D. Molecular Similarities

With advances in molecular biology in the last few decades, researchers seek evolutionary clues at the smallest level: within the molecules of living organisms. Despite the enormous variety of form and function seen in living things, the underlying genetic code – the molecular building material of life – displays a striking uniformity. Almost all living organisms have DNA, and in each case it consists of different pairings of the same building blocks: four nucleotide bases called adenine, thymine, guanine, and cytosine. Using different combinations of these bases, DNA directs the assembly of amino acids into functional proteins. The same uniform code operates within all living things.

These molecules contain more than the master plan for living organisms – each is a record of an organism's evolutionary history. By examining the makeup of such molecules, scientists gain insights into how different species are related. For example, scientists compare the protein cytochrome C from different species. In closely related species, the proteins have amino-acid sequences that are very similar, perhaps varying by one or a few amino acids. More distantly related organisms generally have proteins with fewer similarities. The more distant the relationship, the less alike the proteins.

The idea that species become genetically more different as they diverge from a common ancestor laid the groundwork for the concept of the molecular clock. Scientists know that, statistically, neutral mutations tend to accumulate at a regular rate, like ticks of a clock. Therefore, the number of molecular differences in a shared molecule is proportional to the amount of time that has elapsed since the species shared a common ancestor. This calculation has provided new knowledge of the evolutionary relationship between modern apes and modern humans. The molecular clock concept is controversial, however, and has caused much disagreement between evolutionary scientists who study molecules and those who study fossils. This disagreement arises particularly when the molecular clock time estimates do not agree with the estimates derived from studying the fossil record.

E. Direct Observation

Information about evolutionary processes is also obtained by direct observation of species that undergo rapid modification in only a few generations. One of the most powerful tools in the study of evolutionary mechanisms is also one of the tiniest – the common fruit fly. These insects have short life spans and, therefore, short generations. This enables researchers to observe and manipulate fruit fly reproduction in the laboratory and learn about evolutionary change in the process.

Scientists also study organisms in their natural environments to learn about evolutionary processes – for example, how insects develop genetic resistance to human-made pesticides, such as DDT. While pesticides are often initially effective in killing crop-destroying pests, sometimes the insect populations bounce back. In every insect population there are a few individual insects that are not affected by the pesticide. The pesticide wipes out most of the population, leaving only the genetically resistant individuals to multiply and flourish. Gradually, resistant individuals predominate in the population, and the pesticide loses its effectiveness. The same phenomenon has been observed in strains of disease-causing bacteria that have become resistant to even the most powerful antibiotics. Bacterial resistance forces scientists to continuously develop new antibacterial compounds. Scientists have hoped that curbing overuse of antibiotics might cause the drugs to become effective again. Recent research, however, suggests that bacteria may retain their resistance to antibiotics over many generations, even if they have not been exposed to the agent.