All populations have variation. As I wrote, we can see that as a matter of fact. You can look at individuals, you can have your genome sequenced and my genome sequenced and observe that they will be different. You say I 'need a population with variation': All populations have variation.
(And, again as I wrote, we can see why this arises. With sexual reproduction you will not be a copy of either of your parents. When chromosomes separate errors sometimes occur and so on. We can observe these processes.)
Yes, that is evolution! Except that at some point the difference between the new hens and the original population becomes such that they can no longer interbreed. Then you have a new species. The same thing may happen again to each of these newly separate species, such that there will be further divisions. (Although, of course, most species go extinct).You start with a population of multicolored hens. Then you let them reproduce and select red ones (a recessive trait lets assume, not even present phenotypically in starting population to make things more spectacular), generation after generation, so that in the end you have a hennery all red. You call this evolution? What's new here that wasn't present in the native population, except for a quantitative difference in the pool of genes?
The process is not of one thing turning into something different, but of splitting. All species, the ones alive now and the ones that have gone extinct, all have a common ancestor. That is why we humans share genes with plants and coral and fungi.
As I say, that is what is meant by evolution. Hopefully that has cleared up the main source of confusion.
I'm not sure what you mean by 'complex'. The number and size of genes can increase or decrease. It doesn't follow either that what strikes us as a more complex, or 'advanced' life form has more than a simpler one.After rereading this, it's funny that you even start with a population that is more complex than the final one, as by selecting out genes you simplified the genetic pool. This could be the opposite of evolution, but not worth discussing that.
https://www.boundless.com/biology/textb ... 513-13093/