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.
Darwins theory of evolution is roughly as follows:
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 Darwins research program.
The radical nature of Darwins 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 Darwins 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.
The protective belt: Darwins 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:
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 Lakatoss 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 bats 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:
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.
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
"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
"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.
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.