Hereditary variations are based on particles called genes. Genes retain their identity as they pass from one generation to the next. They do not blend with other genes. Variation in continuously varying traits is largely based on several discrete genes, sometimes many, each of which affects the trait slightly. This is called polygenetic inheritance.
Genes sometimes change to alternative forms.
These changes are called mutations and the alternative forms are called alleles. The rate of these changes is usually very slow. The phenotypic effects of such mutations can range from undetectable all the way to very great. The rate of mutation may be affected by environmental factors but such factors do not preferentially direct the production of mutations that would be favorable to any specific environment. Evolution is a process of change in populations, not in specific individuals.