What is the point of selection step in a genetic algorithm? I'm reading about genetic algorithms, and I'm not sure I understand the point of selection step.Let's say we have a population of size $N$.How many chromosomes should we select using any selection method and what's the point if the population size is fixed ? 

From what I understood till now we select a number of chromosomes (I still don't know how many should we select) and let's say we use Proportionate Fitness selection method and we chose $k$ 'good' chromosomes, then only on those $k$ chromosomes we apply crossover and mutation and let the other unselected ones untouched to the next iteration ? I'm not sure I got it right.Someone can clarify this for me ? 

Thank you !
I think the confusion can be isolated in this sentence: "From what I understood till now we [...] [choose] $k$ 'good' chromosomes, then only on those $k$ chromosomes we apply crossover and mutation and let the other unselected ones untouched to the next iteration?"

Selection plays the role of the hand of death. Unselected organisms correspond to those that died without reproducing. Far from being left "untouched for the next iteration", unselected organisms are discarded. Now, to maintain the population size, we need to create new organisms to replace the discarded ones. This is done by applying mutation and cross-over to the chromosomes of the selected organisms. The next generation will contain a mix of the selected organisms that are left untouched, and mutants of those selected organisms. Biologically, the idea is that the next generation is the organisms that survived and their children. As an optimization technique, the idea is to cull out bad solutions and consider only variations of the "best" current solutions.