A multicellular organism develops from a single zygote, and its phenotype depends on its genotype, which is established at fertilization. Genes are a set of instructions, encoded in the DNA sequence of each organism that specify the sequence of amino acids in proteins characteristic of that organism. The genetic composition of cells can be altered by incorporation of exogenous DNA into the cells. Stability in an ecosystem is a balance between competing effects. The frequency of an allele in a gene pool of a population depends on many factors, and may be stable or unstable over time.
Evolution is the result of genetic changes that occur in constantly changing environments.
Organisms have a variety of mechanisms to combat disease.