On Dec 27, 1831, a wooden ship sailed from Plymouth, England, on a voyage of exploration. Aboard was 22-year-old Charles Darwin, recently graduated and pleased with the dream job he had secured as the naturalist on the HMS Beagle. The four-year trip around the world was in part meant to document the surprisingly different sets of species in separated regions. During the trip, Darwin found many previously unknown mammals, birds, insects, plants, crustaceans, fish and fossils. When he returned to England, his experiences provided the basis for his popular books on geology and natural history. Darwin never left England again. After decades of further study in his home lab and library, Darwin presented a simple but shocking idea that has stood the tests of science and time.

Darwin was a thoughtful observer of nature, who wrote detailed accounts of many living things. Nonetheless, he was a regular person, who even made blunders (like forgetting to label his collections of the famous Galapagos Finches with the names of the exact islands each came from). Although he spent long hours with books and a microscope, he also enjoyed playing billiards, raising pigeons and collecting beetles. He and his wife Emma had 10 children who were witness to his fascination for living things.

The children were so familiar with his intense studies that one of his sons, while visiting a neighbour's house and noting there was no lab, asked, "Where does your father do his barnacles?"

Given the enormous impact on science, and the public ruckus that ensued and still flares up, it might seem what Darwin had to say was strange and complicated. On the contrary, the idea is pure simplicity, and therein is the genius.

Darwin was well aware of the methods of selection used by animal breeders to produce new forms of livestock and pets. Selection of traits works because of the considerable genetic variation present in populations of living things, and because many of the traits of interest are inherited. Darwin saw that such variation exists in natural populations as well. A large surplus of offspring are produced by wild organisms, which are then reduced in number by limited resources, competition, harsh conditions, predators, parasites and pathogens. Only a fraction survives. The survivor traits are passed to the next generation, resulting in a slight shift in the typical traits in the breeding population. Slowly, the traits of those individuals who have the survival edge or some breeding advantage become more dominant in subsequent generations. Given enough time, this can produce new varieties and species. Changes in environmental conditions can result in new species similar to their ancestors in some ways, but better adapted to challenges.

The mechanism, "natural selection," successfully explains changes seen in plants, animals and microbes over time. Darwin's idea was the start of modern evolutionary theory.

By the way, theory means a body of knowledge, not that it is unproven. Someone who says evolution is "just a theory and not a law" has failed to grasp this most basic definition in science.

Evolutionary theory is no longer directed only at understanding how the living world came to its current state. It is opening new doors, including developing new medical technologies based on an understanding of genetic change and evolution of pathogens, and on our own genetic heritage. The science started by Darwin just might save your life.

Dan Johnson is professor of Environmental Science and Canada Research Chair (Evolution and Ecology) at the University of Lethbridge.