## Biological phenomena should be studied in the simplest organisms that exhibit them

One strategy for studying complicated biological phenomena is to reduce them to simpler questions. In biology, this approach means that when we want to understand how a complex organism, like humans, function, it is practical to first table the problem in a more simpler organism that displays the same phenomena.

The principle of minimum complexity - that is, the study of complex biological phenomena in simpler organisms - can be applied thanks to the fact that living organisms share a common ancestor (Figure 20.1). Thanks to Charles Darwin (1809 – 1882) and his followers, we know that in the long-term evolutionary perspective, more complex forms of organisms arise from simpler ones through a combination of the generation of genetic variability (e.g., through mutations or recombination) and the selection of those genetic variants that are best adapted to the given environment as measured by their ability to produce offspring (more information can be found in Chapter 14 – the principle of Evolution). The so-called phylogenetic trees illustrate that all living forms are related to each other (Figure 20.1). Some are more related - located on the same branches of the tree – than others less - located in more distant parts of the tree. During this evolutionary diversification there was (1) preservation of fundamental biological principles that apply to all living forms and (2) specialisation in individual branches, which led to the emergence of a defined taxonomic group or a specific biological species. This unity of life, illustrated by a phylogenetic tree, makes it possible to apply Delbrück's principle who with his collaborators revealed the molecular basis of heredity using simple bacterial viruses (see chapter 14 for more details). In other words, we can formulate models about the functioning of the relevant biological phenomenon based on the study of simpler - model - organisms.