surprising because not only are marine iguanas (Amblyrhynchus cristatus) and land iguanas (Conolophus subcristatus) separate species, they even belong to separate genera.

Marine iguanas have sharp claws that help them grip the rocky seafloor, and they have flat faces that allow them to eat up algae off the rocks.  Hybrid iguanas also have sharp claws and flat faces, but they have never been observed foraging in the sea.  Instead, they forage for food on land like the land iguanas, but unlike the land iguanas, the hybrid iguanas can use their sharp claws to climb trees and cactus in search of food.

Hybrid iguanas show no defects that would impede their survival, but they do not seem to be capable of reproducing, although some scientists think they might turn out to be fertile.  In fact, many hybrid animals have proven to be capable of reproducing.

Howard Snell, a biologist at the University of New Mexico, was the first person to notice the hybrid iguana on South Plaza in 1977, and he saw that the hybrid iguana had features that were intermediate between the marine iguana and land iguana.  From 1977 to 2000, Snell has seen as many as 16 hybrids.  They are so rare that many naturalist guides who have visited South Plaza many times have never seen one.

Luckily, our group saw one hybrid iguana shortly after we had landed on the small wooden dock on South Plaza and then climbed up the lava rocks.  Snell has identified one hybrid individual who was often near the wooden dock.  And since iguanas can live long lives (30  years or more), it's possible that we saw the same individual.

So here's another example of how the evolution of life forms can depend on the unique natural history of a small island.  South Plaza provides the "Goldilocks conditions" that are "just right" for producing hybrid iguanas.  First, the island is so tiny that there is not enough room for land iguanas and marine iguanas to live in separate habitats.  Since their territories overlap, they frequently come into contact, and thus  they have many opportunities for interbreeding.  The second condition is that on South Plaza the breeding seasons of land iguanas and marine iguanas overlap: when male marine iguanas are ready to mate, the female land iguanas are at the end of their breeding season.

If animals from different species can crossbreed to produce hybrids, and if some of these hybrid animals can be fertile, this challenges the traditional belief of Biblical creationists that the Creator created all species to be eternally separate and fixed.  For example, in Jean-Jacques Rousseau's Emile, the Savoyard Vicar argues that the complex order in living nature proves the existence of God as the intelligent designer of that order, and part of that order is the existence of species that are eternally separate.  The Vicar insists: "The generation of living and organized bodies is by itself an abyss for the human mind.  The insurmountable barrier that nature set between the various species, so that they would not be confounded, shows its intentions with the utmost clarity.  It was not satisfied with establishing order.  It took certain measures so that nothing could disturb that order" (Bloom trans., p. 276).

Early in his life, the naturalist taxonomist Carolus Linnaeus (1707-1788) agreed with this traditional belief that the natural order of created species could never be disturbed: "there are as many species as the Infinite Being created in the beginning."  But as he worked more in the classification of species, he discovered plants that were hybrids, some of which were capable of reproducing, and thus becoming a new species.  He concluded: "it is impossible to doubt that there are new species produced by hybrid generation."  For this, he was denounced by clergy who accused him of blasphemy in denying the Biblical account of God's special creation of species.

Later, creationist naturalists like Joseph Gottlieb Kolreuter (1733-1806) and Carl Friedrich von Gartner (1772-1850) studied hybrids to try to prove that Linnaeus was wrong, arguing that the hybridization of two species could not produce a new species, because God had specially endowed hybrids with sterility. 

In his chapter on "Hybridism" in The Origin of Species, Darwin argued against "the view commonly entertained by naturalists . . . that species, when intercrossed, have been specially endowed with sterility, in order to prevent their confusion."  He pointed out that the degree of sterility was innately variable in individuals of the same species, and that some species could cross with other species and produce fertile hybrids.  Darwin did not explore, however, the possibility that hybridization could be a means for the evolution of new species.

Over the past 150 years, most biologists have recognized the evolution of new plant species through hybridization, but they have generally assumed that this is uncommon for plants, and that it never happens for animals.  And yet, in recent decades, the evidence for the evolution of hybrid animals has  become too extensive to ignore.  Biologists like Matthew Arnold of the University of Georgia and James Mallet of the University College London has led the intellectual movement for recognizing the evolution of animal species by hybridization (Mallet 2007; Pennisi 2016).

This new evidence for evolutionary speciation through hybridization subverts the once long-accepted "biological species concept" of Ernst Mayr, which assumes that reproductive isolation is the critical element of any species, so that members of a species breed only with other members of their species.  The evidence for hybridization shows that the barrier to interspecies reproduction is much weaker than Mayr believed.

Once two species mate and produce fertile hybrid offspring, evolution can occur in two ways.  If the hybrid mates back with a member of the parent species, new DNA is introduced into the genome of the parent species, which is called introgression.  If hybrids mate among themselves and reproduce, this creates a new species.  Introgression introduces much more genetic variation more quickly than is possible through random mutation, which means that hybridization can help organisms adapt better and more quickly to changing environments.

Darwin introduced the image of the evolution of species as a tree of life, as in this famous page in his notebooks, where he drew a tree of life and wrote "I think."

 

Some devoted Darwinians have even had this tattooed onto their bodies!  But if the idea of the evolution of new species by the hybridization of old species is correct, then we need to draw connections between the branches of this tree, so that the tree of life would become more like a web of life.

The Galapagos hybrid iguana is not a clear example of this, insofar as there is doubt as to whether it can be fertile.  A better example is the Galapagos finches as studied by the Grants on Daphne Major.  In 1981, the Grants noticed the arrival of an unusual male finch that they called "Big Bird."


            "Big Bird," a Hybrid Darwin's Finch on the Galapagos Island of Daphne Major

He had a big head.  He weighed 28 grams, instead of the 18 grams typical for male finches.  And he sang an unusual song.  He probably originated on Santa Cruz Island from the mating of a cactus finch and a medium ground finch.

At first, Big Bird and his offspring consorted with the medium ground finches on Daphne Major.  But then after a severe drought from 2003 to 2005 killed 90% of the finches on the island, the two surviving Big Bird descendants and their 26 offspring crowded together in one corner of the island, and they breed just among themselves, which suggests that they are becoming a separate species.  Because of their intermediate-size beaks, they can crack certain seeds that other birds can't.  The unusual song that they sing also separates them from other birds.  So the effect of hybridization on evolution here is partly cultural--the social learning of song.

Recently, the Grants have been cooperating with geneticists to identify the genes responsible for the size and shape of finch beaks, which explains the genetic basis for beak variations and for the evolution of the intermediate-size beaks of the hybrid finches (Lamichhaney et al. 2015).

So the descendants of Big Bird are beginning to look like a new species that evolved through hybridization, although the Grants are unwilling to say that this really is a new species.

Hybridization has occurred not only among iguanas, finches, and many other animals, but also among human beings.  In 2010, analyses of the nuclear DNA of ancient and living humans showed that they carried traces of DNA from Neandertals and from archaic humans from Denisova Cave in Siberia.  It seems that Europeans and Asians have inherited 2% to 6% of their nuclear DNA from Neandertals.  And people living in Southeast Asia have inherited about 5% of their DNA from the Denisovans.

If this evolution by hybridization is true, this refutes the traditional creationist claim that all species have been created by God to be eternally separated.  It also subverts the argument of people like Leon Kass that genetic engineering--moving genes from one species to another--should elicit moral repugnance as an unnatural act of "playing God."  After all, it appears now that nature has been engaged in genetic engineering for millions of years.  And we can still see that evolutionary genetic engineering in action in the island laboratories of Galapagos.


REFERENCES

Lamichhaney, Sangeet, et al.  2015.  "Evolution of Darwin's Finches and their Beaks Revealed by Genome Sequencing." Nature 518 (19 February): 371-75.

Mallet, James. 2007. "Hybrid Speciation." Nature 446 (15 March): 279-83.

Pennisi, Elizabeth. 2016. "Shaking Up the Tree of Life." Science 354 (18 November): 817-21.

1 comment:

Roger Sweeny said...

There are millions of Americans (and even more millions of Europeans) who feel moral repugnance about genetic engineering because it is humans "playing nature." Of course, to many of them, nature is a god.