19^th Century Evolutionists, and Support for Darwinian Evolution

The physician Erasmus Darwin, grandfather of Charles Darwin, offered in his ‘Zoonomia’ some evolutionary speculations, but they were not further developed and had no real influence on subsequent theories. The Swedish botanist Carolus Linneus devised the hierarchical system of plant and animal classification that is still in use in a modernized form. Although he insisted on the fixity of species, his classification system eventually contributed much to the acceptance of the concept of common descent.

The great French naturalist Jean-Baptiste Lamarck held the enlightened view of his age that living organisms represent a progression, with humans as the highest form. From this idea he proposed, in the early years of the 19th century, the first broad theory of evolution. Organisms evolve through eons of time from lower to higher forms, a process still going on, always culminating in man. As organisms become adapted to their environments through their habits, modifications occur. Use of an organ or structure reinforces it; disuse leads to obliteration. The characteristics acquired by use and disuse, according to this theory, would be inherited. This assumption, later called the inheritance of acquired characteristics, was thoroughly disproved in the 20th century. Although his theory did not stand up in the light of later knowledge, Lamarck made important contributions to the gradual acceptance of biological evolution and stimulated countless later studies.

Darwin’s Supporters and Predecessors:

Ernst Haeckel, the German biologist, was influenced both by the German idealistic tradition and by the works of Darwin. After reading Origin of Species, Haeckel became one of the more prolific and vociferous supporters of evolution, but was less supportive of natural selection as the mechanism by which evolution occurred. Haeckel was certainly an evolutionist but less so a Darwinian.

An extremely common misperception is that natural selection and evolution are the same thing. In fact, Haeckel is one of many thinkers who believed that all species were historical entities (lineages) but did not share Darwin's enthusiasm for natural selection as the main mechanism for generating the diversity of the biological world. Haeckel instead believed that the environment acted directly on organisms, producing new races. The survival of the races did depend on their interaction with the environment, a weak form of natural selection. Haeckel's mechanism of change required that formation of new characters diagnostic of new species occurred through progressive addition to the developmental trajectory. For example, most metazoans go through a developmental stage called a gastrula -- a ball of cells with an infolding that later forms the gut. Haeckel thought that at one time an organism called a "gastraea" existed that looked much like the gastrula stage of ontogeny. This hypothesized ancestral metazoan gave rise to the rest of the multi-cellular animals.

Haeckel also gave significant popular support to evolutionary theory with his elaborate embryo set drawings, in which embryos of different organisms were drawn side by side at different developmental stages, to show that their evolutionary history (phylogeny) was recapitulated by their embryological development.

The "law of recapitulation" has been discredited since the beginning of the twentieth century. Experimental morphologists and biologists have shown that there is not a one-to-one correspondence between phylogeny and ontogeny. This was in part shown when it was realized that Haeckel had ‘adjusted’ many of his embryo drawings to increase the apparent similarities. Although a strong form of recapitulation is not correct, phylogeny and ontogeny are intertwined, and many biologists are beginning to both explore and understand the basis for this connection.

"Lamarckism" or "Lamarckianism" is now often used in a rather derogatory sense to refer to the theory that acquired traits can be inherited. What Lamarck actually believed was more complex: organisms are not passively altered by their environment. Instead, a change in the environment causes changes in the needs of organisms living in that environment, which in turn causes changes in their behavior. Altered behavior leads to greater or lesser use of a given structure or organ; use would cause the structure to increase in size over several generations, whereas disuse would cause it to shrink or even disappear. This rule -- that use or disuse causes structures to enlarge or shrink -- Lamarck called the "First Law" in his book Philosophie zoologique. Lamarck's "Second Law" stated that all such changes were heritable. The result of these laws was the continuous, gradual change of all organisms, as they became adapted to their environments; the physiological needs of organisms, created by their interactions with the environment, drive Lamarckian evolution.

While the mechanism of Lamarckian evolution is quite different from that proposed by Darwin, the predicted result is the same: adaptive change in lineages, ultimately driven by environmental change, over long periods of time. It is interesting to note that Lamarck cited in support of his theory of evolution many of the same lines of evidence that Darwin was to use in the Origin of Species. Lamarck's Philosophie zoologique mentions the great variety of animal and plant forms produced under human cultivation (Lamarck even anticipated Darwin in mentioning fantail pigeons!); the presence of vestigial, non-functional structures in many animals; and the presence of embryonic structures that have no counterpart in the adult. Like Darwin and later evolutionary biologists, Lamarck argued that the Earth was immensely old. Lamarck even mentions the possibility of natural selection in his writings, although he never seems to have attached much importance to this idea.

It is even more interesting to note that, although Darwin tried to refute the Lamarckian mechanism of inheritance, he later admitted that the heritable effects of use and disuse might be important in evolution. In the Origin of Species he wrote that the vestigial eyes of moles and of cave-dwelling animals are "probably due to gradual reduction from disuse, but aided perhaps by natural selection." Lamarckian inheritance, at least in the sense Lamarck intended, is in conflict with the findings of genetics and has now been largely abandoned -- but until the rediscovery of Mendel's laws at the beginning of the twentieth century, no one understood the mechanisms of heredity, and Lamarckian inheritance was a perfectly reasonable hypothesis. Several other scientists of the day subscribed to the theory of use and disuse -- in fact, Erasmus Darwin's evolutionary theory is so close to Lamarck's in many respects that it is surprising that, as far as is known now, the two men were unaware of each other's work.

In several other respects, the theory of Lamarck differs from modern evolutionary theory. Lamarck viewed evolution as a process of increasing complexity and "perfection," not driven by chance; as he wrote in Philosophie zoologique, "Nature, in producing in succession every species of animal, and beginning with the least perfect or simplest to end her work with the most perfect, has gradually complicated their structure." Lamarck did not believe in extinction: for him, species that disappeared did so because they evolved into different species. If this goes on for too long, it would mean the disappearance of less "perfect" organisms; Lamarck had to postulate that simple organisms, such as protists, were constantly being spontaneously generated. Yet despite these differences, Lamarck made a major contribution to evolutionary thought, developing a theory that paralleled Darwin's in many respects. Rediscovered in the middle part of the 19th century, his theories finally gained the attention they merited. His mechanism of evolution remained a popular alternative to Darwinian selection until the beginning of the 20th century; prominent scientists like Edward Drinker Cope adopted Lamarckianism and tried to apply it to their work. Though his proposed mechanism eventually fell out of favor, he broke ground in establishing the fact of evolution.

Support for Darwinian evolution:

Embryology and Comparative Anatomy:

Darwin and his followers found support for evolution in the comparative study of embryology. Chordates develop in ways that are remarkably similar during early stages, but they become more and more differentiated as the embryos approach maturity. The similarities persist longer between organisms that are more closely related than between those less closely related. Common developmental patterns reflect evolutionary proximity. Lizards and humans share developmental patterns inherited from their common ancestor; the inherited pattern was modified only as the separate descendant lineages evolved in different directions. The common embryonic stages of the two creatures reflect the constraints imposed by this common inheritance, which prevents changes that have not been necessitated by their diverging environment and way of life.

Human and other non-aquatic 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.

A close evolutionary relationship between organisms that appear drastically different as adults can sometimes be recognized by their embryonic homologies. Barnacles are sedentary crustaceans with little apparent likeness to such crustaceans as lobsters, shrimps, or copepods. Yet barnacles pass through a free-swimming larval stage, the nauplius, which is unmistakably similar to other crustacean larvae.

Embryonic rudiments that never fully develop, such as the gill slits in humans, are common in all sorts of animals. Some, however, like the tail rudiment in humans, persist as adult vestiges reflecting evolutionary ancestry. The most familiar rudimentary organ in humans is the appendix. This worm-like structure attaches to the cecum, which is located at the point where the ileum and colon join. The human vermiform appendix is a functionless vestige of a fully developed organ present in other mammals, such as the rabbit and other herbivores, where a large cecum and appendix store vegetable cellulose to enable its digestion with the help of bacteria. Vestiges are instances of imperfections that argue against creation by design but are fully understandable as a result of evolution.

Comparative anatomy investigates the homologies, or inherited similarities, among organisms in bone structure and in other parts of the body. The correspondence of structures is typically very close among some organisms--the different varieties of songbirds, for instance--but becomes less so as the organisms compared are less closely related in their evolutionary history. The similarities are less detailed between mammals and birds than they are among mammals, and less yet between mammals and fishes. Similarities in structure, therefore, not only manifest evolution but also help to reconstruct the phylogeny, or evolutionary history, of organisms.

An explanation of why most organismic structures are not perfect is also revealed by comparative anatomy. Like the forelimbs of turtles, horses, humans, birds, and bats, an organism's body parts are less than perfectly adapted because they are modified from an inherited structure rather than designed from completely "raw" materials for a specific purpose. The imperfection of structures is evidence for evolution and against design.

Paleontology and the Fossil Record:

Radioactive dating indicates that the Earth was formed about 4,500,000,000 years ago. The earliest fossils resemble prokaryotic microorganisms such as bacteria and blue-green algae; the oldest ones appear in rocks 3,500,000,000 old. The oldest (post Ediacaran) animal fossils, about 700,000,000 years old, come from small wormlike creatures with soft bodies and skeletal plates (the small shelly fauna). Numerous fossils belonging to many living phyla and exhibiting mineralized skeletons appear in rocks about 570,000,000 years old (Burgess shale). These organisms are different from organisms living now and from those living at intervening times. Some are so radically different that paleontologists have created new phyla in order to classify them. The first craniate chordates appeared about 400,000,000 years ago; the first mammals less than 200,000,000 years ago. The history of life recorded by fossils presents compelling evidence of evolution.

The lower jaw of all non synapsid tetrapods contains several bones, that of mammals only one; the other bones in the diapsid jaw evolved into bones now found in the mammalian ear. Paleontologists discovered two transitional forms of therapsids with a double jaw joint--one joint consisting of the bones that persist in the mammalian jaw, the other composed of the quadrate and articular bones, which eventually became the hammer and anvil of the mammalian ear.

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 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 continents of the world have their own distinctive fauna and flora. In Africa there are rhinoceroses, hippopotamuses, lions, hyenas, giraffes, zebras, lemurs, monkeys with narrow noses and non-prehensile tails, chimpanzees, and gorillas. South America, which extends over much the same latitudes as Africa, has none of these animals but has different ones: pumas, jaguars, tapirs, llamas, raccoons, opossums, armadillos, and monkeys with broad noses and large prehensile tails.

These vagaries of biogeography are not due solely to the suitability of the different environments. There is no reason to believe that South American animals are not well suited to live in Africa or those of Africa to live in South America. The Hawaiian Islands are no better suited than other Pacific islands for Drosophila flies, nor are they less hospitable than other parts of the world for many absent organisms. In fact, although no large mammals are native to the islands, pigs and goats have multiplied there as wild animals since being introduced by humans. This absence of many species from a hospitable environment in which an extraordinary variety of other species flourishes can be explained by the theory of evolution, which holds that species can exist and evolve only in geographic areas that were colonized by their ancestors.

Molecular Biology:

It is now known that DNA and the enzymes that govern all life processes hold information about an organism's ancestry. This information has made it possible to reconstruct evolutionary events that were previously unknown and to confirm and adjust the view of events that already were known. The precision with which events of evolution can be reconstructed is one reason the evidence from molecular biology is so compelling. Another reason is that molecular evolution has shown all living organisms, from bacteria to humans, to be related by descent from common ancestors.

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.

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 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.

© 2001 Patrick Mellor, University of Oxford.

1. Metazoans: Animals which have many cells and have a digestive cavity - back
2. Ontogeny: Life history of individual organism - back
3. Protists: Single and multicellular organisms that are plant-like, animal-like and fungi-like - back
4. Chordates: Large phylum comprising the animals that possess a rod of flexible tissue, which is protected in higher forms by a vertebral column - back
5. Homologies: Similarities in DNA or protein sequences between individuals or between species - back
6. Prokaryotic: Having cells that lack membrane-bound nuclei - back
7. Phyla: A primary division of a kingdom, as of the animal kingdom, ranking next above a class in size - back
8. Synapsid: Extinct reptile having a single pair of lateral temporal openings in the skull - back
9. Tetrapod: Having four feet, legs, or leglike appendages - back
10. Diapsid: Any of various reptiles having a skull with two pairs of temporal openings and including the lizards, snakes, crocodiles, and dinosaurs - back
11. Therapsids: Extinct mammal-like reptiles found inhabiting all continents from the mid Permian to late Triassic - back
12. Prehensile: Adapted for seizing, grasping, or holding, especially by wrapping around an object - back
13. Nucleotides: The basic building blocks of nucleic acids - back

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