Biological evolution is defined as descent with modification. Descent with modification refers to the passing on of traits from parent organisms to their offspring. This passing on of traits is known as heredity, and the basic unit of heredity is the gene. Genes hold information about every conceivable aspect of an organism: its growth, development, behavior, appearance, physiology, reproduction. Genes are the blueprints for an organism and these blueprints are passed from parents to their offspring each generation.
The passing on of genes is not always exact, parts of the blueprints may be copied incorrectly or in the case of organisms that undergoe sexual reproduction, genes of one parent are combined with the genes of another parent organism. Individuals that are more fit, better suited for their environment, are likely to transmit their genes to the next generation than those individuals that are not well-suited for their environment. For this reason, the genes present in a population of organisms is in constant flux due to various forces—natural selection, mutation, genetic drift, migration. Over time, gene frequencies in populations change—evolution takes place.
There are three basic concepts that are often helpful in clarifying how descent with modification works. These concepts are:
- genes mutate
- individuals are selected
- populations evolve
Thus there are different levels at which changes are taking place, the gene level, the individual level, and the population level. It is important to understand that genes and individuals do not evolve, only populations evolve. But genes mutate and those mutations often have consequences for individuals. Individuals with different genes are selected, for or against, and as a result, populations change over time, they evolve.