A distinct kind of organism with characteristic size, behavior, and habitat that remains constant from year to year. Endangered Species - any species that is in danger throughout all or a significant portion of its range. Extinction - the state in which all members of a group of organisms have disappeared from a given habitat, geographic area, or the entire world. In order to study extinciton and endangerment of species, one needs to know the basic biological theories and facts that cause a species to change. The main thread common to all life is DNA and genetics.
All organisms have genes, which consist of alleles, forms of gene coding for one single trait.
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.