Basics of Evolution - Korzh O.P. - 2006

Part II. MACROEVOLUTION

Chapter 13. Features of Phylogenesis

13.4. Rates of Evolution in Groups

Paleontological data indicate that the rates of development vary among representatives of different taxonomic groups. Alongside rapid changes (taking a few thousand or even hundreds of years), there are slow ones: certain forms have not acquired new features, even at the species level, over hundreds of millions of years. All this cannot be explained solely by external or internal causes, as there are Examples of both rapid and slow changes under the exact same environmental conditions, as well as varying speeds of evolutionary transformations in related groups.

So-called phylogenetic relics demonstrate an extraordinary stability of their Organization—they show no significant differences from their predecessors that lived millions (and in some forms, hundreds of millions) of years ago. These include the coelacanth (Latimeria), lungfishes (Dipnoi) and cartilaginous ganoids (Chondrostei), the tuatara (Sphenodon punctatum), crocodilians (Crocodilia), monotreme mammals; and among plants, the ginkgo (Ginkgo biloba), along with A large number of other primitive organisms.

The existence of relatively young geographical formations (islands and lakes that began to be actively colonized by organisms only in the post-glacial period) points to the possibility of exceptionally high rates of speciation among certain groups of organisms. For instance, Lake Baikal provides examples of adaptive radiation among certain groups of fish (Comephoridae, Cottocomephoridae), amphipods (family Gammaridae), Mollusks (families Baicaliidae and Benedictiidae), and several other forms. Among the amphipods of the lake alone, there are over 250 endemic species grouped into 37 genera, 35 of which are endemic. A similarly fascinating fauna is found in certain East African lakes, which, like Baikal, formed during the Tertiary period. Lake Tanganyika (equatorial Africa) is distinguished by a rich ichthyofauna comprising 40 genera and over 140 species of cichlid fish; Lake Victoria harbors an extraordinary number of endemics among fish of the genus Haplochromis, while in Lake Edward, the differentiation of cyprinodonts (Cyprinodontia) is taking place. All these endemic forms underwent exceptionally large-scale evolutionary transformations over a relatively short period (no more than 15–20 thousand years). Similar examples can be cited for terrestrial organisms of insular ecosystems, whose evolutionary rates sometimes turn out to be even higher.

However, other members of the fauna in these very same lakes show no propensity for speciation whatsoever (in some forms, not even new subspecies are formed). Furthermore, the African lakes Albert and Rudolf, despite having the same tectonic origin as the aforementioned ones, offer no examples of adaptive radiation for any groups of organisms. Similarly, groups that undergo adaptive radiation in certain environments do not exhibit species richness in other parts of their range. Those same freshwater amphipods have fewer species across almost the entire Eurasian continent compared to the representatives of Lake Baikal alone.

One of the problems in assessing the evolutionary rates of groups is the Selection of appropriate criteria. This primarily concerns the time scale over which all these transformations occur: aside from astronomical time, biological time (number of generations) is also distinguished. However, biological time does not always work: only astronomical time can be applied to fossil species, and even then with significant margins of error. Sometimes modern forms also provide examples of exceptions to the rule of deepening evolutionary transformations under rapid generation turnover.

Thus, one of I.I. Schmalhausen's paradoxes states that under normal conditions, other things being equal, The rate of evolution should be determined by the absolute population size of a given species (population), population density, and the rate of generation turnover. Therefore, maximum rates of evolutionary transformation might be expected in widely distributed small organisms with rapid sexual maturation and high fecundity (rotifers, bryozoans, small crustaceans, mites, aphids; among vertebrates—small lizards, geckos, insectivores, rodents, and certain other forms).

At the same time, none of these forms can be classified as progressive (discussed in more detail below). On the other hand, progressive animal representatives are predominantly large in size and have undergone significantly faster evolutionary transformations.

The scientist explains this paradox by the fact that in relatively large animals, the main form of the Struggle for Existence is intraspecific, which provides a certain degree of protection for such organisms from damaging environmental influences. The improvement of Homeostasis and the reduction of Environmental Impact on these forms substantially increases their competitiveness, while small population sizes allow even minor positive gains of individual organisms to be picked up by natural selection. Therefore, in such organisms, the value of the individual significantly increases, as each can contribute to evolutionary transformations. In small animals, the intensity of damaging factors (interspecific and constitutional forms of the struggle for existence) is extremely high, which hampers The formation of new useful traits; the probability of survival for individual organisms possessing useful traits in this case turns out to be quite low, which leads to relatively slow rates of adaptation.

Consequently, biological time does not always work even in neontology, let alone paleontology; it varies with the rate of modification of individual Organs and the transformation of groups as a whole (different forms of progress are discussed below).

Today, it is believed that the only reliable criterion for determining the rate of phyletic evolution is the number of newly emerged species within the respective group.

Based on the duration of existence of various genera of organisms, G.G. Simpson proposed distinguishing three categories:

1) bradytelic - their development time exceeds 250 million years;

2) horotelic - developing up to several tens of millions of years;

3) tachytelic - young genera whose existence time is less than a million years.

Reflections Aloud

The historical development of major taxonomic groups has always sparked considerable interest among various specialists, since the frequent absence of transitional forms and the impossibility of reliably distinguishing ANALOGOUS AND HOMOLOGOUS organs greatly complicate the understanding of inter-

relationships between different Representatives of the organic world. The recognition of such developmental trends as allogenesis and arinogenesis (arogenesis) suggests that there still exist certain differences between speciation and the developmental process of major taxonomic groups (which often occurs in the form of arogenesis). Making definite generalizations at this level is difficult, as almost no groups develop identically, and testing these assumptions is virtually impossible.