* To discuss modern evolutionary theory, it is essential to begin with its origins in Darwin's ORIGIN OF SPECIES. Although it no longer has any serious competitors, it is also useful to review alternatives that have been proposed.
* From antiquity, the general belief in the West was that the Earth had existed for a few thousand years, with all the creatures created at the start and remaining unchanged to the present day. It wasn't until the beginnings of modern science that this static mentality began to erode. The voyages of discovery conducted by Europeans from the 15th century on led to an explosion of knowledge about the plants and animals of the Earth, and in the 18th century, the Swedish naturalist Carolus Linnaeus (1707:1778) engaged in a massive effort to catalog them all.
In the "Linnaean" system, organisms were organized as distinct "species" or different animals that couldn't interbreed, sometimes with a number of "breeds" or "races" or "varieties" (the last term generally meaning plants) associated with a single species. A number of species could be organized into a "genus" (for example, the genus of cats or "felines"); with a number of genera becoming an "order" (felines, canines, bears, and the like becoming the order of "carnivores"); a number of orders becoming a "class" (carnivores, rodents, primates, and so on becoming the class of "mammals"); and classes becoming members of "kingdoms" (either "animal" or "plant").
Linnaeus was not the first to attempt such a classification, but he was the one who succeeded -- his work bolstered by his excellent writing skills, his charismatic ability to influence others, a global network of naturalists feeding him information, and an obsessive thoroughness. The Linnaean taxonomic classification system, with its scheme of Latin names for organisms (Canis lupus for "wolf", for example), remains in effect to this day, with some additions, such as subdivisions of the kingdom of animals and plants as "phyla" (the class of mammals being organized with reptiles, birds, fish, and amphibians as the phylum "vertebrates", or creatures with spinal cords). Sometimes plant phyla are referred to as "divisions".
There was little in the system of Linnaeus that challenged the long-standing notion that species had been created in the past in the same forms as they existed in the present. His neat categorization of different species, which by modern standards was remarkably if by no means perfectly accurate, might well have led someone surveying the Linnaean organization "tree of life" to ask the question: why do these tidy familial groupings exist? Why should there be families of cats, of owls, of bats? If they were all actually created, the amount of diversity within each family was surprising. Why have so many clearly different species of owls -- and, incidentally, why did particular species of owls tend to be associated with different locations?
The French naturalist George-Louis Leclerc, Comte de Buffon (1707:1788) suggested that modern species were derived or "evolved" from different ancestral species, though Buffon was very limited in his proposal, saying that, for example, all the different species of cat were derived from one ancestral species of cat, with no connection between the evolutionary tree of cats and the corresponding tree of wolves and dogs. As far as how or why this evolution occurred, Buffon could only offer fuzzy speculations.
* Buffon wasn't the only scholar of his era to tinker with vague "evolutionary" concepts, but the next generation of scientists generally rejected such ideas -- though they didn't disappear. A French naturalist, Jean-Baptiste Pierre Antoine de Monet, Chevalier de Lamarck (1744:1829), proposed a theory of evolution that actually included a mechanism, something of a first. The simplest way to explain his theory, now known as "Lamarckism", is to consider the well-known example of the giraffe. As Lamarck saw it, the giraffe with its extraordinarily long neck arose from ancestors with shorter necks. In their attempts to reach the higher branches of trees, giraffes stretched their necks as far as they could; this stretched neck would be passed down to their progeny, which would have a longer neck that they stretched farther still.
In the same way, the children of a blacksmith with powerful arms would also have a powerful build, which they would enhance in turn; animals in cooling climates would acquire heavier coats of fur and then pass them down to their offspring. Lamarck's theory effectively proposed a form of "directed" evolution, based on two linked ideas: that adaptations were obtained as a direct response to the "needs" and "strivings" of an organism; and these adaptations were then passed down to progeny. In modern times, the second notion -- "inheritance of acquired characteristics" -- is much more heavily emphasized as Lamarck's legacy than the first.
Lamarck's thinking had little influence. Georges Cuvier (1769:1832) was Buffon's intellectual successor in French naturalism, though absolutely not his heir. Cuvier used his undoubted brilliance and his powerful position in academia to emphatically reject evolution, insisting that as per tradition the species were fixed, having been created and then remaining unchanged to the present. He did admit that there had been changes in species over the history of the Earth. Fossils, the stony remains of the skeletons of ancient organisms, had been long known, but had been dismissed as the remains of freaks or monsters.
Cuvier took them perfectly seriously as evidence of past life -- for example, he examined the skulls of the extinct elephants that would become known as "mammoths" and proclaimed them unarguably different from living elephants. Significantly, Cuvier identified particular types of fossils as associated with particular buried layers of geological deposits, or "strata". He still believed that nature was basically static and unevolving, that the strata were associated with distinct eras that ended in global catastrophes that wiped out species, with a new set of species created in their place. This concept became known as "creationist catastrophism" -- the Maker having implemented all the creatures of each era, and then destroyed them after their time was done.
Over the following decades, fossil evidence of strange beasts and lost worlds of the past began to pile up, with the succession of such worlds increasingly obvious in the fossil record. The geologist Adam Sedgwick (1785:1873), a Cambridge don who would be one of the founding fathers of modern geology, probed the earth of Wales and characterized the strata he found. Others were engaged in similar exercises, and in 1841 John Phillips (1800:1874), a nephew of William Smith, published a scheme in which he organized the "eras" of the past as identified by the fossil species observed in the strata. In 1845, Sedgwick wrote that there was clearly a "progressive development" of forms over eras and periods, but rejected evolution, interpreting the sequence as evidence only of creationist catastrophism.
Other scholars were beginning to question that doctrine -- notably Sir Charles Lyell (1797:1875), another one of geology's founding fathers. Lyell had started out studying to be a lawyer, but he had taken classes from Buckland, a flamboyant lecturer, and got the rock bug. Lyell was inclined towards conservatism about evolution, but in terms of geology he became something of a scientific radical, taking on the catastrophist status quo.
Lyell obtained his inspiration from the work of the Scots scholar James Hutton (1726:1797), who had rejected the idea of a catastrophic history of the Earth, instead proposing that eras of vulcanism -- volcanic activity -- would raise materials from underground to directly or indirectly form islands and continents -- which would then gradually settle into the Earth, to melt and provide another burst of vulcanism. He believed that the Earth lived in a "steady state", with "no trace of a beginning, no prospect of an end." Hutton, in short, had replaced a world a few thousand years old with one that had existed in "deep time".
Hutton's notions were more speculative than rigorously scientific, but Lyell, casting aside catastrophism, was inspired to flesh them out into a much more detailed scientific theory, basing his reasoning on the new geology described by William Smith, Adam Sedgwick, and others, and incorporating the "footprints" of the past as found in fossils. Lyell described his "uniformitarianism" or "gradualism" in the monumental three-volume work PRINCIPLES OF GEOLOGY, published from 1830 into 1833, which envisioned a world of vast time, cyclically reshaped by slow processes of geology.
Evolution was finally put on a solid basis by the work of Charles Robert Darwin (1809:1882), a son of an upper-class English family. Darwin was interested in naturalism, having studied under Sedgwick. From 1831 into 1836, he accompanied the Royal Navy brig HMS BEAGLE on a global mapping and survey voyage, in an unpaid position as a naturalist. During the voyage, Darwin investigated the geology and biological diversity of distant lands, coming to the conclusion that the species of the Earth had indeed evolved, all descended from common roots over deep time.
On his return to England, Darwin married and settled down to a life as a gentleman naturalist, spending over two decades fleshing out his evolutionary ideas. It wasn't until 1859 that he decided to go public -- he wouldn't have done it then, except for the fact that he faced being trumped by a young field naturalist named Alfred Russel Wallace (1823:1913). The result was Darwin's book ON THE ORIGIN OF SPECIES, one of the most important publications of the 19th century.
Darwin had worked from observations of the succession of species in the geological record; the relationships of species as originally established by Linnaeus; the geographic distribution of species over the Earth; the variation of plants and animals in domestication -- generally through what he called "sports", what we would now call "mutations"; and the equivalent variability of wild species to construct his theory. He had observed how breeders could create new varieties of plants and animals through cross-breeding, using "artificial selection" to work towards a desired variety. Very well; a similar process took place in nature, with simple survival honing variations in species to result, in deep time, in new species, a process he called "natural selection".
In hindsight, Darwin had established a "unified theory" of biology. While there was considerable resistance to his theory of evolution from those who clung to traditional creationism -- there always would be, creationism continues to creep along in the 21st century -- he obtained adherents in the scientific community immediately. Not all scientists were impressed, however, Darwin himself admitting there was much more to be learned about evolution.
Darwin was particularly weak when it came to the mechanisms of heredity, of the inheritance of features of organisms from parent to offspring. Darwin had only the sketchiest ideas of how heredity worked, devising an incoherent notion of "pangenesis" as an explanation; even his allies found his thinking on the matter dubious. He did not learn of a research paper, published in 1866 by an Austrian monk named Gregor Mendel (1822:1884) that described the patterns of heredity in crossings of pea plants. The paper made little impact on the research community.
The ignorance of the mechanisms of heredity was not really a blow to Darwin's theory; it simply left it without support on that basis, at least until the matter was better understood. More troublesome was the fact that the physicist William Thompson, later Lord Kelvin, suggested from his analysis of the cooling of the Earth that the planet was no more than a few hundred million years old. Darwin admitted that didn't seem like enough "deep time" to permit the evolution of all the species of the Earth; geologists believed Thompson was wrong, but didn't have any way to challenge his figurings.
The result was that by the end of the 19th century, Darwin's theory was in a state of dilapidation. The science community generally accepted evolution; new fossil finds since the publication of THE ORIGIN OF SPECIES had done much to reinforce the idea. However, there was great doubt that Darwin had come close to explaining how it worked. Fortunately, at that same time, new revelations restored life to Darwin's ideas.
BACK_TO_TOP* The first key to the revival of evolutionary theory was the discovery of radioactivity by the French physicist Anton Henri Becquerel (1852:1908), who found that some materials emitted a mysterious radiation that would fog photographic film. In a few years it would be understood that radioactivity was caused by the energetic breakdown of the atoms of the material themselves, and that the energies produced by such breakdowns were huge -- as given by the famous formulation of the German-American physicist Albert Einstein (1879:1955), "E=MC^2".
The physicists had discovered an energy process that allowed the Universe to be much older than Lord Kelvin had asserted. In addition, since different radioactive materials have different and highly specific decay rates or "half-lives", radioactivity gave geologists a tool to actually estimate the age of many mineral samples. "Deep time" had been restored; not only was the Universe now old enough to allow evolution to take place, it was also possible to provide reasonable hard dates on the fossil record.
The second key rested in the first steps towards the understanding of heredity. It was known at the time that organisms such as plants and animals cells have a central "nucleus"; in the 1880s, microscopic analysis showed that the cell nucleus contained threadlike structures that were eventually named "chromosomes". There were hints that the chromosomes stored hereditary information, but nobody had much idea of the details.
There is no evidence that Mendel heard of this discovery before his death. Progress forward depended on a Dutch botanist named Hugo de Vries (1848:1935). In the mid-1880s, de Vries began to investigate the breeding of flowers; by that time, it was known that chromosomes were involved in cell replication: they existed in pairs that split up in the germ cells of parents, a process known as "meiosis", and then recombined in the egg cells of progeny.
De Vries and others believed, at the time on somewhat circumstantial evidence, that the chromosomes were carriers of hereditary information. De Vries also suggested, correctly, that different traits were found as "units" positioned on the chromosomes. He had no idea of the precise nature of these "units", but he did give them the name "pangenes", in homage to Darwin's theory of pangenesis. De Vries published his conclusion in 1900. He had resurrected the work of Mendel, bringing it into the mainstream of research.
A Danish botanist named Wilhelm Johannsen (1857:1927) gave the then-hypothetical unit of heredity the name "gene" (by stripping down "pangene"), which the British biologist William Bateson (1861:1926) used to mint the word "genetics" for the study of heredity. Later, the entire set of genes for an organism would become known as the "genome".
The first intensive work on chromosome-based genetics was performed by a meticulous American experimentalist, Thomas Hunt Morgan (1866:1945), at Columbia University in New York City, who began work in 1909 on the heredity of the little Drosophila melanogaster fruitfly. They bred very rapidly, and had traits that were easy to observe; Morgan found that they followed much the same general patterns of inheritance as Mendel had observed in pea plants. Morgan deduced that the genes that he presumed existed were strung like beads on a string along a chromosome, and that the recombinations of chromosomes were likely to switch genes around. He was able to map particular genes associated with fruitfly traits on their chromosomes.
* Armed with a coarse but workable understanding of the mechanisms of heredity, in the postwar period, biologists were able to establish in the postwar period a much more rigorous model of evolutionary processes. The pioneers in this effort were the Scots biochemist John Burdon Sanderson Haldane (1892:1964), the English geneticist Ronald A. Fisher (1890:1962), and the American geneticist Sewall Wright (1889:1988). In a series of research papers published from 1924 into 1934, Haldane provided neat mathematical analyses that showed genetics provided the underlying nuts and bolts for evolution.
Haldane started with the example of the peppered moth. The pale-colored peppered moth was long known to British insect collectors, but in 1848 one was found with a dark coloration. In 1896, the British naturalist J.W. Tutt (1858:1911) discovered that while the pale peppered moth was still predominant in the English countryside, the dark peppered moth was overwhelmingly predominant in sooty industrial areas. The conclusion was that the dark peppered moth was better camouflaged for life in industrial areas, allowing it to avoid predators -- birds in particular -- and giving it a selective advantage over the pale peppered moth.
Haldane applied statistical methods to the story of the peppered moth, pointing out that if dark camouflage gave it even a modest survival advantage in an industrial environment, the dark form would outbreed the pale form and predominate within a fairly small space of generations. He also showed that simple mutations couldn't do the job, since it would demand that every fifth moth born to pale parents be a dark mutant, which was absurd; natural selection had to be involved.
Fisher complemented Haldane's work by performing statistical analysis on how mutational change propagated through a population of organisms. The culmination of his work was his GENETICAL THEORY OF NATURAL SELECTION, published in 1930, in which he mathematically demonstrated how a small mutational change would propagate through a population, the rate of propagation being proportional to the advantage of a change, and also showed how shifts in the environment would lead to shifts in the makeup of the population.
Haldane and Fisher were both theoreticians, which made them suspect to practical field biologists, but Wright came from a practical background, and was able to sell field biologists on the new synthesis of traditional evolutionary thought and genetics. His major selling point was an easily visualizable model: the "adaptive landscape" or "fitness landscape". In a 1932 paper, Wright suggested that evolution might be compared to a landscape with peaks and valleys, corresponding to adaptations providing good and poor fitness respectively. Mutations in species would cause organisms to migrate around the landscape in a blind fashion, one step per generation, guided by the "terrain" to drift up peaks and away from valleys as fitness improved.
We can visualize evolution as an "evobot" negotiating its way over the fitness landscape. It should be emphasized that the fitness landscape is a sheer abstraction. The overall "fitness" of an organism is a complicated balance of a great number of factors, reflecting a "fitness peak" that is a composite result of many environmental pressures, or "selection pressures". In addition, the factors are not necessarily fixed, with changes in climate, introduction of competitors and predators, and so on shifting the fitness peaks and valleys around.
There is an implication in this model that if an organism reaches a fitness peak, it is likely to stay there indefinitely, enduring down through the generations with little change. The fact that the landscape can, in fact is almost certain to, shift, with the peaks moving or disappearing, ensures ongoing change.
However, Wright pointed out another mechanism to show why organisms didn't remain static indefinitely: "genetic drift". If mutations are occurring in organisms all the time, then they don't really occupy a static position in the fitness landscape anyway, with the locus jittering around at random due to hereditary "noise". Wright suggested was that small, isolated populations, being more easily influenced by such "noise", might be the precursors of new species, a notion that appealed to field biologists because it was what they had observed.
Genetic drift also had the implication, not particularly emphasized at the time, that evolution wasn't entirely controlled by selective forces, that there might be evolutionary changes independent of them, a population undergoing change that neither provided a real advantage nor caused any real harm. This concept, which Darwin had actually hinted at himself, would become known as "neutral evolution".
* Haldane, Fisher, and Wright influenced a next generation of evolutionary biologists, including the Ukrainian-American field naturalist Theodosius Dobzhansky (1900:1975), the German-American zoologist Ernst Mayr (1904:2005), and the paleontologist George Gaylord Simpson (1902:1984).
Dobzhansky followed up Morgan's lab experiments on the Drosophila melanogaster fruitfly with field studies of the related wild Drosophila pseudoobscura fruitfly, which confirmed the general studies of Morgan. Dobzhansky determined that the variability of the wild fruitflies he caught was much greater than anyone had anticipated, and that the variations were generally distinct to each population of flies. Dobzhansky was observing at the chromosomal level the branching tree of life postulated by Darwin.
While Dobzhansky was an excellent field biologist, he was also interested in theory, and was in particular taken with Wright's notion of an adaptive landscape. In 1937, he published the influential book GENETICS & THE ORIGIN OF SPECIES, the title of which explicitly married Mendel and Darwin. In his work, Dobzhansky discussed in detail the impact of geographic or other factors that led to isolated populations, and also emphasized the "hidden variability" implicit in recessive genes.
Following up from Dobzhansky's work, in 1942 Ernest Mayr published SYSTEMATICS & THE ORIGIN OF SPECIES. In his book, Mayr described species as simply inbreeding populations that were reproductively isolated from other groups, with issues of physical distinctions, such as morphology, regarded as irrelevant in that context. Mayr described the fragmentation of a population into several reproductively isolated groups, what he called "emergent species" with the term "adaptive radiation".
Mayr suggested what he called the "founder principle", which had a clear debt to the thinking of Sewall Wright: small isolated groups could undergo rapid evolution, since genes providing adaptive improvements didn't have to propagate through a large population -- and if geographic or other barriers between the small population and a larger, less well adapted population broke down, the new variant might come into competition with the parent stock and quickly replace it.
George Simpson, as a paleontologist, was familiar with the fossil record, which clearly identified a succession of forms. However, this progression of forms had proven misleading, since it gave the impression of straight-line progress to increasingly refined forms, suggesting pre-Darwin evolutionary mechanisms such as Lamarckism. Simpson was able to show that the fossil record was full of side branches and dead ends, just as Darwin had predicted it should be. The seeming straight-line progress was simply due to looking backwards over the line of evolution from its end and disregarding the branches.
Simpson also noted that the fossil record was marked by confusing discontinuities in its history. In his 1944 book MODE & TEMPO IN EVOLUTION, Simpson interpreted this seemingly contradictory evidence by suggesting that evolutionary change was not a completely smooth process, instead being marked by rapid starts, long intervals of stability, and dead ends that vanished from the Earth in extinctions. The idea didn't catch on at the time, but the notion of "punctuated equilibrium" would eventually be generally accepted.
By the end of the 1950s, the work of Dobzhansky, Mayr, Simpson, and others had established a thoroughly renewed evolutionary theory, the "Modern Synthesis", grounded in a level of rigor and detail that would have astounded even the meticulous Charles Darwin. Disputes over the fundamental elements of evolutionary theory disappeared, with few contesting Dobzhansky's simple claim that "nothing in biology makes sense except in the light of evolution." Without evolutionary theory, the species of the Earth would be no more than entries in a catalog that could provide no real insights into their origins and relationships, making their characteristics no more than arbitrary.
* While biologists refined the Modern Synthesis, biochemists were zeroing in on the precise molecular mechanisms of heredity, discovering that it was encoded in a molecule named "deoxyribonucleic acid (DNA)" that was found in chromosomes. That was puzzling, since DNA didn't seem to be a very diverse molecule. It basically seemed to be a polymeric chain of four "nucleotide" molecules: adenine, thymine, cytosine, and guanine (A, T, C, and G). The challenge touched off a race to determine the actual structure of DNA.
The race was won by a duo at Cambridge University in the UK, including a young American biochemist named James Watson (born 1928), who was investigating DNA in Cambridge, England, working in collaboration with a British physicist named Francis Crick (1916:2004). Working with clues, notably X-ray analysis of DNA crystals, in 1953 Crick and Watson developed their famous "double helix" model of DNA, with DNA actually made up of two linked polymer chains, composed a pair of linked polymer chains, conceptually resembling a ladder twisted into a spiral. The sequence of the nucleotide "bases" in the DNA molecule clearly defined the genome, providing instructions for the assembly of proteins from their amino-acid building blocks, as well as control functions.
Over the next decades, the understanding of DNA and what it could do emerged into the science of "genomics", in which genomes were decoded and assessed to determine their effects, as well as the relationships of organisms. Genomics became one of the most powerful tools of evolutionary science, allowing the relationships of the Earth's grand family of life, and insights into their past evolutionary history, to be nailed down.
BACK_TO_TOP* As noted earlier, Darwin had not been the first to introduce a scientific theory of evolution; he had been preceded decades before by Jean-Baptiste de Lamarck, who had envisioned adaptations as arising in a "directed" fashion due to the "needs" and "strivings" of organisms, with these adaptations passed down to following generations. Lamarckism is a superficially attractive idea that tends to linger in comic books and the like; it was a popular idea even in the scientific community up to about World War II, but it was effectively dead at that late date and now survives only in a ghostly fashion on the scientific fringe. In terms of what we know about biology today, Lamarck's notions are completely unrealistic.
An organism's capabilities are essentially fixed by its genome at conception. This is not to say that an organism is as rigid in structure as any machine rolling off the production line, only that the organism is dealt a particular set of cards at the outset and its life will be a game played with those cards. The organism has no way of adding cards to the genetic hand it is dealt; its progeny will be dealt a new hand of cards, reshuffled in sexually-reproducing species through sexual recombination, and with occasional variations due to mutation. The parent has little or no control over how the cards are dealt, and the hands passed off to its offspring are not necessarily an improvement on the hand originally dealt to the parent.
Even if an organism did acquire new biological features, it has no real means of passing them down to the next generation. The mechanisms by which genes code an organism are complicated enough; it's very difficult to suggest in any detail a mechanism that could "edit" the genome in the germ cells of an organism to track changes in form of the mature organism. This "editor" system would have to have some "blueprint" of how the organism was put together, and would have to "scan" the entire organism to see how it conformed to this blueprint. When the modifications due to the efforts of the organism to, say, stretch its neck were uncovered in the scan, the editor would have to then make the appropriate updates in all the genomes of the germ cells. The genome is far more like a recipe than it is like a blueprint, and the alterations would not be straightforward. This system isn't ruled out by the laws of physics, but it would be extremely complicated in its implementation.
A similar observation could be made about the origins of innate instincts. In Lamarckian terms, instincts in animals arose from behaviors learned by their ancestors and passed down, but that would imply some scheme for translating the memory of the ancestor into genetic codes. In any case, if such an editor system existed, there would certainly be some biochemical evidence for it -- but no evidence has been found for a trace of it.
Worse, acquired characteristics are not necessarily good; if an animal loses an eye, how would the editor know that was something undesireable? The editor would have to be able to make value judgements, determining what new features amount to improvements and which amount to defects. In MET, the value judgement is automatic: if a change improves the odds of survival and procreation, it's retained, otherwise it dies out.
In addition, while it is possible to imagine a giraffe getting a longer neck by its efforts, anyone who's ever worked as a problem-solver knows that problems in themselves do not necessarily suggest obvious solutions, that obtaining solutions may require considerable mental gymnastics, and there may be different possible solutions. Would the editor dream up a list of different possible solutions, and then perform value judgements to figure out which was best? If it just came up with a solution at random and tried it, that would sound something more like natural selection at work. Carrying this thought further, how could Lamarckism create a complicated organ like the eye, or even a simple eyespot? How could a blind creature "strive" to see? The editor would have to be capable of considerable imagination.
* There's a bit of "fine print" in this overall condemnation of Lamarckism. There has been some disputed evidence that acquired immunities in a parent can be passed down to offspring by some mechanism, which doesn't seem at all implausible. If it does happen, it would be an example of limited inheritance of acquired characteristics at work. It is definitely known that a mother will pass down a sample of the bacteria in her gut to her baby at birth, and since the mother may have acquired new and "improved" bacteria since her own birth, that would amount to inheritance of acquired characteristics -- at least through a symbiotic "end run". Indeed, it is becoming increasingly obvious that the evolution of multicellular organisms can be heavily influenced by their "hitchhikers", which have become a hot topic in biological research.
Another "fine print" item that has become more prominent is "epigenetics", the study of environmental influences on inheritance. Heredity is of course not just a matter of the genome, it also involves cellular machinery interpreting the genome -- and of course cells involved in reproduction can be altered by environmental processes, resulting in distinct differences in the next generation of an organism. We've been learning many fascinating things about epigenetics in recent years, but though some have made a great fuss about it, claiming it "revolutionizes" our views of evolution, that seems exaggerated. Nobody is particularly surprised by the concept that the environment in which cells exist can alter their expression of the genome.
None of these qualifications present any challenge to MET. They can only be thought of as "Lamarckian" in the vaguest sense, doing nothing to support the idea that giraffes got a longer neck by stretching it out and then passing the stretch onto their offspring. That is not what is observed. None of the "fine print" amounts to anything that suggests MET needs to be rethought.
BACK_TO_TOP* Another dead competitor to Darwinian evolution was "saltationism", in which a new species arose as the offspring of another species. There is a simple and immediate objection to the idea: where is an organism with sexual reproduction that has emerged in one radical step going to find a mate?
Saltationism also suffers from subtler problems. Establishing a new species requires a single major genetic change or a set of minor genetic changes. As far as major genetic changes go, there are certainly "macromutations" that can have highly significant effects, but such macromutations tend to produce freaks -- for example, flies that have legs growing where their parents had their antennas.
Ronald Fisher pointed out that any viable species is a complex, well-tuned organism; the odds of a tweaky little random change -- such as an increment in size or minor change in coloration -- of improving the tuning is, if by no means certain, fair to good. In fact, if a color change due to a mutation can be either lighter or darker and the darker color change provides a selective advantage, then the odds of obtaining an advantage in this particular case are 50:50.
In contrast, the likelihood of a major random change amounting to an improvement, a "hopeful monster", is very low. Completely changing the coloration of an animal at random is unlikely to improve its camouflage. A simple doubling in size, without the needed complementary modifications of the organism, is not likely to give it any advantage -- human giants, the result of thyroid gland malfunction, are impressive but live uncomfortable and short lives, humans not being adapted to be that big. Taking a small step in the dark is likely to work out much better than a big leap.
As a rule, a new species arises through a set of genetic changes. However, these genetic changes have to be tuned by natural selection. The likelihood of having a large number of simultaneous genetic changes that produces a viable organism greatly different from its parent is vanishingly small. As the well-known modern biologist Richard Dawkins (born 1941) famously put it: "However many ways there are of being alive, it is certain that there are vastly more ways of being dead."
Darwin himself rejected saltationism by famously saying in chapter 14 of THE ORIGIN OF SPECIES that "natura non facit saltum" -- "nature does not make leaps". Saltationism is like rolling a hundred dice in a shaker and expecting to have them all come up 6. This would happen on the average on an interval that would make the lifetime of the Universe seem like the blink of an eye. Random changes could only produce a viable organism by individual small changes, each tuned by natural selection and accumulating over time. MET is more like rolling a hundred dice, setting aside every die that comes up 6, then rolling the remaining dice again until there are no dice left. That might take an hour.
* As with Lamarckism, there is some "fine print" to the argument against saltationism. If the probability of a drastic macromutation being beneficial is slight, that means that there's still a long-odds bet that it actually will be. Functional macromutations may well be a very infrequent but potentially significant feature in the evolution of life. Some believe that it was such rare evolutionary leaps -- the bigger the leap, the more proportionally rare its occurrence -- that were responsible for some of the major innovations in the evolution of life.
There are also some mutations with large-scale effects that aren't all that troublesome and which are clearly demonstrated by the evidence. As discussed later, there are master control genes that direct the construction of an organism from its various structural "subunits", and sometimes they can increase or decrease the number of subunits -- for example add or delete a new set of ribs. Snakes have hundreds of ribs, far more than their four-legged lizard ancestors, having obtained these ribs through mutations in control genes that specified the construction of more rib subunits. Dawkins calls such mutations "stretched jetliner" mutations, along the lines of the way jetliners are often redesigned for higher capacity simply by adding a fuselage "plug" before and after the wings.
As Dawkins points out, in such mutations the basic arrangement of the organism is retained, just as it is in a stretched jetliner, with no requirement for any major reorganization of the organism to compensate for the change. In fact, the discovery of master control genes has suggested that evolutionary modifications along the general lines of stretched jetliner mutations are actually more general and important than previously thought.
Saltationism is still mostly wrong in any case -- natural selection remains in effect, in fact the screening of macromutations by natural selection is very severe, with the disastrous macromutations weeded out by immediate doom. Certainly nobody is proposing that one species gives birth to an entirely new species. The importance of occasional macromutations in the evolution of life on Earth remains argued; but even if such a notion was widely accepted, MET would be able to accommodate the idea of rare useful macromutations with no major adjustment.
It is true that polyploid plant hybrids -- that is, featuring combined sets of chromosomes from two or more different parent species -- are a new species born in one generation, since with their merged sets of chromosomes they are no longer able to reproduce with either of their parent species. However, although much was made of polyploid hybrids by some biologists in the past, they are now generally regarded as an interesting but peripheral issue. Saltationism now only survives in comics, with mutations providing impossible super-powers for bands of "X-Men" and their supervillain adversaries. It seems appropriate to keep it there.
BACK_TO_TOP* A few other alternative scientific evolutionary theories have been proposed but have failed to impress anyone; they need not be discussed here. There's nothing to rule out some new mechanistic theory coming down the road in the future, but it's difficult to discuss a theory that doesn't exist yet. There's also no reason to think that MET cannot accommodate major new concepts -- after all, it's done so in the past.
Of course it can be, and often is, said that maybe there's some other new theory we haven't come up with yet that does a better job than MET. That's completely unarguable, but equivalent to saying that if pigs had wings, they could fly. The only answer is: "We'll be honestly intrigued if you can find a winged pig, and, not joking, entirely fascinated to watch it fly."
There is no credible challenge to MET in the modern era; however, it still remains the most bitterly criticized of all scientific theories, due to the strenuous efforts of creationists. From the publication of ORIGIN OF SPECIES, religious extremists have angrily rejected evolution, saying it denies the existence of a divine creator. That isn't really true; like any science, it simply says: "The sciences have no need for that hypothesis."
Creationists have never been able to get any real traction against evolutionary science. In desperation, they have attempted to imitate science -- taking as their starting point the work of the Reverend William Paley (1743:1805), an 18th-century English cleric and scholar, who published the book NATURAL THEOLOGY in 1802.
NATURAL THEOLOGY suggested that the elaborations of the species of the Earth and the complexity of their organs argued for the work of a creator. Paley, for example, pointed to the complexity of the eye as an example of an object that could not have existed unless some cognizant higher power had specified it. He argued that if one found a watch, even if it was broken, its complexity and orderly structure would have to imply a "watchmaker", and the complexity and orderly structure of organisms similarly implied a "Divine Watchmaker" -- or more generally, what might be labeled a "Divine Craftsman".
Paley's ideas were consistent with traditional creationism, but it was the last gasp of "natural theology" -- which had been thoroughly deflated by the Scots scholar David Hume (1711:1776) a half-century before, who pointed out its weaknesses:
Darwin accepted Paley in his youth, but would later find Hume's questions far more penetrating -- Darwin eventually saying that he had "believed what I could not understand and what is in fact unintelligible." Hume, incidentally, had substantial influence on Darwin; there are clear cribs from Hume's work in Darwin's writings.
In any case, Paley simply dismissed Hume's arguments without seriously addressing them. Modern creationists persist in Paley's argument, attempting to dress it up in modern clothes by invoking contemporary science and mathematics. In the 1990s, the result of creationist activity was a movement of sorts titled "Intelligent Design" that claimed to go beyond Paley and establish a "new science". It was no more than a windy subterfuge, transparently fake science, a web of evasions over long-familiar creationist arguments.
The reality is that the science community always has and always will overwhelmingly regard creationism as a joke, to the extent any attention is paid to it at all. Intelligent Design was never more than a subterfuge; it wasn't science, it was a snide parody of science -- with the same resemblance to credible science as Inspector Clouseau has to credible criminal investigators.
Intelligent Design rhetoric lingers, but it's long gone stale, having become nothing more than another kit of flimsy tricks in the creationist toybox. A fair case can be made for dismissing creationism as an irrelevant nuisance, judging it as silly as claiming as a principle of engineering that elaborate machines such as airplanes or personal computers can only keep operating thanks to the magical intervention of invisible gremlins. Incidentally, people who work with machinery on a professional basis, and know how persnickety machines can be, find the idea of gremlins in the works intuitively plausible. However, few such folk ever claim that gremlins actually exist.
In any case, despite creationism's lack of credibility, it has a certain undeniable appeal to intuition, and generations of creationists have been single-mindedly dogged in sniping at evolutionary science on every pretext. They have been very successful in spreading confusion over and misperceptions of MET, and like it or not, a detailed explanation of MET has no alternative but to address the popular fallacies in circulation on the subject.
Yes, it does sound defensive to address creationist arguments, but creationists are too loud to be ignored, and as a biologist named H. Allen Orr once put it, physicists would sound just as defensive if they had to stand up for the law of gravity in court every few years. To be sure, there is valid reason to respond to such arguments; creationists accuse scientists of "confirmation bias" in their rejection of creationism, saying that scientists are only hearing what they want to hear and shutting out criticisms.
Very well, then it is appropriate to address such criticisms and demonstrate they're smoke instead of fire. It's not that hard to do; besides, resolving such popular misunderstandings can actually yield useful and interesting insights, not to mention a certain amount of humor. Intelligent Design certainly was and remains a joke, but it does pose an interesting question: what does the phrase "Intelligent Design" really mean?
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