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Written by Tsirha Adefris

Natural Selection and Extinction

Abstract

 

This section presents an argument that is overly apparent but is hoped to be a novel commentary on key evolutionary mechanisms: Natural selection and extinction. It is claimed here that these two processes may be described as a single and not an alternate process in nature. The process of natural selection requires a deeper verification while the fossil record is adequate proof for extinction. Therefore, establishing the unity of the two processes can help provide a comprehensive as well as a succinct evaluation of evolutionary processes in general.

 

 

INTRODUCTION

Extinction is caused by rapid changes in the environment, epidemic diseases, overwhelming numbers of predators or being outsourced by an invasive species. The individuals that live through this change do not necessarily have superior genes, but rather advantageous genes to better be adapted for survival in the newly transitioned environment. These individuals then reproduce to make a generation that is genetically different than the previous generation and more capable of surviving in the changing environment. Further genetic differences caused by mutations in the genome causes the original species to split into two species after many generations have passed. (Jablonski, 2005)

Darwinian “teleology” has been referred to and explained by adherents as resulting from either: The failure of morphology to be accepted as a formal science (Ghiseline, M. T. 1994), or Darwin’s lack of an intrinsic explanation for evolution and consequent focus on natural selection as a major driving force in evolution (O’Grady, R.T. 1984).

Raup, D (1994) is foremost among practitioners who have raised the issue of the process of extinction being relegated to minor importance by most neo-Darwinians.  Weins and Slaten (2012) also defend the premise that extinction events require the attention of biologists and suggest that Raup represents paleontologists who cannot fail to examine the process, bearing in mind the prevalence of various geologically confirmed extinction events.

David Raup (1994) directly contrasts extinction with speciation.  Niles Eldridge (1991) who wrote a volume on extinction regards the process as occurring at the nexus of speciation. Perhaps it is no surprise that I am here equating extinction with natural selection itself.  I am not alone in this.  The “scaling down” of the concept of natural selection to mean “the elimination of the unfit” thus equating selection and extinction has been suggested. (Smith, 2012).However, (Darwin|Darwin & Wallace, 1859) Wiens and Slatten (2012) relegate extinction to a general “background” loss of reproductive fitness through long term “negative selection” pressure.

Darwin (1859) in Origins inextricably links extinction with natural selection as two sides of a single process. Did Darwin equate the two? He describes extinction as “that gratuitous mystery”. Clearly Darwin’s concern was the origin of species by natural selection.   It is not intuitive to link selection with speciation and the two are described as cause and effect by Darwin.  However, an examination of current literature, confirms wide acceptance of a direct relationship between species origin and extinction. Beer (2009).

David Raup (1994) who takes the view that extinction is integral to the evolutionary process defends that extinction is the other side of speciation. This view is shared by Niles Eldredge. Extinction processes that have merited close examination by evolutionary biologists and are discussed at length are negative selection, background extinction, evolutionary suicide, and species selection. Webb (2003) has classified all these under what he terms “Darwinian extinction” All share the commonality of being linked up with natural selection in an indistinguishable manner.

He appears very close to the argument presented in this paper in describing what he terms “Darwinian Extinction”. The difference between his treatments of extinction as of many types with one type being caused by natural selection, hence termed “Darwinian”.

I am proposing that not only one type of extinction processes is caused by selection but that extinction, the disappearance of populations, lineages, and clades from the biosphere occurs when these fails to be selected. Thus, selection and extinction are two sides of the same coin. Eldredge and Gould have admittedly described the theory of speciation by punctuated equilibrium events as being inspired by dialectical materialism. I have taken Eldredge’s “extinction at the nexus of speciation” on the ultimate dialectical journey to suggest that extinction is but one with selection as it interpenetrates it.

The premise:  a species A is competitively better suited to its environment….it has a reproductive advantage over species B not competitively suited which will have a reproductive disadvantage, therefore “nature” selects A over B. It is remarked in the literature that B is “negatively” selected.

Over time, in the above scenario B can become extinct with there being no need to assume A has speciated.

Darwin was interested in the origin of species. Adaptive radiation as his Galapagos finches show is his legacy to the mode of evolution.  The above scenario shows us that extinction has the upper hand in the ecological relationship. Darwin was all the while aware of the intertwined nature of selection and extinction.

I am forced to imagine that he was actually considering one and the same thing. Allopatric speciation is the nexus of selection and extinction.

 

The full title of the “Origin” is revealing. “On the origin of species or the preservation of favored races by natural selection in the struggle for life”.  The key to understanding is” the preservation of favored races” …Clearly here is meant the races “favored” by nature as in the case of the breeder who artificially selects.

 

Paleontology forces us to look at points in time when species fell into and out of favor with nature. 

Extinction events, named mass extinctions are recorded over regions of the geologic column.  As remarked above, extinction indeed has the upper hand. It is copious. We do not consider “speciation events” in mass as we do the above …. Adaptive radiations are the equivalent of our undefined speciation events.

 

Darwin’s adherence to the use and disuse theory did not create an obstacle for his theory. However, the mathematical precision of Mendel’s explanation has led to the synthesis which has put “change in allele frequency” as the very definition of evolution.  In addition, we now work with The Hardy Weinberg Law. The mathematics of genetics superimposed on the assumption of natural selection.

 

The material of evolution has been described as phenotype external not genotype. It is suggested that clade-level extinction may provide a key mechanism for explaining negative or hump-shaped relationships between clade age and diversity, and the prevalence of ancient, species-poor lineages in numerous groups (Pyron & Burbrink, 2012)

 

Diversity results through both the processes of species origination and extinction. However, studies of extinction have tended to focus on mass extinctions, despite the fact that the background extinction represents a greater loss in terms of the absolute number of extinct taxa. In order to identify what factors, affect this rate of background extinction. Most clades decline to extinction in a “driven” manner—that is, faster than expected by chance alone.

 

Ecological speciation can occur in either geographically isolated populations (allopatric) or in settings with no physical barriers to gene flow (sympatry or parapatry). When gene exchange is physically impossible, the conditions under which reproductive isolation can evolve are nonrestrictive: allopatric speciation can be driven by strong or weak divergent selection, sexual selection, uniform selection, or even stabilizing selection. It may occur quickly under divergent selection or extremely slowly under uniform or balancing selection.

 

Extinction processes that have merited close examination by evolutionary biologists and are discussed at length are negative selection, background extinction , (Jablonski, 2005) evolutionary suicide, and species selection. Webb (2003) has classified all these under what he terms “Darwinian extinction” All share the commonality of being linked up with natural selection in an indistinguishable manner. He articulates his opinion thus:

One factor left off the usual list of extinction mecha-

nisms is natural selection. The process by which natural

selection leads to extinction has variously been termed

“self-extinction” (Matsuda and Abrams 1994), “evolution

to extinction” (Dieckmann et al. 1995), and “evolutionary

suicide” (Gyllenberg and Parvinen 2001). I prefer to call

this process “Darwinian extinction” to indicate that the

evolution is by natural selection. Darwinian extinction is

most likely to be an important evolutionary force in species

selection. (Webb, C.:2003)

 

He appears very close to the argument presented in this paper in describing the term “Darwinian Extinction”. The difference between his treatments of extinction as of many types with one type being caused by natural selection, hence termed “Darwinian”.

My argument goes against the grain of what Webb has formulated. I am proposing that not only one type of extinction processes is caused by selection but that extinction, the disappearance of populations, lineages, and clades from the biosphere occurs when these fails to be selected. Thus, selection and extinction are two sides of the same coin. (Gould & Eldredge, Punctuated equilibria: the tempo and mode of evolution reconsidered) (Gould, Punctuated equilibrium in fact and theory, 1989) have admittedly described the theory of speciation by punctuated equilibrium events as being inspired by dialectical materialism. I have taken Eldredge’s “extinction at the nexus of speciation” on a hypothetical leap to suggest that evolution is the solution between the related conflicts of natural selection and extinction.

Wiens and Slaten (2012) define the mechanism of background extinction as a typically long-term, multi-generational loss of reproductive fitness. Extinction is here is regarded as ubiquitous as natural selection in biology.

 

To examine the possibility of the claim thus stated four lines of evidence are investigated.

These are:       

       1. Charles Darwin’s detailed references to extinction.

       2.Ronald Fisher’s genetical theory of natural selection.

       3.Examples from the Hominid fossil record

       4.The ecological process of endemism

Finally, a model is suggested that makes use of the evolutionary mode and tempo of punctuated

equilibrium.

 

DARWIN ON NATURAL SELECTION AND EXTINCTION

Darwin’s major contribution as widely popularized is his description of the processes related to the origin of species. Natural selection is described by Darwin as the driving force in evolutionary change. The phenomenon of extinction is also given more than a mere mention by Darwin.

As detailed by Raup (1994), who argues about the importance of the study of extinction mechanisms by biologists, there are several entries in Darwin’s Origin of Species where extinction is described as co-occurrent with natural selection.

“As in each full stocked country natural selection necessarily acts by the selected form having some advantage in the struggle for life over other forms. There will be a constant tendency in the improved descendants of any one species to supplant and exterminate in each stage of descent their predecessors and their original parent (Origins)”.

The above and other similar statements have generally been interpreted to describe a speciation -extinction continuum.

To identify extinction as identical to natural selection, I have chosen the two key processes that are linked with natural selection and evaluate their bearing on extinction.

Variation and competition are the two processes that lead to natural selection. In an understanding of these processes, variation can be subsumed under the characteristic of each species and competition can be explained by a version of the predator-prey model exactly as described by Darwin.

            Variation

Species and individuals within species possess a unique DNA fingerprint. The process of extinction, whereby one species is replaced by another directly causes the profusion of variation.

Among individuals of a species, novel variants are maintained and followed by the consequent extinction of less favored forms (Origins).

We have the example of the variation in the hominid fossil record where the extinction of the lone species presumed to be Homo habilis has been followed by variant species that extend from Europe to Asia.

            Competition

The predator prey model is an apt description of the outcome of competition resulting in the likely extinction of one of the competitors.

The model is viewed as a natural balance where in the absence of prey, predator population goes down and during the increase of predator population the prey population will be subject to extermination.

Darwin has used this example to illustrate natural selection and it is also an ideal manner to represent how extinction may act. Leigh Van Valen (Valen, 1977) in his Red Queen's Hypothesis,  described that any gain in fitness by one unit of evolution is balanced by losses in fitness by others and that this is most easily justified by equating realized fitness with control of trophic energy.

 

Adaptation

Adaptation is an ecological process which figures in evolutionary change. Favorable environments as well as unfavorable environments are the testing grounds for both natural selection and extinction. A thorough understanding of the nature of phenotypic variation will never be achieved without an analysis of the scope and usual causes of maladaptation. (Crespi, 2000).

Natural selection is characterized as a theory of adaptation. The origin of flowering plants theoretically resulted in providing the adaptive background for Primates to appear. The same process has caused the background extinction of the Dinosaurs.

The removal of the Dinosaurs provided an econiche in which the Primates adapted. The Primates, in their early history display a suite of adaptations selected for an arboreal existence. The geological column that represents Primate evolution and the diversification of the order itself is evidence for adaptive selection operating together with extinction.

Trends, long-term directional tendencies in evolution, are traditionally interpreted as selected for and adaptive. Alternatively, trends may be unselected effects of characters and processes within species: the effect hypothesis. Thus, adaptations of organisms, varying among species, were selected for immediate fitness, but they may also incidentally determine different speciation and extinction rates and trends (Vrba E. S., 1983)

 

The Genetical Theory of natural Selection

Ronald Fisher was an early mathematician and statistician who advanced the connection between natural selection as envisioned by Darwin and Mendelian genetics. (Fisher, 1930)

Fisher's genetical theory states that natural selection may be described as an increase in the fitness of an organism and is equal to the organism's genetic variance in fitness at the time.

Since the statistical variance is the average of the differences of observations from the mean, describing fitness using the statistic as in Fisher above renders natural selection identical to extinction.

Although Fisher lent support to the discredited eugenics program, his genetical theory has been given credit in putting evolutionary theory on a firm genetic basis. Fisher’s genetical theory had created a long-lasting controversy with Sewall Wright’s theory of adaptive landscapes. (Frank, 2011)

The Fossil Record

Evidence that may be ascribable to the unity of natural selection and extinction in the examination of the Hominin fossil record is the multiplicity of taxa that range over time replacing one another.

The process of natural selection is behind the appearance of the earliest hominin such as Ardipethicus. However, the paucity of the remains of the genus is evidence that extinction has just as important a hand in its history. While the early Hominin was making its appearance, it was guided by both by selection and extinction.

Endemism

Endemism has been described as the analysis of a global biological system involving processes on both a shorter-term ecological time scale and a longer-term evolutionary time scale. (Anderson, S. 1994). Endemism is currently couched under a warning call for extinction risk. To examine the concept as it relates to life processes, a comparison may be made with the archaeology of seriation. In seriation: styles originate, expand, and disappear. In life species appear, diversify, and become extinct. The suggestion here is that the process that drive species diversification is driven by the tandem processes of selection and extinction.