In 1969 physicist David Bohm argued against reductionism and made the point that perhaps physics is like background noise with only occasional moments of order, while biology is like music, organized in a hierarchy that is capable of continual development.
Any theory of evolution is three-tiered. The first level asks about what it is that evolves - what are species? The second level asks about how evolution occurs - how does one species become two? How does this lineage splitting happen? Finally, why does evolution happen at all? A causal explanation must cover both ontogeny and phylogeny because they are nested, irreversible evolutionary processes. Jack points out some of the problems with ecological definitions of species. He describes some of the historical versions of Ernst Mayr's biological species concept and the problems with sexual or morphological criteria. If one species gradually changes into another, how do we tell when speciation, in terms of reproductive isolation, has occurred? Anagenesis is the slow transformation of one species into another, perhaps more common in breeding programs than in nature. Cladogenesis is the splitting of one lineage into two, and it may happen rather quickly. In the early 1970s, Eldredge and Gould offered a theory of punctuated equilibria, where small, isolated populations diverged rapidly after long periods of stability. They were attacked for this minor variation on Darwinian theory.
What drives evolution? Random mutations (most of which are considered deleterious or neutral) and natural selection don't offer enough. Not long before I arrived at UBC, young systematist Dan Brooks, along with his colleague at the Univ. of Kansas, Ed Wiley, began work on a controversial book, Evolution as Entropy. Their theory built on the expanded role of the Second Law of Thermodynamics as proposed by Ilya Prigogine. Evolutionary phenomena are irreversible processes, they pointed out, and these have a common source - the Second Law. Organisms are self-organized, dissipative structures, but instead of dissipating entropy like molecules floating randomly at equilibrium, they are more like the "sentences in a language," able to generate surprising, new forms from within biological constraints, all tied together by their evolutionary history. These ideas didn't make young Brooks at all popular with his neo-Darwinian colleagues. He organized seminars where students read through drafts of their book. Much of Brooks and Wiley's ideas were based on information theory, related to organization, complexity and increasing complexity. As the entropy of the universe increases, so does the entropy expressed through evolution, but the products of evolution never dissipate maximum entropy. A certain amount of information is necessary to describe a system, and systems also contain and express information. In biology, the amount of information expressed by a system will always be less than the information it contains. The more complex a system is, the more the possible messages that can be generated, and the more information expressed.
As complexity increases, the number of different specific messages that can be generated increases. A bird is more complex than a bacterium, but a bird is also more complex at a later point in time than an earlier point because its history must be included in the characterization. An old tree may appear simpler than a young one, until the history of the tree is considered. A plant extracts information from its DNA to build cell walls, but the information needed to make these walls can't be found in the walls themselves. To a large degree, biological systems are defined by their organization. Organization and its change through time can be quantified most easily in an organism by changes in correlations among its parts. Matter is transformed during development and becomes incorporated, along with genetic recombination and mutation, into the information systems of individuals and species, thereby expanding the information, and so the complexity. Once a certain level of complexity is reached, the species bifurcates, it speciates, and one species becomes two.
I found these ideas exciting because they linked biology to the other natural sciences without reducing biology to trivia. Ontogeny and phylogeny are the nested hierarchical levels expressing unidirectional, irreversible flows of time. These features, not alleles, fitness or reproductive output, are the earmarks of evolutionary events.