Atheism

Atheism ("a-theism") is the belief that there is no God.

The ancient Christians were called atheists because they did not believe in the Classical gods (for example Zeus and Aphrodite, also known as Jupiter and Venus).

Parodies of Theism

Russell's Teapot

For the purpose of argument, Bertrand Russell asserted without giving any proof, that a teapot orbits the Sun somewhere in space between the Earth and Mars: he can't be proved wrong, so should we believe him? From the Russell's Teapot entry at Wikipedia :–

Russell's teapot is an analogy, formulated by the philosopher Bertrand Russell (1872–1970), to illustrate that the philosophic burden of proof lies upon a person making unfalsifiable claims, rather than shifting the burden of disproof to others.

So it isn't up to atheists to disprove that God exists: atheism is the null hypothesis.

Pastafarianism

A modern version of Russell's Teapot is the Flying Spaghetti Monster. "Believers" call themselves "Pastafarians" and claim privileges given to members of religions, such as tax privileges and licence to wear special headgear. They attack religious beliefs of all kinds, and especially illogical arguments advanced in defence of religious beliefs, with light-hearted parody. See an example at Unworthy Arguments.

Confused thinking about Atheism and Evolution

* Atheism is disbelief in gods, or disbelief in the One God of the Bible. The first Christians were called atheists because they did not believe in the Greek and Roman gods.
* Evolution means unfolding — for example we might talk about the evolution of Mozart's musical style during his career. As used in Biology, it refers to the evolution of species — the kinds of living creatures.

Wikipedia is a good place for more formal definitions of the many senses in which Atheism and Theism are used, and of Evolution in Biology.

No necessary relationship

Clearly, there is no necessary relationship between theism and the observation that there are different kinds of living creatures; but there is overlap for many individual believers who see the hand of God in nature. Some believers in God who accept mainstream biology describe their beliefs as "theistic evolution" or "evolutionary creationism", while others reject these terms as being logically incoherent. See here for an outline of these viewpoints.