Basics of Evolution - Korzh O.P. - 2006

Part II. MACROEVOLUTION

Chapter 12. Evolution of Ontogeny

12.1. Features of Ontogeny in Various Taxonomic Groups

Ontogeny (from Greek on, genitive ontos — being + genesis) refers to the individual development of an Organism from the egg stage to death, or, in the case of reproduction by fission, from one Cell Division to the next (transition to a new state).

Ontogeny is based on the sequential and irreversible unfolding and Selection/27.html">Realization of Genetic information encoded in the regulatory structures of the germ cell, driven by external and internal factors. It is this developmental plasticity that enables subsequent evolutionary changes in organisms. The term "ontogeny" was introduced into scientific use by E. Haeckel in 1866.

Development in Multicellular Organisms performs two main Functions: 1) it ensures a diversity of cell types and determines The sequence of events within each generation; 2) it "accounts" for the continuity of life during the transition from one generation to the next. The first function comprises the processes of differentiation (The Emergence of cellular diversity), morphogenesis (the Organization OF THE resulting Cells), and growth (increase in size). The second function boils down to reproduction (Fig. 12.1).

Class="center">

Fig. 12.1. The course of ontogeny using the frog as an example (after S. Gilbert, 1993)

A distinction should be made between the sexual process and reproduction — phenomena that can exist independently of each other. Reproduction is the process by which new individuals arise, whereas the sexual process involves the creation of new Gene combinations derived from two individuals. Organisms that reproduce by simple fission lack a sexual process. Conversely, Protozoa and Bacteria can undergo a sexual process without reproduction. In bacteria, gene exchange between different individuals occurs via sex pili, or fimbriae (sex villi); in paramecia, the sexual process is carried out via conjugation, while reproduction occurs through simple fission.

When these two processes are combined, it is referred to as sexual reproduction. This combination gives rise to two progressive features:

1) Meiosis, through which the diploid chromosome set is reduced to haploid;

2) a mechanism for recognizing individuals of opposite sexes (specialized regions on cell membranes with specific components).

While lifespan is limited, the fundamental difference between unicellular and multicellular organisms lies in the fact that in unicellular organisms, each cell can theoretically give rise to new individuals by preserving its substance (biological immortality). In contrast, multicellular organisms undergo a segregation of the germline cells, which give rise to Gametes and ensure the appearance of offspring. Somatic cells are mortal and incapable of participating in sexual reproduction. Thus, the transition to multicellularity was accompanied by increased complexity, a longer lifespan, and the loss of immortality.

The course of ontogeny has certain differences in plants and animals (Figs. 12.1, 12.2). This applies both to The ratio of individual Stages of Ontogeny, the alternation of life cycles, and differentiation processes. In animals, despite the wide variety of ontogenetic types, four key processes are paramount:

1) Cleavage — a series of extremely rapid mitotic divisions that result in the distribution of the zygote's cytoplasmic volume among smaller daughter cells (blastomeres);

2) Gastrulation — cellular movements that lead to The formation of a multicellular embryo and the differentiation of cells into three germ layers. The ectoderm gives rise to the epidermis and The Nervous System; the entoderm forms the digestive tube, its associated Organs, and the Respiratory system; and the mesoderm develops into the majority of Internal Organs;

3) Organogenesis — the interaction and migration of individual cells, resulting in the formation of organs;

4) gametogenesis, which is initiated by primordial Germ Cells (their formation takes place during the Cytology/cytology/16.html">Early stages of differentiation).

Fig. 12.2. Life cycle using the fern Polypodium sp. as an example (after I.V. Grushvitsky and S.G. Zhilina, 1978)

Thus, in animals, the MAIN STAGES OF differentiation (excluding modular animals) occur during embryonic development. In plants, morphogenesis and growth prevail over differentiation and take place in the postembryonic period. The fundamental difference in the organization of unitary versus modular organisms can be considered the primary cause of these distinctions. The divergent development of most unitary organisms (the "creation" of the organism during the Embryonic period followed by its subsequent functioning) is contrasted with the convergent development of modular species (where the formation and functioning of the organism occur cyclically and simultaneously).

Among animals, the greatest differences are observed between organisms with direct development and those whose ontogeny includes metamorphosis. The Biological Significance of the larval stage lies in the partitioning of the functions of feeding, growth, dispersal, and reproduction among different developmental phases. Furthermore, if larvae and adults inhabit different environments, this promotes an expansion of ecological tolerance and a more thorough utilization of resources.

Metamorphosis refers to the set of processes that leads from larval organization to the adult form. During this process, specialized adaptations for a larval lifestyle disappear and imaginal organs are formed (Fig. 12.3). The behavior of the animals changes accordingly. Specific conditions are required for metamorphosis to take place.

Fig. 12.3. Metamorphosis illustrated by The life cycle of a butterfly (after J. Mitchell, 1984):

1 - caterpillar; 2 - pupa; 3 - imago

It should be noted that differences in ontogenetic patterns can be traced even among quite closely related groups. According to L.P. Poznanin, finding two animals with identical developmental stages is virtually impossible. These differences are clearly exemplified by birds and mammals, which exhibit altricial (imaturonate – born immature) and precocial (maturonate – born mature) types of ontogeny. In the former case (passerine birds, felines, canines, etc.), the newborns are incapable of independent existence, often blind and lacking fully developed Sensory Organs (Fig. 12.4). Without parental care, these young cannot survive, although they possess certain adaptations; for instance, nestling birds can lower their BODY Temperature AND enter a state of cold torpor if parental brooding is absent. According to I.A. Shilov, parental brooding is actually required not so much to prevent the chicks from freezing, but rather to intensify their metabolic processes.

Fig. 12.4. One-day-old chicks (after A.V. Mikheev, 1996):

1 - tawny pipit (Anthus campestris), altricial group; 2 - eastern imperial eagle (Aquila heliaca), intermediate group; 3 - grey partridge (Perdix perdix), precocial group

In precocial birds and mammals, the young are capable of independent locomotion, feeding, and other viability-sustaining reactions immediately after birth. Naturally, they still require parental care, yet these offspring can go without direct supervision for prolonged periods. For example, domestic chicks and other precocial hatchlings can forage for themselves, while most ungulates are able to follow their mothers over considerable distances within a few hours of birth. At the same time, different species exhibit varying degrees of developmental capacity As a result of differences in their ontogeny.

Significant differences are also apparent in the duration of both ontogeny as a whole and its individual stages. Overall life span varies so drastically across groups of organisms that identifying any universal pattern is difficult. Multicellular organisms range from ephemeral forms that exist for only a few weeks (such as Penicillium) to extremely long-lived species surviving for several millennia (e.g., sequoia, yew). Among animals, the ratio between the durations of different ontogenetic periods becomes particularly significant. A comparison of precocial and altricial types of ontogeny reveals that an extended Postnatal period of "helplessness" in young birds and mammals contributes to a higher development of their cognitive abilities in adulthood. The most striking example of this is humans, who experience the longest such period in the animal kingdom.