Editor’s note: Feb. 12 is Charles Darwin’s birthday, and on that date the CVPost began a six-part series on the life of the British naturalist whose theory of evolution transformed scientific views of the natural world. The series is written by Wil Taylor, chair of the biology department at the University of Wisconsin-Eau Claire. As a paleontologist and educator, Taylor has a lifelong passion for the study of evolution. His study has taken him, among other places, to the Galapagos Islands and Darwin’s country estate at Downe, England.
By Wil Taylor
For the CVPost
Charles Darwin’s The Origin of Species is one of the most influential books ever written. It also is one of the least read influential books ever written.
So let us consider just what is in The Origin.
It starts with pigeons.
Why pigeons? Because soon after returning home from his voyage on the HMS Beagle in 1835, Darwin begins breeding them, in keeping with a popular hobby in 18th century England.
Pigeons are a logical way for Darwin to begin his work because they are familiar to him and to the public. More importantly they are highly variable, so it is possible, using selective breeding, to produce some very odd birds indeed. For a sampling and some appreciation of diversity among pigeons, including 70 breeds that begin with the letter B, see http://en.wikipedia.org/wiki/List_of_pigeon_breeds.
Darwin wonders if humans can produce such tremendous diversity with selective breeding (a.k.a. artificial selection), might nature be acting on natural diversity to select those best fit for the environment? This is the core of Darwin’s mechanism, which he named “natural selection,” as a parallel to artificial selection.
In The Origin of Species, Darwin discusses some examples of natural selection, then introduces a now widely used metaphor of a “tree” of ancestral relationships. This “tree of life” is the only illustration in the book.
Darwin then admits that his entire theory will not hold water if the beneficial traits of a successful organism are not passed on to its offspring. These ideas surrounding genetics and inheritance were virtually unknown at the time. Interestingly, in the first few years of the The Origin’s existence, a monk in what is now the Czech Republic, Gregor Mendel, would work out the key points of genetic inheritance. But the significance of his work would not be recognized until after Darwin’s death.
One other soft point in Darwin’s theory is that natural selection is a slow process; tiny changes accumulating over thousands of generations are needed to explain the diversity in the living world. So Darwin needed lots and lots of time.
At the time, however, no one had any idea how old the Earth was. Most scientists rejected the literal Biblical interpretation that the Earth was created in 4004 B.C. But beyond that, there were no good ways to determine the Earth’s age.
Darwin takes a crack at it by considering things like how long it must take for cliffs to erode, but really he has no more insight on this than anyone else.
He then considers the fossil record, which was not well established at the time. But he does note that the simplest forms of life appear in the lowest (oldest) geologic strata (as his ideas would predict), and that the fossil record does contain organisms that combine features of several modern groups, which is also predicted by his “tree of life.”
But Darwin reserves his main attention for the subject that is so familiar to him, and which constitutes the single most compelling line of his reasoning: the distribution of organisms, also known as biogeography.
Darwin’s argument goes something like this.
Why are there no elephants in Hawai’i? It seems like a fine place for elephants to live. If the Creator put organisms where they would do well, why not elephants in Hawai’i? It seems haphazard.
Darwin argues that there are many facts of species distribution that make little sense under the assumption of Special Creation. But they make good sense if three key factors are at play:
· A species has a definite site or region of origin.
· A species achieves a broader distribution by migration.
· A species becomes modified and gives rise to descendant species as it migrates.
To bolster this scenario, Darwin illustrates some examples, many drawn from his experiences on the Beagle.
Deserts in both Africa and South America are inhabited by plants with a particular set of characteristics. They have thick stems that store water, small or no leaves, spines, shallow roots, and other drought-adaptive features.
But the African plants are members of one plant family (Euphorbiaceae), while those in North and South America are members of the cactus family. Why would the Creator make different families of plants for exactly the same environments on two different continents?
This is well explained by the three factors listed above.
Secondly in The Origin, Darwin shows that barriers to migration seem to influence the distribution of organisms.
Creatures on either side of the Central American isthmus are different, though the environments are the same. Why?
Vast expanses of ocean also are a barrier to migration. Identical marine environments next to South America and those on islands in the Pacific are populated by very different organisms.
The only exceptions to this, as he observed on the Galapagos Islands, are organisms that can migrate long distances. Some birds are the same, especially those that normally fly long distances. The only mammals (with one exception) on the Galapagos are bats.
There are no amphibians on these islands, but there are reptiles. Reptile eggs could survive transport on mats of vegetation (which is still the way they are thought to have gotten there), but amphibian eggs could not withstand the salt water.
Thirdly, Darwin shows that organisms in the same region share an affinity, but are not quite the same; they are not the same species.
There is a rodent called the vizcacha that lives on the plains of Argentina. It has an alpine relative in the mountains nearby and two aquatic relatives. None of these are related to the beaver or the muskrat of North America. Why not?
Darwin’s explanation is that the ancestor to all three was there to provide the breeding stock in South America.
So, repeatedly, the one thing Darwin is arguing for – and which he admirably succeeded in showing in The Origin – is that the natural order of things is better explained by “descent with modification” (his euphemism for what scientists now call evolution) than by Special Creation.
The final few chapters of The Origin deal with how a number of actively researched and discussed subjects of the period are well explained by descent with modification caused by natural selection. Some of the subjects include how organisms are classified; the similarity of the embryos (the earliest stages of life before birth) of all vertebrates; and “vestigial” organs, such as the tiny bones inside snakes where their legs would be if they still had them.
An especially forceful one of Darwin’s arguments involves the front limbs of vertebrates.
There is an exact correspondence – the same number and position of bones – in the front limbs of dogs, whales, moles, birds and humans. This is despite the fact that these limbs are used for running, swimming, digging, flying and grasping, respectively.
What kind of an engineer would design limbs of such wildly differing purposes, but limit himself to exactly the same raw materials (bones)? It would be a silly thing for the Creator to do. So Darwin argued that the Creator did not do this, and instead created a natural process that would do the work without interference.
The fact that all these seemingly disparate subjects can be explained by one simple set of ideas is one of the things that gives The Origin such power. In subsequent years, this principle would be named “consilience,” and evolution is often cited as its most vivid example.
Near the end of his landmark volume, Darwin cryptically mentions humans by saying, “Much light will be thrown on the origin of man and his history.” But his intent from the start was to avoid discussing humans as part of his theories in The Origin. He would go on to publish an entire book on human evolution in 1871.
Darwin’s final words in The Origin book are famous and worth reading:
“There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone circling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved.”
This is also the only appearance in the book of the word “evolve” in any form.
In his waning years, Darwin’s health continued to decline, but his intellect, curiosity, and productivity did not. He wrote an entire book on human emotions which was among the first books to feature printed photographs.
As a botanist, I especially appreciate the investigations Darwin undertook on plants. He conducted experiments on plant movements, partially aided by his lab assistant and son, Francis. And in 1875 he published a book detailing his investigation of plants that eat insects.
Darwin also published an entire volume on flower variation in orchids. This book discussed in detail how orchids interact with insect pollinators to avoid self-fertilization. He demonstrated that orchids have a bewildering array of structures that allow them to entice and conscript different kinds of insects in moving pollen around, but all the different flowers that do this use the same combination of four flower parts. In this sense, it is the plant equivalent of using the same forelimb bones to achieve very different functions in vertebrates.
Darwin died on April 19, 1882, at the age of 73. He was buried in Westminster Abbey, close to Sir Issac Newton.
In his lifetime Darwin was surprised that his writings on human evolution were not more controversial. So why are they so controversial today?
Next week.
Next Week: Part VI – Darwin’s ideas today