A New Taxonomy of Selection-Types
Author: Devaraj, M
Abstract:The author proposes a new categorization of selection based upon the method of origination of the species. Proposed is the classification of independently-originating species (integrative selection) and dependency-induced species (competitive selection). The supporting arguments are included and well as examples of how such a classification system works and why it is a more evolutionary revealing system.
Keywords: dependency-induced origin, independent origin, evolution, competitive selection, integrative selection, sexual selection, mutation, taxonomy.
Selection today, has been accepted as one of the mechanisms of evolution despite the mutationist-selectionist arguments of yesteryears. The mechanisms of selection however have not been properly discriminated or distinguished except in their effects on their subject population. The reason for this is that selection mechanisms are assumed to be individualistic and varied without any conformity or underlying basis for categorization.
Selection has been divided into the following three categories based on end effect:
Selection however has not been classified on the basis of its origin or on the basis of its effect on the individuals in a population since the general idea is that populations and not individuals which are the determinants of the evolutionary progress of a species. This has been assumed because of the limited life span, the non-variability of the genotype, and the limited spatial influence of the individual.1
However it has been observed by the author that selective mechanisms can be divided into groups which conform to these three categories based on their end effects.
The author suggests that the "type of origin" be used as a basis for classifying selective mechanisms. It can be seen that there are two origins for selective mechanisms.
(1) The first type of origin is "dependency-induced origin." It can be observed in most species that there are requirements on either the co-constituents of the environment and/or on the stability of the environment. These dependencies or requirements of the species lead to competitive selection. Any simple requirement such as food, reproductive partner, etc., leads to competition between individuals for their attainment. This type of selective mechanism has a differentiative effect on a population.
Though certain selection mechanisms originate independent of the subject species, their selective action is due to the presence of a dependency on the part of the Species .(Pollinating agents, seed dispersal agents selection of sexual partners etc). These mechanisms are classified into dependency-Induced (competitive) selection mechanisms since the selective criteria is determined by the dependency and not the selection mechanism. Therefore their effect is also consistent with competitive selection in that their effect on a population is differentiation and not integration
(2) The second type of origin of selective mechanism is that of "independent origin." That is, the mechanism arises independent of any behavioral aspects or requirements of the subject population. Being the prey of another species, environmental calamities, and other effects which occur independent of the subject population constitutes this category of selection. These mechanisms are integrative in their effect on a population.
Here the criterion for selection is determined by the selection mechanism.
Generally most species suffer both types of selection mechanisms. However there are differences in the levels to which they experience one kind of relationship or another, and these differences in levels give rise to the varying speeds of evolution that species undergo.
Method of Classifying Population Interactions into the Proposed Selection Mechanisms
To classify different selection mechanisms into these two categories, population interactions between species and environmental conditions can be categorized in the following manner.
Interaction between populations has been classified2 as:
A relationship that is beneficial to a species is therefore considered to be dependency-inducing and the origin of the competitive selective mechanism. In such a consideration the dependency-induced population would be the predator species which is dependent on the prey, the parasite species which is dependent on the host, and so on.
Competitive relationships (interspecific) despite having an adverse effect, are similar to intraspecific dependency-induced competition since they are also dependency-induced. Just as there is a competition between individuals within the species community (in a dependency-induced selection mechanism) for a particular limited object or resource, which will adversely effect one or more of the individuals, there can be two species competing for a limited resource or object . The only difference is that rather having the same taxometric classification, the taxa is higher . It therefore belongs in the classification of dependency-induced selection mechanisms.
Apart from interspecific relationships, other origins of competitive selection mechanisms are environmental factors and would therefore be of independent origin. The stability or periodicities of environments induce a dependency in species which allows for competitive relationships to occur intraspecifically.
The unpredictability of environment is also a selective mechanism and is an independently-originating selective mechanism.
Regarding the calculation of the stability of environmental factors, geographical stability (i.e. seismic events), ecological stability, and isolation levels are those which determine the level of stability for an environment. Climatic stability, periodicity of events such as seasons, etc., also must be considered.
It can be reasoned that an important mechanism which is the outcome of a stable or cyclic environment is sexual selection. In absence of a stable environment, sexual selection is reduced or altered in varying degrees depending upon the extent of instability.
Even if sexual selection exists in an unstable environment, the selective pressure of the environment (in this case, the instability, which is independent of the current species' origin) will ensure that a genotype/phenotype which is favored by sexual selection is not continually selected.
Generally genotypes which are fit for sexual selection do not possess sufficient fitness for independently-originating selection mechanisms. This is probably because sexual selection and generally competitive selection, are selective for a larger Individuals while the independent mechanisms are selective for a smaller Individuals.2
The level to which the environmental instability dominates as a selective mechanism over the competitive selection mechanisms is dictated by the extent of the instability of the environment. In other words, the influence of sexual selection in a population is directly proportional to the level of stability of their environment.
Generally, it can be observed that any prey population is more in numbers than its predator population. Predation can therefore be seen to be an independently-originating selection mechanism. Since prey populations experience the independently originating selection mechanism, predation, the following reasoning can be made:
(1) In any population, predatory or prey, a parental type with a high fertility character with will have more offspring and hence the genotype has more chance of dominating the genefrequency of the population.
In a case of predation as a selective mechanism, it is obvious that selection will favor genotypes with a higher fertility rate for the species being preyed upon. Even in the simple sense the genotype with a higher fertility rate will survive predation is higher than one with a lower fertility. Hence, it can be seen that prey populations will definitely have a high fertility character.
In order to demonstrate the effect of independently-originating selection mechanisms on a population, let us assume a population with the following characters. Haploid with an independent selection coefficient of 0.3%, means that in every new generation of the species, 30% of the individuals are lost due to independent selection before the attainment of sexual maturity.
Let us assume that there are two genotypes in the population for the character of fertility. One which is gives a higher fertility per individual is denoted by F and the other for normal fertility is denoted by f . Assuming the fertility ratios as being fixed at 2:1 and the gene frequencies of the two genotypes at an arbitrary value such as 0.25 for the genotype F and 0.75 for the genotype f, assuming the lack of any dependency-induced selection which will be selective of the survival traits such as superiority in competition for resources (assuming that only independent selection such as predation is occurring). (Dr. S, I have deleted the probability calculation based on count of individuals since the final values obtained might seem to be unrealistic. However this failure was not due to any fault of the theory but due to the considerations I had to give to the selection coefficient and the fertility ratios. Maurice)
Using genefrequency estimates:
0.25F : 0.75f #1
assuming a fertility ratio between the two genotypes as being 2:1 (F:f), the frequency ratio of the succeeding generation would be:
(0.25X2)/(0.5+0.75)F : (0.75X1)/(0.5+0.75)f
0.4F : 0.6f # 2
and assuming a selection coefficient of 0.3 acting on the population, the outcoming frequencies will lie inbetween the two extremes of:
0.1/(0.1+0.6)F : 0.6/ (0.1+0.6)f and 0.4/(0.1+0.6)F: 0.3/(0.1+0.6)f
which are the ratios of:
0.14F : 0.86f and 0.57F : 0.43f
Assuming that there has been a numerically same number of individuals eliminated from both genotypes, let us assume a ratio of:
0.25/(0.25+0.45)F : 0.45/(0.25+0.45) f
(As according to the assumption that 50% of the selection acts on genotype F which means 0.4 (Eq. #2) minus 0.15 is 0.25 and 50% of genotype f, which means 0.6-0.15=0.45)
0.36F : 0.64f #3
Now this population reproduces with the same fertilities as before:
(0.36X2)/(0.72+0.64)F : (0.64X1)/(0.72+0.64)f
which shows that the frequency of the next generation would be:
0.53F : 0.47f
This frequency ratio shows an increase in the frequency of the F genotype from which it can be concluded that in the event of a genotype with higher fertility in a population subject to non specific (Random, without criteria) selection, the particular genotype has more chance of influencing and dominating the population.
These types of selection which act to select fertility are independently-originating selection mechanisms. It can said that as such, it can be seen that the selection which acts on the population with the two genotypes of fertility, is independent of any character of the subject population and selects independently of any character. It is not necessary for the selective mechanism to require fertility as a fitness trait. If the selective mechanism originates independently, and if the subject population does not have any intense dependencies, the independent selection will select for fertility.
As an example where Independent selection does not favor fertility due to the presence of a dependency is the case of viceroy butterfly Limenitis archippus and the Peppered Moth Biston betularia. The dependency of the Viceroy butterfly on the Monarch butterfly Danais plexippus (Batesian Mimicry) will cause independent selection to select for the better mimic instead of fertility.
(2) It can be seen that since high fertility is selected in a prey species and predatory species usually do not possess any direct selection mechanism for fertility and instead are subject to competitive selection which is usually not selective of fertility. This is because competitive selection causes selfishness at the level of individuals and their genotypes. In fact, a lower fertility rate may be desirable so that the species population does not exceed the prey resources. Hence prey species will have to be numerically superior compared to their predator types
(3) Since fertility is higher in the prey groups (neglecting the levels of genetic polymorphism), the number of variations within a given period of time will be more for a prey species than the predatory species.
(4) The higher the number of variations for a selective mechanism to act on, means that there will be a much higher speed of adaptability for prey species.
From this, the conclusions that can be made about the prey populations (populations experiencing independently-originating selection mechanisms) have the characteristics of high speed of adaptability, better adaptability, and a higher fertility rate.
While predation is given as an example here, it can be considered with any other type of independently-originating selection mechanism. The same reasoning applies.
The terms predator and prey are also intended to accommodate the benefactors and beneficiaries of a relationship. An example of a population who's size based on ecological relationships is the relationship (Batesian mimicry) between the Monarch butterfly Danais plexippus and the viceroy butterfly Limenitis archippus1 where the mimic is the viceroy butterfly who is found to have a smaller population compared to the Monarch butterfly (the model). The palatable viceroy butterfly is dependent on the unpalatable Monarch butterfly.
This dependency makes the viceroy subject to a reduced population size, because when the viceroy has the larger population, it will prove deleterious to both specii since relatively fewer unpalatable monarchs will not prove to be a significant deterrent. When there is a larger population of palatable viceroy butterfly, the monarch will also get predated on despite its inedibility. This shows that the viceroy which should have originally been more subject to predative selection, has evaded this selection successfully due to its mimicry. Such a reduction of predative selection not only brings down the fertility rate, but dependency-induced selection further reduces this selective factor by the adherence to the Monarchs wing pattern and not fertility. Hence if any genotype for higher fertility occurs, it will be selected out unless the individual shows a better ability to mimic. Since the selective factor will always be the ability to mimic, even in a condition where fertility is also a factor, selection will always sideline the fertility character in successive generations.
Though the method of fertility determination is different in this case, it can be seen from various case studies and by logical reasoning, that a generalization can be made that the benefiting species are always reduced in population size and fertility more than their benefactors.
It can be reasoned that, when the selection originates independent of dependencies of the subject species as its effect is to influence the species as a whole and since to achieve fitness it is not necessary to compete between individual genotypes but only to increase the frequency of the successful genotype to survive the independently of the originating selective mechanisms. When the selection is competitive, it encourages differentiation between individuals.
It has also been observed that there is a type of speciation which occurs within a population without any geographical isolation. This is called as Sympatric speciation.3 The origin of Sympatric speciation can be shown to originate as a result of competitive selection.
Competitive selection occurs when numerous individuals of a population compete for a common requirement, for example food. Under these conditions of abundance, this type of selection plays a passive role of differentiating between types of adaptations in individuals aimed at acquiring the required objective. For example, assuming that a particular population has two phenotypes, one being more successful over the other in obtaining food, the competition does not extinct the second phenotype but rather adaptive changes are incorporated for survival such as smaller size, slower metabolism, or a wider diet.
When there is a reduction in the availability of the object, (sporadic changes) the selection mechanism acts directly increasing the distances between the successful adaptations in various directions.
This is achieved by selecting the individuals which have adapted best to their respective environments. When this happens, individuals with lesser fitness levels are less directional in their adaptations and are then eliminated by the selection mechanism. This is the origin of Sympatric speciation.
From this, it can be seen that the dependency-induced competitive selection mechanisms are not supportive of individuals conforming to the population genefrequencies for intermediate phenotypes. In other words, competitive selection is not integrative of the genefrequency.
As opposed to competitive selection, in the case of independently-originating selection mechanisms, the mechanisms do not act between individuals but on the individuals. If the individual is not fit to survive the mechanism, it is eliminated. So any surviving individual can be assumed to possess a character or characters that are successful in overcoming the selective mechanism. Hence such a character can be found to increase in frequency within the population over repeated selection within the whole population, as do other characters since there is no conflict between the different adaptive directions, such as in the case of competition generated adaptations. For example if a population has two adaptations to evade predation such as higher fertility and camouflage, these two adaptations are not conflicting since they do not interfere with each other in their methods of being successful against the selective mechanism.
However if both directions of adaptations are based on conflicting requirements, homogeneity of the population will not be achieved. For example if one method of evading predation is by camouflage requiring a particular geographical condition such as vegetation and the other method is by faster mobility which requires flatlands, differentiation will occur in the population which will be accented over repeated selection and hence lead to distancing in both genetic and geographical distances. This is the origin of Allopatric speciation speciation which is occurring due to geographical barriers.3 However, each of the two differentiated populations will show a homogeneity of genefrequency of their respective population because of the integrative effect of the independently-originating selection mechanism. Here the differentiation is not caused by the selective mechanism but by the dependency of the group on a particular situation.
The categories of selection referred to in this article are independently-originating selection mechanisms and dependency-induced selection mechanisms. The author suggests that for easier reference, independently-originating selection should be referred as integrative selection and dependency-induced selection be referred to as competitive selection.
From the above arguments, the following summation can be made about the effects of the integrative selective mechanisms and competitive selective mechanisms.
Integrative selective mechanisms:
Competitive selection mechanisms:
Implications and Areas of Influence
This proposal of categorizing selection based upon the type of origin has a wide ranging influence on the many aspects of evolutionary research. Some areas of influence are:
1. Present Classification of Selection: Stickberger, Monroe W. The Kinds of selection. In Evolution. Jones and Bartlett publishers,1990; Pages 441-443.
Influence of Individuals and Population on the Evolutionary Characteristics of a Species: Stickberger, Monroe W. The Neo-Darwin synthesis. In Evolution. Jones and Bartlett publishers, 1990; Pages 418,419.
Batesian Mimicry: Population Ratios Between Model and Mimic: Stickberger, Monroe W. Evolution. Jones and Bartlett publishers; 1990. Page 440.
2. Pianka, E.R., r-k selection. In Evolutionary Ecology, 4th Edition, Harper and Row, New York; 1988.
3. Barton, N.H., and B. Charlesworth, , Genetic revolutions, founder effects and speciation. Ann.Rev.Ecol.Syst.,1984. 133-164
Journal Home Page