Maybe you remember what is a species from the school, but in fact, it is not easy to distinguish one species from another. In this topic, we will discuss what exactly is a species, what are its criteria, and whether species is a real thing or just a convention.
What is a species
The term "species" was first used by the English scientist John Ray in 1693. The standard definition states that a species is a group of living organisms that are similar in certain respects. This is a rather vague definition, and many still dispute how it is possible to distinguish between different species. Despite the fact that this term is used in all modern theories, not only of evolution but of biology in general, the question of the actual existence of a species is extremely debatable.
It is not a problem for us to understand that our own Maine Coon and our neighbor's Persian cat belong to the same species — Felis catus or domestic cat. Despite all the differences, they share the same body structure, both show predatory behavior, know how to purr and meow, and if the neighbor does not close the door, then these two cats will have offspring. This is exactly how a species is defined — as a basic unit of biological taxonomy of living organisms and a set of organisms close in origin, similar in morphological and physiological characteristics, capable of freely interbreeding with each other and producing fertile offspring. This is a good definition, but it has a number of limitations. What about those organisms that produce non-fertile offspring (for example, a horse and a donkey)? Or, for example, with unicellular protozoa, which do not have sexual reproduction at all?
Besides, if we keep talking about cats, another species — the European wild cat or Felis silvestris can do the same things that a domestic cat can do, including giving common offspring with a domestic cat. That is why the debate about whether the domestic cat is a separate species or a subspecies of a wild cat continues — and the domestic cat either stands out as a separate species or becomes a subspecies. So in reality, despite the fact that the classification of the species allows us to better understand the structure of the world and that recent advances in genetics have made the species definition a little clearer, it is still a conventional division.
Criteria of a species
So, we found out that the species is a convention. This does not help in understanding the evolution and structure of the living world, so we will analyze the principles by which species are distinguished in detail. Most often, in order to establish the existence of a species, all or most of these principles must be followed.
1) Morphological principle. It includes the similarity of the external and internal structure of individuals. Species that are evolutionarily distant from each other are not difficult to distinguish (no one confuses an elephant and a hippopotamus), but in the case of closely related species, this can be difficult (try to tell the difference between an Asian and an African elephant offhand!).
2) Cytogenetic principle. Each species is characterized by a unique karyotype — a set of chromosomes, characterized by their number, size, centromere position, staining pattern, and so on. A human normally has 46 chromosomes, and a cat has 38. However, there are situations when distant species have similar karyotypes (for example, the lion has the same 38 chromosomes).
3) Molecular principle. There are molecular differences between species, primarily related to differences in the DNA sequence of related proteins. Even within related species, there are differences in the genetic structure of the DNA, which ultimately results in different protein structures, even if they perform a similar function. The appearance of these differences can be influenced by accident (for example, an error that crept in at some point, which does not affect the viability of the organism and population and therefore remains) and natural selection (an error in the genome made it possible, for example, to obtain a more effective protein at the output, which helps individuals reproduce better).
Now scientists can read genome sequences, and on the basis of these data, build phylogenetic trees of organisms — reconstructions of the paths of evolutionary divergence, based on establishing the sequence of replacements in DNA. If you look at the picture below, representing the feline phylogenetic tree, you can see how long ago the ancestors of big cats and all the rest diverged — therefore, despite the common origin, there are so many differences between tigers and domestic cats. But the European wild cat and the domestic cat have diverged quite recently, and the boundaries of the species between them are still blurred.
In the DNA of any organism, there are evolutionarily conserved sections, they are vital and remain unchanged (for example, the gene encoding the muscle protein actin — it is very similar in most organisms). There are also variable regions, and sometimes they are so variable that they can differ even in individuals of the same species. Scientists were able to deduce the existence of so-called genetic markers that change most often, and it is clearly visible. To distinguish one species from another, they look for the presence of differences in the structures of these genes, and when a certain level of these differences is "achieved", it can be possible to speak of a divergence of species.
4) Biochemical principle. This principle is often used to identify bacteria, since their metabolic pathways are extremely diverse, while morphologically bacteria practically do not differ from each other, it still can be helpful in other species' research. Even within species that are similar to each other — for example, plants — separate families can be distinguished according to the type of their metabolism: for example, they distinguish a special crassulacean acid metabolism (CAM) — unlike most plants, representatives of Crassula absorb carbon dioxide at night, and not during the day.
5) Ecological principle. This is the ecological niche of the species — a set of optimal conditions and environmental factors for the species. Each species occupies its own ecological niche, and usually, two species with the same niche cannot coexist in the same ecosystem.
6) Geographical principle. Each species has its own distribution area. This, however, is one of the most controversial criteria: the ranges of different species may overlap, break off, or a species may even cover the entire planet (for example, humans or the same domestic cats). However, it is the geographic principle that often "helps" species to separate when for some reason the common range becomes torn apart (for example, a peninsula has turned into an island). Species begin to diverge, do not mate with each other, and after some time may become separate new species.
7) Physiological principle. This, firstly, includes the physiology of reproduction: the age of puberty, the duration of pregnancy, the number of cubs, the duration of feeding, and so on.
8) Ethological principle. This principle takes into account the peculiarities of species' behavior. Foremost, the peculiarity of behavior during the mating season, the nesting period, and care for offspring — sometimes even with the coexistence of closely related species in the same territory, it is the behavior that prevents them from interbreeding, despite the fact that this is physiologically possible.
Conclusion
So, it is assumed that a species is a set of organisms close in origin, similar in morphological and physiological characteristics, capable of freely interbreeding with each other and producing fertile offspring. Not all scientists agree with this definition of the species and believe that the species is a convention. Nevertheless, to recognize a group as a species, there are several principles — morphological, cytogenetic, molecular, biochemical, ecological, geographical, physiological, and ethological. Neither a species nor an individual can be a unit of evolution. This requires a population — a small group of representatives of the same species living in a certain area. A population undergoes stabilizing selection, accumulates "beneficial" mutations, and sometimes experiences gene drift, which through evolution can help it become a separate species. There is a constant exchange of genes between non-isolated populations.