Ageing of the phylogenetic line

Changes in time in the character of cladogenesis within a certain phylogenetic line are a striking and simultaneously difficult-to-explain macroevolutionary phenomenon. The paleontological record contains a number of phylogenetic lines that first continued in a state of evolutionary calm for a longer time after their formation, underwent speciation with only minimal frequency and their members also accumulated only a minimum of anagenetic changes. This was followed by turbulent evolutionary activity with splitting off and mutual phenotype diversification of a large number of species in a short period of time. On the phylogenetic tree, this phase appears as sudden radiation of a large number of phylogenetic lines from practically a single point. The period of radiation is again followed by a period of evolutionary calm, with a minimum of new branches, and evolution occurs only within the formerly split-off branches and tends to have the character of adjustment of the adaptation to the conditions of the environment rather than the creation of new diversity. In this period, specialized forms emerge in the line, frequently with large body dimensions that readily become extinct during the next mass extinction event. The last phase in the existence of a phylogenetic line from the viewpoint of its cladogenesis consists in evolutionary stagnation and decline – the species undergo speciation more slowly than extinction and thus the entire line gradually dies out. In some cases, rejuvenation can occur in any phase of the phylogenesis of the line, i.e. new radiation occurs in one of its branches and lines formed during this event then gradually replace most of the lines formed during the previous radiation (Fig. XXVI.2).

            The described process was sometimes interpreted in the past as maturing and ageing of the phylogenetic line. However, it is very probable that this is actually only a superficial analogy with the life cycle of a biological individual. In the life of a biological individual, i.e. in the section defined on the one hand by its birth and on the other hand by the physical destruction of its body, there actually do exist physiological processes determining the alternation of developmental phases, during which the viability of the individual and its ability to reproduce first increase and then decrease, with a proportional change in the probability of its death. However, this is not the case in the life of a phylogenetic line. Only individual species are formed and become extinct (and thus it is, at the very least, theoretically quite possible that the individual species can age, see XXVI.5), but the phylogenetic line encompassing all the species of organisms on the Earth lasts without interruption from the moment of emergence of life on this planet and only gradually branches over time. The individual lines (phylogenetic sublines) do, of course, disappear through extinction; however, the newly formed branches have, at the moment of their formation, the same age as all the branches existing on the Earth at the given moment. The individual taxa, monophyletic sections of the phylogenetic line defined on the phylogenetic tree by a taxonomist on the basis of an important trait, i.e. on the basis of the attained level of anagenesis, can, of course, emerge and disappear. The evolutionary emergence of a new taxon corresponds not to the formation of a new individual, but only to the formation of the relevant new anagenetic traits in a species within a long-existing phylogenetic line. Simultaneously, various taxa are delimited on the basis of very diverse traits. There is absolutely no reason to assume that the formation of basically an arbitrary trait would automatically initiate the process of ageing of the relevant phylogenetic subline. However, the anagenetic traits themselves, or rather the complex of anagenetic traits that characterize a new taxon, can age, e.g., can freeze, see below.

            It is highly probable that the “life cycle” of a taxon is not a result of its ageing and real changes in the traits of the relevant species. It is more probably a manifestation of mathematical laws following from the existence of fluctuations in speciation rates within a single phylogenetic line and also a consequence of the manner in which we obtain input data. A taxon that did not undergo at least one period of radiation has a very low chance of surviving in nature long enough for us to be able to observe it in the paleontological record and thus of being included in our considerations. There is low probability that a taxon will be subject to constant radiation; radiation is apparently always caused by some specific, relatively rare anagenetic change (see XXVI.2.1). The probability of radiation of a certain line is simultaneously apparently negatively affected by the overall biodiversity in the environment (see, e.g., the king of the hill effect, XXII.5.5, and the discussion of the model of the ecological network, XXII.7). As a consequence, a global balance is apparently maintained between the rate of speciation and the rate of extinction. However, this means that a taxon that does not undergo radiation in time becomes extinct. Obviously, once a taxon has become extinct, it can no longer undergo radiation and can thus not return to the paleontological record.

An alternative explanation is provided by the theory of frozen plasticity, according to which all evolutionary lines gradually age (and thus increasingly old lines emerge) {15429}. Old lines have reduced probability of evolutionary thawing and formation of new fundamental anagenetic features – and thus important evolutionary radiation, as more and more traits that were originally capable of thawing following peripatric speciation become permanently frozen. This phenomenon could explain why, for example, the great majority of disparities, the great majority of animal strains evolved in the Cambrian and, since that time, the number of basic body plans has tended to decrease on the Earth {12783}.

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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