Evolutionary Theory

Last modified on Friday, October 02, 1998, by Malcolm R. Forster
(Extracted in part from "Philosophy of Biology" by James G. Lennox)

Fact versus theory:

Fact of evolution = the fact that evolution has occurred.

Theory of evolution = an explanation about how and why evolution has occurred.

Note: Darwin did help establish the fact of evolution. However, the fact of evolution was already accepted by some prominent biologists prior to Darwin, so this is not his most important claim to fame.

Darwin’s theory of evolution is roughly as follows:

  1. The struggle for survival: Biological organisms have more offspring than can possibly survive.
  2. Inheritability: Biological organisms inherit some of their traits from their ancestors and pass them on to their descendents.
  3. Variation: The inheritable traits of biological organisms vary, even within the same species.
  4. Differential fitness: Some inheritable traits will be more advantageous than others in the struggle for survival.

Therefore, there has been and will continue to be, on average, a (natural) selection of those organisms that have advantageous traits that will lead to the evolution of species.

This is what Lakatos would call the hard core of Darwin’s research program.

The radical nature of Darwin’s theory

The fact of evolution is radical enough, especially in light of the extremely recent arrival of homo sapiens. It questions the primacy of our place of the universe. However, the Darwin’s theory is even more radical than that. Darwin delayed publication of his theory for many years; in fact he only published when he discovered that Wallace was about to publish the same theory. The probable reason for his delay was a fear of the controversy his theory would provoke.

  1. Darwin’s theory undermines one of best theological arguments for the existence of God: If you come across a watch, and observed its intricate design, then it is reasonable to infer the existence of a watchmaker. If you come across a biological organism that has an even more intricate design, then it is reasonable to infer the existence of a Creator. This is called the argument from design. Evolutionary theory provides an alternative explanation. Moreover, the imperfection of many designs is evidence for evolution and against design.
  2. Darwin’s theory does not imply that evolution is progressive. Evolution has no predetermined direction. The direction of evolutionary change depends on the local environment at the time. There is no progression from inferior to superior organisms implied by the theory. In fact, immoral traits like ruthlessness and violence are often rewarded in evolution. In particular, there is no implication that homo sapiens is superior to other species. There is no final cause directing evolutionary change. Evolution is not teleological or goal directed.
  3. Darwin’s theory supports a materialistic world view (nothing to do with materialism in the consumer sense). The view that humans have souls, while animals do not, finds no place in Darwin’s theory. It supports the view that the only things in the universe are material things.

The protective belt: Darwin’s theory predicts (or postdicts) that evolution has occurred. However, the theory, by itself, does not say which traits are inheritable, nor how they vary, or the way in which resources are limited, or how the different traits aid in survival, or how all these factors change over time.

A model of a particular episode or instance of evolutionary change will add specific details to the hard core assumptions, concerning:

  1. the range of inheritable traits in a biological population(s).
  2. the environment, and how it changes over time.
  3. the relative benefit that these traits confer to the members of the populations possessing them in the various environments (fitness values).

Accommodation versus prediction: Some of these details may be introduced as parameters (e.g., fitness parameters), which are inferred backwards from facts to be explained. But if all of these details are inferred from the facts to be explained, then the theory is merely accommodating the facts, and there are no predictions (or postdictions). In that case, evolutionary theory would be pseudoscientific according to Lakatos’s demarcation criterion.

The positive heuristic, if it exists, should suggest how these details should be filled in should the most plausible assumption fail to accommodate the observational facts.

Definition: A homology is a similarity between or amongst different species that is the result of their common ancestry. There are at least 3 kinds of homologies: (a) Structural homologies such as the similarity between the bone structure in a bat’s wing and our forearm. (b) Behavioral homologies arising for instincts inherited from a common ancestor. (c) Protein homologies in which the sequences of amino acids that for a protein are similar because of common ancestry. Similarities that do not arise from common ancestry, such as the wing of a bird and the wing of an insect, are called analogies.

Kinds of evidence for evolution:

  1. The fossil record. The facts of evolution, or the story of evolution has been pieced together mainly from the fossil record. Example 1 is an example of Darwin’s theory may explain such evidence.
  2. Homologies in living species. If we look carefully at living species we see surprising and unexpected similarities (Lakatos’s novel facts?) between disparate species. These may range from similar bone structures in limbs of whales and bears, to similarities of the instinctive behavior of water fowl living in quite different areas of the globe, to similarities and differences in protein sequences in living organisms.
  3. Artificial selection. Breeding experiments that show that selection can transform a population from one kind to another. This provides limited support for inheritability and variation. It does not provide evidence for the other postulates.
  4. Experimental evidence from genetics. There is now a lot of biochemical evidence about DNA and the mechanisms of inheritance. This helps provide auxiliary hypotheses about the source of inheritable variation, and the mechanisms of inheritance. For example, it could help support Assumption 3 in Example 1.

Let's at an example of evolutionary explanation.

Example 1: The evolution of horses.

Support: There is independent fossil evidence confirming that grasses evolved and become abundant at that time.

Support: Look at modern species and assume that the same physical facts apply for all time. Also note that there was another horse lineage which continued to grace on leaves and did not evolve an elongated tooth.

Support: Look at modern horse species to check for variation and check for inheritability through artificial breeding experiments.

Support: It is almost a matter of logic to say that a horse that can eat leaves or grass has more food available to it than a horse that can only eat leaves.

Support: Look at fossil evidence for grasses; compare "grazability" of modern grasses.


  1. It is very hard to think of all the auxiliary assumptions need to deductively entail the fact to be explained.
  2. Most of the assumptions have independent sources of support. It will not matter if some of the assumptions have no independent support, since some degree of accommodation is allowed.
  3. Some of the assumptions talk about other instances of evolution. Does this beg the question? No, at worst it shows that the explanations need to be evaluated as a whole. That is why Darwin's Origin was convincing in a way that previous arguments for evolution were not--it tied together many different explanations that appealed to the same core assumptions.
  4. What matters is that various facts are tied together in a surprisingly tidy way, which is hard to explain as "arising from chance".

Figure 1: Phylogeny based on differences in the protein sequence of cytochrome c in organisms ranging from Neurospora mold to humans.

"The Theory of Evolution: Patterns and rates of species evolution: MOLECULAR EVOLUTION: The molecular clock of evolution." Britannica Online.



Figure 2: Rate of nucleotide substitution over paleontological time. Each dot marks (1) the point at which a pair of species diverged from a common ancestor and (2) the number of nucleotide substitutions, or protein changes, that have occurred since the divergence. The solid line drawn from the origin to the outermost dot gives the average rate of substitution.

From F.J. Ayala, E. McMullin (ed.), Evolution and Creation (1985)

"The Theory of Evolution: Patterns and rates of species evolution: MOLECULAR EVOLUTION: The molecular clock of evolution." Britannica Online.


 VESTIGES OF EVOLUTION: "Human and other nonaquatic embryos exhibit gill slits even though they never breathe through gills. These slits are found in the embryos of all vertebrates because they share as common ancestors the fish in which these structures first evolved. Human embryos also exhibit by the fourth week of development a well-defined tail, which reaches maximum length when the embryo is six weeks old. Similar embryonic tails are found in other mammals, such as dogs, horses, and monkeys; in humans, however, the tail eventually shortens, persisting only as a rudiment in the adult coccyx."
"The Theory of Evolution: The evidence for evolution: EMBRYONIC DEVELOPMENT AND VESTIGES" Britannica Online.

BIOGEOGRAPHY: "Darwin also saw a confirmation of evolution in the geographic distribution of plants and animals, and later knowledge has reinforced his observations. For example, there are about 1,500 species of Drosophila vinegar flies in the world; nearly one-third of them live in Hawaii and nowhere else, although the total area of the archipelago is less than one-twentieth the area of California. There are also in Hawaii more than 1,000 species of snails and other land mollusks that exist nowhere else. This unusual diversity is easily explained by evolution. The Hawaiian Islands are extremely isolated and have had few colonizers; those species that arrived there found many unoccupied ecological niches, or local environments suited to sustain them and lacking predators that would prevent them from multiplying. In response, they rapidly diversified; this process of diversifying in order to fill in ecological niches is called adaptive radiation."
"The Theory of Evolution: The evidence for evolution: BIOGEOGRAPHY" Britannica Online.

MOLECULAR EVOLUTION: A remarkable uniformity exists in the molecular components of organisms--in the nature of the components as well as in the ways in which they are assembled and used. In all bacteria, plants, animals, and humans, the DNA comprises a different sequence of the same four component nucleotides, and all of the various proteins are synthesized from different combinations and sequences of the same 20 amino acids, although several hundred other amino acids do exist. The genetic "code" by which the information contained in the nuclear DNA is passed on to proteins is everywhere the same. Similar metabolic pathways are used by the most diverse organisms to produce energy and to make up the cell components.

This unity reveals the genetic continuity and common ancestry of all organisms. There is no other rational way to account for their molecular uniformity when numerous alternative structures are equally likely. The genetic code may serve as an example. Each particular sequence of three nucleotides in the nuclear DNA acts as a pattern, or code, for the production of exactly the same amino acid in all organisms. This is no more necessary than it is for a language to use a particular combination of letters to represent a particular reality. If it is found that certain sequences of letters--planet, tree, woman--are used with identical meanings in a number of different books, one can be sure that the languages used in those books are of common origin.

Genes and proteins are long molecules that contain information in the sequence of their components in much the same way as sentences of the English language contain information in the sequence of their letters and words. The sequences that make up the genes are passed on from parents to offspring, identical except for occasional changes introduced by mutations. To illustrate, assume that two books are being compared; both books are 200 pages long and contain the same number of chapters. Closer examination reveals that the two books are identical page for page and word for word, except that an occasional word--say one in 100--is different. The two books cannot have been written independently; either one has been copied from the other or both have been copied, directly or indirectly, from the same original book. Similarly, if each nucleotide is represented by one letter, the complete sequence of nucleotides in the DNA of a higher organism would require several hundred books of hundreds of pages, with several thousand letters on each page. When the "pages" (or sequence of nucleotides) in these "books" (organisms) are examined one by one, the correspondence in the "letters" (nucleotides) gives unmistakable evidence of common origin.

The arguments presented above are based on different grounds, although both attest to evolution. Using the alphabet analogy, the first argument says that languages that use the same dictionary--the same genetic code and the same 20 amino acids--cannot be of independent origin. The second argument, concerning similarity in the sequence of nucleotides in the DNA or the sequence of amino acids in the proteins, says that books with very similar texts cannot be of independent origin.

The evidence of evolution revealed by molecular biology goes one step further. The degree of similarity in the sequence of nucleotides or of amino acids can be precisely quantified. For example, cytochrome c (a protein molecule) of humans and chimpanzees consists of the same 104 amino acids in exactly the same order; but differs from that of rhesus monkeys by one amino acid, that of horses by 11 additional amino acids, and that of tuna by 21 additional amino acids. The degree of similarity reflects the recency of common ancestry. Thus, the inferences from comparative anatomy and other disciplines concerning evolutionary history can be tested in molecular studies of DNA and proteins by examining their sequences of nucleotides and amino acids.

The authority of this kind of test is overwhelming; each of the thousands of genes and thousands of proteins contained in an organism provides an independent test of that organism's evolutionary history. Not all possible tests have been performed, but many hundreds have been done, and not one has given evidence contrary to evolution. There is probably no other notion in any field of science that has been as extensively tested and as thoroughly corroborated as the evolutionary origin of living organisms.
"The Theory of Evolution: The evidence for evolution: MOLECULAR BIOLOGY" Britannica Online. <http://www.eb.com:180/cgi-bin/g?DocF=macro/5002/24/11.html>

Homology, in biology, similarity of the structure, physiology, or development of different species of organisms based upon their descent from a common evolutionary ancestor. Homology is contrasted with analogy, which is a functional similarity of structure based not upon common evolutionary origins but upon mere similarity of use. Thus the forelimbs of such widely differing mammals as humans, bats, and deer are homologous; the form of construction and the number of bones in these varying limbs are practically identical, and represent adaptive modifications of the forelimb structure of their common early mammalian ancestors. Analogous structures, on the other hand, can be represented by the wings of birds and of insects; the structures are used for flight in both types of organisms, but they have no common ancestral origin at the beginning of their evolutionary development. A 19th-century British biologist, Sir Richard Owen, was the first to define both homology and analogy in precise terms.
"homology" Britannica Online.

Figure 3: The labellum of the mirror ophrys (Ophrys speculum). The colouring so closely resembles that of the female wasp Colpa aurea that males of the species are attracted to the flower and pick up pollen during their attempts at copulation.

E.S. Ross

DOBZHANSKY: Between 1920 and 1935, mathematicians and experimentalists began laying the groundwork for a theory combining Darwinian evolution and Mendelian genetics. Starting his career about this time, Dobzhansky was involved in the project almost from its inception. His book Genetics and the Origin of Species (1937) was the first substantial synthesis of the subjects and established evolutionary genetics as an independent discipline. Until the 1930s, the commonly held view was that natural selection produced something close to the best of all possible worlds and that changes would be rare and slow and not apparent over one life span, in agreement with the observed constancy of species over historical time.

Dobzhansky's most important contribution was to change this view. In observing wild populations of the vinegar fly Drosophila pseudoobscura, he found extensive genetic variability. Furthermore, about 1940 evidence accumulated that in a given local population some genes would regularly change in frequency with the seasons of the year. For example, a certain gene might appear in 40 percent of all individuals in the population in the spring, increase to 60 percent by late summer at the expense of other genes at the same locus, and return to 40 percent in overwintering flies. Compared to a generation time of about one month, these changes were rapid and effected very large differences in reproductive fitness of the various types under different climatic conditions. Other experiments showed that, in fact, flies of mixed genetic makeup (heterozygotes) were superior in survival and fertility to pure types.
"Dobzhansky, Theodosius" Britannica Online.

INDUSTRIAL MELANISM: Melanism refers to the deposition of melanin in the tissues of living animals. The chemistry of the process depends on the metabolism of the amino acid tyrosine, the absence of which results in albinism, or lack of pigmentation. Melanism can also occur pathologically, as in a malignant melanoma, a cancerous tumour composed of melanin-pigmented cells.

Melanic pigmentation is advantageous in many ways: (1) It is a barrier against the effects of the ultraviolet rays of sunlight. On exposure to sunlight, for example, the human epidermis undergoes gradual tanning as a result of an increase in melanin pigment. (2) It is a mechanism for the absorption of heat from sunlight, a function that is especially important for cold-blooded animals. (3) It affords concealment to certain animals that become active in twilight. (4) It limits the incidence of beams of light entering the eye and absorbs scattered light within the eyeball, allowing greater visual acuity. (5) It provides resistance to abrasion because of the molecular structure of the pigment. Many desert-dwelling birds, for example, have black plumage as an adaptation to their abrasive habitat.

"Industrial" melanism has occurred in certain moth populations, in which the predominant coloration has changed pale gray to dark-coloured individuals. This is a striking example of rapid evolutionary change; it has taken place in less than 100 years. It occurs in moth species that depend for their survival by day on blending into specialized backgrounds, such as lichened tree trunks and boughs. Industrial pollution, in the form of soot, kills lichens and blackens the trees and ground, thus destroying the protective backgrounds of light-coloured moths, which are rapidly picked off and eaten by birds. Melanic moths, by their camouflage, then become selectively favoured. "Industrial" melanic moths have arisen from recurrent mutations and have spread via natural selection.
"melanin" Britannica Online.