Species selection is currently considered to be an important evolutionary mechanism, which could be responsible for the existence of some macro-evolutionary trends. Consequently, it will be further discussed in the chapter devoted to macro-evolution (XXVI). In species selection, competition occurs between species and entire developmental lines as to which of them will most probably undergo speciation (split off daughter species) and will be less likely to suffer extinction. Phylogenetic lines containing species that will most probably split off daughter species or that will not rapidly suffer extinction, will be evolutionarily successful in the long term even if the properties that are the cause of more frequent speciation, e.g. low mobility of its members, are disadvantageous from the standpoint of survival of individuals within the species.
It is quite possible that a number of very important traits occurring in modern organisms, e.g. sexuality (Stanley 1979), emerged just because of species selection. On the other hand, only individual selection could be responsible for the formation of all complex adaptive traits (inter-allele selection in sexually reproducing organisms – see below), which is the only known evolutionary mechanism that is capable of forming a complicated adaptive biological structure or function through gradual accumulation of minor changes leading to optimization of the relevant structure or function. The main handicap of species selection compared with individual selection lies in the small number of units that can compete together and the small space in time for the multistage evolution of more complicated traits. While, in individual selection, an enormous number of individuals compete together, the number of species that exist in a given territory at a given instant is substantially more limited. Simultaneously, the lifetime of an individual is much shorter than the period of existence of the species, so that intraspecies individual selection has sufficient time to accumulate a number of suitable changes, gradually leading to the formation of a certain complex adaptive trait, even if the species were capable of evolutionarily responding to selective pressure only for a certain time after its formation and not for the entire time of its existence (see XXVI.5). In contrast, the average time of existence of a species is approximately several million years, so that the entire period of existence of life on Earth (3.5 – 3.8 billion years) or even just the entire period of existence of macroscopic multicellular organisms (700 – 800 million years) could not easily encompass a sufficient number of “generations” of subsequent species.
In contrast, the advantage of species selection over individual selection lies in the fact that it has, so to speak, the final word in relation to fixation of a certain trait. A trait that can be in any way advantageous from the standpoint of the individual will finally disappear from nature if its existence leads to the extinction of the species whose members bear this trait. Human intelligence is certain advantageous from the standpoint of individual selection. However, if human beings kill themselves off in an atomic war, at the end of the day, the last laugh will be had, for example, by far less intelligent moles.
Compared to group selection, species selection has two great advantages. The first of them is resistance to invasion of an alternative form of the trait. If a certain trait is advantageous from the standpoint of the group and disadvantageous from the standpoint of the individual, then this trait will disappear from nature in the majority of cases as, sooner or later, individuals without this trait from another population will enter a prospering population of its bearers and will finally predominate through individual selection. In contrast, traits advantageous from the standpoint of the species and disadvantageous from the standpoint of individuals cannot disappear from nature in this way as reproduction isolation of the individual species will prevent invasion of the bearers of a trait of one species into the population of a second species.
A further advantage of species selection compared to group selection lies in the fact that the genetic variability of species need not necessarily be less than the genetic variability of the individuals of these species. For populations, the situation is far less favourable from the standpoint of the effectiveness of selection. The individual populations are formed by large groups of individuals and thus, quite necessarily, any property of various populations, i.e. the average properties of its members, varies less between populations than this property varies amongst individuals in the framework of the entire species. Simultaneously, the effectiveness of selection depends on the amount of variability amongst the individuals that are the subject of selection.