Err, actually species evolve because of random chance. Whether that particular permutation of genetic strands survives and multiplies depends on the situations it has to overcome, which is also random to an extent.
I’ve uploaded this for anyone who needs it. It’s not anti-religious (like some of this author’s work is), but I think it will get you up to speed on the theory of evolution if you haven’t gotten up to date on it so far. It’s really excellent and thoughtful work. I for one am not religious, but I think this book demolishes the creationist point of view while also getting people (religious or not) interested and passionate about real science.
Even more accurately, its a fact the life has become more diverse and complex over time.
(an aside) Its actually somewhat mistaken to say the evolution happens because of natural selection. Natural selection causes a reduction in genetic diversity (by killing off the weak and stupid), while evolution is the result of new genes, an increase in diversity.