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An Evolution Primer

v1.0.6 / 01 jul 12 / greg goebel / public domain

* In 1859, the English naturalist Charles Darwin published ON THE ORIGIN OF THE SPECIES, which proposed that the different species of life-forms on the Earth all descended through a tree of branching paths from common ancestors. Darwin proposed that the process was directed by "natural selection" -- in which random changes in the forms of organisms from generation to generation were weeded out by simple survival, with the "unfit" dying out and the "fit" continuing to propagate. This document provides a brief survey of Darwin's concepts and the modern evolutionary theory (MET) derived from them.

a question of origins


[1] THE ROOTS OF EVOLUTIONARY THOUGHT
[2] CHARLES DARWIN
[3] THE ORIGIN OF SPECIES
[4] GREGOR MENDEL
[5] THOMAS HUNT MORGAN / THE MODERN SYNTHESIS
[6] MODERN EVOLUTIONARY SCIENCE
[7] COMMENTS & REVISION HISTORY

[1] THE ROOTS OF EVOLUTIONARY THOUGHT

* Before the era of modern science, the general belief in the West was that the Earth had been around for a few thousand years, with all the organisms on the planet created at the start and remaining unchanged to the present day. This belief was reflected in what was called the "Scala Natura (Scale of Nature)", in which the world was arranged in a scale of increasing refinement, with the Earth at the bottom, plants above that, then the beasts, with humans at the top of the hierarchy -- though humans were subordinate to the other-worldly angels. This arrangement did not imply any relationship between the various levels of the scale other than one of rank. The ranks had been fixed at the Creation and would remain fixed until Judgement 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 of the plants and animals of the Earth, and in the 18th century, the Swedish naturalist Carolus Linnaeus (1707:1778) conducted a massive effort to catalog them all.

Carolus Linnaeus

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. There were corner-cases as well, for example crossbreeds or "hybrids" between species that were sterile -- such as mules, which were a cross of a horse and a jackass. 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").

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 next generation of scientists generally rejected such ideas. The influential Georges Cuvier (1769:1832), Buffon's intellectual successor in French naturalism, emphatically rejected the proposals of evolutionists, insisting that as per tradition the species were fixed, unchanged from the day of Creation.

The idea of evolution didn't go away. 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.

the neck of the giraffe

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.

Cuvier would have none of it, and since he was well-known and powerful while Lamarck was not, Cuvier's views carried the day. Cuvier 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 proclaiming 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.

* Cuvier was a pioneer in the study of fossilized species, what is now known as "paleontology", and in the wake of his studies, fossil evidence of strange lost worlds of the past began to pile up, with the succession of such worlds increasingly obvious in the fossil record. Ideas began to form that the Earth had existed through a series of "eras", with such notions taking clear shape through the work of Sir Charles Lyell (1797:1875), one of founding fathers of the science of geology.

Sir Charles Lyell

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 would raise materials from underground that would 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's notions were more speculative than rigorously scientific but Lyell, casting aside catastrophic notions, fleshed them out into a much more detailed scientific theory. Lyell described his "uniformitarianism" 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.

Lyell, though not by nature a radical, was proposing a radical idea. Catastrophism suggested periodic interventions by a Creator; uniformatism saw the world as endlessly shifting by slow changes through geological processes taking place in vast time. Although Lyell didn't realize it at the outset, that notion of "deep time" would also provide stage on which organisms could themselves shift by slow increments, evolving from one form into another.

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[2] CHARLES DARWIN

* Charles Robert Darwin (1809:1882) was born to a privileged English family of the town of Shrewsbury, near the border of Wales. When it came time for young Charles to settle on a career, he originally planned to follow in the footsteps of his father and become a doctor. However, after two years of study at the University of Edinburgh, he decided that he had no stomach for the medical profession -- watching a surgery in the days before anesthetics was unpleasant -- and so he went to Cambridge to take preliminary education for the Anglican clergy. This career path was not guided by any particular religious conviction, it was just that a life as a vicar was regarded as suitable for a gentleman. Charles was interested in naturalism, spending considerable time collecting beetles and the like, and a vicarage would give him leisure time to pursue his interests.

Charles Robert Darwin

At Cambridge, he was exposed to the writings of naturalists of the time, one prominent influence being NATURAL THEOLOGY, published in 1802 by the British naturalist and theologian William Paley (1743:1805). The Reverend Paley's NATURAL THEOLOGY proposed that the elaborations of the species of the Earth and the complexity of their organs argued for the work of a Higher Power who had created them.

Darwin graduated with modest honors in early 1831, and though he was supposed to go on to further studies for the clergy, he decided instead to take up an offer to go on a voyage around the world on the Royal Navy brig HMS BEAGLE. The BEAGLE was on a research mission, focused on mapping and survey, and Darwin was taken aboard as an unpaid naturalist. The BEAGLE departed England at the end of 1831; the journey was planned to last two years, but the ship would not return for five.

While on the voyage, Darwin read through Lyell's PRINCIPLES OF GEOLOGY, and it gave him food for thought as he inspected the evidence of gradual geological change that he observed at the BEAGLE's various stops around the globe. He was also struck by the diversity of the different species of organisms that he found, particularly by what he found in the early fall of 1835, when the vessel made landfall in the Galapagos Islands, off the coast of Chile. The creatures of the Galapagos were distinctive, even unique in many ways. There were sea-going iguanas, the only sea-going lizards Darwin had ever heard of, with a similar species of iguana that lived on land. There were oversized tortoises unique to each island.

Of particular importance in hindsight, Darwin described "a most singular group of finches, related to each other in the structure of their beaks, short tails, form of body and plumage ... " Some had small beaks for eating insects, some had big beaks for crushing seeds, and there was a range of variation in size. In fact, they so differed in appearance and habits that Darwin hardly thought them related, and was astonished when his samples were inspected by expert ornithologists later and he was told they were all finches.

Darwin's finches

Darwin had been becoming increasingly convinced of Lyell's gradualism, that the Earth changed its forms in a slow and gradual fashion. His stay in the Galapagos gave him the, for the moment latent, seed of an idea that the same ideas might apply to organisms as well -- that species evolved by slow changes over time.

* After his return to England in the fall of 1836, Darwin cast about in social circles of London for a time, wrote his memoirs of the trip, and began to assemble his notes and ideas. The memoirs were published in 1839 under the title of THE VOYAGE OF THE BEAGLE, the book was widely read, and he became a well-recognized and respected figure in the naturalist community. He married his cousin Emma Wedgwood in 1839, to then settle down in at a country estate in Down, north of London. Both sides of the family were wealthy and so he had no need to work for a living; Darwin became a full-time naturalist while he raised a big family.

He gradually began to assemble the enormous pile of observations he had acquired during the voyage of the BEAGLE into a theory of the origins of species. Beginning in 1837, Darwin had begun writing up a set of notebooks, with the pattern of his ideas coming together in a fit of note-writing. The fact that different families of species were found in different geographic locations suggested that each family was derived from a common ancestor, which had arrived in that locale in the past, with the "family resemblance" of members of the family, such as the birds of the Galapagos, also suggesting derivation from some common ancestry. Darwin knew from the variation in domestic animals, such as dogs or pigeons, that organisms were mutable, capable of changing greatly in form from generation to generation. Lyell's catastrophism gave a timescale on which such changes in species might take place, but that left one big problem: what mechanism focused the changes?

Thomas Malthus

The answer had come to him in 1838, after reading the ESSAY OF THE PRINCIPLE OF POPULATION, published in 1798 by the economist Thomas Malthus (1766:1834). Malthus suggested that human populations tend to increase faster than their food supply, with the limit on the food supply finally putting a cap on the numbers of individuals. Darwin now saw matters clearly: species competed to eat, to reproduce, to avoid predators, and those more successful in the game would dominate while the less successful would die out. If any chance variations occurred in specific individuals that made them more successful in this "struggle for existence", those changes would be passed on, with changes gradually accumulating to the extent of creating new species. Darwin called the concept "natural selection", as a foil to the "artificial selection" used to create new domestic varieties of plants and animals.

In Darwin's view, there was no fixity of species. Instead of a particular species being created according to a fixed template, in modern terms like a specific model of car or other vehicle, a particular species was an interbreeding population of similar organisms, with a range of variation around a set of average characteristics. Over time, pressures on that population, due to competitors, predators, changes in climate, and so on, would shift the average characteristics of that population, or in other words gradually modify a species. If a particular species was split into two populations -- by some geological change that creates a barrier, migration of some members of that population, or some variation in habits that isolated a subgroup of the individuals from the rest -- then in time the average features of those two populations would vary enough to make them distinct, ultimately resulting in new species. This was exactly what he saw with the finches of the Galapagos.

Like Lyell's ever-changing Earth, species themselves shifted in a very gradual fashion over "deep time", with those less fit to deal with the struggle for existence dying out and those more fit propagating and expanding their population. In fact, the ever-changing Earth implied such adaptation of species since it forced them to adapt to altered environments. The branching tree of species described by Linnaeus was not just an accident: it reflected the branching of species into other species through the game of natural selection, with the "transmutation" of species randomly occurring as though through throws of the dice and the results then being subjected to the test of survival and propagation.

There was, in Darwin's view, no Higher Power as the Reverend Paley had seen specifically guiding the creation of different species -- except for the Grim Reaper, who consigned the unfit to the discard pile of extinction. Darwin understood the public would find this idea shocking, and so he knew he would need to make a very persuasive case. He decided that no stone available to him would be left unturned in bolstering his theory, and that he would not go public with his ideas until he had nailed them down thoroughly.

* By the end of 1857, Darwin had been tinkering with evolutionary ideas for two decades, but even then he didn't feel ready to go public. He had distributed a draft on the subject to close friends and correspondents, including Sir Charles Lyell and Thomas H. Huxley (1825:1895), a biologist. Reactions were mixed; Lyell did accept that evolution occurred, but had problems buying off on natural selection. Huxley was more impressed, telling Darwin: "How extremely stupid of me not to have thought of that!" Huxley tried to persuade Darwin to stop sitting on his work, saying that he was likely to be trumped by somebody else if he didn't. Darwin was still in no hurry, but then a wild accident of chance intervened.

T.H. Huxley

Darwin had been corresponding with Alfred Russel Wallace (1823:1913 -- note the unusual spelling of his middle name), a young naturalist who was then performing field studies in what is now Indonesia. In early 1858, Wallace also came up the notion of evolution by natural selection, and decided to send a paper outlining the idea to Darwin so that he might review it, and pass it on to Lyell if it seemed to have merit. Darwin was in an uncomfortable position, neither willing to give up credit nor slight Wallace's work. After consulting with his scientific colleagues, on 1 July 1858 Darwin presented Wallace's paper to the Linnaean Society of London, an organization of naturalists -- and also presented two papers of his own the subject. The whole arrangement was a bit dodgy but Wallace, an easy-going sort, swallowed whatever annoyance he might have felt and only recorded his pleasure at being given such consideration by prominent scientists.

The 1858 presentation didn't attract much attention, likely because it was so brief and casual. Darwin had been planning to write an absolutely comprehensive work on evolution by natural selection, but the cat was out of the bag, so in a year's time he threw together a book with what he had. The book was published on 1 November 1859 under the title of ON THE ORIGIN OF SPECIES BY THE MEANS OF NATURAL SELECTION. Not surprisingly, it became known by the simpler title of THE ORIGIN OF SPECIES. It would prove to be one of the most important books ever written.

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[3] THE ORIGIN OF SPECIES

* Although Darwin, a plodding and finicky man, thought THE ORIGIN OF SPECIES was a sketchy work compared to what he wanted to write, it comes across as very thorough. He presented his argument in a well-structured fashion, working through the basic elements of the theory:

The evidence for variation under domestication was obvious. As Darwin showed, humans had long exploited the malleability of organisms to raise new breeds of dogs, farm animals, and crop plants through selective breeding.

He also pointed out that there was a considerable variation of species in nature. Although there was no immediate evidence to believe the malleability of life in the wild was as great as that in domesticated breeds, there were obviously related families of organisms that exhibited a great deal of diversity. In addition, he noted that species tended to exhibit local variations, with experts able to tell what locale a particular specimen was from. This was the case with the Galapagos tortoises, with residents of the islands easily able to determine which island a tortoise came from.

Galapagos tortoise

* That said, he presented his notion of natural selection. The ideas of Malthus suggested that organisms propagate so rapidly that they would outstrip their food supply, unless their numbers were checked. That led to the core concept of natural selection, that those species whose variations gave them an advantage in the "struggle for existence" would survive and prosper, while those that didn't would die out. It was a simple idea, a mere suggestion that natural forces could perform selection to lead to new forms of organisms, just as an animal or plant breeder used selective breeding -- artificial selection -- to obtain new breeds or varieties.

Along with the notion of natural selection, Darwin also introduced the concept of "sexual selection". The elaborate tailfeathers of a peacock obviously served no useful purpose for individual survival and were hard to explain. However, since only males had the fan of tailfeathers, that suggested that sex had something to do with it. Under sexual selection, peahens were attracted to peacocks with more elaborate tails, and so peacocks with bigger tails tended to propagate more than those with smaller tails, ultimately leading to a riot of elaboration.

Even at the outset Darwin realized that simple individual survival -- the ability to catch prey or the ability to avoid becoming prey, the ability to survive on available food, the ability to fight off disease and stand extremes of climate, or in other words the obvious elements of the struggle for existence -- wasn't all there was to the matter. For example, he observed the interdependence of flowers and pollinating insects such as bees, with each unable to survive without the other -- what in a later time would be called a "symbiotic" relationship, implying that the two lines had evolved in a mutual fashion.

Bees also often had a hive organization in which the death of an individual, such as the death of a bee after using its stinger against an enemy, would contribute to the survival of the hive as a whole. In more general terms, Darwin pointed out that "selection may be applied to the family, as well as to the individual" -- a concept which would turn out to be trickier than it sounds but which is now accepted as "kin selection".

* In sum, evolution by natural selection implied that a species isolated into two branches by geography or habit would gradually diverge through random variations into two separate species, and that the overall history of species could be diagrammed as a branching tree, with similar species arising from recent branches and less similar species arising from older branches. Darwin knew that the boundaries between species tend to be indistinct. Wolves can interbreed with dogs -- in practice usually the larger ones, since wolves tend to regard small dogs as prey -- and produce fully functional offspring. Lions and tigers can produce offspring, but they are generally sterile. More widely differing species cannot interbreed at all. Wolves and dogs are still close together on the taxonomic tree, implying they only split apart recently. Lions and tigers are farther apart, which suggests they split apart some longer time ago.

Darwin emphasized that natural selection is a very slow process, that the changes in a species would happen by slow degrees, generally not perceptible from one generation to the next. Natural selection was also necessarily opportunistic and short-sighted: no changes that didn't provide an immediate, if possibly slight, advantage to each generation would be promoted over time. Of course, changes that neither harmed nor aided the fitness of a species were also possible under the scheme, though they wouldn't be promoted by selection. Darwin pointed out examples of intermediate forms of adaptations he had come across to reinforce the notion of gradual change -- for example, a poisonous snake he had seen in South America that would shake its tail to make noise even though it had no rattles on its tail. Rattlesnakes had acquired modified scales on the tail to provide a distinctive rattling sound.

He was also fascinated by the patterns of instinct, which in insects and similarly unintelligent beasts was invariably inflexible and unchanging -- what in our modern age would be characterized as robotic, executing as commanded by a fixed program. Darwin felt that natural selection could also account for the instincts of creatures such as ants who nobody would think capable of anything resembling real thought.

In addition, instincts covered a wide range of variability, just as natural selection might assume, with Darwin pointing to the wide variation in the structure of bird's nests as a bit of evidence. He used as another example the neat hexagonal honeycombs of most honeybee hives, describing a Mexican bee species that didn't produce such a neat structure, as if it were not as far down the evolutionary path of instinctive behaviors. Along the same lines, he observed that paper wasps also built nests full of hexagonal cells -- but made of paper, not wax, which suggested the "parallel" evolution or at least modification of the same scheme in the two different species.

* Darwin noted that the way different species varied often followed clear patterns. For example, cave-dwelling fish obviously derived from fish from the outer world generally lost their eyes, such organs being irrelevant to survival in continuous darkness, in fact a liability because of their vulnerability to injury and infection. Natural selection, in its short-sighted way, simply threw the eyes away even though they were something that might come in handy in changed circumstances. It wasn't a question of all the blind creatures being related, either, because in many cases they were clearly from different stocks.

Another aspect of the case Darwin made was the flexibility of life. Why were there woodpeckers in South America that lived on the pampas, where there were no trees? Why were there crabs on Indian Ocean islands that ate coconuts, with specialized claws to tear them open? If these creatures had all been designed, their forms were arbitrary; they only fell into a perceptible pattern with the assumption that they had arisen through a branching process following purely opportunistic paths.

coconut crab

* Darwin acknowledged that the geological record for the evolution of life was necessarily sketchy, but he also pointed out that the fossil record clearly demonstrated a succession of forms consistent with an evolutionary concept. Darwin insisted that organisms would evolve by small and usually indistinguishable steps, rejecting the idea that it happened in abrupt transitions. This concept, known as "saltationism", essentially asserted that an individual of one species would give birth to individuals of a new species. As noted, Darwin believed the process of change was much more gradual.

Along with the geological evidence, Darwin pointed out that the geographical distribution of species was consistent with evolution. For example, why were there clearly different families of monkeys in South American and Afro-Asia? There are no baboons in South America, no marmosets in Africa. It seemed that the two families had split off at some time in the distant past and branched out along two different paths, what Darwin called a "principle of divergence".

Finally, Darwin concluded his argument by pointing out that the organizations of organisms into clearly related families as established by Linnaeus was absolutely consistent with the concept of evolution by natural selection, with the mechanism clearly accounting for the branching paths of that organization. Darwin also pointed to the transitions in the development of embryos to show that they hinted at relationships between organisms.

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[4] GREGOR MENDEL

* THE ORIGIN OF SPECIES proved very influential even over the short term, with Thomas Huxley acting as such an aggressive advocate that he became known as "Darwin's Bulldog". Within decades, evolutionary ideas became much more the norm than the exception within the scientific community. However, Darwin's notion of evolution by natural selection didn't take the world by storm, many scientists preferring to believe in variations on Lamarckism and saltationism.

There were two big problems with Darwin's work. The first was that in the late 19th century physicists, assuming that stars were simply radiating away the heat energy they had acquired in their formation, showed stars could only shine for tens of millions, maybe hundreds of millions of years, making the Universe much too young to provide time for modern species to have evolved by natural selection. The second was that Darwin lacked any clear idea of "heredity", of how the characteristics of parents were passed down to the next generation.

The general assumptions with regards to heredity at the time was that cross-breeding different organisms resulted in a melding of characteristics, sort of like mixing two shades of paint together. This was to an extent common sense: a child with, say, a black father and a white mother would be expected to have coloration and features -- traits -- somewhere between the two. Darwin himself was inclined towards this view, until it was pointed out that it was lethal to the idea of natural selection: any variations from the norm of a population would be diluted in a few generations back to the norm. If a white man were to be shipwrecked on an isle inhabited by black folk and adopted into the fold, then in few generations the physical traits of that white man on his descendants would simply fade away.

No variation meant no evolution by natural selection. Actually, it was obvious to Darwin and everyone else that blending was a bit too simple in itself to be the answer. For example, children might be brown-eyed or blue-eyed but never any mix of the two, and to really confound things, a blue-eyed child might have parents who both had brown eyes. Darwin never had a real clue of how heredity worked, and he went to his grave in 1885 without knowing that the problem had been effectively solved two decades earlier.

Gregor Mendel

* Gregor Johann Mendel (1822:1884) had been born in Moravia, then part of the Austro-Hungarian Empire and now part of the Czech Republic, as Johann Mendel. At age 21, he became a member of a Catholic Augustinian monastery in Brno, acquiring the name Gregor. Religious conviction didn't have a great deal to do with this career choice; the monastery was more or less an agricultural research station, and he saw it as a place where he could live a comfortable existence and pursue scientific studies.

In the mid-1850s, Mendel began a set of studies on plant breeding. As his test subjects, Mendel selected pea plants, which would prove to be almost ideal for his purpose. He identified seven different features or "traits" of the plants -- round or wrinkled peas, yellow or green pea interiors, purple or white flowers, and so on -- and started crossbreeding the plants.

pea plants

The first observation that he made was that if plants with a specific opposed trait were crossed -- for example, one plant with round peas crossed with one plant with wrinkled peas -- then all the results of the first generation of the cross all had the same single trait, with no mixing of traits -- for this example, they would all have round peas. The same principle applied to all seven traits, with crosses yielding yellow pea interiors, or purple flowers, or puffed up seed pods -- but not green pea interiors, white flowers, or deflated seed pods.

Now Mendel crossed the results of each crossing among themselves, and found out something fascinating: the second generation did not give uniform results, and the way it didn't was distinctive. Crossing the hybrid pea plants with round peas produced a second generation of pea plants, some with round peas and some with wrinkled peas. However, the ratio of the plants with round peas to those with wrinkled peas was a distinct 3:1. The same applied to the second generation of all the other hybrids. This was obviously not a pattern that resulted from chance.

Mendel concluded that the traits were encoded by what he called "factors", but are now called "genes". For example, pea plants with round peas had a factor that could be labeled "R", while pea plants with wrinkled peas had a factor that could be labeled "W". How the genes were actually constructed, Mendel had no idea. He did realize that each parent contributed one gene for the trait -- "R" or "W" -- to its children:

   RR
   WR
   RW
   WW

He also realized that in the cases where both factors were found, one factor would be "dominant" and be expressed, while the other would be "recessive" and wouldn't. In this case, "R" was dominant, so the only case in which "W" was expressed was "WW", which gave the 3:1 ratio Mendel observed. Mendel carried on this idea to crossing multiple different traits, and found patterns entirely consistent with his ideas about heredity.

Mendel completed his work in 1864 and published the results in 1866. It was one of the most important scientific documents of the 19th century -- and it was completely ignored. Mendel died in 1884, a respected and influential local figure, but not one recognized by those who knew him personally as an important scientist.

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[5] THOMAS HUNT MORGAN / THE MODERN SYNTHESIS

* By the beginning of the 19th century, Darwin's ideas were increasingly out of fashion, but two events brought them back into style with a vengeance. The first 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 believed. 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. Not only was the Universe now old enough to allow Darwinian evolution to take place, it was also possible to provide reasonably hard dates on the fossil record.

* The second event was the rediscovery of Mendel. At the turn of the century, a number of researchers performed experiments along the lines of that performed by Mendel, and forgotten paper was retrieved from the archives, where it had been gathering dust. The rediscovery of Mendelian "genetics", as the field became known, didn't do Darwin much good over the short term. Mendel had simply outlined fundamental principles, much more work had to be done to flesh his ideas out in detail, and confusion reigned for the better part of two decades.

The fogs began to clear away when careful genetic studies were performed by a meticulous American experimentalist, Thomas Hunt Morgan (1866:1945). Starting in 1909, he began to perform crossbreeding studies on the little fruitfly Drosophila melanogaster, breeding them in rows of small milk bottles on rotten bananas in his "fly room" at Columbia University in New York City. Morgan didn't believe Mendel's work at first, but then he began to obtain and crossbreed flies with unusual mutant characteristics -- for example, a male fly with white eyes, instead of the normal red eyes.

Drosophila melanogaster fruitflies

Morgan and his students were not only able to obtain Mendelian patterns of inheritance with such mutations, they were also able to link the mutations to specific features of the fly's "chromosomes", the threadlike structures found at the center of all the fly's cells. The conclusion was that the genes for these features were found in the chromosomes, though nobody still had any clear idea of how genes were actually constructed.

* It would fall to Morgan's scientific successors to dovetail Mendelian genetics to Darwinian evolutionary concepts. 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 ten research papers published from 1924 into 1934, Haldane provided neat mathematical analyses that showed genetics provided the underlying nuts and bolts for evolution. Fisher complemented Haldane's work by performing statistical analysis on how mutational change propagated through a population of organisms. 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.

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). By the 1950s, they had fleshed out the thinking of the previous few decades to establish the "modern synthesis" of evolutionary science.

In the meantime, biochemists had been hunting for the biochemical basis of the gene, and had zeroed in on a molecule known as "deoxyribonucleic acid (DNA)" commonly found in chromosomes as the agent of heredity. In 1953, a young American biochemist named James Watson (born 1928) and a British physicist named Francis Crick (1916:2004) unraveled the famous "double helix" structure of DNA, and over the next decade biochemists determined the basic rules of genetics at the biochemical level. Within another decade, tools were developed that would eventually lead to the deciphering of complete genetic codes -- "genomes" -- of organisms, providing detailed insights into their evolutionary history.

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[6] MODERN EVOLUTIONARY SCIENCE

* In the 21st century, the modern synthesis is the generally accepted doctrine of evolutionary science. None of the other competing theories have survived.

Lamarckism was a popular idea up to about World War II, but it was effectively dead at that late date. Lamarckism's notions of directed evolution and inheritance of acquired characteristics are simply not compatible with what we know about the structure of organisms. The genomes of organisms are fixed at birth, and though organisms may adapt to changed circumstances, they can only do so within the limits of the genetic "cards" they have been dealt. To the extent that they do adapt, they have no mechanism for passing their adaptations on to the next generation. That would imply some sort of "editor" that could scan the organism for changes and update the organism's germ cells -- sperm and eggs -- to reflect those changes. That might not be impossible, but it would be complicated, and if such an "editor" system existed, it would be very noticeable. Biochemists haven't found a trace of it.

As far as saltationism goes, it amounts to the idea that an entirely new and viable species, involving significant changes, could arise in a single generation. Ronald Fisher pointed out that a working 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 contrast, the likelihood of a major random change amounting to an improvement, a "hopeful monster", is poor. Completely changing the coloration of an animal at random is unlikely to improve its camouflage.

Saltationism envisions a new species arising from a set of simultaneous mutations, while the modern synthesis envisions a new species gradually arising from occasional mutations over time, with the changes screened by natural selection. Saltationism is like rolling a hundred dice in a shaker and expecting to have them all come up 6. Given one throw a minute, all day, every day, it would take far longer than the age of the Universe for this to happen. In reality, evolution is more like rolling twenty dice and setting aside every die that comes up 6, then rolling the remaining dice again until there are no dice left. Given one throw a minute, it predictably takes about an hour. No other scientific theory of evolution has been able to compete with the modern synthesis.

* Modern evolutionary science rests on a major body of apparent evidence. All dogs are genetically wolves; human selective breeding has demonstrated the mutability of species by generating a vast range of forms from the wolf template, ending up with creatures, such as pekinese, that nobody would mistake for wolves. Crop plants have in some cases gone through similar drastic modifications. It is startling to realize that a few thousand years ago, corn looked pretty much like an ordinary sort of grass. Now it features hugely distorted heads with heavy stalks to support them. Corn can no longer disperse seeds on its own and would die out in a year or two if humans didn't take care of it.

Of course, these are examples of artificial selection, but all natural selection amounts to is saying that the culling of a species by environmental pressures could produce the same sort of results. Species are so mutable that zoo-keepers trying to preserve rare animals find it difficult to maintain captives that really match their wild cousins. Animals that are happy with being captives tend to breed much more easily than those that aren't, and so zoo animals tend to become increasingly tame from one generation to the next -- which still doesn't mean that it's a good idea to walk into a tiger cage.

Evolution is particularly obvious in the case of pathogens, the most notorious example being "antibiotic-resistant bacteria". The introduction of antibiotic drugs in the middle of the 20th century provided medicine with a powerful set of weapons against dangerous bacterial infections, but even at the time the inventors of antibiotics knew that bacteria would evolve to defeat the antibiotics, and now we are suffering from an ever-rising tide of bacteria that shrug off drugs that would have killed them off neatly thirty years ago. Another interesting example was the discovery in 1975 of a bacteria that could digest nylon.

As far as more elaborate organisms go, insects are well-known to have quickly evolved resistance to pesticides like DDT. For another example, metal electric power towers that are clad in zinc to resist corrosion will form zinc deposits in the soil that kill normal grasses and other plants. In fact, most such plants grow perfectly well around the towers -- but on examination they are strains that can tolerate high levels of zinc. Try to bring in plants that grew up far away from a tower and they will die.

* These are small-scale examples of evolution in action, but no one has identified any real obstacle to large-scale change through the same processes that produce the most elegant adaptations. Consider the "sea dragon", a species of species of seahorse that looks exactly like a clump of seaweed. Could such a creature have arisen from an ordinary sea horse over many generations, gradually picking up mutations that refined its resemblance to seaweed until it acquired such perfect camouflage?

leafy sea dragons

Sure, why not? Early on, the ancestral seahorse had only a slight resemblance to seaweed, but that would be enough to give it a slight advantage, since a predator wouldn't be able to see it from as far away or under poor viewing conditions.

It should be emphasized again that evolution works on populations; "a fish" doesn't mean a single fish, it means a population of interbreeding fish. That population will have a variation in characteristics around an average value, with the distribution of those variations making up what could be loosely visualized as a bell-shaped curve, just as there's a bell-shaped curve of the distribution of heights of adult human males. There is an entire population in which there is a continuous range of appearances, with one end of the range clearly looking less like seaweed and the other end of the range more like seaweed. The fish on the end of that curve that looked less like seaweed would be slightly more vulnerable to predators to those that looked more like seaweed, causing the mean of the curve to very gradually shift towards the component of the population that looked more like seaweed. The end result is a fish that performs a spectacular impersonation of seaweed.

* It must be emphasized that the sea dragon is not trying to look more like seaweed -- it has no ability to achieve such a change through an act of will. The population of sea dragons simply undergoes effectively random mutations; those individuals with mutations that make them look more like seaweed survive better and crowd out those whose impersonation isn't as good. Evolution is an unguided process, it has no direction except increased fitness to the environment. It does not produce a solution by any plan, it produces any solution that happens to work.

Such solutions are improvisations, based on pre-existing features of organisms. For a particularly vivid example, there are two broad classes of bottom fish: skates and rays versus halibut and other flatfish. They are only similar in a general fashion; they couldn't otherwise be much more different, and the way in which they differ points straight to evolution.

Skates and rays are relatives of sharks. Sharks tend to be relatively flattened fishes, and so once they took up bottom-feeding habits there was an evolutionary tendency to become flattened further to allow them to hug the bottom for concealment and defense. The rays seem to be an elegant design for a bottom-feeder. Now consider the halibut and other flatfish. The halibut is a bony fish, like trout or herring, and the mark of such fish is that they are flattened vertically, with a cross section that is narrow and tall. The simplest way for such a sort of fish to become an efficient bottom-feeder was to turn on its side, and that's exactly what the halibut did.

flatfish

A halibut actually starts life as a more or less ordinary bony-fish fingerling, but as it matures it turns sideways and one eye migrates to the other side of its head. This is a totally inelegant approach to becoming a bottom-feeder, what an engineer would call a "kluge" and an ugly one at that, but it works and works well enough. Evolution can achieve elegance but cares nothing about it, "works well enough" does the job.

Another interesting example is the fact that a bee using its stinger kills itself by doing so. Not only does this seem poorly designed in itself, a bee that could perform multiple stings -- live to fight another day -- would clearly have an advantage over its kamikaze cousins. Unfortunately, as far as bees were concerned, the one-shot suicidal sting has proven "good enough" -- it's a "cost-effective" solution, intruders that get stung learn the lesson well enough, and the loss of worker bees doesn't impose enough of a cost to drive evolution towards bees capable of multiple stings.

* The evidence of such opportunism supports evolution. suggesting emergence of "whatever works" by "trial & error" processes. So does the fact of "convergent evolution". Sometimes evolution produces organisms that look very similar, or have similar features, but have widely different evolutionary roots and differ greatly in their details. Porpoises look much like sharks, but it's obvious that air-breathing porpoises used to be land animals. One big giveaway is that most whales and porpoises still have a set of vestigial, completely useless rear leg bones in their tails. Of course, if they hadn't been land animals at some time in the past, they wouldn't have to come to the surface to breathe on a regular basis.

vestigial leg bones of beaked whale

On the other hand, the "whatever works" nature of evolution can lead to entirely different implementations of organisms or their features that perform the same functions. Imagine a large grass-eater of the grasslands whose defense is speed, escaping a predator by running away as fast as possible. One approach is the antelope, with four hoofed, long legs. Australia came up with a different approach, the kangaroo, with twin strong rear legs for bounding and a heavy tail for balance and additional boost.

* 21st-century paleontology has continued to provide support for evolutionary science. With access to many high-quality fossil beds, hardly a month goes by without a major new fossil discovery. 21st-century "genomics" has provided an even more powerful boost to evolutionary theory.

The coding of the genome can be regarded as something like a "serial number" -- a very long serial number -- that is passed down through lines of descent, with occasional changes in the serials from generation to generation. The similarity of these serials can be used to establish family relationships between different species: two species with similar serials are closely related, two species with dissimilar serials are not closely related.

Most genomes are big, so it's cumbersome to use entire genomes to investigate the relationships of organisms. The usual practice is to identify a number of small segments of the genome and use them as "serials"; multiple segments have to be used because there are unlikely but not impossible sources of confusion in any one genetic pattern, and a "majority vote" using different segments filters out the noise.

With the power of DNA analysis, relationships that were once obscure become clear. An analysis of the relationships between the members of the cat family used 30 different sequences and came up with some surprising findings, for example that cougars and cheetahs were very closely related, and that housecats were more closely related to cougars than they were to bobcats.

Is the giant panda a relative of the raccoon, coati mundi, and so on? Or is it a very unusual bear? DNA says it's a bear. DNA analysis has not only shown, from analysis of old feathers, that the extinct dodo bird was a species of pigeon; it was also more closely related to certain existing species of pigeons than those species are related to other species of pigeons.

In sum, as far as the scientific community is concerned, few will disagree with Theodosius Dobzhansky's famous assertion that "nothing in biology makes sense except in light of evolution", any more than the study of Latin makes sense without accepting the existence of the Roman Empire. Those who appreciate Darwin's founding work can only regard it as ranking in the top list of the accomplishments of modern civilization.

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[7] COMMENTS & REVISION HISTORY

* I wrote an extended document on evolutionary theory titled INTRODUCTION TO EVOLUTION, but even as I was writing it, I knew that its length and detail would turn off nontechnical readers with only a casual interest in the subject. As a result, once the document was completed, I stripped it down and used it to construct this one-chapter primer. Since this primer is just a grossly abridged version of the parent document, I do not cite sources here -- see the parent document for a list of sources as well as more details.

* Revision history:

   v1.0.0 / 01 jan 08 / Originally A DARWIN PRIMER.
   v1.0.1 / 01 mar 08 / Some polish, changed to AN EVOLUTION PRIMER.
   v1.0.2 / 01 jun 08 / Minor grammatical corrections.
   v1.0.3 / 01 oct 09 / Deemphasis of references to Darwin.
   v1.0.4 / 01 jun 10 / Extended the illustrations set.
   v1.0.5 / 01 aug 10 / Minor corrections.
   v1.0.6 / 01 jul 12 / Review & polish.
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