How does natural selection lead to evolution?

Evolution describes the divergence of species by natural selection. It is best to think of this at the level of the gene pool. The gene pool is all the genes contained within a population. The gene pool is affected by selective pressures which are environmental challenges that cause one genetically determined trait to be favoured over another. Lets go back a little. In a population there is variation, meaning that not everyone is a clone of one another but some genes are found in some and not in others. New genes arise by mutation. Mutation is happening all the time, and it is necessary for the continued survival of a species. The mutated gene will most likely decrease the fitness of the individual (we can discuss why this is ) but in very rare cases it might confer an advantage to the organism. Increased fitness means the individual is more likely to survive and breed successfully, meaning that it is more likely to pass on its genes to the next generation. The gene structure of the population will change over generations because the gene that increases fitness will proliferate untill all the members of the population have that gene. This means there comes a generation where all or almost all the members can be traced back to one ancestor, where that mutation arose. This change in gene structure happens because the environment selects for it; for example it could be a mutation that leads to denser hair and a mutation that leads to more subcutaneous fat which means that in a suddenly sub zero climate these individuals are more likely to survive upto reproductive age because they retain body heat better. They're more likely therefore to pass on these genes. The species evolves because there is a widespread adaptation to the new sub zero climate. However, consider the same two genes within a population where the climate has reached a new normal of around 30C, maybe these individuals are more likely to die of heat exhaustion so these gene decrease the fitness of the individual. Hence evolution is less about the genes themselves and more about the interaction between the genes and the environment, this describes natural selection.