Evolutionary biologist Ernst Mayr (1904-2005) liked to stress that basic evolutionary change is a two-step process1. The first step is the production of genetic variation in every generation. In sexually reproducing organisms genetic variation is created by random mutations in the DNA of eggs and sperm cells and also by genetic recombination. New genes may also be introduced into a population when an individual of the same species migrates over from another population. The essential characteristic of this step is that nothing, other than chance, guides the production of genetic variation.
The second step of evolutionary change is the choosing of the genotypes that will produce the next generation. This is where natural selection operates, while chance plays a less important role.
These mating soldier beetles (Chauliognathus pennsylvanicus, family Cantharidae) are in the process of generating variation. Soon the chromosomes in their eggs and sperm cells will recombine and generate the genes of their offspring. Any mutations in the parent beetles' DNAs will also be passed on to their offspring.
All the while, the beetles are under the ever watchful eyes of natural selection, which means that the individuals that are good at getting food, avoiding predators and other dangers or finding a mate are more likely to survive and leave offspring than those that are not as good.
Learn more about evolution at these sites:
Tree of Life
1. Mayr, E. 1963. Page 128 in Populations, species and evolution; Mayr, E. 1988. Page 98 in Toward a new philosopy of biology.