How and why does the population change over time using natural selection

Explanation: Natural selection is a cause of evolution. So over time, organisms can start to evolve due to artificial selection. Natural selection, sometimes referred to as "survival of the fittest," is a mechanism for the evolution of a population to become better adapted for survival in a specific environment. This means that organisms with traits that are best suited for the environment will pass these traits to their offspring.

from my research i have to the conclusion that decomposers, such as fungi, bacteria and invertebrates, play a critical role in nutrient cycling and without them the earth's ecosystem may not support life as we know it. elements such as carbon, nitrogen and phosphorus exist in limited amounts on earth, although huge amounts of carbon are being released into the atmosphere from underground sources now by humans. it is still extremely important that other nutrients be retained by the process of re-use. decomposers are organisms that are able to chemically break down dead material and make these elements available for use by other organisms. without this breakdown of dead material, dead bodies would pile up. so decomposers also play a role in clean up also. without them, life might not continue as we know it and dead material would overwhelm the earth.    

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.