* In chapter 4 of THE ORIGIN OF THE SPECIES, Darwin outlines his core concept of evolution by natural selection. In chapter 5, he begins his effort to justify the concept by outlining "laws of variation", with mixed success.
* On the first page of chapter 4 of THE ORIGIN OF SPECIES, Darwin focused on the notion of evolution by natural selection:
BEGIN_QUOTE:
Can the principle of selection, which we have seen is so potent in the hands of man, apply in nature? I think we shall see that it can act most effectually.
Let it be borne in mind in what an endless number of strange peculiarities our domestic productions, and, in a lesser degree, those under nature, vary; and how strong the hereditary tendency is. Under domestication, it may be truly said that the whole organisation becomes in some degree plastic. Let it be borne in mind how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to their physical conditions of life. Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should sometimes occur in the course of thousands of generations? If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind?
On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection.
END_QUOTE
In more modern terms, Darwin was saying that any organisms with a selective advantage would tend to dominate their populations, while those with a selective disadvantage would gradually fade out. Darwin added that "variations neither useful nor injurious would not be affected by natural selection", essentially describing what is now known (in much more detail) as "neutral evolution" -- a point that often tends to be forgotten or at least underplayed, that the spontaneous tendency of organisms to vary at random necessarily produces a good deal of arbitrary variation in form and function, only limited by the tendency of selection to trim it back. Evolution, in effect, "jumps on its horse and goes off in all directions at once", some of the directions working out better than others.
He also suggested that "a change in the conditions of life, by specially acting on the reproductive system, causes or increases variability ... and this would manifestly be favourable to natural selection, by giving a better chance of profitable variations occurring ..." In this, Darwin was hinting at the "evolution of evolution", or what could be called "evolvability", the idea that evolution could actually result in adaptations that allowed organisms to evolve more rapidly.
In any case, Darwin not only proposed that natural selection could shape organisms as did artificial selection, he emphasized again that over the longer run -- nature having vastly more time available -- it would do a better job:
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Man can act only on external and visible characters: nature cares nothing for appearances, except in so far as they may be useful to any being. She can act on every internal organ, on every shade of constitutional difference, on the whole machinery of life. Man selects only for his own good; Nature only for that of the being which she tends. Every selected character is fully exercised by her; and the being is placed under well-suited conditions of life.
Man keeps the natives of many climates in the same country; he seldom exercises each selected character in some peculiar and fitting manner; he feeds a long and a short beaked pigeon on the same food; he does not exercise a long-backed or long-legged quadruped in any peculiar manner; he exposes sheep with long and short wool to the same climate. He does not allow the most vigorous males to struggle for the females. He does not rigidly destroy all inferior animals, but protects during each varying season, as far as lies in his power, all his productions. He often begins his selection by some half-monstrous form; or at least by some modification prominent enough to catch his eye, or to be plainly useful to him.
Under nature, the slightest difference of structure or constitution may well turn the nicely-balanced scale in the struggle for life, and so be preserved. How fleeting are the wishes and efforts of man! How short his time! And consequently how poor will his products be, compared with those accumulated by nature during whole geological periods. Can we wonder, then, that nature's productions should be far "truer" in character than man's productions; that they should be infinitely better adapted to the most complex conditions of life, and should plainly bear the stamp of far higher workmanship?
It may be said that natural selection is daily and hourly scrutinising, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages, and then so imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they formerly were.
END_QUOTE
Darwin's remarks here tend towards a bit of overstatement. As discussed later, evolution works on populations, the fate of individual organisms only being relevant in terms of population averages. In practice, the winnowing of "good" versus "bad" in that population may not be particularly drastic; however, Darwin was still correct, in that even a subtle winnowing of a large population over a long period of time ends up demonstrating the power of selection.
Natural selection could only act, Darwin pointed out, for the benefit of the organism, though he added that in "social animals it will adapt the structure of each individual for the benefit of the community" but only if "each in consequence benefits from the selected change." What Darwin firmly rejected was the old notion, which still lingers today, that natural selection would promote changes in one organism purely for the benefit of another:
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What natural selection cannot do, is to modify the structure of one species, without giving it any advantage, for the good of another species; and though statements to this effect may be found in works of natural history, I cannot find one case which will bear investigation.
END_QUOTE
* There were cases where natural selection didn't seem to be very logical, for example the gaudy oversized tail of the peacock -- incidentally, the feathers grow out of the peacock's back, technically speaking they don't constitute a tail, and purists call it a "train" as in "bridal train". The peacock's tail was pretty and whimsical, something that looked much more like a liability than an asset. How could natural selection account for it?
Peahens don't have elaborate tails, which suggested the peacock's tail had some sexual purpose. Darwin puzzled over this and came up with the notion of "sexual selection":
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Thus it is, as I believe, that when the males and females of any animal have the same general habits of life, but differ in structure, colour, or ornament, such differences have been mainly caused by sexual selection; that is, individual males have had, in successive generations, some slight advantage over other males, in their weapons, means of defence, or charms; and have transmitted these advantages to their male offspring.
END_QUOTE
Peahens preferred to mate with peacocks with the most spectacular tails. Over time that meant that peacocks with ever more spectacular tails would be produced, with the process reinforced by producing peahens that liked spectacular tails. The cycle would continue until the peacock's tail started to become a liability. In modern times, it has been suggested that the peacock's tail may not be just useless showing-off for the peahens: it is also a sign to the peahens that the male is healthy, since a male who isn't in good condition will have a bedraggled tail. Some skeptics feel the "handicap principle", as it is known, is putting too fine a point on matters, but there are cases where it makes perfect sense.
In any case, although sexually-selected features tend towards the bizarre and unique, not specifically shared between different species, they are not any particular problem for evolutionary theory. It might seem like a lot of overhead just to reproduce, but the reply is: just reproduce? As Darwin pointed out in the comment on "success in leaving progeny", reproduction is the payoff of survival, mere survival of an organism being mostly pointless to the long-term evolution of a species if it doesn't reproduce.
* To Darwin, the effects of natural selection were highly evident. Consider the competition between wolf and deer: the leanest, fastest wolf would tend to catch the most deer, while the most alert and quickest deer would be best suited to escape wolves. Wolves that lived in different environments -- the mountains versus the plains -- would have to deal with different terrain and different prey, and over time the two wolf stocks would be subtly altered in different directions by their different lifestyles.
Similarly, although plants originally bred by tossing their pollen to the wind, insects that collected that pollen as a useful food source would take it to another plant of the same species, cross-breeding the plant. Plants that had adaptations to encourage this process would propagate more effectively than just dumping pollen to the wind, which is an inefficient and scattershot process. Over time, increasingly elaborate relationships between plant and pollinator emerged. Darwin saw all such processes of adaptation as very slow and gradual:
BEGIN_QUOTE:
Natural selection can act only by the preservation and accumulation of infinitesimally small inherited modifications, each profitable to the preserved being; and as modern geology has almost banished such views as the excavation of a great valley by a single diluvial wave, so will natural selection, if it be a true principle, banish the belief of the continued creation of new organic beings, or of any great and sudden modification in their structure.
END_QUOTE
Today, Darwin's notion of the slow and infinitesimal adaptation of organisms is regarded as mostly right, but with possible exceptions. He was correct in that drastic changes in organisms are much less likely to do an organism any good than small changes: a series of baby steps in the dark is much less likely to come to disaster than a giant leap. In fact, if there is a mutation that changes fur color to darker, the odds are about 50:50 that it will provide an advantage.
Still, the fact that it's a matter of the odds suggests that major mutations may occasionally have had significant effects in the evolution of life. Over a very long period of time, events of very low probability become certain, if still infrequent. Organisms also have "developmental genes" that perform high-level functions, for example "grow an eye" or "grow a leg", with the details performed by other genes -- and in some cases large-scale changes are not troublesome. Snakes lost their legs, then acquired greatly lengthened ribcages through adding commands to "grow more ribs". However, such considerations have to be regarded as "fine print", not demonstrations that Darwin was on the wrong track.
* Darwin commented that sexual reproduction seemed to be important in the evolution of life, but he had only vague ideas as to why:
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I have collected so large a body of facts, showing, in accordance with the almost universal belief of breeders, that with animals and plants a cross between different varieties, or between individuals of the same variety but of another strain, gives vigour and fertility to the offspring; and on the other hand, that CLOSE interbreeding diminishes vigour and fertility; that these facts alone incline me to believe that it is a general law of nature (utterly ignorant though we be of the meaning of the law) that no organic being self-fertilises itself for an eternity of generations; but that a cross with another individual is occasionally -- perhaps at very long intervals -- indispensable.
END_QUOTE
Darwin here is merely toying with vague and incomplete ideas. He did not seem to realize, at least as far as this citation indicates, that microorganisms tend to be "asexual reproducers" -- that is, they don't need to have sex with another member of their species to reproduce, they just generate copies of themselves through splitting in two or budding or the like. Large organisms are, in contrast, generally "sexual reproducers", requiring two parents to generate offspring.
Why sex? What advantage does it provide? Given the modern understanding of genetics, it is clear that sex promotes genetic diversity. Genes and the genome were compared here earlier to cards in a deck of cards; but it would be a better analogy to say that the genome of a sexually-reproducing organism consists of two decks of cards, one deck contributed by each parent. Our genome is embodied in stringlike structures called "chromosomes" at the center of our cells; while asexual reproducers are generally "haploid", with only one set of chromosomes, sexual reproducers are generally -- with exceptions too confusing to belabor here -- "diploid", with chromosomes in matched pairs, one of the pair contributed by each parent.
Most of the genes in these paired chromosomes are the same, but not all of them. Some genes have different variants or "alleles", with different alleles having different effects -- different coloration, for example. When two different alleles of the same gene are contributed by the parents, as a rule one is "dominant" and is expressed, while the other is "recessive" and remains invisible. The only way a recessive gene can be expressed is if an organism obtains the same recessive gene from both parents.
In sexual reproduction, the two parallel sets of chromosomes in a parent are scrambled more or less at random, to produce a new single set of chromosomes -- it's like taking the two decks of cards and selecting cards from them at random to come up with a new single deck. The two parents provide their single sets of chromosomes to create a new organism; in terms of the analogy, they both pass their new single deck of cards on to create a new double deck. The scheme is adapted to promote genetic diversity. In some cases the new organism will be dealt a bad "hand", the worst genes of both parents, and will be eliminated by natural selection; in others, it will obtain a good "hand", the best genes of both parents, and will predominate.
Establishing that sex promotes genetic diversity is all very well, but that leads to the question of why genetic diversity is a good thing. Certainly Darwin was unambiguously right in his belief that inbreeding is a bad thing, a fact that breeders of his time were well aware of. Since recessive genes aren't expressed, harmful recessive genes can be carried on from generation to generation, remaining invisible and not harming their host. Inbreeding makes it more likely that an organism will receive a harmful recessive gene from both parents, ensuring that the gene will be expressed. This is why inbred groups of humans tend to suffer from rare genetic disorders.
However, that really doesn't explain why sex is advantageous in the first place. Recessive genes are not a problem in asexual reproducers. Among haploid asexual reproducers, if a gene undergoes a mutation that renders it harmful, that gene will be expressed in an organism that possesses it, with selection weeding the organism out. There are no recessive genes in asexual reproducers, and so the harmful gene cannot "hide out" for generations, to pop up later as a genetic booby trap.
So what advantage does sex provide? There are a number of explanations, not all mutually exclusive, none absolutely satisfactory in all respects. The most popular idea in modern times is that it provides protection from disease. Microorganisms reproduce very quickly, and so they evolve rapidly. Large multicellular organisms do not reproduce quickly. Some microorganisms parasitize large multicellular organisms, and such "pathogens" are guaranteed to crack the "combination lock" of the immune defenses of a species -- except if the combination is "scrambled" on a generational basis by sexual reproduction. While microorganisms can get away with being asexual reproducers, large, long-lived organisms need sex to prevent the microorganisms from wiping them out. Sexual reproduction, then, is important to the "evolvability" mentioned above, a feature that allows organisms to evolve more rapidly, frustrating the pathogens that afflict them.
This scenario is certainly supported by evidence. Wild bananas are full of seeds; domesticated bananas are effectively seedless, and they can only be propagated by transplanting cuttings, making them asexual reproducers. The most popular type of banana up to the 1950s was the "Gros Michel", but it was mostly wiped out by a fungal infection. The current popular banana is the "Cavendish", which is now being threatened by a mutant strain of the same fungus. The asexual bananas are at a disadvantage relative to the fast-reproducing fungus, and over the long run any one banana variety may well be doomed.
* Darwin did certainly recognize that cross-breeding among the population of individuals of the same species tended to maintain the characteristics of that population. That led to the insight that if a population was split, say by a portion of it migrating over a mountain range when conditions was harsh, the two populations would undergo a gradual "divergence of character". In modern terms, genetic changes in the two populations would cause them to "genetically drift", becoming more and more dissimilar over time. That was why species that had been transplanted to islands over time by various natural accidents tended to be unique to the particular islands they inhabited.
This "divergence of character", often seen between related but mutually isolated populations gave a diagram of the emergence of new species a branching character, an "evolutionary tree" that not merely resembled but matched the trees of classification of organisms. There is a tendency to think of evolution in terms of linear progress, with "primitive" ancestors leading by nice smooth steps to "improved" descendants, giving the impression that evolution by natural selection occurs in clear directions through some sort of guidance or "momentum". That's misleading; evolution only works in the broad general direction of improved fitness to the environment. Instead of working along a nice neat line of progress, it features species emerging on different branches, diversifying into different forms and opportunistically taking up different ways of life.
Many of these branches die out; Darwin pointed out as a necessary consequence of evolution by natural selection that many species would inevitably go extinct. If there were new species arising at all times that featured improved adaptations to their environment, then some of the older species would not make the grade in the struggle for existence and disappear from the face of the Earth.
Darwin ended chapter 4 with an eloquent summary of his ideas:
BEGIN_QUOTE:
If during the long course of ages and under varying conditions of life, organic beings vary at all in the several parts of their organisation, and I think this cannot be disputed; if there be, owing to the high geometrical powers of increase of each species, at some age, season, or year, a severe struggle for life, and this certainly cannot be disputed; then, considering the infinite complexity of the relations of all organic beings to each other and to their conditions of existence, causing an infinite diversity in structure, constitution, and habits, to be advantageous to them, I think it would be a most extraordinary fact if no variation ever had occurred useful to each being's own welfare, in the same way as so many variations have occurred useful to man.
But if variations useful to any organic being do occur, assuredly individuals thus characterised will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance they will tend to produce offspring similarly characterised. This principle of preservation, I have called, for the sake of brevity, Natural Selection. Natural selection, on the principle of qualities being inherited at corresponding ages, can modify the egg, seed, or young, as easily as the adult. Amongst many animals, sexual selection will give its aid to ordinary selection, by assuring to the most vigorous and best adapted males the greatest number of offspring. Sexual selection will also give characters useful to the males alone, in their struggles with other males.
Whether natural selection has really thus acted in nature [is considered in more detail later]. But we already see how it entails extinction; and how largely extinction has acted in the world's history, geology plainly declares.
Natural selection, also, leads to divergence of character; for more living beings can be supported on the same area the more they diverge in structure, habits, and constitution, of which we see proof by looking at the inhabitants of any small spot or at naturalised productions. Therefore during the modification of the descendants of any one species, and during the incessant struggle of all species to increase in numbers, the more diversified these descendants become, the better will be their chance of succeeding in the battle of life. Thus the small differences distinguishing varieties of the same species, will steadily tend to increase till they come to equal the greater differences between species of the same genus, or even of distinct genera.
We have seen that it is the common, the widely-diffused, and widely-ranging species, belonging to the larger genera, which vary most; and these will tend to transmit to their modified offspring that superiority which now makes them dominant in their own countries. Natural selection, as has just been remarked, leads to divergence of character and to much extinction of the less improved and intermediate forms of life.
On these principles, I believe, the nature of the affinities of all organic beings may be explained. It is a truly wonderful fact -- the wonder of which we are apt to overlook from familiarity -- that all animals and all plants throughout all time and space should be related to each other in group subordinate to group, in the manner which we everywhere behold -- namely, varieties of the same species most closely related together, species of the same genus less closely and unequally related together, forming sections and sub-genera, species of distinct genera much less closely related, and genera related in different degrees, forming sub-families, families, orders, sub-classes, and classes. [...]
The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth. The green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species. At each period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have tried to overmaster other species in the great battle for life. The limbs divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was small, budding twigs; and this connexion of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups.
Of the many twigs which flourished when the tree was a mere bush, only two or three, now grown into great branches, yet survive and bear all the other branches; so with the species which lived during long-past geological periods, very few now have living and modified descendants. [...] As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications.
END_QUOTE
BACK_TO_TOP* Chapter 4 of THE ORIGIN OF SPECIES is Darwin at his most insightful. In chapter 5, he ends up probing at the bounds of his knowledge again, trying to make sense of the patterns of variations that arise through evolution, but with meager success. Indeed, early in the chapter he makes what in retrospect was a major blunder:
BEGIN_QUOTE:
I think there can be little doubt that use in our domestic animals strengthens and enlarges certain parts, and disuse diminishes them; and that such modifications are inherited.
END_QUOTE
While it's sometimes overlooked now, Darwin never completely rejected Lamarckian principles. We know now that organisms are handed a particular genome when they are conceived; they are issued a deck at the outset, and they cannot modify that deck in any specific way. The deck, or half of it for sexual reproducers, will be passed down to descendants unchanged, aside from the occasional mutation. To be sure, the development of offspring is heavily dependent on the environment established by parents -- in particular, animals capable of learned behavior can train their offspring, for example in the art of hunting, to give them an advantage. However, this "acquired advantage" has nothing directly to do with any modifications of the genome from generation to generation.
Darwin here seems to have at least partly missed the implications of his own theory. Those organisms with the best-developed "parts", as he called them, that gave them an advantage would tend to dominate the populations of their species, and gradually supplant those with more poorly-developed parts. Simply exercising those parts had little to do with it, except to the extent that it made it more likely that animals that did so would survive and propagate.
The reason that parts would be lost through disuse was less obvious. If the organism had a lifestyle that made no use of particular use of some of its parts, then those parts would not be subjected to "selection pressures". Such parts would either not be altered in any way advantageous to the organism, becoming useless as its other parts evolved, or would be actively "broken" by mutations. This is known as "relaxed selection" -- when selection pressures are relaxed on particular features of organisms, the genes controlling those features will sooner or later be broken by mutations, and natural selection will not prune organisms with those broken genes from the population. We have a gene for synthesizing vitamin C, but it is badly broken and nonfunctional; our ancestors had plenty of access to fruits and vegetables that provided all the vitamin C they needed, so when the gene for producing it broken, its absence made no practical difference. Cats, which are strict meat-eaters, have a functional vitamin C gene because they have no real source of vitamin C in their diet.
Darwin didn't think Lamarckism could do the whole job, believing that it was a secondary influence along with natural selection. In reference to the fact that there were many flightless beetles that had nonfunctional wings, he commented that "the wingless condition of so many [...] beetles is mainly due to natural selection, but combined probably with disuse." Darwin was reluctant to discard the "training wheels" of Lamarckism, despite the fact that it was as redundant to his theory as wings were to flightless beetles.
Darwin did, however, understand the notion of "negative selection", in which a part that has become a liability tends to wither away under selection pressures, for example the eyes of burrowing mammals, which were generally non-functional or not very functional:
BEGIN_QUOTE:
As frequent inflammation of the eyes must be injurious to any animal, and as eyes are certainly not indispensable to animals with subterranean habits, a reduction in their size with the adhesion of the eyelids and growth of fur over them, might in such case be an advantage; and if so, natural selection would constantly aid the effects of disuse.
END_QUOTE
This circumstance, as Darwin pointed out, also applied to a wide range of cave creatures -- they were almost always blind.
Other comments in the chapter seem unremarkable in hindsight. Darwin observed on the fact that organisms tended to adapted for particular climates and didn't always do well when transplanted to regions that didn't match their normal environments. He discussed how variations in the features of organisms tended to be systematic, with changes in one feature corresponding, directly or inversely, to changes in others. He noted how exaggerated features of organisms, in particular sexually-selected features, tended to be highly variable among related species -- in fact, sexually-selected features are notorious for their whimsical appearance.
Darwin did have an interesting comment on the emergence of partially striped coats, reminiscent of the zebra, in horses and mules, suggesting it as evidence of descent of horses and their relatives from common ancestors:
BEGIN_QUOTE:
For myself, I venture confidently to look back thousands on thousands of generations, and I see an animal striped like a zebra, but perhaps otherwise very differently constructed, the common parent of our domestic horse, whether or not it be descended from one or more wild stocks, of [wild horses or asses] and zebra.
He who believes that each equine species was independently created, will, I presume, assert that each species has been created with a tendency to vary, both under nature and under domestication, in this particular manner, so as often to become striped like other species of the genus; and that each has been created with a strong tendency, when crossed with species inhabiting distant quarters of the world, to produce hybrids resembling in their stripes, not their own parents, but other species of the genus.
To admit this view is, as it seems to me, to reject a real for an unreal, or at least for an unknown, cause. It makes the works of God a mere mockery and deception; I would almost as soon believe with the old and ignorant cosmogonists, that fossil shells had never lived, but had been created in stone so as to mock the shells now living on the sea-shore.
END_QUOTE
The last comment at least indirectly reflected on the concept of "Omphalos creationism", advanced by a British naturalist named Philip Henry Gosse in his 1857 book OMPHALOS: AN ATTEMPT TO UNTIE THE GEOLOGICAL KNOT. The book tried to reconcile evidence for an old Earth as revealed by geological evidence with the young Earth described in scripture -- by claiming that it had simply been created to look old, with fossils and geological strata in place at the outset. "Omphalos" was from the Greek for "navel", incidentally, referring to the idea that Adam and Eve had been created with navels even though they had not been born in the conventional sense of the word.
The idea was not well received, since it made the Creator a "cosmic prankster" who created false clues to trick humanity, and it acknowledged that the evidence for an old Earth (or, by implication, evolution) was valid -- but then tried to invalidate it by claiming it was all a fabrication, using much the same approach as employed by modern conspiracy theorists who, when confronted with inconvenient evidence, dismiss it as having been "faked by the conspiracy." Omphalos creationism tends to linger on even today, though it is generally seen as a tactic of desperation.
Anyway, at the end of the chapter Darwin admitted that his "ignorance of the laws of variation" was "profound". He felt that future researchers would address the issue, and believed that his concepts were nonetheless valid:
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Whatever the cause may be of each slight difference in the offspring from their parents -- and a cause for each must exist -- it is the steady accumulation, through natural selection, of such differences, when beneficial to the individual, that gives rise to all the more important modifications of structure, by which the innumerable beings on the face of this earth are enabled to struggle with each other, and the best adapted to survive.
END_QUOTE
In this judgement, even with fine print accumulated since 1859, Darwin remains correct. Darwinian evolution can be minimally formulated as "variation + selection"; Darwin necessarily focused on selection because he had only the coarsest understanding of variation. We know far more and are continuing to learn more about processes of variation, but despite occasional attempts to proclaim the downfall of Darwin's concepts on that basis, nothing has been discovered that undermines the essential, but not exclusive, role of selection.
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