The Edinburgh Geologist
Issue no 38

Evolution and genetics

by Angus Harkness

In studies of evolution, the thinking of morphologists and geneticists sometimes seems unconnected, this despite a series of linking reviews (Maizels & Ashburber, 1994; Roberts et al, 1996; Lang, Gray & Burger, 1999). The word, evolution, appears increasingly in the Annual Review of Genetics. For amateur geologists, such papers may provide patterns which are sometimes understandable.

The need for living systems to use efficiently and thus conserve energy (Atkinson, 1977) prompted my suggestion that any mobile organism, especially when life emerged from the support of water, would tend to evolve further to reduce unnecessary weight including the amount of genetic controlling material, DNA, in cellular nuclei and mitochondria (Harkness, 2000). This has to be balanced against increase in the amount of DNA per cell associated with evolutionary progress (see below).

Plants, which were first to emerge from the water and colonise the land, have large amounts of DNA per cell in their nuclei and in their mitochondria. However expressed (as micrograms per cell or per nucleus, or as millions of base-pairs in the genome), the figures show a pattern consistent with this hypothesis. Plants have about 5-200 micrograms of DNA per million cells, most amphibia 5-15 micrograms of DNA per million cells,  most other animals 1-2 micrograms of DNA per million cells, but mammals have about 5 micrograms of DNA per million cells (see Sober (1973) and references cited therein). These early data have now been refined by sequencing the chains of DNA but, so far, in only six nucleated species, two yeasts, a plant, a worm, a fruit fly and man (Bork & Copely, 2001). In addition, the fruit fly has removed from its nuclei inactive genes (pseudogenes) at about seventy times the rate in man.

However, the DNA in the energy producing subcellular mitochondria has been sequenced for about 20 years. The coding in the DNA is by sequences of molecules of purine and pyrimidine bases. Sequences for mitochondrial DNA for 86 animal species are available (Lang, Gray & Burger, 1999). These animal mitochondrial DNAs are small, about 16,000 base pairs. Fungi, 6 species, have about 20,000-100,000 base pairs and plants, only 2 species, have 200,000 to 300,000 base pairs. These figures suggest that about 1000 genes (control assemblies) have been lost from mitochondria during the establishment of the mitochondrial genome. The mechanisms include migration of mitochondrial genes to the nucleus and takeover of mitochondrial control by nuclear genes. The mitochondrial genes that are left in man largely specify big scaffolding proteins which are difficult to move into mitochondria. The limited data in plants suggests evolution has gone in the opposite way.

Evidence from plants shows that larger amounts of DNA are associated with biological success. Plants frequently use multiple sets of genes, polyploidy (Otto & Whitton, 2000). In ferns, about 2-4% of changes producing species involve polyploidy; in flowering plants, chromosome doubling may 'propel a population into a new adaptive sphere'. Isolated gene reduplication is used successfully in animals to produce new control proteins while maintaining the essential original protein.

From the evidence outlined above, it would seem unwise to regard DNA with no known function at present as junk left over from history.

The sequence of evolution largely derived from geology is important in arranging and then understanding events in biology. How can we test the above hypothesis? A test might be based on the increase in cell size associated with polyploidy in plants and some invertebrates. Does an increase in cell volume in an earlier strata precede the appearance of a new closely-related species further up the geological column?

References

Atkinson, D.E., 1997. Cellular energy metabolism and its regulation, Academic Press, London, 293p.

Bork, P. & Copely, R., 2001. Filling the gaps, Nature, volume 409, pp. 818-820.

Harkness, R.A., 2000. Evolutionary problems in Australia, The Edinburgh Geologist, volume 34, pp. 22-25.

Lang, B.F., Gray, M.W. & Burger, G., 1999. Mitochondrial genome evolution and the origin of eukaryotes, Annual Review of Genetics, volume 33, pp. 351-397.

Maizels, N. & Ashburner, P., 1994. Genomes and evolution, Current Opinion in Genetics and Development, volume 4, pp. 797-938.

Otto, S.P. & Whitton, J., 2000. Polyploid incidence and evolution, Annual Review of Genetics, volume 34, pp. 401-437.

Roberts, D.McL., Sharp, P., Alderson, G. & Collins, M.A., 1996. Evolution of microbial life, Cambridge University Press, 299p.

Sober, H.A., 1973. Selected data for molecular biology, Handbook of biochemistry, 2nd edition, CRC Press, Cleveland, Ohio, p H110 (see also 3rd edition).

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Angus Harkness is a biochemist with an amateur interest in geology. He has been a Fellow of the Society since 1993 and was a member of Council. He has worked on biochemical genetics for the Medical Research Council, the University of Edinburgh and the National Health Service.

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