Evolution ("by non-random natural selection"):
- First, you need an imperfect replicator. A "replicator" is anything that automatically makes copies of itself. Self-replicating structures are found in nature, and not all of them involve life. An imperfect replicator is something that makes copies of itself but makes small mistakes every now and then.
- When you have an imperfect replicator making imperfect copies of itself, there is going to be variety in the copies. Some of the variations (the aforementioned "mistakes," or "mutations") are going to be harmful to the replicator, some are going to have no statistically significant effect (this type of mutation is called a "neutral" mutation) and some are going to be beneficial.
- In the environment in which the replication occurs, there are many variables, such as temperature, the composition of the atmosphere, the types of terrain (or air or water) in which the replication occurs, etc. The environment also changes sometimes.
- As mentioned, some mutations are harmful. What is meant by that is that some mutations make the new replicator less likely to succeed at copying itself or even surviving until it can copy itself. Neutral mutations have no significant effect either way. Beneficial mutations make the replicator more likely to copy itself successfully and to survive in functioning condition.
- These different varieties of the replicator are part of the environment in which they "live." In other words, what they do changes their environment (e.g. by making resources more scarce by using them up or by changing the shape of the terrain.) Thus, a variety that was previously average at copying itself might be more or less well suited to another environment, and thus it might become a "weaker" or "stronger" replicator without changing itself, as a result of the change of the environment.
- When there are different varieties with different characteristics, there is competition. The same environment serves as the background for the "life" of all types of replicators, so when resources are scarce or threats emerge, some are going to lose out, while others will be more capable of survival and successful replication. This is natural selection.3
- Now that there is selection among the different types of replicators, the statistical trend will be that newer replicators are going to be on average stronger (where "stronger" means simply "more capable of survival and successful replication") than their ancestors. From this stronger generation, new generations with new mutations will emerge, and they will also be subject to competition and selection. Note that it is by no means random which individuals get "selected" for survival and which are doomed to die without sufficient offspring to keep the line alive. Instead, it is the environment that determines which traits are beneficial, and thus the development (or "evolution") of the replicators over time is not a random process.
- Sometimes, some lineages (or "families") will be separated from the main group, by migration or by some natural events (such as a river changing course as a result of a flood.) These "families" will then go on to "live" in their own environment, evolving (by random mutation and non-random natural selection) into a direction that will usually be different from the one that their cousins in the main group are taking. This is called "speciation," though that term is mostly relevant when discussing "species."
1: The word "theory" has two very distinct meanings. The one that is used in common parlance is synonymous to "conjecture." That sense of the word "theory" is very weak, in the sense that not much weight is placed on the credibility of that theory. In science and academia, "theory" has a very different meaning. To quote Wiktionary, a scientific theory is "[a] coherent statement or set of ideas that explains observed facts or phenomena, or which sets out the laws and principles of something known or observed; a hypothesis confirmed by observation, experiment etc." It is that sense in which evolution is referred to as a "theory," just like the theory of gravity, Einstein's theory of general relativity and the germ theory, which is the model that describes how microscopic organisms, such as viruses and bacteria, cause diseases. When an opponent of evolution asks why we don't call it the "fact of evolution," the correct answer is two-fold. 1: Yes, we do; and 2: we also don't call "germ theory" "germ fact," because the word "theory" refers to the entire framework, so "theory" is a more precise word for this function. If it had no evidence to back it up, it would be called "the evolution hypothesis" or "the evolution conjecture." 2: "Abiogenesis" is the term used for the emergence of life from non-life. Similar to how there is no distinct point at which one species can be said to have split from its ancestor (as any pair of organisms from that lineage would be able to interbreed if they were a sufficiently low number of generations and mutations apart,) there is no clear point where "non-life" became "life." Instead, it's a gradual process and the only reason we can make a distinction is that we can compare life at an advanced state (such as a cell) to replicators at a low level of complexity. Where we draw the line is a matter of definition, and it will never be very clear. 3: Darwin was the first to use the contrast between "natural" and "artificial" selection as a tool for explaining how natural selection works and what the time-frame is like. "Artificial selection" refers to any situation where there is an intelligent agent deciding which individuals in the sample (or collection of organisms) get to breed and which individuals they are paired with. If you want a huge cow, you need a normal flock and time. What you do is, you pick the largest ones and allow them to breed (well, not with cows, but you know.) The small ones, you remove from the process. If you keep doing this, generation after generation, you'll eventually end up with cows that are on average much larger than the ones you first had, as you will have artificially created an environment where the size of a cow is the primary trait by which it gets selected. Humans have been doing this with cows for over 10 000 years (which is longer than the age of the Earth, according to Young Earth Creationists.) Similarly, dogs have been domesticated for possibly as long as 33 000 years and at least for 15 000 years, and all current varieties have been produced via artificial selection from wolves. Natural selection differs from artificial selection in that there is no plan and no intelligent agent doing the selection. Instead, selection can be driven by other organisms or by other features of the environment. A change in the environment, such as an ice age, can cause extinctions. In the surviving species, they causes sometimes dramatic changes, when traits that in the past were very successful can become harmful, and traits that were neutral or harmful in the past suddenly (or gradually) start surviving better and are thus selected for. When the environment changes again, so do the conditions for survival, and again the species changes, possibly back the direction it used to be. A more direct means of natural selection is provided by other organisms: parasites and predation. If, for instance, there was an ant with a red, round gaster, it might sometimes be mistaken for a berry by birds. Thus, the ants with the most berry-like gaster would be selected against, and so the species would on average evolve a smaller gaster with a different colour. 4: The eye is often chosen as the prime example of "irreducible complexity," mainly because it serves as a platform for a delicious quote from Darwin, which is always mined without context. In The Origin of Species, Darwin gives this quote, followed by a detailed explanation of how the eye evolved. Quote mining target in teal, explanation in black:
This thread is for discussion about evolution. I also want to use this thread as a platform on which those who don't know much about evolution and those who don't believe it at all can ask their questions and I'll try to answer them. Of course, I'm no expert, either, but evolution is such a simple concept that I think even an amateur can defend it, especially as I have read very much about it. There obviously are people on these fora who know much more about evolution than I do, and I'd be pleased to have them here, too. In this first post, I didn't really discuss the evidence for evolution. I think I'll probably try to talk about different types of evidence as the questions start to build up, as I don't want to inflate this OP any more. The reason I let it balloon to this size is that there are some very common misconceptions that result in the same questions every time: "how come monkeys aren't giving birth to humans?" and the afore-mentioned "what use if half an eye?" and so on. I wanted to get rid of the most predictable problems before we begin. Note that I haven't proofread every part of this post, as it simply is so overwhelmingly huge that I can't find the energy for it now that I'm done writing it (in one session!) so there are bound to be mistakes, especially as I am not a native speaker of English. Please be so kind as to point out any mistakes and we'll have an OP that's easy to read and interesting. So, any questions? Anyone wanna elaborate on some of the points I made in my OP? Have you perhaps seen or read something interesting that has to do with the topic? Have at it!
edited 5th May '12 10:53:40 AM by BestOf
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