User:Igor233/Evolution issues

This section covers current scientific issues regarding the process of biological evolution.

Darwin proposed an elegant and simple method, natural selection, whereby mutational changes that benefited organisms would become more prevalent in a population and might eventually become universal in a species. The method depended on natural variation between characteristics possessed by individuals in a population to provide selectable phenotypic differences. Subsequent theorists, primarily based on data acquired prior to 1950, (See Neo-Darwinism and including modern evolutionary synthesis hereinafter classical theory) held that evolutionary propagation and retention of organism mutational changes could only occur if the change resulted in benefit to the ability of individual organisms (including immediate descendents) to survive or reproduce. Further, such changes had to be expressed in that they resulted in a phenotypic change that plausibly affected the probability that the possessing individual would survive or reproduce.

Apparent Conflicts with Classical Natural Selection Theory

Although the vast majority of observed organism characteristics conform to classical natural selection theory in that they plausibly assist individual organisms in surviving or reproducing there are some very widely observed organism characteristics that appear to conflict with the individual benefit requirement.

Life Span Variation and Semelparous Species

The very wide variation in life spans between even very similar species led to the conclusion that life span was being substantially internally self-limited by the particular design of the organism, a violation of the classical natural selection concept. Semelparous species and others that die suddenly following reproduction appeared to be more explicit examples of self-limited life span. Darwin suggested in later editions of Origin that these observations could be explained by the existence of some undiscovered compensating benefit associated with the adverse life span limitation. (Neo-Darwinism requires compensating individual benefit.) Peter Medawar (1952) and others subsequently proposed various competing theories of gradual mammal aging that were compatible with classical theory but did not address semelparous organisms. (See Evolution of ageing.)

Altruism

Altruism involves observation of animal behaviors that apparently violate the individual benefit rule (e.g. an animal risking its own life protecting an unrelated member of its species).

Sexual Reproduction

Sexual reproduction, because of the relative reproductive uselessness of males and other factors also appears to represent an individual disadvantage relative to asexual reproduction and yet was evolved and retained. In many species the male provides individually beneficially protection and nurturing functions. However in reptiles and some other species the male provides no supporting function. In these species it would be a reproductive advantage if all animals (and not just females) could produce young. Sexual reproduction also seems to be logically inconsistent with classical theory. A sexually reproduced organism is less likely to resemble its parent than an asexually reproduced (clonal) organism, seemingly detracting from the evolution process.

Mating Rituals

Organism characteristics that seem to self-limit reproduction without compensating individual benefit also appeared. These include mating rituals that act to generally delay mating. For example, the Bighorn Sheep has a mating ritual that results in substantial risk to the individual sheep and also tends to delay mating for (on average) several years. Individuals that did not possess the ritual behavior would appear to have a reproductive advantage.

Puberty Age

Age at which reproductive capacity first exists (puberty) appears to be developmentally unnecessarily late in many species, especially in the male. In species in which the male does not provide protection or nurturing of young, a younger male puberty age would appear to represent an individual advantage.

Genetics Issues

The process of biological inheritance is central to the propagation of mutational changes in evolution. A consequent issue concerns the degree to which relatively recent (1950 to present) discoveries regarding the process of inheritance in sexually reproducing species (see genetics) should affect thinking about the evolution process. Many aspects of the genetic design of a complex organism (such as the particular location (locus) of a gene on a chromosome, introns, transposons, and junk DNA) are thought to have no effect on the organism's phenotype and therefore nominally do not result in or affect an evolvable (selectable) property under classical theory. However, all of these aspects, through genetic linkage and other mechanisms plausibly affect the inheritance process and thereby potentially affect the evolution process. For example, the concept of mutational robustness suggests that characteristics having identical phenotypic effect could vary in their susceptibility to mutational alteration by virtue of their particular underlying genetic design thus affecting the evolution process. This sort of analysis suggests that classical propagation models may be inadequate, especially regarding sexually reproducing species, and increases the plausibility of group selection and other concepts that violate classical theory.

Alternative Evolutionary Process Concepts

Beginning in 1962 a formal scientific disagreement has existed regarding the definition of evolutionary benefit as it applies to the process of evolution. Various adjustments to classical natural selection theory have been proposed in efforts to better explain the apparent conflicts. All the alternatives consider individual benefit to be the most important factor in influencing the evolution process but consider that other factors may also be significant.

Group Selection

Various group selection theories (1962 and later) (including kin selection (1963) and multilevel selection) have been developed contending that benefit to groups (beyond immediate descendents) can compensate for individual disadvantage and thus allow evolution and retention of an organism characteristic that produces a net individual disadvantage. Corresponding theories regarding the group benefits of conflicting observations including altruism, aging as a self-limitation, etc. then appeared. Neo-Darwinists continue to contend that group selection is impossible because of propagation issues and propose their own explanations for the apparent conflicts.

Evolvability

Other theorists (primarily since 1996) contend that characteristics that benefit the ability of an organism to evolve (increase evolvability) can also evolve and be retained despite having a net individual disadvantage. Corresponding evolvability theories of aging (see Evolution of ageing) and other apparently individually adverse characteristics appeared. Neo-Darwinists reject evolvability as a form of group selection.

Gene Oriented Theories

The Selfish Gene theory and other "gene oriented" theories also attempt to explain some apparently individually adverse observations, primarily altruism.

Current Status

Current bioscientists can be divided into two groups based on their beliefs regarding the evolution process:

• Neo-Darwinism completely explains the evolution process. All of the small minority of apparent conflicts and logical issues can be explained within neo-Darwinism.
• The totality of observed conflicts and logical issues requires at least some adjustment to neo-Darwinism. There is no agreement within this group as to which, if any, of the proposed adjustments is valid.

Generally, classical theory ( neo-Darwinism or modern synthesis) provides a simpler and more plausible propagation scheme and benefits from the conformance of the great majority of observations as well as long tradition. Group selection and evolvability provide better matches to the minority of non-conforming observations but suffer from less developed propagation concepts. In one form or another issues regarding the evolution process have persisted for 150 years and no scientific agreement is expected in the near-term.