Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

Interspecies comparison studies indicate that cells in the diploid phase are usually larger than cells in the haploid phase (Cavalier-Smith 1978).From this, it follows, amongst other things, that they have a smaller ratio of their cell surface to their cell volume.Under conditions where the metabolism and cell growth are limited by supply of nutrients across the cell surface, it is advantageous for the organism to spend most of its life cycle in the haploid phase, in which the transport of nutrients across the relatively larger cell surface is more efficient (Lewis 1985).On the other hand, if cell growth is limited by a process that is not connected with the relative size of the cell surface or a process whose intensity is inversely proportional to the size of the surface, it will be more advantageous for the organism to spend a greater part of its cell cycle in the diploid phase.For example, in organisms living in a strongly hypotonic or, on the other hand, in a strongly hypertonic environment, it can hold that, the smaller the relative surface, the less the energy that that cell must expend to pump water or salt from the cell or into the cell; under these conditions the life processes of larger cells are energetically more efficient.In this case, it can be expected that the organism will spend the most time in the diploid phase.

In a great many situations, although not always, the diploid phase is also more advantageous from the viewpoint of resistance of cells to mutations and damage to genetic information (Long & Michod 1995; Jenkins & Kirkpatrick 1995).In a haploid organism, a mutation that inactivates a product of a gene that is vital for life is usually lethal.All the genes are present in two copies in diploid cells, so that loss of one of them does not generally have fatal results for the organism (Orr 1995).It will apparently not be accidental that the haploid yeast Schizosaccharomyces spends most of its life cycle in the G₂-phase, while in nature almost always diploid Saccharomyces spends most of its time in the G₁-phase.

In some cases, the second copy can act not only as a backup for replacement of damaged function, but also as a template for repair of the damaged copy.If such repair actually occurs, following cell division the relevant mutation will be present in a haploid cell and can fully manifest its lethal effect in the haploid state.From the standpoint of the affected cell this is fatal, but elimination of mutated cells can be advantageous from the viewpoint of the species.  Passage through the haploid phase can provide an additional “quality control” for the nuclear genome; cells with incomplete genetic information can be eliminated from the population in this phase by natural selection.Although ripe haploid gametes have most genes temporarily inactivated in the members of a great many taxons, probably to   limit inter-gamete competition in the body of multicellular organism (see XIV.3.1), testing of the functioning of the haploid genome can, however, already occur at the level of the haploid precursors of the sex cells.

Alternation of the ploidy phases of the life cycle in multicellular organisms is called metagenesis. Very frequently the two ploidy phases differ in their means of reproduction, where the haploid gametophytes form gametes and the diploid sporophytes form spores.In some organisms, the gametophyte phase is more important, i.e. larger, morphologically more complicated and longer-lasting (mosses and lichens); in others, the sporophyte phase predominates (angiosperm plants), while the two phases do not much differ in other organisms (ferns,  Pteridophyta) (Jenkins 1993; Mable & Otto 1998).Alternation of phases with sexual and nonsexual reproduction also occurs in some animals (especially Turbellaria and Cnidaria).This process is also designated as metagenesis in this case, although the bodies of both the  sexual and nonsexual phases are composed of diploid cells under these circumstances.