Let's now talk about the processes by which evolution works. This is fairly obvious stuff, but bear with me as I point out areas for improvement, expansion, and reference to humans. The first and foremost principle of evolution which made Darwin well-known is Natural Selection. Again, this links back to a previous principle, that of "all life that cannot exist does not", so many of the principles flow into one another. An important note to make about natural selection is that it is a retroactive process, which means that it works backwards. It does not predict the future, or selection pressures, but instead it acts as a sweeper of life that is no longer self-supporting when faced with a new reflection of its environment. For example, the reflection of a sudden and prolonged lack of food for say, dinosaurs, is put simply a dinosaur without food, and this is a non-supporting life form. Therefore, it becomes extinct.
The principle of Natural Selection states that all living organisms must be adapted to their environment, be it a natural environment such as a lake that has taken a thousand years to form, or an artificial environment such as an incubator for premature babies. Of course, the incubator is adapted to the baby more than the baby is adapted to the incubator, so adaptation works both ways, especially in the case of humans who are able to manipulate their environment to a greater extent than wolves or lillies. But let's save the topic of human evolution for later, and focus firstly on natural environments, because it is those which result in the evolution of life capable of manipulating its environment to a great degree, such as humans.
Natural environments are defined as environments which cause life forms to exist, rather than be caused by life forms themselves. For example, a classroom cannot be defined as a natural environment because it has been made by humans. However, a desert is a natural environment for snakes. The key characteristics of natural environments which take part in evolution are called selection pressures. Selection pressures are the changing properties of an environment which cause life forms to have to respond. These, too, are natural or artificial. As a human, you will be more aware of the artificial selection pressures than of the natural ones. For example, you might be less worried about a storm than your debt. However, in natural environments the selection pressures are natural too, if they are a cause of the environment itself.
Natural selection pressures such as increasing temperature or floods, can impact on living organisms so that most are unable to survive. This automatically removes those organisms' genes from the gene pool by their death, so that "naturally", only the few that happened to have a mutation which favoured their survival are left over. They multiply, so that their advantageous mutations are passed on, and future generations are resistant to the change that happened in the past, that is the increase in temperature or flooding. This doesn't stop here, because selection pressures change - although at different rates in different environments at diferent times. For example, sharks are thought to be the best adapted species on Earth, and they have been around for millions of years. So even though their environment may have many changing properties, these are not selection pressures, because the sharks are already well-adapted to most environmental changes.
On the other hand, there are species which experience changing adaptations frequently, due to their ever-changing environment. For example, certain species of insect have been discovered to have had wings 10 million years ago, lost them 5 million years ago, and now have their wings back again. They have always had the gene responsible for wing development, but this gene was turned off 5 million years ago as a result of their environment and its selection pressures at that time. If some insects happened to have had no wings, and the selection pressure involved predating birds eating them during flight, then those with no wings would have survived and passed on their turned off wing-gene.
Natural Selection is a retroactive process, as we have seen, which involves the death of ill-adapted life, and the propagation of the leftover life which has survived the selection pressures. What about foreseeing of selection presures, and planning ahead? Is there such principle of evolution?
Yes, there is. Let's now move on to the proactive selection process, Sexual Selection in the following Post.