Why Darwin Matters (4 page)

Darwin’s “one long argument” was with the theologian William Paley and the theory Paley posited in his 1802 book,
Natural Theology: or, Evidences of the Existence and Attributes of the Deity, Collected from the Appearances of Nature
. Sound eerily familiar? The scholarly agenda of this first brand of Intelligent Design was to correlate the works of God (nature) with the words of God (the Bible). Natural theology kicked off with John Ray’s 1691
Wisdom of God Manifested in Works of the Creation
, which itself was inspired by Psalms 19:11: “The Heavens declare the Glory of the Lord and the Firmament sheweth his handy work.” John Ray, in what still stands as a playbook for creationism, explains the analogy between human and divine creations: If a “curious Edifice or machine” leads us to “infer the being and operation of some intelligent Architect or Engineer,” shouldn’t the same be said of “the Works of nature, that Grandeur and magnificence, that excellent contrivance for Beauty, Order, use, &c. which is observable in them, wherein they do as much transcend the Efforts of human Art and infinite Power and Wisdom exceeds finite” to make us “infer the existence and efficiency of an Omnipotent and All-wise Creator?”
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Paley advanced Ray’s work through the accumulated knowledge of a century of scientific exploration. The opening passage of Paley’s
Natural Theology
has become annealed into our culture as the winningly accessible and thus appealing “watchmaker” argument:

In crossing a heath, suppose I pitched my foot against a stone, and were asked how the stone came to be there. I might possibly answer, that, for any thing I knew to the contrary, it had lain there forever. But suppose I had found a watch upon the ground, and it should be enquired how the watch happened to be in that place. The inference, we think, is inevitable; that the watch must have had a maker; that there must have existed, at some time and in some place or other, an artificer or artificers who formed it for the purpose which we find it actually to answer; who comprehended its construction, and designed its use.

But life is far more complex than a watch—so the design inference is even stronger!

There cannot be design without a designer; contrivance without a contriver. . . . The marks of design are too strong to be got over. Design must have had a designer. That designer must have been a person. That person is GOD.
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For longer than we have had the theory of evolution, we have had theologians arguing for Intelligent Design.

After abandoning medical studies at Edinburgh University, Charles Darwin entered the University of Cambridge to study theology with the goal of becoming a Church of England cleric. Natural theology provided him with a socially acceptable excuse to study natural history, his true passion. It also educated Darwin in the arguments on design popularized by Paley and others.
⁹
His intimacy with their ideas was respectful, not combative. For example, in November 1859, the same month that the
Origin of Species
was
published, Darwin wrote his friend John Lubbock, “I do not think I hardly ever admired a book more than Paley’s ‘Natural Theology.’ I could almost formerly have said it by heart.”
¹⁰
Both Paley and Darwin addressed a problem in nature: the origin of the design of life. Paley’s answer was to posit a top-down designer—God. Darwin’s answer was to posit a bottom-up designer—natural selection. Natural theologians took this to mean that evolution was an attack on God, without giving much thought to what evolution is.

Ever since Darwin, much has been written about what, exactly, evolution is. Ernst Mayr, arguably the greatest evolutionary theorist since Darwin, offers a subtly technical definition: “evolution is change in the adaptation and in the diversity of populations of organisms.” He notes that evolution has a dual nature, a “ ‘vertical’ phenomenon of adaptive change,” which describes how a species responds to its environment over time, and a “ ‘horizontal’ phenomenon of populations, incipient species, and new species,” which describes adaptations that break through the genetic divide.
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And I’ll never forget Mayr’s definition of a species, because I had to memorize it in my first course on evolutionary biology: “A species is a group of actually or potentially interbreeding natural populations reproductively isolated from other such populations.”
¹²

Mayr outlines five general tenets of evolutionary theory that have been discovered in the years since Darwin published his revolutionary book:

1.
Evolution:
Organisms change through time. Both the fossil record of life’s history and nature today document and reveal this change.

2.
Descent with modification:
Evolution proceeds through the branching of common descent. As every parent and child knows, offspring are similar to but not exact replicas of their
parents, producing the necessary variation that allows adaptation to the ever-changing environment.

3.
Gradualism:
All this change is slow, steady, and stately. Given enough time, small changes within a species can accumulate into large changes that create new species; that is, macroevolution is the cumulative effect of microevolution.

4.
Multiplication:
Evolution does not just produce new species; it produces an increasing number of new species.

And, of course,

5.
Natural selection:
Evolutionary change is not haphazard and random; it follows a selective process. Codiscovered by Darwin and the naturalist Alfred Russel Wallace, natural selection operates under five rules:

A. Populations tend to increase indefinitely in a geometric ratio: 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024 . . .

B. In a natural environment, however, population numbers must stabilize at a certain level. The population cannot increase to infinity—the earth is just not big enough.

C. Therefore, there must be a “struggle for existence.” Not all of the organisms produced can survive.

D. There is variation in every species.

E. Therefore, in the struggle for existence, those individuals with variations that are better adapted to the environment leave behind more offspring than individuals that are less well adapted. This is known as
differential reproductive success
.

As Darwin said, “as more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either
one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life.”
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The process of natural selection, when carried out over countless generations, gradually leads varieties of species to develop into new species. Darwin explained:

It may be said that natural selection is daily and hourly scrutinising, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until the hand of time has marked the long lapses of ages, and then so imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they formerly were.
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The time frame is long and the changes from generation to generation are subtle. This may be one of the most important and difficult points to grasp about the theory of evolution. It is tempting to see species as they exist today as a living monument to evolution, to condense evolution into the incorrect but provocative shorthand that humans descended from chimpanzees—a shorthand that undercuts the facts of evolution.

Natural selection is the process of organisms struggling to survive and reproduce, with the result of propagating their genes into the next generation. As such, it operates primarily at the local level. The Oxford evolutionary biologist Richard Dawkins elegantly described the process as “random mutation plus non-random cumulative selection,”
¹⁵
emphasizing the non-random. Evolution is not the equivalent of a warehouse full of parts randomly assorting themselves into a jumbo jet, as the creationists like to argue. If evolution were truly random there would be no biological jumbo jets.
Genetic mutations and the mixing of parental genes in offspring may be random, but the selection of genes through the survival of their hosts is anything but random. Out of this process of self-organized directional selection emerge complexity and diversity.

Natural selection is a description of a process, not a force. No one is “selecting” organisms for survival or extinction, in the benign sense of dog breeders selecting for desirable traits in show breeds, or in the malignant sense of Nazis selecting prisoners at Auschwitz-Birkenau. Natural selection, and thus evolution, is unconscious and nonprescient—it cannot look forward to anticipate what changes are going to be needed for survival. The evolutionary watchmaker is blind, says Dawkins,
pace
Paley.

By way of example, once when my young daughter asked how evolution works, I used the polar bear as an example of a “transitional species” between land mammals and marine mammals, because although they are land mammals they spend so much time in the water that they have acquired many adaptations to an aquatic life. But this is not correct. It implies that polar bears are
on their way
(in transition) to becoming marine mammals. They aren’t. Polar bears are not “becoming” anything. Polar bears are well adapted for their lifestyle. That’s all. If global warming continues, perhaps polar bears will adapt to a full-time aquatic existence, or perhaps they will move south and become smaller brown bears, or perhaps they will go extinct. Who knows? No one.

Evolution is a historical science, and historical data—fossils—are often the evidence most cited for and against it. In the creationist textbook,
Of Pandas and People
—one of the bones of contention in
the 2005 Intelligent Design trial of
Kitzmiller et al. v. Dover Area School District
, in Dover, Pennsylvania—the authors state: “Design theories suggest that various forms of life began with their distinctive features already intact: fish with fins and scales, birds with feathers and wings, mammals with fur and mammary glands. . . . Might not gaps exist . . . not because large numbers of transitional forms mysteriously failed to fossilize, but because they never existed?”
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Darwin himself commented on this lack of transitional fossils, asking, “Why then is not every geological formation and every stratum full of such intermediate links?” In contemplating the answer, he turned to the data and noted that “geology assuredly does not reveal any such finely graduated organic chain; and this, perhaps, is the gravest objection which can be urged against my theory.”
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So where
are
all the fossils?

One answer to Darwin’s dilemma is the exceptionally low probability of any dead animal’s escaping the jaws and stomachs of predators, scavengers, and detritus feeders, reaching the stage of fossilization, and then somehow finding its way back to the surface through geological forces and unpredictable events to be discovered millions of years later by the handful of paleontologists looking for its traces. Given this reality, it is remarkable that we have as many fossils as we do.

There is another explanation for the missing fossils. Ernst Mayr outlines the most common way that a species gives rise to a new species: when a small group (the “founder” population) breaks away and becomes geographically—and thus reproductively—isolated from its ancestral group. As long as it remains small and detached, the founder group can experience fairly rapid genetic changes, especially relative to large populations, which tend to sustain their genetic homogeneity through diverse interbreeding.
Mayr’s theory, called
allopatric speciation
, helps to explain why so few fossils would exist for these animals.

The evolutionary theorists Niles Eldredge and Stephen Jay Gould took Mayr’s observations about how new species emerge and applied them to the fossil record, finding that gaps in the fossil record are not missing evidence of gradual changes; they are extant evidence of punctuated changes. They called this theory
punctuated equilibrium
.
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Species are so static and enduring that they leave plenty of fossils in the strata while they are in their stable state (equilibrium). The change from one species to another, however, happens relatively quickly on a geological time scale, and in these smaller, geographically isolated population groups (punctuated). In fact, species change happens so rapidly that few “transitional” carcasses create fossils to record the change. Eldredge and Gould conclude that “breaks in the fossil record are real; they express the way in which evolution occurs, not the fragments of an imperfect record.”
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Of course, the small group will also be reproducing, following the geometric increases that are observed in all species, and will eventually form a relatively large population of individuals that retain their phenotype for a considerable time—and leave behind many well-preserved fossils. Millions of years later this process results in a fossil record that records mostly the equilibrium. The punctuation is in the blanks.