What Darwin Got Right (and Wrong) About Evolution

Anagenesis is the technical term for an evolutionary change in a group in which one species replaces another but branching into separate species does not take place. It can be argued that as a species travels through time, it continually adapts to its environment. The traits of individuals that do not survive long enough to reproduce fade from the species. Over time, observable changes (in size, coloration, or other traits) might appear as natural selection operates within the species. Hundreds of generations later, the species will be different from what it once was, but no new branches of the species’ evolutionary path will have been created.

Speciation, the creation of new and distinct species in the course of evolution, is simply an extension of anagenesis, but with branching allowed. Speciation also involves natural selection, but it is most easily seen in populations. If one or more populations are isolated from the rest of a species over many generations (and members of each isolated population breed only with one another), each population may become distinct from the original species. Each isolated population may face a unique set of environmental conditions that the population will need to adapt to. If so, each population may evolve differently. A population in a warmer environment might develop adaptations to survive the heat, whereas a population in a colder, wetter environment might develop a different set of adaptations to deal with colder, wetter conditions. After many generations, adaptations may arise in members of one population that might keep them from breeding successfully with members of other populations. These adaptations could be physical (such as changes in size, coloration, or body chemistry), or they could be behavioral (such as in the development of courtship dances or mating calls). Over time, the differences between the isolated populations may become so great that each population becomes a new species.

One of the hallmarks of good theory construction is the use of separate lines of evidence as proof. To lend support to his theory of natural selection, Darwin took examples from biogeography, paleontology, embryology, and morphology. He noted several examples of “closely-allied species” (that is, closely related species that likely descended or branched off from a common parent species) inhabiting the same territory or adjacent territories. He noted that different zebra species were found together on the plains of East Africa and, in perhaps his most famous example, that several living species of Galapagos finches co-occurred in the Galapagos Islands—a cluster of isolated islands in the eastern Pacific Ocean. The pattern of such closely related species in space supported the idea that these species had a similar origin. Darwin also noticed patterns of closely related species clustering in time. The fossil record showed several examples of similar-looking species occurring next to one another in the same layer or in successive layers of rock. Evidence of the influence of natural selection also appeared in developing embryos, where structures observed during the early stages of development of the higher vertebrates (fishes, amphibians, reptiles, birds, and mammals) resembled the structures of more-primitive animals.

Darwin also leveraged morphology (that is, the general aspects of biological form and arrangement of the parts of a plant or an animal) to support his theory. Taxonomy, the classification of different forms of life, is rooted in the observable traits that group individual living things into species, genus, family, and so on. Generally speaking, the more traits different forms of life share, the closer their evolutionary relationship is. Through the process of taxonomy (which involves comparing the observable traits of living forms with the same kinds of traits in fossils), one can develop a decent understanding of the ways different lines of plants, animals, and other forms of life emerged across time.

During the 19th century the Bible (not the fossil record) was widely considered the primary authority on Earth’s age. It held that Earth was only about 6,000 years old. Most scientists of the time, however, acknowledged that Earth was certainly older. By the early 1860s, just a few years after On the Origin of Species was published, Scottish engineer and physicist William Thomson (later, Lord Kelvin) pointed out that Earth loses heat by thermal conduction and that geologic processes may have changed as a consequence. Furthermore, Thomson concluded that this cooling placed an upper limit on Earth’s age, which he believed to be less than 100 million years old. This notion was soon embraced by many other scientists, including Darwin—in part because his own son, George, who was an astronomer, had also calculated Earth's age as being many tens of millions of years old. Darwin did not think that 6,000 years was enough time for life to have diversified and evolved into its various forms according to natural selection theory. A span of 100 million years, however, seemed more plausible to him. Although Darwin seems to have been on the right track about Earth’s age, modern instruments have shown that Earth is 4.5 billion years older than William Thomson’s (and George Darwin’s) calculations.

Although Darwin’s theory of natural selection was basically correct, in the late 1860s he proposed a theory that was very wrong. That theory—”pangenesis”—was an attempt to explain variation among individuals in a species. Offspring in sexual species display a mix of traits from both of their parents. Siblings look different from one another, but they also share features. Largely on the basis of Austrian botanist Gregor Mendel’s work, we know that traits are produced by genes—specifically, alleles (any one of two or more genes that may occur alternatively at a given site on a chromosome). Genes make up the DNA blueprints of all life-forms, determining such physical traits as eye color and the risk of developing certain diseases. According to Darwin’s pangenesis, however, “gemmules” were the seeds of cells, supplied by each parent during conception. Gemmules were produced by all the organs and other structures in the body of each parent. The gemmules from the mother and the father would mix with one another in the fertilized egg. If there were enough of these seed cells and if they developed in the proper way, the offspring would be healthy and viable. Birth defects, such as an underdeveloped organ, resulted either from a lack of gemmules provided by that same organ in the parents’ bodies or from a linkup between the wrong gemmules to build that organ. Darwin also posited that children bore a stronger resemblance to one parent than to the other because the gemmules coming from one parent may be stronger, better adapted, or more numerous than those coming from the other parent. But Darwin’s cousin Sir Francis Galton, in an experiment using rabbit blood, failed to find gemmules, so the theory was dismissed.