Basics of Evolution - Korzh O.P. - 2006

Part I. MICROEVOLUTION

Chapter 10. Speciation as the Core Phenomenon of the Evolutionary Process

10.3. Sympatric Speciation

By sympatric speciation (from the Greek syn meaning together and patris meaning homeland), we mean The process of new species forming within the geographical ranges of their ancestral species, that is, without the involvement of geographical isolation.

For a long time, many scientists questioned its possibility or, like E. Mayr, believed that The Emergence of any species could be explained entirely through allopatric speciation. The primary counterargument against this mode of speciation was The Challenge of how a new species could establish genetic independence while sharing the exact same range as the ancestral species. Today, sympatric speciation is recognized just as widely as allopatric speciation.

The mechanisms of sympatric speciation are more diverse than those of allopatric speciation. This is because sympatric speciation is driven by some form of biological isolation. When the pressure exerted by this isolation is sufficiently strong, and the new form is not eliminated by the negative effects of natural Selection, the differences can accumulate over time, allowing the new form to attain species status.

In plants, this form of speciation is quite frequently triggered directly by genetic isolation, most commonly taking the form of polyploidy (Fig. 10.2). Changes in hereditary material can occur most rapidly through autopolyploidy, which involves a multiple increase in the chromosome number.

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Fig. 10.2. Polyploidy in black nightshade (Solanum nigrum) (after K.M. Sytnyk, 1986):

A - general view of the plants; B - chromosome sets; 1 - diploid (36 Chromosomes); 2 - tetraploid (72 chromosomes); 3 - hexaploid (108 chromosomes); 4 - octoploid (144 chromosomes)

Polyploid forms are characterized by larger overall body sizes and proportions, accelerated growth rates, enhanced vitality, and increased fertility. It is believed that this may be one of the reasons why polyploid forms are so widespread in harsh environments (such as polar latitudes and mountainous regions). Well-known Examples include cultivated plants such as cotton, sugarcane, potatoes, bananas, chrysanthemums, and dahlias.

Another equally common process driving The formation of new species through relatively rapid shifts in heredity is allopolyploidy, which arises from the Hybridization of multiple species followed by an increase in chromosome number.

One of the most human-significant plant groups originating through this pathway is the genus wheat (Triticum), a large proportion of whose members are of hybridogenic origin.

Distant hybridization plays a definite role in speciation, as evidenced by the widespread presence of hybrids in nature. One such example is the sorbuscotoneaster (x Sorbocotoneaster), discovered in southern Yakutia not merely as isolated specimens, but as an entire population that has colonized a significant territory (covering about 300 km). The hybrid Water/144.html">Origin of the domestic plum (Prunus domestica) from the cherry plum (P. divaricata) and the blackthorn (P. spinosa), as well as the raspberry (Rubus maximus) from two species—R. idaeus and R. speciosa—has been experimentally proven.

Among animals, the emergence of new species via polyploidy also occurs, though examples are far scarcer than in plants. It is generally thought that polyploidy in animals is restricted to parthenogenetic forms (although such forms are now known even among vertebrates). Hybridization is much more common, occurring in fish, birds, and especially invertebrates.

Experiments under artificial conditions have established that a significant number of species produce fertile offspring when crossed. In general, interspecific hybrids are known to exist across all mammalian genera, with a considerable portion of them being fertile (seven interspecific hybrid types are known in goats, four in both canids and camelids/llamas, while intergeneric hybrids even occur among cattle).

Among birds, hybrid forms are quite common even in natural conditions, as mentioned previously. These examples indicate that genetic isolation is frequently a secondary phenomenon.

Closely linked to polyploidy and hybridogenesis is METABOLISM/2.html">THE CONCEPT OF abrupt (saltational) speciation. For some time, scientists debated the possibility of rapid speciation—where a new species arises within a single generation. Polyploids or hybrid forms, which immediately acquire genetic isolation from their parental forms, might seem to support this idea. At the same time, we must recognize that these forms can initially be considered only as potential species. The development and realization of this potential is a historical process driven by the operation of all the evolutionary mechanisms discussed earlier.

Thus, on the one hand, we cannot deny the reality of the rapid formation of fundamentally new forms through hybridogenesis or polyploidy; on the other hand, these forms can only become true species after a certain period of time (provided that conditions allow it). Therefore, even these pathways of speciation cannot be equated with instantaneous saltational speciation. The appearance of new hybrid or polyploid forms can merely accelerate the overall pace of evolution.

It should be noted that the potential avenues for sympatric speciation do not end there. Furthermore, unlike in plants, the predominant forms of isolation in animals are ecological and morphophysiological. Initial Stages of such speciation are exemplified by the facts discussed earlier in the section on biological isolation. Specifically, this applies to various ecological races (such as the spring and winter fish populations of many species, or cuckoos adapted to different host species).

Consequently, the defining feature of sympatric speciation is that some form of biological isolation serves as the primary driver behind the formation of a new species.

In some cases, genetic isolation between the new form and the ancestral form arises instantly; in others, it develops only gradually over time. Once established, the new species "solves the problem" of its geographic range, and may eventually become allopatric relative to its ancestral form. This further complicates the Analysis of the origins and distribution patterns of particular species.