First of all, god damn it that this thread already got to 2 pages before I even looked at the OP for the first time.
Your question demonstrates a misunderstanding of the theory of evolution. Don't worry, most people have the same misconception as you do. So, let's go slow.
First of all, you need to understand that the smallest biological unit to be able to evolve is a population. Individuals can't evolve. Families can't evolve. It takes a whole population of individuals to evolve.
Forget everything you've seen in movies like Evolution starring that guy from the x-files or the underworld franchise. Evolution takes many many many steps and many many millions of years to have any effect that would be noticable.
So, let's zoom in and look at a population in a short amount of time. Suppose we have a population of googoogaga. A googoogaga is a species of flightless birds that live on an island near the lost continent of Atlantis. One day, an individual googoogaga is born with a genetic mutation that makes his head slightly larger than other males. When he grows up, he is able to use his head to knock out competitive males and so he's able to mate with more females.
2 million years later, it is now instintual for googoogagas to fight each other with their heads to compete for females. Where are all the googoogagas that didn't have big heads? Did they die off? No. They just simply failed to produce offsprings over many generations and thus that trait no longer exist in googoogagas.
Then the inhabitants of Atlantist experiment with fusion weapons and conviniently blows up the continent, sending millions of tons of dust into the atmosphere. Surrounding areas face extended periods of winter. Plant life begins to perish.
Having a big head actually requires a lot of energy to maintain. So, over many generations, googoogagas with big heads can't eat enough food because of the long winters and so googoogagas with smaller heads begin to flourish over the ones with big heads.
I hope you're following me here. The point is evolution happens fluidly over long periods of time and that entire populations evolve, not just any single groups of individuals. Unless a populations are separated by geography or whatever else, they will continue to interbreed and evolve together.
What I just described above is just 1 dimension of evolution. There are many dimensions. I highly encourage you to start reading and take courses. Like everything else in life, it's not as simple as posing a single question expecting a life changing answer.
I'm an engineer. I sure hope nobody will ever ask me how I go about designing a steel frame. I can't even begin to describe the process to a non-engineer and not have them all confused.
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