note: I wrote this after I got into a bit of a debate with a creationist at a social visit. I said I’d e-mail him. I started typing, and the result was a 1400 word essay. Do I think this will convince him? Not at all. But here, I hope, is an introduction to evolution for laypersons. I know, there are no footnotes, but I can provide them. A lot of good stuff was gleaned from the talk.origins archives. Comments and suggestions would be appreciated.
Evolution, by definition, refers to a change in the frequencies of genes over time.
A gene is a “patch” of DNA that codes for a certain function, or contributes to a certain function, within an organism. So, for instance, there are several genes that contribute to the size and shape of your nose. Changes in these genes mean a change in the size and shape of your nose. Not all people have the same mix of genes, as you can readily tell by looking at the noses of people around you. One gene can have several versions (called alleles).
The way for a gene to change or for new genes to form is through random mutation. This is an incredibly common process — every day new cells are formed in your body that harbour mutations. Most mutations are deleterious, that is, they are not “good” for the organism’s viability in general. Others are “neutral,” that is, they make no change on their own. And a few are “beneficial” — they increase the chances of the organism’s viability. When a deleterious mutation occurs, it often results in the death of the cell or the organism. This is how we function. This doesn’t always happen. Cancer, for instance, is an example of deleterious mutations that go unchecked by a body’s immune system.
Certain genetic mutations may seem to be deleterious, but are also beneficial. For instance, “full” sickle cell anemia in black people is a very dangerous and life-threatening disease. However, having “half” sickle cell traits offers protection from malaria.
What evolution refers to is the change in the frequency of alleles (gene versions) in the gene pool of a certain population over a period of time. Genes and genetic mutations (that is, new alleles) are propagated over several generations. Deleterious mutations aren’t always propagated (but sometimes, they are, as in sickle cell anemia).
What, exactly, selects for the genes that will be passed on? This is referred to as selection. Selection can be natural or artificial. Artificial selection occurs everyday in labs and farms, by deliberate and specific human intervention. For instance, the Canola oil we consume is a result of artificial selection and controlled breeding. On the other hand, natural selection is what occurs in nature: Certain genes are extant and propagate because they allow an organism to better adapt to its surroundings.
For instance, bacteria and viruses are constantly evolving — that is, their gene pools are constantly changing. In the mid-20th century, penicillin was seen as a wonder drug to take care of bacterial infections — it works by breaking down the cell walls of gram-positive bacteria and preventing repair. This means the bacterial cells pretty much burst open when they try to reproduce. Wide use of penicillin, coupled with bacterial genetic mutations, meant that eventually strains of bacteria developed that were able to resist penicillin. Whereas penicillin was originally a very potent drug, it is now almost useless against bacteria acquired in hospitals — because these are remarkably resistant to penicillin. That’s evolution.
Every year, you need to get a different flu shot. That’s because the influenza virus mutates and spreads its mutations rapidly. Scientists then have to find out the new strains, develop antivirals, and mass produce them. When random mutations become too wide and unpredictable — or unmanageable — then you have an influenza epidemic on your hands. That’s evolution.
In the late 19th century in Britain there was a moth population (peppered moths) that was for the most part largely light coloured — it blended in well with its surroundings. Due to the development of factories and the soot that resulted from them and accumulated on trees, these light coloured moths stuck out. Birds ate them more and more. The dark coloured ones, hitherto in the minority, soon found that they were doing better than the light coloured ones, and gradually became the majority. Their alleles (the ones that allowed them to be darker) had increased in frequency in the gene pool of the population over time. That’s evolution.
Evolution then is an empirically proven fact. So far, so good — organisms continuously adapt to their surroundings because of natural selection.
Where is the controversy? Speciation — that is, the development of “new” species. Differences in species means, essentially, that for genetic, ecological, behavioural or geographical reasons they remain in “reproductive isolation” — one species does not mate with another in natural conditions.
What happens here, in speciation, is just the same as what we have been talking about so far as evolution. Allele frequencies change, geographical and geological events occur over time to the point that reproduction between two organisms from separate populations doesn’t occur. (This doesn’t necessarily mean that it can’t happen. It just means that it doesn’t happen. For instance, tigers and lions are two separate, distinct species. But they can mate and produce offspring — not the most healthy and viable offpsring. Essentially, this points to the fact that, although functionally and genetically distinct, they are closely related.) We have to remember that “species” is an arbitrary category invented by humans to understand the world around them.
Let me give you an example of how a speciation event may occur: A certain population of an organism is separated goegraphically (geographical isolation), let’s say population B gets transported to an island. This means that there is no interbreeding between population A and population B.
Because of natural genetic changes that occur over long spans of time, and also because of sociological and behavioural changes, there will be a marked difference between population A and population B. Even if they are introduced to each other and are biologically capable of reproducing, behavioural changes in mating procedure and considerable visual differences will probably mean that they will not (under natural conditions) mate and reproduce. Just like tigers and lions.
Given even more time in geographical isolation, it is possible that the genetic differences are so varied that, although population A and population B can reproduce, their offspring cannot. Like horses and donkeys, they can produce mules — but mules are sterile and cannot reproduce.
Given yet more time in geographical isolation, population A and population B, while resembling each other to some degree, may not be able to mate and reproduce at all. Kind of like gorillas and chimpanzees.
I should disclaim here that I am not saying that gorillas and chimpanzees have had more time to evolve than donkeys and horses, who have had more time to evolve than tigers and lions. Nor am I saying that they evolved due to geographical isolation (there are other ways of speciation events occurring). I am simply pointing out possibilities — all of this does not have to have happened over a relative time scale — but it did happen over a long amount of time and with various forms of isolation, not just geographical. To get ideas of time scales it is best to consult with zoologists who specialize in these respective animals.
Has speciation been observed directly? Yes, in laboratory experimentation with worms and flies and also with plants — and in nature as well.
Further evidence for speciation events comes from the fossil record. While the fossil record will never be complete due to geological considerations, it has contributed to showing the evolution of several organisms in remarkable detail (for instance, the horse’s evolution from a much smaller animal is well documented and substantiated by geological evidence). Anatomy shows us homologous and vestigial structures that are remnants of genetic history (such as cartilagenous hind leg appendages in whales). Some of the most compelling evidence of evolution comes from the genomes of various animals. Comparing a human’s DNA to a rat’s, for instance, shows considerable differences — but also some interesting similarities. Compare a human’s DNA to a chimpanzee’s, and the similarities become far more significant and numerous than the differences. Humans and chickens share a lot of genetic material, but chickens and reptiles share a lot more.
There is a lot of evidence for evolution, from all kinds of disciplines of science: Geology, paleontology, anatomy, cell biology, genetics, etc. The evidence is so overwhelming that evolution is a matter of scientific consensus.
While it may seem that organisms exist in immutable forms, they clearly do not. DNA changes every day, all over the world, in all kinds of ways. Gene frequencies change over time — and we have had billions of years to work with it (the Earth itself is about 4.5 billion years old). Evolution — the change in gene frequencies over time — is an indisputable fact.