Evolution (I)
Guest Essay by
Brian van der Spuy, of the Netherlands:
What it is, and some evidence for it.
Ask some of your friends what they understand under the word "evolution", and you'll be surprised how many different answers you get. "It's the idea that we all come from monkeys." "It's a theory that tells us where fossils come from". "It's a fact, not a theory". "It's only a theory, not a fact." And so on.
. In my experience, there are few aspects of science so thoroughly misunderstood as evolution, so in this article I shall attempt to clear up some of this confusion. In the next article, I'll make some notes about about one specific theory of evolution, namely "natural selection".
What should really be cleared up is what exactly is meant by the word evolution, and the rest will be easy! Broadly speaking, people can mean several different things when they use this word, and I'll discuss four of these, each under its own heading.
1. The fact of evolution, or does it really occur?
Evolution can be simply defined as the change of organisms over time, or more specifically, the change of the frequencies of genes in populations over time. Let me illustrate this by an example: suppose that in a population of animals, 50% of the individuals possess a gene that codes for the manufacture of a certain protein. Suppose that at some later time, we find that 60% of the population now possess that gene. If this is the case, then evolution has occurred by definition, since the frequency with which a gene occurs in the population, has changed.
In the title of this section, I ask the question whether evolution really occurs, and the answer is an unequivocal "yes". A change in the gene frequencies of populations can be, and is quite routinely observed. Evolution is an observed fact. [The various theories try to account for the observed facts. JKH]
The book in which Charles Darwin first proposed his theory of evolution is provocatively called The origin of species, and one might well ask whether new species can come into existence via simple changes in gene frequencies. For most people, it is easy to see how say, fruit flies, could evolve eyes of a different color over time, but many have difficulty accepting that new species, unable to interbreed with the parent species, can evolve. But this part of evolution is also fact. During the twentieth century, such speciation events have been observed so many times that biologists are always slightly puzzled when people say "the evolution of new species is only a theory." The fact is that speciation is so common that no biologist is surprised to observe it anymore; in fact, in many instances it is expected, and the researcher would be surprised if it doesn't happen. So what is all the debate about? I'll discuss that in the next section.
2. The theory of evolution, or why does it occur?
We have established that evolution does occur, and that new species can arise in this way. But in science, we do not so much gather up collections of facts, as try to understand those facts and explain why they are the way they are. When it comes to evolution, this is an especially difficult process, because organisms and ecosystems are very complex and it is difficult to unravel chains of cause and effect.
The fact of evolution has been accepted long before any widely accepted theories as to its mechanism have been proposed. Even in Ancient Greece, proto-evolutionary theories were proposed by philosophers. Early in the nineteenth century, Jean Baptiste de Lamarck (1744 - 1829) proposed that evolution occurs by the inheritance of acquired characteristics. According to this, any physical characteristics that an organism acquires during its lifetime, can be passed on to its offspring. If an ape spent lots of its time swinging in trees, it would eventually acquire a strong torso and arms. Lamarck would propose that the ape's offspring would inherit these characteristics, and because it started out well adapted for a life of swinging in trees, it might well spend even more time doing this than its father, in the process acquiring even stronger arms and torso which it would pass along to its offspring. In this way, we would eventually end up with modern apes, with their immense barrel chests and powerful hands and arms. (Note: I don't know whether Lamarck himself actually ever used this example; it is only an illustration of his ideas).
Lamarckian ideas are not accepted today. Even in his own time, the flaws in his ideas were quite apparent. For instance, Jewish boys have been circumcised for millennia, yet they are never born circumcised. So it didn't look like acquired characteristics could in fact be inherited. And even if they could, what would prevent detrimental characteristics, like tumors, broken bones and disfigurements from being inherited as well? It didn't look as if Lamarckian principles could explain the origin of species very well.
The theory that is now the most widely accepted was proposed by Charles Darwin and Alfred Russell Wallace and is known as natural selection. Since it forms the subject of my next article, I am not going to say much about it now. For the moment, the only point I am trying to make here is that evolution as such is a fact, and the only aspect of it that is at all debated, is which mechanism is responsible for it. Most biologists are of the opinion that natural selection plays by far the major role.
3. Common descent, or "even if evolution occurs, are we really descended from monkeys?"
So now we have established that evolution does occur, and let us for the moment accept that it occurs by natural selection. This still does not prove that all living forms on the planet are descended from one or a few original forms.
Contrary to popular opinion, the main evidence that all present life forms share one or a few common ancestors does not come from fossils. Fossil evidence is useful to help us reconstruct specific evolutionary lines of descent, and the fact that such lines can be constructed is useful evidence in itself, but the main evidence that there are such lines, comes from living organisms.
When it comes to common descent, there are really two things which we would like to establish: 1) Are modern organisms really descended from previous - and different - ones, and 2) do all modern organisms have a common ancestor? These two questions are interrelated, and I shall treat them together.
How can we, in principle, know that organisms, or anything at all for that matter, have a history? Let me rush in where angels fear to tread, and use an analogy. If you go into shop and look at a brand new bicycle on display, it is really difficult to say anything about how it came into being. A bicycle is a marvel of engineering, and even more so a new one, so we can't really infer how and by which steps it was made, or even if any steps were involved. Maybe the engineer just called it into existence.
. But go to a third world country, and take a look at some of the bicycles you see there, and it becomes immediately apparent that they must have been constructed bit by bit. The frame might have been made by one manufacturer, the two wheels by two different others, and the tyres by yet another. Look closely and you see that while the cycle is now red, it was originally painted blue. The front brake is still working like it should, but the mechanism of the back brake has been modified. The bell isn't working anymore, but it is still there. The owner has painted it bright purple as a decoration. The point is that this bicycle, unlike the brand new one, very visibly has a history that we can trace by all the weird little quirks in its structure. It is very difficult to imagine that a bicycle engineer would have designed a bicycle to look like this, but it is easy to see how someone working in his back garden with limited resources, would have cobbled the thing together, ingeniously using whatever spare parts he could get hold of. What we want to look for in nature is the biological equivalent of a third world bicycle, little strange and unexpected features of organisms that would speak of their history. Such examples abound, and one can write whole books about it. Here, I shall just mention a few; the idea being not so much to make a comprehensive list as to show what kind of evidence can be observed in living organisms that points to their evolutionary past.
But they have a further quirk that points to an evolutionary history. In order to manipulate shoots of bamboo, pandas have a "thumb" on each front paw. Anatomical studies of this structure clearly shows that it is not a digit, like in the case of human thumbs. It is in fact constructed from one of the panda's wrist bones, while the front paw still has all five its digits intact. The panda's thumb is not really as efficient as a "real" thumb would have been, but it works well enough. Still, it seems an awful lot like this structure is not the kind of thing an engineer would have designed, unless he wanted us to think that it had evolved!
More recent evolutionary history is also sometimes strikingly visible in embryos. As an example: most of the largest whales on earth are baleen whales, i.e. instead of teeth, the have structures called baleen plates in their mouths, with which they filter plankton from the water. But they are thought to be descended from whales that had teeth, and in one stage of their development, the whale embryos also do in fact have embryonic teeth which are later lost.
Abiogenesis, or where did life come from in the first place?
Theories of abiogenesis attempt to explain how the first living cells could have come into existence from non-living matter. Since evolution is defined as the change in life over time, abiogensis strictly speaking has nothing to do with evolution. From a purely evolutionary perspective, the first cells could have formed from non-living chemicals, been placed on earth by aliens or have fallen from the blue sky, and it would make no difference to theories of how it developed and changed from then. But since many people often use the word evolution when they actually mean abiogensis, I'll include some notes on it here. However, the subject of abiogenesis is really more in the fields of chemistry and biochemistry than biology as such, and since I am no chemist, I cannot make more than a few brief statements.
Until around the middle of the nineteenth century, it was widely believed that animals such as fleas, flies and other vermin could spontaneously arise from organic matter. By around 1900, this idea had been quite thoroughly debunked though, through the work of Louis Pasteur and others, and these days when we talk about abiogensis we mean something entirely different.
In the 1950's, a famous experiment was done by Harold Urey and Stanley Miller in which atmospheric and weather conditions believed to have existed on the primitive earth were simulated. Within a short time, organic molecules such as amino acids, the building blocks of proteins, had spontaneously formed from inorganic substances such as water, carbon dioxide and ammonia. It became clear that the basic building blocks of life form quite easily.
But going from basic building blocks to even primitive cells is a long jump. It is now generally thought that the first living entities were acellular molecules capable of making copies of themselves. Such molecules can be synthesized in the laboratory, and it is also known that RNA, a molecule similar to DNA and which can also carry the genetic code, can replicate and direct some chemical reactions without the enzymes that are usually necessary when DNA is used as genetic material. Experiments have also shown that certain inorganic substances such as clays can perform some of the functions normally performed by enzymes in the cell, and other experiments have shown that under the right conditions, oils similar to the substances that make up the cell's outer membrane will spontaneously arrange themselves into little cell-like structures.
Most biologists are therefore of the opinion that the first living entities on earth arose spontaneously from non-living material.
And this is where I shall end this article. In the next, I shall take a look at the most important evolutionary theory, namely the one known as natural selection.
Evolution links: (to return, click on
"minimize" or "eXit".