audubonguides

Last week’s entry covered how scientific names work and describe a given organism’s place in the evolutionary framework. This week, I’ll build both back and forward on that, going from the origins of the system to what modern molecular genetic research has told us about how life fits together (there are a few surprises)! We’ll look briefly at the careers of several biologists, from the 18th century to the present day, who have shaped our understanding of life.
We actually need to start more than a century before Darwin published On the Origin of Species, with Swedish physician and botanist Carolus Linnaeus (sometimes written as Carl Linnaeus or Carl von Linne). Linnaeus’ great contribution to biology was his two books Systema Naturae (1st edition 1735, but the influential edition is the 10th, of 1758), and Species Plantarum (1753), which introduced the binomial (genus and species) method for naming animals and plants, respectively. Linnaeus is also responsible for a significant portion of the system of names above the genus level, although some of those levels are later additions.
In addition to having invented the system of nomenclature we use today, Linnaeus named approximately 7,700 species of plants and 4,400 species of animals. Many of his names are still in use today, including such familiar organisms as the Harbor Seal (Phoca vitulina), the Red Maple (Acer rubrum) and even ourselves (Homo sapiens) – in 1959, his successors honored Linnaeus by designating him as the lectotype for humans – the formal description of human beings is “the animal species that includes Linnaeus”.
While Linnaeus was a great field biologist, leading expeditions throughout Sweden for most of his life, he rarely left Sweden, and never during the period when he classified thousands of species. Most of his classifications were based on material his students sent him from around the world. Students of Linnaeus traveled to an amazing variety of places (several sailed with Captain Cook, for example) and sent back specimens for Linnaeus to classify and name. He also came amazingly close to evolution without ever understanding it, and, in fact steadfastly denied the possibility that any sort of evolution could exist. Looked at through an evolutionary lens, it seems amazing that anyone could have completed this brilliant work of classification without becoming aware that organisms were related to each other. Linnaeus placed humans, monkeys and apes together in the Primates, while always maintaining that each species was separately created in an unrelated event, and that his system was merely a method of organization – he was famous for saying “God created, Linnaeus organized”. Linnaeus came amazingly close to evolution several times without actually seeing it – in addition to correctly grouping humans with other primates for the first time, he was also the first to recognize that both bats and whales are mammals. He seems in all those cases to have been acutely aware of biological relationships, yet unable to make the mental leap from separate creation to an evolutionary paradigm that would permit these species to actually BE related.
After Linnaeus designed the modern system of biological classification and hugely expanded the number of known species, many scientists began to wonder whether each species was, in fact, a separate creation. While evolutionary ideas were beginning to be discussed by the beginning of the 19th century, nobody had worked out a system by which evolutionary change could occur. It was thought that since the descendants of any given species were always of the same species, the species were immutable. The chicken and egg problem is the classic example of this – a chicken is what lays an egg, but is also what hatches from one, so how does a non-chicken ever lay a chicken-producing egg? The answer to this was uncertain until the middle of the 19th century, and was the fundamental impediment to a workable theory of evolution.
In 1831, a young naturalist named Charles Darwin set off on a long sea voyage aboard HMS Beagle, a survey ship of the Royal Navy. Darwin was actually aboard primarily as a social companion to Captain FitzRoy, because English captains of the time were of a much higher social class than their crews, and forbidden to socialize with them. Darwin’s secondary function aboard the Beagle was to keep notes on geology and natural history, a position in which he excelled, and which would later bring him fame. As Darwin traveled around the world observing the plants and animals, he was able to see relationships between them – the data point that was directly in front of Linnaeus and many other earlier biologists, but that none had satisfactorily explained. It took him over 20 years from the first rough sketches of the idea of evolution to the publication of On the Origin of Species, a timeframe that seems less surprising when you consider that the Origin is so beautifully thought out and written that it is still referred to today, 152 years after its first publication in 1859.
Starting while he was still aboard the Beagle on the five-year voyage, and refining his ideas over many years at home in England, Darwin developed the theory of evolution by natural selection. Different members of a species are not identical – they possess slight, but important variations. According to Darwin, and to 150 years of biology since he published On the Origin of Species, individuals which have variations favorable to survival will be more likely to survive to reproductive age, and to leave offspring behind, which will carry the (initially randomly occurring) variation. This view of species as essentially mutable – individuals vary, and those which have variations which benefit their survival will leave more offspring, enhancing their contribution to the next generation – is diametrically opposite to Linnaeus’ view that each species is perfect and unchanging, created by God for a specific purpose.
In Darwin’s view, species diverge from a common ancestor when any of several circumstances occur. Two populations of the same species could become separated (one moves to a new habitat), where different variations lead to better survival. The variations are present in some members of both populations, but one form leads to better survival in one location, while a different form is advantageous in the other. Eventually, in the absence of contact between the populations, they might diverge into separate species. Alternatively, a change in environment for part of a population might mean that a different set of variations was most successful, leading to a distinct population that may eventually diverge far enough to form a new species. A third possibility is that species could diverge through competition with each other, rather than through environmental change. For example, a bird with a mid-sized beak could evolve towards a small-beaked and a large-beaked species even in the absence of environmental segregation, if the smallest-beaked birds could take advantage of one food source, and the largest-beaked ones another, while a bird with a mid-sized beak was a disadvantage in exploiting either source. All of these possibilities have been confirmed by research over the last 150 years.
Darwin built on Linnaeus’ classification of species, bringing relatedness to Linnaeus’s order. A Linnaean view would say “these two species in the same genus are similar, therefore they belong next to each other, like library books on a shelf”. Darwin found the mechanism to say “they’re not just similar, they’re related to each other and share an ancestor, and that’s why they’re next to each other”.
Darwin’s great insights were the importance of subtle variations and time. Until shortly before Darwin, the Earth was believed to be only a few thousand years old, leaving time for a fairly limited number of generations of most species. Geologists in the half-century before Darwin were revising the age of the Earth steadily upward, giving substantially more time for change to occur – in Darwin’s day, there wasn’t an agreed-upon age of the Earth, but estimates were from hundreds of thousands of years to hundreds of millions or more (the currently accepted estimate is slightly over 4.5 billion years).
An older Earth gave time for subtle variations to accumulate – a non-chicken didn’t have to lay an egg from which a chicken hatched. Over many generations, a near-chicken could lay an egg that produced a slightly nearer-chicken, and, even today, a chicken doesn’t lay an egg that produces exactly the same chicken (as a matter of fact, from the standards of what Linnaeus first named Gallus gallus (a chicken), some of what shows up on today’s large poultry farms may be perilously close to being non-chicken, a different species).
While Darwin was formulating his grand idea, there was no concept of genetics in the intellectual discourse at all. People had long known that children tended to look like their parents (and that children who looked exactly like the wandering peddler who passed through town nine months before their birth indicated something had gone on). Nobody had provided any mechanism for why this was true, however.
At almost exactly the same time Darwin was working out the mechanisms of evolution, an Austrian monk named Gregor Mendel was studying heredity. While Darwin was studying the world as it is on a broad scale, gathering ideas from a round-the-world voyage, Mendel was the great small-scale experimentalist. He planted thousands of pea plants over several years, keeping track of the parents of each plant, and of the characteristics of the offspring. Mendel worked out how characteristics are passed from generation to generation.
Mendel was the first to discover that every organism receives one copy of each gene from each parent, and that it is random which of a parent’s two copies a given offspring receives. He also developed the concepts of dominant and recessive traits – a dominant trait appears if you get at least one copy of its gene (from either parent), while a recessive trait is only visible if each parent contributes a copy of its gene (if one parent contributes the dominant version of the gene, and the other the recessive, the offspring will have the dominant appearance, not a blend of the two). Mendel had worked out most of the basic math of inheritance, at least for traits that are controlled by a single gene (many complex traits are influenced by several genes, which makes the situation more difficult).
Unfortunately, and unlike Darwin, who was one of the best-known scientists of the 19th century within his lifetime, Mendel’s work was largely forgotten until well after his death. He presented his work as primarily about hybridization, without going into the much larger implications of inheritance in general. As a monk, and later abbot of his monastery, he was not able to give his scientific work the wide publicity it deserved. Other scientists dismissed Mendel as a monk breeding peas in a garden, not realizing until many years after his death that he had discovered the first principles of the modern science of genetics.
Mendel’s genetics was promptly forgotten about for 35 years, as Darwin’s evolution raised firestorms of criticism, largely from a religious establishment that did not want to hear that humans are close relatives of apes, preferring instead to view humans as direct descendants of God. By Darwin’s death in 1882, the battle was essentially over, at least in scientific circles. Just about every biologist had accepted that Darwin was right – debate would continue, and still does, on the precise mechanisms of evolution, but Darwin’s basic idea of evolution through natural selection was, and still is, essentially unchallenged. Many religious scholars also came to accept Darwin’s ideas relatively quickly – he was buried at Westminster Abbey, with many of the senior clergy of Britain in attendance.
Mendel’s ideas would take much longer to achieve general acceptance – his great paper was published in an obscure location, and discussed at a local natural history society. Shortly afterwards, his work faded into obscurity, where it remained for 35 years. Around the turn of the 20th century, several biologists began to independently work out the laws of heredity, and, in the process rediscovered Mendel’s writings. It was quickly accepted that Mendel had, in fact, pioneered this work in the mid-19th century.
Through the first half of the 20th century, the thrust of evolutionary thought was applying Mendelian genetics to the study of Darwinian evolution, leading by the middle of the century to the understanding of evolution that, on a basic level, is current today.
Mendel’s work only said that some material was passed from parent to child that carried genetic information – it made no assumption as to what that material could be. That would have to wait for nearly a full century, until James Watson, Francis Crick and colleagues worked out the structure of DNA in the early 1950s. The structure of DNA (and with it, how genetic information was passed on a molecular level) caused another revolution in our broader-scale understanding of the relationships between organisms. Building on the original work on the structure of DNA, recent biologists have been able to decode significant portions of the genetic code, presenting a clearer picture of evolutionary relationships among species. As we have come to understand the genetic basis of relationships between species, the emphasis of taxonomy has shifted from groups of physically apparent relatives (how Linnaeus constructed his classifications nearly 300 years ago) to groups of genetically closest relatives, called clades. Clades can be nested within each other, just as Linnaeus’ genera nested within families, families within orders, and so on. Clades of roughly the same extent as Linnaean groups are still called by the Linnaean name, so a Linnaean family that more or less encompassed a molecularly defined clade would still be called a family.