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Evolution

Introduction

Fossils provide a window into the past, and even early in human history the occurence of fossils has stimulated people to strive for a better understanding of the Earth. How is it possible that fossil shells are found high up in the mountains, hundreds of miles away from the coast? Why do certain fossils appear in one rock layer and not in the other? How is it possible that so many fossil lifeforms are no longer present today?

These, and many other questions, each havecaused landslides in human thinking throughout history. They stood at the cradle of notions as stratigraphy and the build-up of layers, plate tectonics and mountain formation, and the notion of an Earth that is much older than the human mind can possibly comprehend. But perhaps the most fundamental shift in our thinking is related to our origin, and to the origin of all life on Earth: the notion of evolution.

 

Change of species and extinction

Throughout geological history, we constantly see the coming and going of species. In successive geological strata we find all kinds of new fossil life forms, most of which to disappear again rather quickly from the fossil record when looking at younger layers. In some cases, there seem to have been instances of mass extinction, such as at the end of the Permian or the Cretaceous. This image conflicted with the idea of an unchanging and perfect divine creation, as was assumed in many cultures. The notion that species are variable and that they can arise and go extinct again is an important insight, that paved the way to an understanding of how exactly this came about. This process is now known as evolution.

The insight that species were actualy succeeded by other species throughout geological history was, at least for some, not convincing enough that species themselves were changeable. It offered no explanation of what happened exactly and what underlying mechanisms were responsible for the changes. The catastrofist Georges Cuvier was convinced that relatively calm periods alternated with heavy catastrophes, which would provide an answer for both the geological landscape and the sequence of species in the fossil record. According to Cuvier, individual species, however, were unchanging. And it was here that this briljant anatomist, was eventually proven wrong.

"Why has not anyone seen that fossils alone gave birth to a theory about the formation of the Earth, that without them, no one would have ever dreamed that there were successive epochs in the formation of the globe."

G. Cuvier, 1831

An alternative to catastrofism was uniformitarianism, introduced by James Hutton and refined by Charles Lyell, amongst others. It stated that throughout geological history the Earth has been formed by the same natural phenomena as we see nowadays. This implicated not only that the Earth was ancient, but also that the current natural laws and their influence on living beings throughout history have always been the same. The realization that geological history had lasted much longer than what had been presumed so far, was  crucial for the evolutionary way of thinking: the change of species, after all, took a lot of time.

Before Darwin, various scientists had presented  incomplete or erroneous models of how species would have changed. One of the most well-known theories is by Jean-Baptiste Lamarck, which suggested that acquired properties could be passed over the generations. Although this theory is now obsolete, it is one of the first elegant hypotheses on how evolution might work.

 

Darwin and Wallace

It was Charles Darwin who presented a solid theory about the evolution. Thanks to the family fortune Darwin was able to focus throughout his life on his passion, the study of the different facets of biology. The accuracy with which he carried out his research turned out to be his greatest asset; he would carry out carefull experimental research for years before eventually publishing his insights.

The fact that Darwin was a brilliant scientist is underlined especially by the amount of thorough, fundamental work he published, besides his well known ideas on evolution. His publication about barnacles, for example, was one of the most crucial works about this group ever and it is to a large extent still considered to be scientifically relevant. He has pioneered with his work about the movement of plants (mainly vines and carnivorous plants), behaviour, dispersion and migration of animals, all substantiated by numerous ingenious trial designs. He was one of the first to point out the importance of earthworms within ecosystems. His model for the growth of coral reefs for the first time offered an explanation about their occurrence and shape.

Very decisive in the life of Darwin was the journey he undertook as a naturalist aboard the HMS Beagle (1831-1836). It is common knowledge that his visit to the Galapagos Islands during this trip was crucial for the development of Darwin's later insights. Also inspiring, however, was the collecting and research of the fossil mammalian faunas of South America which Darwin carried out earlier. Here, too, Darwin found fossils of animal species which so far were unprecedented. To Darwin it was clear that these were extinct species.

On the Galapagos Islands, Darwin encountered a large-scale evolutionary experiment, even though at that time he was not aware of its significance: several volcanic islands that were successively created in a relatively short period of time, at a reasonable distance from the mainland but not too far to enable colonization, and at a sufficient distance from each other in order for most animal groups to ensure a good geographical isolation. For these reasons, different types and variations from common ancestors could develop on each island.

Many of these species were clearly different from, yet related to, species of the South American mainland. The most well-known example are the so called Darwin’s finches, which developed divergent shapes of their beaks on different islands, depending on their diet. Also clear to Darwin were the variations between the different spot birds and lizards, in addition to many other organisms.

Darwin started a large collection of animals and plants which he collected on several islands, but the significance of the above-mentioned patterns was hardly noticed by Darwin during his visit. He didn’t document systematically what specimen he had collected on which island, which is crucial to understand how species differ between the islands. Without a doubt he regretted this very much at a later stage. After his homecoming, Darwin understood the significance of these variations all too well, and through some connections he still managed to purchase some systemetically collected specimen to substantiate his ideas. Darwin, who had been on the road for nearly five years and who had been seasick nearly all this time,  consequently never set foot outside of the British isles again.

 

'I think...' with a rudimentary sketch of an evolutionary tree (Notebook B, p 36)

 

The idea that species were mutable was at that time well known (but only recognized by few). Darwin's big idea is not that he showed that species were mutable, but that he was the first to propose a mechanism that could explain these changes, namely the variable inheritance of properties and natural selection. But perhaps his greatest merit is that he – mainly due to the fact that he was unsure of himself – developed this idea during several decades with great care, a Titans work which ultimately led to his most iconic publication: "On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life" (1859).

"[...] At last gleams of light have come, & I am almost convinced (quite contrary to opinion I started with) that species are not (it is like confessing a murder) immutable. Heaven forfend me from Lamarck nonsense of a “tendency to progression” “adaptations from the slow willing of animals” &c,—but the conclusions I am led to are not widely different from his—though the means of change are wholly so— I think I have found out (here's presumption!) the simple way by which species become exquisitely adapted to various ends. [...]"

C. Darwin, quote from a letter to J.D. Hooker, January  11th, 1844

It was not only the realization that his work would have a big impact on the established conservative thinking of that time, but also the fact that it would be less than welcome in the religious corner, which prompted Darwin to keep working on his book until the idea was sufficiently substantiated to resist fierce criticism. Darwin hesitated to publicize for many years, until in June 1858 he received an essay from a befriended scientist, Alfred Russel Wallace, who had arrived at the idea of natural selection on his own account.

Darwin saw the relevance of his life's work collapse and Lyell quickly organised a joint presentation by Darwin and Wallace for the Linnean Society. Neither Darwin nor Wallace was willing to quibble for the right of recognition, and they both continued  their scientific work with a mutual respect for each other. Soon it became clear that where Wallace had come up with the idea of natural selection in a very creditable way, Darwin had already worked it out and tested it thoroughly, and had already found connections with insights from other scientific disciplines.

"[...] The numbers that die annually must be immense; and as the individual existence of each animal depends upon itself, those that die must be the weakest—the very young, the aged, and the diseased,—while those that prolong their existence can only be the most perfect in health and vigour—those who are best able to obtain food regularly, and avoid their numerous enemies. It is, as we commenced by remarking, “a struggle for existence,” in which the weakest and least perfectly organized must always succumb. Now it is clear that what takes place among the individuals of a species must also occur among the several allied species of a group. [...]"

A.R. Wallace, quote from a letter to C. Darwin, june 1858

The publication of "On the Origin", and later also of "The Descent of Man, and Selection in Relation to Sex" brought a rarely seen paradigm shift in scientific thinking. The idea that man as a species was an animal amongst the animals, tob e grouped with primates, stood at odds with the anthropocentric worldview of man as a supreme creation. From a theological point of view it was unacceptable. The controversy about this has long left the scientific arena, and by now there is a broad scientific consensus around the validity of the developed theories about evolution.

 

Neo-Darwinisme and genetics

Although most of Darwin's insights still apply today, for Darwin himself there was still some essential scientific knowledge missing, resulting in a number of questions forcably remaining unanswered. One of the most fundamental missing puzzle pieces concerned the underlying mechanism of evolution. Although Darwin was not aware of this, the cornerstones were already being laid by Gregor Mendel, who studied the heritability of traits in plants (which eventually resulted in the genetics). With the discovery of DNA and the development of molecular genetics in the following century, the last missing building blocks for a solid theory of evolution were provided. The combination of the Darwinian insights with modern genetics is called neo-Darwinism.

 

Phylogenetic family tree, based on RNA-analysis (redrawn from NASA)

 

For Darwin, the foundation of an evolutionary process was descent with modifications of inheritable properties, in combination with a selection process. Darwin concluded that there is a "struggle for existence" between individuals in a population. Only the best adapted individuals will survive ("survival of the fittest"). In a natural ecosystem we speak of natural selection, but breeders of pets or plant cultivators basically use the same process. In those cases we call it artificial selection. A fundamental difference is that man selects targeted to promote certain properties (e.g. milk production, disease resistance, color, etc.), while natural selection is a spontaneous process.

Natural selection is an unconscious selection on fertility: the capacity of an individual to pass genes on to the next generation. This often leads to the misconception that selection pressure effects the individual, the group, the population or the species, while this is not the case. As an individual, and thus the properties of an individual (including the ability to survive and to reproduce) develops in function of the expression of the genetic material, in a specific context (both the environment and the presence of other genes) the selection pressure occurs at the level of the genes (and of groups of genes).

Nowadays, we know that by far most molecular changes in the genetic code originate from neutral processes, i.e. genetic modifications that have no significant influence on the fertility of their bearers. This is called 'genetic drift'. Two populations that are genetically isolated fro each other, drift apart (genetically speaking), even when there is no selective pressure on these populations. Natural selection does play a role in evolution, but this role is limited.

Basically, evolutionary insights can be applied to any system in which a form of inheritance of information can be linked to random modification and selection. Thus, evolutionary thinking is sometimes applied to describe the emergence and change of ideas in a society. After all, ideas are passed (whether or not changed), and picked up or rejected in a selection process. The simplicity of the theory of evolution, combined with its strong explanatory power, is what makes it to be one of the most revolutionary scientific discoveries ever. This inspired the evolutionary biologist Dobzhansky in 1973 to remark that ' nothing in biology makes sense except in the light of evolution '.

 

Evolution and the fossil record

From the above explanation one would expect that fossils have played a role in the formation of the current views around evolution. The fossils themselves, however, play just a modest role in recent research around evolution. Although fossils literally tell the story of how life has evolved, the fossil record itself is immensely patchy, which causes all kinds problems. It's like telling the story of a movie based upon three fragments of 1 second each. Disciplines such as molecular genetics, evolutionary biology and population ecology offer a much more detailed understanding of the mechanisms of evolution and speciation at different scales. It is much easier to tell the story if you have parts of the movie script.

An important observation is that the fossil record integrally corresponds to what you would expect on the basis of evolutionary biology: in successive layers, species follow each other up  in a logical manner, and pass on specific characteristics which have evolved within the group, on to the next groups. Properties that are less prominent become obsolete or disappear altogether. Some groups change little over longer periods of time, which can be explained by a status quo in environment variables and their adjustment tot his situation.

Other groups change relatively dramatically over short periods of time, often caused by big changes in environmental conditions. Periods of increased extinction are followed by the intake of the vacated ecological niches. Also biogeographical patterns are recognizable. Thus, the distribution of fossils is often associated with models of continental drift, through which one can recognize the phases of colonization and migration, and so on.

Sometimes fossils give a more complete picture of the changes that take place within the group. Over a period of time, so called lineages can be recognized in different layers with similar faunas; a gradual transition from one form to another. These lineages can provide important insights into the evolutionary gradient, and help to link different fossil groups.

It is a cliché that paleontologists are looking for a 'missing link', a fossil that is an intermediate form between other fossil forms. The term ' missing link ' is a highly mediatized understanding, which in reality is both confusing and meaningless. For example, let’s consider a hypothetical group, say species A, which in the course of evolution passes on to become species B. If this transition takes place over a period of 2 million years, with an average generation time of 2 years, we have about 1 million generations to go from A to B. When exactly did species A  become species B?

When we look at 1 generation, we see that the parents are of the same species, and the children are of the same species. Nowhere between A and B we see a generation where parents from species A have children from species B. The transition from A to B runs gradually, through small adjustments. The distinction between types A and B in the fossil record is only marked because of the missing fossils in the intermediate layers: 1 million missing links, so to say. From this point of view, it could be concluded that each fossil which is discovered, was a missing link until its discovery.

"Nothing in biology makes sense except in the light of evolution"

Theodosius Dobzhansky (1973)


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