Darwin's Theory of Evolution

Notation:

• This is something predicted or explained by the theory

This is an assumption, presupposition, hypothesis, component, law, or explanatory strategy of the theory

The distinction is sometimes a bit arbitrary. While I find it useful for expository purposes, it probably should not be taken too seriously.

Since the main purpose of this class is not learning biology, but how scientific theories originate and become accepted, I focus not on present-day evolutionary theory but the theory Darwin originally presented. I shall say a bit about the modem synthesis at the end, and I shall probably slip into anachronism a few times, but, by and large, it is my intention to stick to the original Darwinian theory.

Darwin sailed as a naturalist on the Beagle for five years, from 1831 to 1836. He observed some things he found striking (Darwin's finches, Galapagos islands). On his return to England, he, by his own account, spent five years on background reading and study, before working on an explanation. His theory was more or less complete by 1844, and he spent the next decade and a half writing it up carefully. One suspects that he, like Copernicus, might have published only on his deathbed, but Wallace came up with a similar theory, and Darwin was forced to publish (a benefit of the structure of scientific reward). He calls the Origin of Species an "abstract," but it was (first edition) over 500 pages. The full version, never completed, ran to over 700 pages.

Since species were widely thought to have been independently created in Darwin's day, the big news was that species have an origin (hence, the title), but the theory is much broader: the title might be, The Origin of
Kingdoms, Phyla, Classes, Orders, Families, Genera, Species, and Varieties (134).
The terminology suggests the tree of life, and the theory explains why there is such a thing.

• Tree of life (134)
• Form of trees (134–135)

Theories committed to independent generation or allied ideas, including intelligent design, cannot explain those phenomena.

Darwin took his main innovation to be "insight into the means of modification and coadaptation" (96). He derives it from his study of the results of human breeding of domesticated plants and animals (96). We learn from that study that

Selection:

Living creatures are subject to, usually small, variations.

Such variations are frequently inheritable in quite specific ways:
     They can apply only to specific parts of the life cycle (114–115).
     They can apply to only one sex (115).

Breeders systematically prefer, whether consciously or unconsciously, some variations over others.

Since such effects are cumulative, they explain

• The origin of domestic varieties.

Examples: pigeons, dogs, horses, chickens, ducks, rabbits, cattle, apples, sheep, camels. The variations between domestic breeds are frequently as large as the differences between natural species. (Example, mine, not Darwin's: Chihuahuas and Saint Bernards.) Intermediate forms are often known.

Natural selection:

All living things are in a constant struggle to survive and reproduce. Of necessity most fail.

As a result, even the most trivial advantages are critical.

Since selection effects are cumulative over huge time periods, constant, and efficient, they explain

• The origin of natural varieties.

If that is indeed the origin of natural varieties, then it must be the case that

• Every stable feature of a natural variety is of direct benefit to the survival or reproductive capacity of the organisms of that variety.
• Natural varieties will be highly coadapted.
  

Thus, distinct varieties will be suited for different conditions of life.

• Protective coloration is an obvious example (113–114).

Since species that have more members will have more variations (just because there are more chances); since species that live in many areas and hence are exposed to many different conditions in which specialized changes might be useful; and since common and widely diffused species must already have some advantages in survival and reproduction

• Dominant species (those that are common, that have wide ranges, or are widely diffused within their range) will have the most varieties.
  

Since the book is "only an abstract" Darwin offers no evidence for the claim, but he in fact had compiled extensive tables.

• Coevolution of species will be common. (Flowers and bees (118&ndash119). Bees and orchids. Ants and aphids.)

So far, everything I have mentioned is compatible with intelligent design, though none of it is predicted or suggested by intelligent design. What is even worse for intelligent design, it suggests no alternative explanations.

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But Darwin goes further: he extends his claims up the tree of life. At levels above varieties, the situation of the traditional intelligent design theory is much worse: Advocates of intelligent design must reject Darwin's explanations, and have no alternative.

The same process that forms distinct varieties, operating over a long period, forms genera, over an even longer period, families, orders, classes, phyla, and even kingdoms.

Thus,

• Well-marked varieties are incipient species (106, 124).
• There is no definite distinction between varieties and species (106–107).

There are often disputes about whether two related groups are to be classified as distinct species or mere varieties of a single species. If they are classed as distinct species, then the "single species" becomes a genus, and the indefiniteness propagates up the classificatory hierarchy.

• If there are many species of a particular genus in a given area, then those species will have many varieties (107).
• Dominant species will usually be members of large genera,
  therefore

Large genera will tend to became even larger (107).

Why does natural selection through the struggle for existence lead to increasing degrees of difference? The gradual, incremental character of the process might lead one to expect it to result in a hodge-podge of closely related forms, instead of distinct species, themselves grouped into distinct genera, and so up the tree of life. Darwin's explanation is that

More diversified types of organisms can make use of more diversified habitats and so increase in number (125).

More closely related organisms and those with similar forms of life are most acutely competitors for survival (130).

That will apply to organisms with a common food source, ones that use the same types of nesting areas, that grow in the same soil types, that are parasitic on the same hosts, and so forth. As a result of acute competition, when there is a series of closely related varieties in the same area, there will be a tendency for the intermediate ones to become extinct, leaving only the ones at the extremes of difference. Thus, we should expect to find the varieties of a single large species to be arranged in clusters of closely allied varieties, each with a restricted range, and that an analogous fact will be true of large genera (107).

Darwin explains the branching structure of real trees in the same way (135).

Just as dominant species will have the most varieties, since they have the greatest chance at variation,

Dominant genera will have the most species, and so forth, up the tree of life.

Dominant species will, because they vary more and are exposed to more conditions, tend to become more dominant displacing and causing the extinction of other species. The analogous argument goes through at all levels of the tree (133, 134).

As a result,

• We can expect to see fewer genera than species, fewer families than genera, and so forth.
• We can expect organisms from larger habitats to increase in fitness faster than those in small habitats (121–123, 126–127).
  • Organisms on small, isolated islands and in small, isolated bodies of water (freshwater lakes) should exhibit less variety and more ancient forms than those in other places.
    They should be good places to look for "living fossils." Darwin's finches.
  • Transitional habitats should not have unique species (136–139).
    That is a special case of why we should not expect to see a continuum of living forms.
  • Exotic organisms should tend to dominate when introduced from a larger habitat into a smaller one (110).
    Example: Rabbits in Australia.
    The frequent success of exotic species falsifies some of the very few predictions that can be made on the basis of Intelligent Design:
• Darwinian evolution predicts that organisms from large habitats will be more fit than those from small habitats, even in the small habitats; Intelligent Design predicts—if it predicts anything at all—that species will always be the best possible to inhabit their native habitat.
• Darwinian evolution predicts that organisms will not be ideally suited to their respective forms of life, that there will be room for improvement and hence for displacement and extinction by exotics; Intelligent Design predicts—if it predicts anything at all—that species will always be perfectly suited to their native habitat.

This is as good a place as any to list a number of other correct predictions of Darwinian evolutionary theory that Intelligent Design gets wrong.

• Ontogeny recapitulates phylogeny (157).
• Rudimentary organs (157).
• Common design that makes no contribution to fitness (155–156).
• Homologous organs that make different contributions to fitness (146, swim bladder to lung).

The first three items on that list are Darwin's chief kinds of evidence that humans are descended from other species just like any other organism (175, 178)—a point he barely mentions in The Origin of Species, but makes in detail in The Descent of Man.

Sexual Selection

Sexual selection is a special kind of struggle for survival: the struggle between members of a single species of the same sex for mates; a special case of the struggle to reproduce.

Variations may be inherited by only a single sex, and only appear in sexually mature adults (115).

The preference of members of one sex for some characteristic of the other can select for that characteristic.
Example: Bright plumage in male birds (116).

A characteristic that confers an advantage within one sex in competing for mates can select for that characteristic.
Example: The lion's mane (116).

Kinship Selection

Evolutionary theory suggests that the unit of selection is not the individual organism, but the group of genetically related individuals. It may therefore make biological sense for an organism to sacrifice its life or its ability to reproduce to ensure the survival of a closely related individual. British biologist J.B.S. Haldane once joked that he wouldn't give his life to save his brother, only to save two brothers or eight cousins. Darwin used the idea of kinship selection to explain insect colonies with only one female capable of reproduction.

Here is a more controversial, but easier to explain, example of kinship selection. In some species of parrots, there are no obvious, visible distinctions between males and females. Some such parrots mate for life, and half of their unions are homosexual. That is the same fraction you would expect if the parrots mated without regard to gender, and suggests the obvious hypothesis that the parrots can't distinguish their genders either. How is such a thing biologically possible? Parrots raise their chicks in large groups of close relatives. Chicks require a tremendous amount of parental care: They mature slowly, and they must learn the "language" of their community — the calls of parrots are not instinctive, but learned, passed on from one generation to the next. The number of sterile, because homosexual, unions guarantees that the ratio of adults to chicks is twice what it would otherwise be. That does not, of course, directly benefit the sterile couples, but it is a substantial benefit to their close relatives of the next generation. The individual loses out, but the genetic and cultural inheritance of the individual does not.

Objections and Replies

The lack of intermediate forms

The objection is that we should find sequences of gradual change in the fossil record, if change has in fact been a slow accumulation of small bits. Darwin's reply is that even though there is a species that is the common origin of, say, a modern family, all of its descendants have continually modified since its day, and so there is no reason to expect that it will be, in any simple sense, intermediate in form between its modern descendants (148–149). The most we can really hope for is to discover a record of an era in which there is a single species identifiable as a primitive member of the modern family. We do find that, reasonably often. In a few cases, we have discovered quite long closely connected series of fossils. (Horses provide a nice example. Horse evolution in detail, Horse evolution in a single picture, The line of descendence that ends in the modern horse.) That is the most we would expect.

The dearth of fossil evidence

The objection is that the fossil record shows one system of organisms followed by another completely different system with nothing in between. That was Lyell's objection to evolution. Darwin's reply is that fossils form only rarely, under very special circumstances, and so we should expect to find "gaps" and sudden changes in the fossil record (150–151). Modern evolutionary theorists have, however, tended to favor "punctuated equilibrium," a theory that evolutionary changes happen extremely slowly, practically not at all, except following massive changes in climate, geology, mass extinctions, and the like, when it happens very rapidly. That has the result that the fossil record seems much more complete.

Organs of great perfection

This is Paley's problem, the chief "argument" for intelligent design: it is difficult to imagine how eyes (144–145), wings, and the like could have evolved in gradual steps. Evolution requires that they do arise gradually, and that every single step in the process confer added fitness. There are many puzzles of this kind that are the subject of current research, and many examples remain mysterious. There have been some notable successes. Wings, for example, even when not suited to full flight, are suited for gliding (140–141). But the key point for our purposes is that the evolutionary type of explanation is the only game in town: Intelligent Design not only provides no explanation, it cannot provide any explanation. In Popperian terms, Intelligent Design is not falsifiable: "God made it that way" explains anything equally well.

Racism and Sexism in Darwin

Darwin was a product of his time.

Darwin's examples of sexual selection seem to all have males playing an active role, females a passive one. In fact, according to his theory, females aren't passive, they are choosy (116). That was itself, in the Victorian context, more of a role for women than many were willing to grant. It is worth noting that most of his examples are perfectly correct, he just misses ones that don't fit his stereotypes. What the theory of sexual selection actually predicts is that the gender that invests the most energy in child rearing will be the choosy one. The gender that invests the most energy in child rearing is usually the female, but there are species, for example, the seahorse, where the male does most of the child rearing. In such species, it is the male who is choosy. See "Sexual selection :A tale of male bias and feminist denial" for a readable discussion of and references to both the scientific and political issues.

Much of Darwin's terminology in The Descent of Man strikes us today as extremely racist: there is a lot of talk of "savages" and contrast between "the dark coloured races" and the "white and civilized races" (187). Darwin takes the "negro" to have characteristics that suggest some similarity to the gorilla (185). It is therefore worth emphasizing that he took all humans to be of a single species that, according to his theory, the differences between different groups of people were not permanent and divinely ordained (itself a progressive idea), and that most of the occasional apelike characteristics observed in people that he describes are not associated with any racial group. It is also, however, true that any characteristic he takes to be rudimentary that is associated with darkness of skin is described as a racial characteristic, while rudimentary characteristics more common in light-skinned people are described as characteristics of some Europeans, not as, what would be parallel, characteristics of the caucasian—the one that leaps to mind is hairiness (189). Many racists have adopted evolutionary cover for their views, and so it is worth emphasizing that evolutionary theory is no more racist, and provides no more support or evidence for racism, than do the religions that have also been used as a cover for racism.

The Modern Synthesis

Today's evolutionary theory combines Darwin's ideas with knowledge of mechanisms of inheritance: genes and DNA. We use sequencing of DNA to map the tree of life, in addition to the old techniques. DNA tells us how closely related various species are: the more similar the DNA, the more similar the species. Genetic drift and gene frequency within a population have been added to natural selection on single variations as mechanisms of evolutionary change. Punctuated equilibrium has been added to Darwin's gradualism as a picture of the rate of genetic change. Which is more important is a matter of current dispute.

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Evolution in the News

IntelligentDesignMightBeMeetingItsMaker
CreationismIsaReligiousBelief
BirdsOriginatefromDinosaurs: PhotographofNewArchaeopteryxFossil
TheOtherID

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Back to the General Philosophy of Science

As we see, no single piece of evidence "proves" or "disproves" evolution by natural selection. It is firmly established because it suggests observations worth making, which usually turn out well; and provides a unified explanation of many general phenomena that would have escaped notice entirely without the theory to suggest them. No alternative theory has been proposed that has even remotely similar power to structure our observations, experiments, and explanations concerning the natural world.

-- ShaughanLavine - 30 Nov 2005 - 04 Dec 2005