XIV.3 Multicellular organisms exhibit a cycle of alternating gamete and multicellular organism stages; natural selection can operate both among gametes and among multicellular individuals

In unicellular organisms, cells are only rarely differentiated morphologically into microgametes and macrogametes.This is apparently a result of the fact that the unicellular organism and the unicellular gamete are exposed to very similar selection pressures in their environment, which does not permit them to differ much from the common structural plan in their morphologies.However, the situation is somewhat different for permanently attached (sessile) unicellular organisms.A different selection pressure acts on the gametes than on the sessile individual, as they must, at the very least, be capable of looking for one another.In this case, it can be expected that differentiation into microgametes andmacrogametes will occur.

            Differentiation into microgametes andmacrogametes is more or less a general rule in unicellular organisms forming colonies and in true multicellular organisms.The phase of a multicellular, frequently macroscopic organism regularly alternates in the life cycle of the species with a mostly unicellular gamete phase.It is evident that the selection pressures acting on a macroscopic organism and a microscopic gamete are different.Simultaneously, especially microgametes are frequently produced in an enormous excess compared to macrogametes, so that the intensity of their mutual competition to find and fertilize macrogametes can be extremely high.

            The aspect of mezigamete selection warrants closer attention.At first glance, it may seem that its effectiveness must be incomparably greater than the effectiveness of selection at the level of an adult multicellular individual.One sperm from amongst millions participates in fertilization of an egg.However, it should be borne in mind that each gene occurs in the set of sperm derived from a single individual in only two variants, where 50 % of the sperm carry one variant and 50 % the other one.Thus, if the concepts of the theory of interallele competition are considered (see IV.9.1), it is apparent that, from the viewpoint of the individual alleles, this does not involve selection of one in a million but one of two.It is, of course, necessary to point out that new mutations formed during gametogenesis will be selected with an effectiveness of one in a million if they occur in the sperm population with this frequency.s

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more
Draft translation from: Evoluční biologie, 2. vydání (Evolutionary biology, 2nd edition), J. Flegr, Academia Prague 2009. The translation was not done by biologist, therefore any suggestion concerning proper scientific terminology and language usage are highly welcomed. You can send your comments to flegratcesnet [dot] cz. Thank you.