Each organism has two alleles for one trait. The actual alleles are a code that express a genotype, the component genes of an organism that are inherited. As these genes are expressed, they for the phenotype, the visible or expressed characteristics of an organism. An organism's genes are inherited from their parents. So there is variation, because each parent has two alleles and can only pass one to the offspring.
Evolution causes species to change over time.
These changes can cause a species to improve and become more adapted to an environment or it can lead to extinction. In the wild more animals are produced that live to reproduce. It is the process of natural selection that chooses which animals will reproduce. This is how genes become the key point of natural selection, those animals which reproduce pass on their characteristics to the next generation. A biotic community is the existence of each population as well as that of each individual that is governed by some extent by all others.