Big Idea I: Evolution is inherited change within a lineage.

To most people, the term “evolution” means change over time. This is a correct definition when discussing non-biological evolution such as the evolution of the solar system or the evolution of computers. It is also correct in this context, but more is necessary. The biological definition of evolution is inherited change within a lineage.

To fully understand the biological definition of evolution, it helps to consider each part separately.

• Inherited. All organisms reproduce and pass their genes to their offspring. Each new generation inherits their genes and associated traits from their parents.
• Change. The new generation is not an exact replica of the parental generation. Sexually reproducing organisms get half of their genome from each parent, so the result in each offspring is a combination. Parents that are most successful at reproducing contribute more offspring to the next generation, so their genes claim a higher percentage of the new generation. In addition, random genetic mutations occur that change the genetic composition of the next generation and introduce variationVariation:
  The differences among individuals in a population. among individuals.
• Lineage. As each generation reproduces it creates an uninterrupted chain of genetic inheritance from ancestors to descendants.

All together, these three concepts detail the biological definition of evolution. If you follow any lineage of organisms from ancestor to descendant, you will find inherited changes, in other words, evolution.

Bivalves have been evolving for over 500 million years. The bivalves alive today have some similarities but also many differences from those discovered in the fossil record. The similarities between modern and ancient bivalves provide evidence of their common lineage. The differences are evidence of their evolution over time.

For more discussion on this topic, see the sections Why Study Evolution?: Defining “Evolution” and How Evolution Works.