Three major questions in biology: what, how, why?
The growth of biological endeavour has been strongly dependent of the kind of questions which were asked. The first basic and necessary approach is ''what''. The answers are mainly descriptive: what is the anatomy of a frog? What are the species existing in the Hawaiian archipelago? Taxonomy is basically a descriptive discipline, but even the mere inventory of extant species on earth is far from complete and requires further investigations.
A second step in biological knowledge is ''how''. How does this work? How does this develop? That is the major, central approach in modern experimental biology. It is applied to all levels of organisation from molecules to ecosystems, and tries to get answers to questions like: how a gene controls a protein synthesis; how two molecules interact; how the germ layers of an embryo arise from a single cell; how so many Drosophila species coexist in the Hawaiian archipelago.
Experimental, functional analysis is at the core of modern biology, it implies an analytical approach and leads to the major discoveries. Any system investigated is subdivided into parts, which are easier to handle and to analyse. In this respect there is both a temptation and a danger of reductionism. Reductionism will try to explain the functioning of a complex biological system by the mere addition of its known parts, while an increase in complexity always leads to the appearance of new, emerging, unexpected properties. The sequencing of entire genomes is certainly a major progress in biological knowledge, but it will not explain directly the realisation of an organism. Indeed, sequencing is more a ''what'' question than a ''how'' one. Since biological processes are integrated at different levels of complexity (molecules, cells, organisms, populations and ecosystems) and since, at each level, new emergent properties appear, it is both necessary and justified to tackle simultaneously all these levels. There should not be any hierarchy among biological disciplines, but only discrimination between good innovative science and poor repetitive science.
The third kind of question is ''why''. Why are there bacteria and elephants? Why micro-organisms and Metazoa? Why so many (over 1000) Drosophila species in Hawaii? Why species at all? Why is sometimes considered a crazy question, but it is also a pervading interrogation since the beginning of historical times. Why are there living beings on earth? Why are there humans? In the absence of scientific knowledge, philosophers tried to answer these questions and the recurrent explanation in almost all civilisations was creation: there was nothing to understand further. Only the accumulation of what and how answers during the last three centuries has permitted the birth and the growth of another explanation: all living organisms share a common ancestry and have evolved and diversified during almost four billion years of earth history. Biological evolution, not creation, is the general and now well recognised process. Much however still remains to be explained and this is the goal of a mature discipline in life sciences: evolutionary biology. It is a difficult discipline for at least two main reasons. First, as stated above, it must integrate, interpret and explain all kinds of biological information at all levels of integration. Second, being basically an historical approach, it is in many cases not amenable to an experimental analysis. In other words, many hypotheses and interpretations in evolutionary biology cannot be falsified in the sense of Popper (1972). Evolutionists, using a comparative method, are proposing plausible interpretations, often implementing mathematical techniques. These models may be falsified or improved by further observations.
Being based on a principle of inheritance with change over generations in natural populations, evolutionary biology needed to integrate all the information from genetics, especially from population genetics. Finding now the same genes in different species and distant taxa has been a further ''verification'' of the evolutionary theory and has permitted to work out molecular phylogenies, providing an irreplaceable historical information. More recently major conceptual progresses have been made at a higher integrated level, in interpreting the evolution of life histories (Charlesworth 1980, Charnov 1993, Roff 1992, Stearns 1992). This rapidly expanding field, at the crossroads between observations and theories, tries to explain why bacteria and elephants coexist in nature.
During the maturation years of evolutionary theory, embryological information has played a fairly marginal role, in spite of the fact that most extant species are multicellular. Having realised that identical genes are expressed in very different groups and lineages, developmental biologists are now aware that they must integrate evolutionary information in their research, at least the historical and phylogenetic aspects. Evolutionary biologists, on the other hand, will discover the diversity and richness of problems raised by the development of highly complex multicellular organisms. Time is coming for a convergent evolution of the two disciplines.