Evolution is the scientific theory that is intended to explain the diversity of life on Earth. The most basic definition of it is "cumulative change through time". In biology, it says that populations of organisms are subject to change as a result of environmental factors acting on them and causing them to adapt to specific environmental changes.

There are several mechanisms of evolution, but the most common and powerful one is natural selection.    This mechanism is based on several facts and one conclusion based on those facts:

1. All organisms overproduce, making more offspring than are necessary to replace themselves.
2. Populations of organisms tend to remain stable in most environments.
3. There will be variation in the traits and forms of specific organisms in a population.
4. Those variations in organisms that are better suited to a specific environment will most likely help their possessors survive and reproduce themselves.
5. Over time, as environments change, the populations of organisms will change along with them.

Now, there should be nothing controversial about such things, since they are based on perfectly straightfoward observations about organisms that can occur today. But because the implications of evolution strike so deeply at how humans view themselves, some people, using their religious beliefs as the excuse, have argued that evolution is somehow evil or destructive.

Also, because evolutionary theories have been applied so extensively to past events, some people have claimed that evolution is somehow not scientific. But that is ludicrious. ALL scientific theories are based on  clearly defined physical or chemical laws and ALL of them must have application to past events or they are useless, because time itself is a never ending stream of present events going into the past. The only way evolution could not be scientific is if it was not consistent with the scientific laws already established via the scientific method. And there is no evidence that evolution violates any scientific law, despite claims to the contrary by certain anti-evolutionists, who themselves ignore the contraints of the scientific method in reaching their own conclusions about the world they live in.

Mutations are the primary source of variations that appear in populations of organisms, and they are result of random changes in the sequences of genes. Genes are made of DNA, a molecule made of thousands or even millions of repeating units. Chemical or physical reactions may shift or remove one or more of those units, causing the DNA sequence to code for a different trait in an organism than it otherwise would. Because mutations are random in nature, their results are far more likely to weaken the resulting organism, called a mutant, that is born with the new trait. Then the organism will die later as a result of competition with others, or at least it will fail to reproduce itself, and the mutation will disappear along with its possessor when it dies. But occasionally a mutation will occur that actually gives a organism an advantage of some kind over others, and then it will survive, reproduce itself, and its decendants will inherit the beneficial trait. Eventually, nearly all the members of the population will have that trait, and the species itself will have changed. Either that, or the population itself will split in two, with one group having the new trait and the other not having it, and the two groups will begin to live apart, thus establishing a new species.

In addition to environmental factors, another way of changing the population of organisms is via sexual selection. In this, the members of one sex in a population will express a preference for certain traits in members of the opposite sex, resulting in that trait becoming common in the population even though it does not actually contribute at all to the individual organism's survival. The only thing that matters is that the organism reproduce itself, and so a male bird may evolve things like long tail feathers (which may make it more vulnerable to predators) to prove to females of the same species that it is strong enough to provide good genes to the female's offspring. A deer may grow long antlers for the same reason, though the antlers may also provide both an ability to fight with other males for the females' favor, as well as a defense against predators.