Category: Darwin

My letter to Darwin

November 24, 2018 • 10:00 am

Nine years ago, Radio 4 in England invited several of us to write a letter to Charles Darwin, telling the old guy what we think he’d like to know 200 years after his birth in 1809 and 150 years after he published On the Origin of Species.  We read these “Dear Darwin” letters on the air (you can hear them here; the other participants were Craig Venter, Sir Jonathan Miller, Peter Bentley, and Baruch Blumberg). Later, Oxford University Press, the British publisher of Why Evolution is True, published my letter on its website. In honor of the publication of The Origin on this day in 1859, I’ll put that letter below.

My Dear Mr. Darwin,

Happy 200th birthday! I hope you are as well as can expected for someone who has been dead for nearly 130 years. I suppose that your final book, the one about earthworms, has a special significance for you these days. Are the worms of Westminster Abbey superior to the ones you studied so carefully in the grounds of your home at Downe in Kent? They’ve certainly mulched some distinguished people over the years!

But enough of the personal questions: let me introduce myself. I am one of thousands – maybe tens of thousands – of professional biologists who work full time on your scientific legacy. You’ll be happy to know that Britain remains a powerhouse in what we nowadays call evolutionary biology, and your ideas now have wide currency across the entire planet. I work in Chicago, in the United States of America. But even the French have finally reluctantly relinquished their embrace of Jean-Baptiste Lamarck, whose misguided evolutionary ideas you did so much to discredit.

Your Origin of Species turns 150 this year. I just re-read it in your honour and must say that, though you did not always have the snappiest turn of phrase, it really is a wonderfully comprehensive and insightful work. It is remarkable, considering what you did not know when you wrote it, how robust the book has proved over the years. The findings of modern biology, many of them inconceivable to you as you beavered away in your Down House study, have provided ever more evidence in support of your ideas, and none that contradicts them. We have learned a huge amount in the past 150 years, but nearly all of it still fits comfortably into the framework you outlined in The Origin. Take DNA, for example. This is what we call the hereditary material that is passed down from generation to generation. You knew nothing about it – remember how you wished you understood more about how heredity works? Now we have full DNA sequences from dozens of species, each one a string of billions of the four DNA letters—A, T, G and C—each a different chemical compound. What do we find when we compare these sequences, say between a mouse and a human? We see the DNA equivalent of the anatomical similarities – as mammals – that you noted mice and humans share because they are descended from a common ancestor, an early mammal. Strings of As, Gs, Cs, and Ts tell precisely the same evolutionary story as traits like lactation and warm-bloodedness. It is absolutely marvelous that your 150 year old insight on common ancestry should be so relevant to the very latest discoveries of the new field we call molecular biology.

In The Origin, you gave very little evidence for evolution from the fossil record, wringing your hands instead about the incompleteness of the geological record. But since then, the labors of fossil-hunters throughout the world have turned up plenty of evidence of evolutionary change, and many amazing “transitional” forms that connect major groups of animals, proving your idea of common ancestry. You predicted that these forms would exist; we have found them. These include fossils that show transitions between mammals and reptiles, fish and amphibians, and even dinosaurs with feathers—the ancestors of birds! Just in the past few years, paleontologists have unearthed an astonishing fossil, called Tiktaalik, that is intermediate between fish and amphibians. It has the flat head and neck of an amphibian, but a fishy tail and body, while its fins are sturdy, easily able, with slight modification, to give them a leg up when they left the water. The fossil record has given us a direct glimpse of an event of great moment in the history of the planet: the colonization of land by vertebrates. And we have evidence just as convincing for the recolonization of the sea by mammals: the group that gave rise to whales. In The Origin, you were correct in suggesting that whales arose from land animals, but you got it wrong on one point. You thought they may have come from carnivores like bears, but we now know this is not true. Instead, the ancestral whale came from a small hooved animal rather like a deer. And in the last thirty years we have discovered a whole series of intermediate fossils spanning the gap from those ancient deer to modern whales, showing them losing their hind legs, evolving flippers, and moving their breathing hole to the top of their head. Both Tiktaalik and these ancestral whales put paid to the objection, which you yourself encountered, that no transitional form between land and water could possibly have existed.

Perhaps the most remarkable set of intermediate fossils, however, come from an evolutionary transition rather closer to home. In 1871, you more predicted that, since humans seem most related to African great apes, gorillas and chimpanzees, we would find human fossils on that continent. And now we have them—in profusion! It turns out that our lineage separated from that of chimpanzees, our closest living relatives, nearly 7 million years ago, and we have a superb series of fossils documenting our transition from early apelike creatures to more modern human forms. Our own species has become an exemplar of evolution. And we know even more: evidence from our hereditary DNA material has told us that all modern humans came from a relatively recent migration event—about 100,000 years ago—when our ancestors left Africa and spread throughout the world.

The idea you were proudest of was natural selection. That too has had a good 150 years, holding up well as the main cause of evolution and the only known cause of adaptation. Perhaps the most dramatic modern example involves bacteria that are now known to cause disease, including the scarlet fever that was such a plague upon your family. Chemists have developed drugs to cure diseases like this, but now, as you might well predict, the microbes are becoming resistant to those drugs—precisely in accord with the principles of natural selection—for the most drug-resistant microbes are the ones that survive to breed. There are hundreds of other cases. One that will especially please you is the observation of natural selection in the Galápagos finches you collected in the Beagle voyage—now called “Darwin’s finches” in your honor. A few decades ago, zoologists observed a great drought on the islands that reduced the number of small seeds available for the birds to eat. And, just as predicted, natural selection caused the evolution of larger-beaked birds within only a few years. These examples would surely be a centerpiece of The Origin were you to rewrite it today.

All told, the resilience of your ideas is remarkable. But that is not to say that you got everything right. On The Origin of Species was, admit it, a misnomer. You described correctly how a single species changes through time, but you came a cropper trying to explain how one species splits into two. Speciation is a significant problem, because it underpins the branching process that has yielded the tree of life – that extraordinary vision you bequeathed us of the natural world as one vast genealogy. Speciation is the key to understanding how, starting with the very first species on earth, evolution has resulted in the 50 million species that are thought to inhabit our planet today.

You once called speciation the “mystery of mysteries,” but it’s a lot less mysterious these days. We recognize now that species are separated one from another by barriers to reproduction. That is, we recognize different species, like humans and chimpanzees, because they cannot successfully interbreed. To modern evolutionary biologists, studying “the origin of species” means studying how these barriers to reproduction arise. And now that we have a concrete phenomenon to investigate, we are making remarkable progress in understanding the genetic details of how one species splits into two. This is in fact the problem to which I’ve devoted my entire career

I wish I could end this letter by telling you that your theory of evolution has achieved universal acceptance. As you well knew, evolution has proved a bitter pill for religious people to swallow. For example, a large proportion of the American public, despite access to education, clings to a belief in the literal truth of Genesis. You will find this hard to believe, but more Americans believe in the existence of heavenly angels than accept the fact of evolution. Unfortunately, I must often put aside my research to fight the attempts of these “creationists” to have their Biblical views taught in the public schools. Humans have evolved extraordinary intellectual abilities, but sadly these are not always given a free rein by their owners. But this probably won’t surprise you – remember the Bishop of Oxford and his attempt to put your friend Thomas H. Huxley in his place?

You wrote in your introduction to The Origin of Species that

“No one can feel more sensible than I do of the necessity of hereafter publishing in detail all the facts, with references, on which my conclusions have been grounded; and I hope in a future work to do this.”

It seems that, distracted by other projects, you never got around to it, but my own effort along these lines is represented in a book (which I enclose) called Why Evolution is True. It goes further to describe the evidence supporting you than a letter this size ever could, but it’s just one book at just one moment in the history of biology. When I myself am as long gone as you are, somebody else will certainly need to write an update, for the facts supporting your theories continue to roll in, and I wager they will continue to do so.

So, rest in peace, Mr. Darwin, and here’s hoping that the next hundred years will see a steady evolution of rationality in a troubled world.

Your most humble servant,
Jerry Coyne

Teaching Evolution: Darwin: Unity of type and adaptation

March 31, 2018 • 10:30 am

Note from Jerry: Greg plans to run a mini-MOOC here, so if you want some education in evolution, do the readings and answer the questions (to yourself). This is the first installment.

by Greg Mayer

This semester I’m teaching BIOS 314 Evolutionary Biology, an upper level undergraduate course. The students are all or mostly biological sciences majors, and general genetics and biostatistics are prerequisites for taking the course. As a textbook, I use Evolution, by Doug Futuyma and Mark Kirkpatrick, published by Sinauer Associates of Sunderland, Mass., which last year became an imprint of Oxford University Press. This is the 4th edition of the current iteration of this text, which also had 3 editions under an earlier iteration with the title Evolutionary Biology. Doug was my undergraduate evolution teacher, and I took the class with him while he was writing the first edition of Evolutionary Biology.

I hope to say more about the textbook later, but I also have the students read a series of what I regard to be classic papers or extracts– one each week–and these are what I want to share with WEIT readers. Each reading is accompanied by a brief biography and illustration of the author, and a small number of study questions, designed to guide the student in understanding the reading. I sometimes assign these questions as homework essays, or include them on exams. The first week’s reading each time I’ve taught the course this way has been an extract from the Origin. I will, when possible, provide links to the readings. For this Darwin reading, you can use John van Wyhe‘s superb Darwin Online. I’ll be posting a reading every few days. If you want to read along, it will be like taking a guided readings course in evolutionary biology.

Charles Darwin (1809-1882), after studying briefly at Edinburgh to be a physician, went to Cambridge to prepare for the Anglican priesthood. While there, his interest in natural history attracted the attention of his professors, which led to him being invited aboard H.M.S. Beagle as a supernumerary naturalist for that survey vessel’s global circumnavigation (1831-1836). By the time he returned he was already a recognized geologist, and, leaving the ministry behind, he soon established himself as a rising star in all departments of natural history. After study of his Beagle collections convinced him that species were not immutable, he discovered natural selection as a means of modification. Spurred by the later but independent discovery of natural selection by A. R. Wallace, he published his views and evidence in On the Origin of Species in 1859. Within a few years, the whole of the scientific community accepted descent with modification as the key process underlying the phenomena of the organic world, though natural selection as the chief means of modification was not accepted during Darwin’s lifetime. Among his many other works are Zoology of the Beagle (1838-1843, edited by Darwin), Voyage of the Beagle (2nd ed., 1845), Variation Under Domestication (1868) and The Descent of Man (1871). All of Darwin’s published works are available at John van Wyhe’s Darwin Online, http://darwin-online.org.uk/. He is buried in Westminster Abbey, not far from Sir Isaac Newton. The best biography is Janet Browne’s Charles Darwin (vol. 1, 1995; vol. 2, 2002).

 

Reading:
Darwin, Charles. 1859. On the Origin of Species. 1st ed. John Murray, London. Extract from Chap. 13, pp. 434-450, “Morphology” and “Embryology”.

 

Study Questions:

1. Why is the concept of homology crucial for even being able to talk about organic structure? How are homologous organs recognized? What is Darwin’s explanation for homology?

2. How does Darwin’s account of serial homology (the resemblance of parts within an organism, for example, the forelimbs to the hindlimbs, or of a cervical vertebra to a thoracic vertebra) depend on the repetition of parts or segmentation?

3. What is the “law of embryonic resemblance”? How do these resemblance relate to the organisms’ conditions of existence? How does this relation depend on whether the embryo is active and provides for itself?

4. When during development does Darwin suppose most species differentiating characters to become evident? How does this relate to when he supposes the characters to affect the survival and reproduction of their bearers?

The textbook—and misguided—presentation of natural selection

July 17, 2016 • 1:00 pm

I was reading a nice article by Andrew Shtulman* on the most common misconception people have about natural selection (that it involves not differential reproduction among genetically different individuals but the gradual and simultaneous transformation of all individuals in a population), when I came across his presentation of Darwin’s “variational” theory of natural selection. That’s the first view given in parentheses, and the correct one.

This characterization of evolution via natural selection (it’s not really a definition) involves a three- or four-step logical chain. Here’s the way it’s presented in many classes, and the way I used to present it:

  1. Animals and plants produce many more offspring than can survive, ergo there is tremendous mortality in nature.
  2. Animals and plants differ in their traits.
  3. Bearers of some traits leave more offspring, or survive better, than do bearers of others. (For example, those individual moths who match their backgrounds better are less likely to be eaten by birds than are their more conspicuous relatives.)
  4. Some portion of the differences among individuals in these traits will be passed on to the next generation. That is, some of the variation is heritable (“capable of being inherited”).

If these conditions hold, then the population will undergo gradual genetic change, being enriched in the genetic variants that give their bearers greater reproduction and/or survival (“fitness” is the word we use here). This is a variational view of evolution rather than the incorrect transformational view outlined above.  The chain can also be described as “Excess production of individuals + variation in fitness + some heritability of that fitness = evolution via natural selection.”

I hope you’ve followed me so far. If you’ve taken any evolutionary biology, you’re likely to have heard the chain of logic above. It is exactly the chain outlined by Darwin in On the Origin of Species. Darwin, in fact, said that he finally grasped the importance of natural selection when he read Malthus’s “On Population”, which described the overproduction of offspring. For when Darwin realized that the huge excess of young animals and plants must somehow be culled if populations remained at relatively stable sizes, he saw immediately that the culling would probably be based on the traits that individuals had, and if variation in those traits had at least some genetic basis (remember that Darwin was unclear about how genetics worked), it would, over time, produce a predominance of the variants producing those traits.  Curiously, A. R. Wallace also hit on the idea of natural selection after reading Malthus as well.

But there’s one problem with this: one of the points is not necessary for evolution by natural selection. Can you guess which one?

It’s #1—the very point that brought Darwin and Wallace to the brilliant idea of natural selection.

Why isn’t this necessary? While clearly not all offspring survive in any species—otherwise we’d be up to our collective tuchas in rabbits, beetles, or oak trees—natural selection can still cause evolutionary change in a population that is expanding (or decreasing), and in which all offspring survive. Imagine, for instance, that a lizard makes it to a luxuriant island in the ocean, one loaded with food and without predators. Imagine too that every lizard dies only of old age—after it’s already had its offspring. And further imagine that some lizards are better able to digest the local vegetation, and thus are better nourished and leave more offspring than others. Every offspring survives, but more of the next generation will carry those genetic variants that make them better at digestion. Over time, the population will evolve by natural selection, as the carriers of the “good digestion” genes overtake their dyspeptic confrères.  So we have (variational) evolution by natural selection, but every individual that is born survives to reproductive age. And this will work even if individuals are immortal and never die.

Of course the situation I just described is unsustainable: eventually the population of lizards will get big enough that they’ll be competing for food, and might even kill each other. My point, though, was that natural selection can operate independently of a huge mortality—or any mortality—in a population.

Ergo, we can omit #1 above from the characterization of natural selection. When I realized this, I dropped it from teaching.

In fact, the great evolutionary biologist Ronald Fisher made just this point in his famous book The Genetical Theory of Natural Selection (1930). In chapter 2, he objects to the Malthusian point, saying that it not only ignores differential reproduction in favor of differential survival (my example above), but ignores the fact that organisms often overproduce offspring as the very result of natural selection.  The vast overproduction of offspring in some ocean fish, for example, isn’t just a given: it’s likely resulted from the high mortality experienced by tiny fish, due largely to predation. If that’s the case, you have to produce more offspring just so some will survive.

Here’s the quote from Fisher explaining that. Fisher was famous for his dense and often opaque prose, but maybe you can grasp his point:

“. . . it should be remembered that the production of offspring is only excessive in relation to an imaginary world, and the ‘high geometrical rate of increase’ is only attained by abolishing a real death rate, while retaining a real rate of reproduction. There is something like a relic of creationist philosophy in arguing from the observation, let us say, that a cod spawns a million eggs, that therefore its offspring are subject to Natural Selection; and it has the disadvantage of excluding fecundity [offspring production] from the class of characteristics of which we may attempt to appreciate the aptitude. . . [T]he historical fact that both Darwin and Wallace were lead through reading Malthus’s essay on population to appreciate the efficacy of selection, though extremely instructive as to the philosophy of their age, should no longer constrain us to confuse the consequences of that principle with its foundations.”

Fisher was a very smart man. So, if you teach how natural selection causes evolution, you might want to omit point #1 above—or at least qualify it.

Sir_Ronald_Fisher_2
Ronald Fisher (1890-1962)

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*Shtulman, A. 2006. Qualitative differences between naïve and scientific theories of evolution. Cognitive Psychology 52:170-194.