Basics of Evolution - Korzh O.P. - 2006

Part II. MACROEVOLUTION

Chapter 13. Features of Phylogenesis

13.1. Forms of Phylogenesis in Various Groups

The directions of organismal evolution are determined, on the one hand, by the environment in which they develop, and on the other hand, by the Structural Features of the Organism itself (its preceding phylogenesis).

Phylogenesis refers to the historical development of certain groups of organisms, the Evolution of the organic world as a whole, its individual taxonomic groups, and so forth.

Phylogenesis can develop in the form of Primary and secondary patterns: the primary ones include phyletic evolution and divergence, which serve as the basis for other evolutionary transformations, whereas convergence and parallelism are regarded as secondary patterns (Fig. 13.1).

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Fig. 13.1. Diagrams of Various Forms of phylogenesis (after A.V. Yablokov and A.G. Yusufov, 1989):

1 - phyletic evolution; 2 - divergence; 3,4 - synchronous and asynchronous convergence; 5 - parallelism

Phyletic evolution is the transformation of one species into another (within a single phylogenetic Lineage) without accounting for potential branching—that is, a shift in the average traits characteristic of a specific group. Such transformations form the basis for all other variants of group evolution and can therefore be distinguished for almost every species.

It should be noted that not all researchers agree on the necessity of distinguishing this form of phylogenesis. On the one hand, it is viewed as a variant of speciation (anagenesis), while on the other hand, it cannot continue indefinitely for a given group and at a certain stage involves a transition to divergence or Other forms of phylogenesis. To this, it must be pointed out that evolution never stands still, as evidenced precisely by phyletic evolution. Even in relatively stable groups (e.g., phylogenetic relics), a gradual change in forms over time is observed. If development without the splitting of a group into several new ones persists for a short time, there is no reason to discard the proposed term, which helps to better understand the general direction of evolutionary transformations in certain species or larger taxonomic groups as they adapt to specific environmental conditions.

A classic example is provided by the paleontological series of well-studied organism groups. The most famous is the phylogenetic series of horses—the most specialized modern representatives of perissodactyls. This lineage evolved primarily in North America, from where its representatives continually migrated to Europe. Following the migration, the modern horse eventually became extinct in America. The ancestral form that gave rise to the entire horse lineage was a small forest-dwelling creature (*Phenacodus*) that fed on soft tree leaves. Tertiary cooling and a reduction in forest areas, accompanied by the subsequent formation of open spaces, prompted the gradual transition of horse ancestors to steppe-dwelling conditions.

Such a transition could not have occurred without significant structural reorganizations affecting multiple systems simultaneously. First and foremost, living in open spaces necessitated escaping from natural predators, which led to a gradual increase in body size and the transformation of the initial pentadactyl limb first into a tridactyl one (*Eohippus*, *Mesohippus*, *Miohippus*, and other forms), and subsequently into a monodactyl one (*Pliohippus*, modern horse — Fig. 13.2).

Fig. 13.2. Evolution of the equine limb (after H.F. Osborn from I.I. Schmalhausen, 1935):

1 - Eohippus (Early Eocene); 2 - Orohippus (Middle Eocene); 3 - Mesohippus (Early Oligocene); 4 - Miohippus (Late Oligocene); 5 - Hypohippus (Middle Miocene);

6 - Hipparion (Late Miocene - Early Pliocene); 7 - Pliohippus (Late Miocene - Early Pliocene)

Simultaneously, the Fibula undergoes reduction and fuses with the Tibia, and the lower part of the ulna is reduced while its upper part fuses with the radius (oligomerization processes associated with the enhancement of The primary function of the respective Organs).

Significant transformations also affected the dental system of these animals (Fig. 13.3). The transition to grazing (a much tougher diet compared to tree leaves, especially grass hay and seeds) necessitated a reduction in tooth wear (leading to The formation of hypsodont cheek Teeth). The presence of high cusps and ridges covered with cement ensures that as the crown wears down at any level, not only brittle cement but also several layers of hard enamel are exposed, significantly extending the functional lifespan of the teeth.

Fig. 13.3. Transformation of the upper cheek teeth in horses (after H.F. Osborn from I.I. Schmalhausen, 1945):

1 - Hyracotherium; 2 - Pachynolophus; 3 - Anchilophus; 4 - Mesohippus; 5 - Anchitherium

Another well-known example of phyletic evolution is the developmental history of elephants, which evolved as highly specialized forms. The most fascinating changes involve the transformation of the dental system, where the second pair of incisors undergoes hypertrophic development, eventually forming tusks. The gradual shortening of the jaw and the complication of the molar Structure into a lophodont type (Fig. 13.4)—characterized by large chewing surfaces with numerous transverse folds (up to 27 in the Asian elephant, *Elephas maximus*)—led to the so-called horizontal tooth replacement. This mechanism means that the rear teeth erupt only gradually, keeping pace with the wear of the preceding ones and functionally and morphologically replacing them. In modern elephants, only a single molar Functions normally on each side of the jaw at any given time.

Fig. 13.4. Evolution of molars and HEAD shape in proboscideans

(after I.I. Schmalhausen, 1945): 1 - Elephas (modern species); 2 - Stegodon (Pliocene); 3 - Mastodon (Diluvium);

4 - Trilophodon (Miocene); 5 - Palaeomastodon (Early Oligocene); 6 - Moeritherium (Late Eocene)

The ancestral form of elephants is considered to be the Eocene *Moeritherium* (Fig. 13.5), which possessed an almost complete dentition and resembled a small tapir (up to 1 m in length). In *Palaeomastodon*, only tusks remain at the front, with the upper tusks being larger while the lower jaw itself is greatly elongated; apparently, these forms already possessed a fairly large trunk. Subsequently, the elephant lineage split into at least two groups: in deinotheres, the lower tusks began to predominate (while the upper ones gradually regressed), whereas in mastodons and other elephants, conversely, the upper tusks developed. Only the lineage leading to true elephants developed the lophodont type of molar teeth. Other Examples of such phylogenetic series are also known.

Fig. 13.5. Skulls of primitive proboscideans (after Andrews from I.I. Schmalhausen, 1945):

1 - Moeritherium (Late Eocene); 2 - Palaeomastodon (Early Oligocene)

Divergence is the splitting of forms (or traits) resulting in the formation of homologous structures due to differences in environmental conditions acting upon an initially single group (or related groups). Charles Darwin himself predicted that evolution occurs precisely through the gradual divergence of traits accompanied by the extinction of intermediate, less adapted forms. The greater the differences in the habitats of these daughter groups and the longer the divergence process continues, the more pronounced the corresponding divergence becomes.

A clear result of the divergent development of organisms is The Emergence of homologous organs (Fig. 11.1), which share a common origin but are the consequence of organisms adapting to different environmental conditions. I.I. Schmalhausen provides the following example of divergence: while the differences between river and lake trout or various species of frogs are insignificant, trait divergence increases markedly in such related groups as the otter and the marten, the squirrel and the souslik (different habitats), and is even more pronounced between the hedgehog and the bat, or the elephant and the manatee, as well as the bird and the crocodile.

Another example of divergence can be seen in The Development of ungulates (condylarths) and carnivorous forms (creodonts), which were barely distinguishable from one another in the Cytology/cytology/16.html">Early stages of evolution. It is believed that ungulates originate from a specific ancestral group, but their development involved a significant number of parallelisms. Consequently, a modern cow has approximately the same degree of relatedness to a horse as it does to a tiger or any other carnivore. It is widely held that divergence provides the necessary foundation for the development of major systematic groups.

Convergence is the acquisition of similar morphological features by systematically distant forms that happen to inhabit similar environments. Unrelated organisms in this context refer to those connected by a distant degree of kinship (to some extent, all Living organisms are related). A defining feature of convergent development is the formation of analogous organs (which are functionally similar yet possess fundamentally different structural organizations). The more specific the environmental demands placed upon an organism and the longer and deeper the adaptation to that environment, the more pronounced the convergence (Fig. 13.6). The peculiarity of convergence lies in the fact that the similarity affects only external features without extending to internal structures, and is secondary in nature; whereas divergence is based on deep organizational similarity combined with secondary differences dictated by environmental specificities.

Fig. 13.6. Convergent resemblance between marine turtles (order Chelonia) and fossil placodont reptiles (after I.I. Schmalhausen, 1945): 1 - Archelon (Late Cretaceous); 2 - Placochelys (Triassic)

Thus, examples of convergence-based similarity include the reduction of eyes in cave-dwelling animals and the development of flippers in aquatic reptiles and mammals. Convergent resemblance across whole organisms can be seen in the streamlined body shapes of sharks, teleost fish, reptiles (ichthyosaurs), and mammals (dolphins). More closely related organisms can also exhibit convergent forms: the elongation of limbs in cranes and storks, the similarities between the marsupial mole and the common mole, the opossum and the otter, or the marsupial glider, the flying squirrel, and the colugo. In these cases, we are likewise dealing with unrelated forms, as their similar traits were acquired independently even in the early phases of their development. For instance, the hawksbill turtle (*Eretmochelys*) and the leatherback turtle (*Dermochelys*) once shared common ancestors, but these purely terrestrial turtles evolved divergently while still on land, whereas their subsequent transition to an aquatic environment and the development of shared traits should be regarded as a consequence of convergent evolution.

Parallelism is the phenomenon whereby closely related animal species independently acquire similar structures. I.I. Schmalhausen also defines parallelism as the convergence of related forms. A persisting problem here is the relativity of defining the degree of such kinship. In related forms, drawing a clear boundary between parallelism and convergence—and consequently between ANALOGOUS AND HOMOLOGOUS organs—is nearly impossible. At the same time, using the term "parallelism" is considered appropriate because it denotes the independent development of similar adaptations on The basis of homologous structures. Thus, parallelism essentially consists of two processes: prior divergence and subsequent parallel development of shared traits under similar environmental conditions by the respective organisms.

If two related groups of organisms evolve in parallel across many traits, the parallel evolution of these forms is evident.

Examples of parallel development include the two major dinosaur lineages—Saurischia and Ornithischia—which independently underwent increases in body size, a return to quadrupedal locomotion, and the formation of numerous other shared traits. Both pterosaur lineages, pterodactyls and rhamphorhynchs, also evolved in parallel as they transitioned from gliding to true flight, independently acquiring such features as skeletal pneumatization, wing development, a Sternum, and so forth. On the other hand, when comparing pterosaurs and birds, one should speak only of convergence, as these adaptations developed entirely independently and via somewhat different pathways (their wings must be considered analogous organs).

Another instance of parallel development can be seen in toothed and baleen whales; seals, walruses, and eared seals; the single-toed forms of horses (North America) and litopterns (South America), etc. The example of adaptive parallelism in ungulates (Fig. 13.7) highlights the varying degree of organizational reorganization required. For instance, litopterns developed single-toed limbs more rapidly, yet the size of their middle digit remained virtually unchanged. Horses evolved more slowly, but their adaptive gains were accompanied by profound restructuring of their entire anatomical Organization—an increase in digit size, coordinated growth in body size, and so on. It is believed that precisely because of this deeper modification of their overall organization, horses rapidly ousted litopterns from the evolutionary arena as more progressive forms following the interchange of the Americas.

Fig. 13.7. Comparison of The structure of forelimbs (1) and hindlimbs (2) in various ungulates (after G. Simpson, 1983):

A - modern horse (Equus); B - Early Miocene litoptern (Thoatherium)

The phenomena of synchronous and asynchronous parallelism are sometimes distinguished. An example of the former is the development of single-toed forms among ungulates (which occurred almost simultaneously). Asynchronous parallelism involves the development of similar structures in different forms separated in time (it is believed that sabre-toothedness arose in felines at least four times during different geological epochs—Fig. 13.8). However, applying this approach allows us to speak of synchronous and asynchronous convergence as well, since the evolution of a streamlined body shape in dolphins could by no means occur synchronously with ichthyosaurs and other predecessors.

Fig. 13.8. Asynchronous parallelism illustrated by the development of sabre-toothedness in felines (after A. Romer from A.V. Yablokov, A.G. Yusufov, 1989): 1 - Hoplophoneus (Oligocene); 2 - Dinictis (Oligocene); 3 - Smilodon (Pliocene); 4 - Pseudoaelurus (Pleistocene)

Overall, it can be considered that parallel development was initially characteristic of all related forms that subsequently diverged. This is almost a general rule, with the exception of cases where these groups later transitioned to living in drastically different environments. In practice, however, without detailed historical investigation of the relevant forms, it is quite difficult to distinguish The phenomenon of parallelism from other forms of group evolution.