However, the topic of evolution comes up a lot in various situations, and I constantly find that many people are woefully undereducated - or worse, were lied to - so I figured I would clear up just what the theory of natural selection is, in a very broad and layman-friendly sense. First, a couple of definitions:
Evolution: Evolution is a phenomena. It is not a theory - it is an observed occurrence. Evolution happens and to deny is it idiotic. Evolution happens to viruses, evolution happens to bacteria, evolution is evident in our fossil records and DNA. Evolution is change within a species.
Natural Selection: The theory of Natural Selection proposes the mechanism behind the phenomena of evolution.
Theory: In common English, is is unfortunate that the word "theory" has come to mean "guess" or "hypothesis". In the scientific realm, a theory is "a well-substantiated explanation of some aspect of the natural world; an organized system of accepted knowledge that applies in a variety of circumstances to explain a specific set of phenomena; theories can incorporate facts and laws and tested hypotheses" A theory is the highest status an idea can go. A scientific law and a theory are not the same: a law describes a phenomenon while a theory explains why it occurs.
The best example to demonstrate the differences is usually gravity, because most people understand and few dispute gravity. The phenomenon of gravity is what we can feel and observe - we see falling objects, feel ourselves attracted to the earth and we observe planets orbiting stars. The law of gravity is F = G * ((m1*m2)/r2. And the theory that explains gravity is general relativity.
The Theory of Natural SelectionNatural selection is so simple it is breathtaking. From a very short, simple idea flows all the variation of life. Not to get side tracked by atheism, but I find the idea of natural selection to be more awe-inspiring and wonderful than the idea of god. There is true beauty in understanding our world.
Here is an overview of natural selection:
Some mutations will be beneficial
An individual with a beneficial mutation will be more likely to breed
Over time the beneficial mutation will become a dominant trait in the species
Over even longer periods of time the beneficial mutation will be present in all individuals of the species
The key is the third one: that beneficial mutations make an individual more likely to breed. This is where selection pressures come in. A caveat: Individuals do not evolve. Populations do not evolve. It is the gene pool that evolves. Natural selection acts on genes A gene pool is the set of all possible genes and gene combination that an individual in a species can have. At the danger of personifying, a gene's "goal" is to survive in the gene pool forever and natural selection is what chooses the winners.
If a gene is beneficial for an individual of a species it has a better chance of becoming a dominate gene in the gene pool, as individuals possessing that gene produce more offspring and spread that gene through the population over time. If a gene is detrimental to an individual, the chance of the individuals possessing that gene surviving or out-breeding those with beneficial traits is very low. Eventually the gene can be cut from the pool of available genes if all individuals possessing that gene die.
Natural selection can only work on genes that exist in the pool - that is, lifeforms don't start mutating and evolving when pressures change; the beneficial mutation must already exist in at least one individual. If environmental pressures change and no beneficial mutation exists in the population, the population dies out (unless an individual with a beneficial mutation happens to be born, though I suspect this doesn't happen often).
Mutations happen mostly randomly - errors in DNA transcription and repair, occasionally because of viruses or absorption of another lifeforms (in the case of bacteria and other single-celled organisms). Of course, most things that occur "randomly" have an average pace, and we can predict the mutation rate of DNA for a certain species, or even a certain region of DNA.
Side note: Not that it's relevant at all, but I find it fascinating. In electronics, the concept of electrical resistance is very important. The resistance in a circuit affects the current, and resistors placed in a circuit are specially designed of resistive materials to provide a specific amount of resistance. Resistance is caused by electrons randomly bumping into ions and other atoms as they move about in the "sea" of electrons. On a small scale this is extremely chaotic, but on a large scale it happens at such a predictable frequency that we can design a resistor with a very precise amount of resistance.
Selection pressuresWhat makes a trait beneficial or not for an individual?
The selection pressures determine whether a trait (gene or mutation) is beneficial, neutral, or detrimental. Many mutations happen in so-called "junk" DNA - parts of DNA that are inactive and aren't read. Others don't affect things for an individual currently, though if selection pressures change they may become detrimental or beneficial. This could be thing like eye color, scale patterns, ear size.
Selection pressures are varied and numerous - the ones people tend to think about most are climate, disease, available food and predators, but there is also something called sexual selection. Sexual selection usually occurs in species that choose mates - you probably wouldn't see it occurring in bacteria or plants. Sexual selection pressures can actually result in traits that appear to be detrimental to an individual if you consider only environmental pressures. The most commonly used example is flashy male coloration, such as many birds have. Other examples of sexual selection is many birds' propensity to sing, female humans' comparatively large breasts and baboon's bright red buttocks. Even though these traits may be bad for an individual's chances of survival, they help the individual's chances of breeding and thus help the gene's chances of survival.
RecapThis is the basics of the theory of natural selection. Of course there is so much more to say about evolution and natural selection, and lots of myths and arguments to debunk. Perhaps another day, although I covered some of the more common ones in my duo of posts "Arguing with creationists" and "Take Two"
If you take nothing else away from it, take this: Natural selection works on genes, not individuals. Selection pressures are what determines if something is good, bad or neutral for an individual. There is no final goal for natural selection - there is no "best" way of living and reproducing. Each individual species finds their niche or dies out, and it will continue on like this forever. The driving force behind evolution is chance and things are always changing.