ABSTRACT
If we stop for a moment and consider the vast diversity of living things that surround us—over 1 billion species of animals (of which 300–400 million species are beetles), 248,000 species of plants, and about 5,000 species of bacteria—we cannot help but be in awe. What has led to this great diversity? Yet, as more genomes—the complete genetic blueprints of organisms—get sequenced, we find great similarities among very different organisms. For example, humans and the bacterium that causes cholera are 34 percent identical in protein sequence for the gene coding HMG CoA reductase, a metabolic enzyme important in the production of cholesterol in humans. Why are organisms that appear so different really so similar at the genetic level? Evolution by natural selection is fundamental to our understanding of the similarities and differences among all organisms at the many different levels of organization, from gene sequences to physiological processes to complex behaviors. Indeed, as stated by evolutionary biologist Theodosius Dobzhansky, “Nothing in biology makes sense except in light of evolution.”
