Saturday, 27 October 2012

51. Biological Symbiosis and Evolution

Ironically the popular evolutionist’s view that organisms evolve by the accumulation of random mutation best describes the evolutionary process in bacteria. All of the larger, more familiar organisms originated by symbiont integration that led to permanent associations (Margulis and Sagan 2002).

Biological symbiosis means a prolonged living arrangement or physical association among members of two or more different species. Levels of partner integration in symbiosis may vary in intimacy; and integration may be behavioural, metabolic, of gene products, or 'genic'.

An example of symbiosis in action are the microbes that live in a special stomach of the cow, providing the enzymes for the digestion of cellulose. The cow, in turn, provides shelter and nutrition to the microbes. We say that the microbes are the 'symbionts' of the cow.

Biological cells are either prokaryotes or eukaryotes (cf. Part 43). Examples of prokaryotes are: E. coli; blue-green algae or cyanobacteria; and archaebacteria.

Eukaryotes can be divided into four kingdoms:

  • Protoctists (algae; amoebas; ciliates; slime moulds).
  • Fungi (moulds; yeasts; mushrooms).
  • Plants (mosses; ferns; flowering plants).
  • Animals (molluscs; arthropods; fish; mammals). 

The genealogical relationship of all living organisms has three main branches: bacteria, archaea, and eucarya. However, some authors distinguish just two branches, namely bacteria and eucarya, and subdivide bacteria into eubacteria and archaebacteria. The eubacteria and the archaea or archaebacteria are prokaryotes. The eucarya are eukaryotes.

Lynn Margulis is a major proponent of the idea that parasitism and symbiosis were major driving forces in the evolution of cellular complexity. She has been hammering home the point that the main components of eukaryotic cells have descended from independent living creatures which ‘attacked’ the cells from outside. In due course, the attackers and the host evolved a relationship of mutual dependence and benefit. In stages, the erstwhile invading organisms became first chronic parasites, then symbiotic partners, and finally an indispensable part of the host.

The mitochondria are symbionts in both plant and animal cells, as are chloroplasts in plant cells. The evidence for this is that the molecular structures of mitochondria and chloroplasts are indeed very close to certain bacteria.

The emergence of a new life form from such symbiosis is called 'symbiogenesis'.

Margulis marshalled evidence to argue that most of the big steps in cellular evolution were caused by parasites. And that nucleic acids were the oldest and the most successful cellular parasites.

Thus the classical viewpoint that speciation occurs, i.e. new species arise, only as a result of the cumulative effect of mutations etc., has been strongly contested by Margulis. According to Margulis and Sagan (2002), ‘No evidence in the vast literature of heredity change shows unambiguous evidence that random mutation itself, even with geographical isolation of populations, leads to speciation. Then how do new species come into being? How do cauliflowers descend from tiny, Mediterranean cabbagelike plants, or pigs from wild boars?’ Their answer is that species arise largely by the acquisition of entire genomes through symbiogenesis.

Margulis’s stance raised debate. Ernst Mayr wrote an appreciative foreword to the 2002 book by Margulis & Sagan. But the foreword also said this: ‘Speciation – the multiplication of species – and symbiogenesis are two independent, superimposed processes. There is no indication that any of the 10,000 species of birds or the 4,500 species of mammals originated by symbiogenesis.’ Contrast this with the statement of Rachel Nowak (2005): ‘Symbiosis has popped up so frequently during evolution that it is safe to say that it’s the rule, not the exception.’

In biology, studies of evolution are about tracking the changes of life through time. In particular, they are about tracking the origin of species. But what exactly is a species?

The morphological definition. Creatures belonging to a species look alike (dogs look like dogs).

The biological definition. Creatures belong to a species if they can mate and produce fertile offspring. This was introduced by zoologists, and also by botanists.

The phylogenetic definition. Groups of organisms considered to be all descended from the same ancestors are said to belong to the same species.

Margulis & Sagan (2002) rejected the biological and the phylogenetic definition, and suggested something which can accommodate only the morphological definition:

The symbiogenetic definition. 'If organism A belongs to the same species as organism B, then both are composed of the same set of integrated genomes, both qualitatively and quantitatively. All organisms that can be assigned to a unique species are products of symbiogenesis'.

Life originated with bacteria. Bacteria do not speciate. The idea of a species does not apply to them. Bacteria can pass genes back and forth. There is no fixed genome to define the species of any bacteria. Bacteria are prokaryotes.

The first eukaryote emerged by the symbiogenesis of two prokaryotes. The concept of a species can apply only to eukaryotes. It follows that the origin of species occurred long after the origin of life in the form of bacteria. Therefore, the species of all the larger organisms (protoctists, fungi, animals, plants) originated symbiogenetically in the beginning. Nucleated organisms emerged on Earth some 1.2 billion years ago.

Symbiosis can occur only if the arrangement is beneficial to all the partners involved. In a later post, when I shall give you a historical narrative of the various 'energy regimes' in the evolution of our ecosphere, I shall explain what benefits accrued to the partners when the eukaryotic cell evolved from its partner organisms.

There is no reason to believe that symbiogenesis is the only way in which new species can arise. It is a characteristic of complex systems that, often, small changes can have unexpectedly large consequences, including the emergence of new species. Effects of mutations can gradually build up to a stage wherein a sudden bifurcation occurs in phase space (cf. Part 30), and a new species arises. Speciation may well be an emergent phenomenon often. This contention of mine is in disagreement with the statement of Margulis & Sagan (2002) that 'intraspecific variation never seems to lead, by itself, to new species'.

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