My sophomore year at Cornell I took a class on evolution and it had a major impact on my life. The class was more about the cultural relevance and the large-scale pathways of evolution than the biological mechanisms. We discussed a lot of population genetics and had the opportunity to read Darwin's On the Origin of Species. As I was reading Darwin's argument for both evolution as fact and natural selection as its mechanism was, at least to me, flawless. It really painted a picture of natural selection not only being a possibility, but an innevitability.
I've included a quote from the summary of Chapter 4: Natural Selection:
If during the long course of ages and under varying conditions of life, organic beings vary at all in the several parts of their organisation, and I think this cannot be disputed; if there be, owing to the high geometrical powers of increase of each species, at some age, season, or year, a severe struggle for life, and this certainly cannot be disputed; then, considering the infinite complexity of the relations of all organic beings to each other and to their conditions of existence, causing an infinite diversity in structure, constitution, and habits, to be advantageous to them, I think it would be a most extraordinary fact if no variation ever had occurred useful to each being's own welfare, in the same way as so many variations have occurred useful to man. But if variations useful to any organic being do occur, assuredly individuals thus characterised will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance they will tend to produce offspring similarly characterised. This principle of preservation, I have called, for the sake of brevity, Natural Selection. Natural selection, on the principle of qualities being inherited at corresponding ages, can modify the egg, seed, or young, as easily as the adult."Charles Darwin, On the Origin of Species
Here Darwin describes two basic qualities which must be present for natural selection to occur: variations that can be passed on to offspring and a struggle for resources. With these two qualities, there can be no question whatsoever that adaptation occurs and over time, can conceivably add up to drastic changes and apparent speciation.
However, the subject of this website is not evolution as fact. Whether or not you believe that evolution was responsible for creating all the beautiful life we find here on Earth, you can still come to understand certain aspects of natural selection that we will exploit in GP, namely:
- The variations that allow evolution to occur are random: There is no all-powerful agent controlling the way genes mutate. Genes mutate randomly as a function of error of the transcription process, ie DNA makes a mistake when copying itself. This means that any mutation can either be beneficial, damaging, or have little-to-no effect.
- Survival of the fittest: This term gets thrown around a lot and is often misunderstood, but in essence it is really a very simple and powerful explanation of how evolution occurs. The individuals who are best able to survive just naturally do and if they procreate, their genes are passed to future generations. If an individual does not survive, it cannot reproduce and any maladaptive mutations which caused it to be unfit to survive just disappear from the gene pool. Fitness to survive is an incredibly complex measure and depends on many seemingly unrelated factors, but the idea that certain traits give certain individuals a better chance of making it is not too hard to understand.
- Evolution occurs in populations, not individuals: Evolution can only work in populations of several individuals. A single individual or line of individuals cannot evolve, because there is no selection of positive adaptations over negative mutations. Without processing information on the fitness of several individuals, no evolutionary progress occurs.
- Evolutionary progress occurs through the selection of fit individuals and that selection is controlled by the struggle for existence: I always italicize the word progress when referring to evolution, because using it invokes teleological arguments. Evolution has no end game or goal, it is merely a consequence of natural tendencies. That being said, by progress I mean the adaptation of a population to a particular environmental niche. It is more likely that individuals proving to be more fit will contain components of an ideal individual than less fit individuals. Due to limited resources, there is constant compeptition between all individuals, but the ones who are more fit, by virtue of being more fit, are more likely to pass on those beneficial genetic components to offspring. This relationship is the fundamental theory of natural selection.
- A side note- We did not evolve from chimps or any other currently living species. The idea behind speciation is that at one point in history there was a species which eventually split into two species which eventually became today's humans and chimps. Our most recent common ancestor may have been more chimp-like than human-like, but that is not to say that chimps have not experienced any selective pressure since the split.
This is just an extremely basic summary of natural selection. If you would like to know more, I suggest starting by going straight to the source. Read On the Origin of Species and Darwin's other books. They demonstrate a very advanced notion of natural selection.
Now we will begin actually working with GP. It is easiest to understand GP as it relates to the problems it can solve. So let us introduce Three Simple Problems.