BOTANY VOLUME 3 - EVOLUTION AND SYSTEMATICS - 2007

10. EVOLUTION

10.4. Macroevolution

The described joint effects of random Mutations, genetic recombination, Genetic Drift, reproductive isolation, and natural Selection as components of neo-Darwinian views on the evolutionary process are sufficient for understanding intraspecific differentiation and various speciation processes. This level of evolution is termed microevolution. However, there certainly exist rare, singular events with unusually profound evolutionary consequences. These include, for instance, the Water/144.html">Origin of the autotrophic plant Cell via endosymbiosis between a heterotrophic eukaryotic Organism and autotrophic Bacteria, or mass extinctions such as the one at the Cretaceous-Paleogene boundary, likely resulting from an asteroid impact. Endosymbiosis can be likened to a very large mutation, while a mass extinction is essentially nothing more than a sudden and drastic change in selection pressures. Thus, both events fit into the conventional understanding of the evolutionary process. Nevertheless, the question arises: can Examples of evolutionary changes observed predominantly through paleobotany and comparative Morphology over long geological timescales (macroevolution) be explained by the mechanisms of evolutionary changes occurring at the species level?

Neo-Darwinism most often Answers this question in the affirmative: it is generally accepted that the prolonged operation of the aforementioned evolutionary mechanisms is capable of producing the large-scale evolutionary changes observed. The premise underlying this assertion—that currently observed evolutionary mechanisms do not differ from those operating in the distant past—dates back to the English geologist Charles Lyell, who in his Principles of Geology (1830–1833) explained the geological and geomorphological Structure OF THE Earth precisely through THE PRINCIPLE OF actualism, thereby rejecting the catastrophism theory of G. Cuvier popular at the time. Reading Lyell's work during his voyage around the world strongly influenced Charles Darwin.

Examples of macroevolution include profound differences between groups of organisms of high taxonomic rank, the so-called evolutionary trends (major directions of development): the origin, prolonged existence, and extinction of groups of organisms.

The colonization of land in the Ordovician, Silurian, and Devonian by plants adapted to aquatic life is an example of a monumental evolutionary shift that required numerous morphological, anatomical, and physiological changes. Plants required a water-impermeable cuticle to prevent uncontrolled water loss. Because the appearance of the cuticle made water absorption across the entire plant surface impossible, rhizoids and roots emerged as specialized water-absorbing Organs. The necessity of transporting water from these organs—most often embedded in the soil—to other PARTS OF THE plant, even in small plants, became efficient only through the evolution of specialized conducting elements (hydroids, tracheids, vessels). Furthermore, plant parts rising above the ground required strengthening structures. Gas exchange, also hindered by The formation of cuticle, was facilitated by the evolution of Stomata, while internal gas transport necessitated The Development of a system of internal cavities (intercellular spaces). Corresponding to these adaptations to terrestrial life, the morphological and anatomical differences between primary aquatic and primary terrestrial plants are immense. Paleobotany has shown that various structures of land plants appeared sequentially in Earth's history (e.g., the earliest cuticle fragments are known from the Ordovician, plants with stomata from the Silurian, and upright axes with tracheids from the Devonian). Furthermore, all the aforementioned structures can be convincingly interpreted as adaptations. These two facts indicate that THE ORIGIN OF land plants, despite their current major differences from primary aquatic plants, was a gradual process of adaptation to altered environmental conditions, driven by no evolutionary mechanisms other than those already known.

One can repeatedly observe evolutionary trends—the evolution of a trait or a complex of traits proceeding in only one direction over a long period. A particularly striking example of directed evolution at a high taxonomic level is the progressive reduction of the gametophyte generation in the course of land plant evolution, ranging from bryophytes with their gametophyte dominance, through ferns and gymnosperms, to angiosperms, in which both male and female gametophytes consist of only a few Cells. Equally noticeable when comparing different groups of land plants is the increasingly enclosed Nature of the female gametophyte (see 11.2). A possible explanation for evolutionary trends lies in the assumption of directed environmental change and, consequently, directed selection over a long period. However, it is highly questionable whether such an assumption is valid, given that Earth's climate history was rather turbulent with strong climate fluctuations, for instance, in the Quaternary and earlier periods. Another assumption involves accepting a directional trend of evolution toward progress (progressive evolution). With regard to the evolutionary reduction of the gametophyte generation and the associated increasing dominance of the sporophyte generation, one could argue that, given the initial similarity of both generations, one of them is more viable. This is widely believed to be the sporophyte generation because, as a diploid generation—in contrast to the haploid gametophyte generation—unfavorable mutations are expressed only when homozygous. The fact that the trend could be viewed as progressive evolution does not at all mean that Representatives of the early phases of this evolution were unadapted to their environment.

Instead of explaining evolutionary trends through natural selection, an alternative process to neo-Darwinian mechanisms was proposed—species selection. This hypothesis assumes that a trait, such as varying body size, is associated with different rates of speciation or extinction. Thus, species consisting of larger individuals might exhibit a higher speciation rate or a lower extinction rate. Over time, this would lead to an increase in the number of species comprising larger individuals relative to those comprising smaller ones, as well as an increase in the average size of individuals. Importantly, size itself is not directly acted upon by natural selection. The existence of species selection as a process independent of natural selection remains controversial.

The paleontological record clearly shows that groups of organisms arise, persist for varying lengths of time with greater or lesser species diversity, and then either become extinct or survive as very few species. Examples include the Lycopodiopsida or Equisetopsida, which in the Carboniferous were represented by multiple lines of arborescent forms and dominated the vegetation of that era, whereas today relatively few species survive as an admixture to the dominant flowering plants. An explanation for this phenomenon could be that one group, being better adapted, outcompetes another. Two scenarios are possible: on the one hand, progressive traits emerge in an unchanging environment, leading to the survival of the newly formed group of organisms; on the other hand, in a changed environment, groups adapted to the new conditions replace those adapted to the old environment.

The major differences between groups of organisms, evolutionary trends, and the origin and extinction of organism groups can be readily understood

as more or less gradual adaptive changes brought about by natural selection. And although far from every known example of macroevolutionary change easily finds a satisfactory explanation within the framework of known Evolutionary Processes, no more perfect or universally accepted alternative explanations have been found to date.

Many concepts have been introduced to describe the pattern of evolutionary change on a geological timescale. When an evolutionary Lineage changes over time, it is referred to as anagenesis; if it remains unchanged over a long period, it is termed stasis. Evolutionary change in a single direction (an evolutionary trend) is called orthogenesis. The differentiation of a single lineage and its splitting into several new ones is known as cladogenesis. A special case of cladogenesis can be considered adaptive radiation, which is defined as the ecological differentiation of a single lineage. The best-known example of adaptive radiation is the diversification following the initial colonization of an archipelago of islands. Although adaptive radiation usually proceeds relatively rapidly, its rate is not part of the definition of adaptive radiation. The paleontological record typically reveals relatively long periods of stasis punctuated by short phases of intense cladogenesis. This course of evolution is termed punctuated equilibrium.

In the evolution of traits, one can distinguish homologies from convergences and parallelisms¹. Homology is defined here as the correspondence of a trait in different organisms due to its inheritance from a common ancestor. In contrast, traits are termed convergent if they arose in different, distantly related lineages through adaptation to similar environmental conditions. Convergently evolved similar traits can usually be easily identified on The basis of their detailed structure or function. Frequently cited examples include the succulent stems of New World cacti and Old World euphorbs (see Fig. 4.35), or the reduction of the perianth, the development of stamens with versatile filaments and large anthers with dry pollen, stigmas with a large surface area, etc., in wind-pollinated plants of very diverse affinities (see 11.2, pollination). Unlike the above, parallelism involves the independent emergence of a similar trait in closely related lineages. Parallelism can usually be recognized not by the peculiarities of the trait itself, but only through a Phylogenetic Analysis. Such an analysis must demonstrate that the trait shared by two organisms is not directly traceable to their immediate common ancestor. An example of this is common ragwort (Senecio vulgaris) and wood ragwort (S. sylvaticus). The complete or nearly complete reduction of ray florets in the inflorescences of these species occurred independently and, accordingly, in parallel during their evolution from different ancestors with normally developed ray florets. Parallel evolution is underpinned both by adaptation to similar environmental conditions and by the probable canalization of evolutionary change imposed by the CHARACTERISTICS OF THE common ancestor.

¹ Homology can be distinguished from analogy, whereas convergence and parallelism can be contrasted with divergence. — Note by the Editor.