One of the big guiding forces of evolution is what we call natural selection and selective pressure

Different organisms of the same species will have different traits. Some will be taller, some shorter, some darker, some lighter, etc.

And every single trait can fit into three categories.

1) helps the organism survive and reproduce in its environment

2) hurts the organism's survival in its environment

3) has no hurtful or helpful effect at all

And these traits can be passed down to offspring, and these offspring can either take the trait as is, or there might be some sort of mutation or recombination which changes it slightly.

Now, the organisms that have harmful traits, also called deleterious, will have a higher tendency to die off on in that environment, meaning that they won't be able to pass on the harmful trait to the next generation. As a result, only the non-harmful traits get passed on. Over many generations, the species will have a greater and greater concentration of the organisms with non-harmful traits, and fewer and fewer of those with harmful traits, which is how we discuss evolution.