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In a unique environment created by isolation, a changing environmental condition of life known as time, allows genetic variation (from mutation) to occur within a population. If this mutation is immediately displayed, most of this genetic variation is damaging to life; environmental selection eliminates harmful phenotypes and they are lost to future generations. A displayed phenotype may be neither helpful nor harmful; this would be a neutral phenotype. Most commonly (but not always), neutral variation is carried as a recessive allele in the genotype and it is not expressed in the phenotype. This neutral genetic variation has no current value, but is not harmful. Neutral genetic variation may continue in the genotype of a population for a long time. If the genome expresses a previously silent mutation (e.g., with homozygous recessive alleles), it usually expresses a change. Neutral genetic (genotype) variation that had no survival value in the past could eventually gain importance; at a later time, this genetic variation may combine with other forms of genotype variation and express a phenotype with improved survival in a newly-created environment. In Levinton’s chapter on Development and Evolution, he states that “The stuff of phenotypic evolution is small developmental units of relatively low burden on fitness. Some of these, such as doubling of structures early in development, might eventually have strong evolutionary significance.”

Levinton makes a case for small amounts of variation over time as being the most likely to survive. Darwin showed that some variation will be advantageous to survival (in a special isolated environment). Darwin suspected, but did not have today’s evidence, that mutational change, the original source of inherited (genotype) variation is completely random.

Environmental selection’s support for a valuable expressed variation (phenotype) increases the number of offspring that survive under the conditions of life in that environment; environmental selection’s limiting factors have the opposite effect.

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In the struggle for life, within the conditions of life, the costs of living are dear. Life has to “make do” with whatever genetic potential for adaptation that is available (within its genetic variation/natural selection genetic bias).

History has shown that this is often not enough. Additionally, the basic needs of water, energy, and other resources are never enough; populations grow until resources are exhausted. Recall the “struggle for existence” of Thomas Malthus (there is a strong and constantly operating check on population from the difficulty of subsistence). In other words, it takes not only the right genetics, but it takes energy and other costly (environmental) resources to make and maintain a structure (Jared Diamond, The Third Chimpanzee). Structure, formation and maintenance must be cost-effective in terms of survival value (natural selection). In a world of limited resources, there is not a great deal of room for neutral variation in structure (phenotypic).

Environmental selection (the gatekeeper for genetic success) sorts out survivors; this environmental selection results in a natural selection preservation of favorable phenotypes (a change in gene frequencies in a population, owing to fitness of phenotypes’ reproduction or survival among the variants—Levinton).

Phenotype variation is packaged as combinations of traits within one individual organism. Expanding one trait may then be at the expense of other traits (Diamond—3rd Chimpanzee chapter on Why Do We Grow Old and Die?). To be beneficial, this new combination must enhance the overall survival chances of the individual and/or offspring. The most effective expression of the phenotypes in an organism is the most optimum balance of all traits (Diamond—3rd Chimpanzee chapter on Why Do We Grow Old and Die?). One form of primitive hominid (Australopithecus robustus) took a vegetarian pathway towards developing an energy-intensive a large gut. Other forms of primitive man (Homo habilis and erectus) utilized a diet of meat and developed a more energy-intensive large brain;

this followed the pattern of gracile australopithecines with a lighter build (especially skull and teeth).

What happens when (phenotypic) structures are no longer needed in a changed environment? Cave fishes have eyes in varied stages of degeneration. The complex morphological structure that once provided the function of (trichromatic) sight is now only a neutral degenerating phenotypic expression in a sightless world. The genes that formed the structure and function of vision received no environmental indirect support. Without environmental selection, whenever mutation occurred in these genes, the mutations persisted; i.e., were not eliminated.

Structure and function was compromised, irreversibly. In addition, using limited resources that could be used to maintain the (phenotypic) function of sight is not cost-effective for this special environment of total and eternal darkness. The combined effects of environmental selection and natural Genetic Variation 111

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selection genetic bias have favored the redirection of limited resources needed for survival elsewhere. Nearly all phenotype variation has been influenced by the interaction between environmental selection and natural selection genetic bias. Genetic expression and genetic survival are not random events; they are both influenced by environmental conditions. Recall again the icefish that lost the function of its globin genes, due to loss of environmental support of the globin-bearing phenotype. Specifically, environmental selection did not eliminate the mutation that caused this gene to malfunction; the non-functioning gene was passed to offspring while resources were directed elsewhere.

Genes can be added; genes can be duplicated and modified. In a process that is duplicated today by genetically-modified crop technology, bacteria may have combined genetic material to create new life variations, different and more complex than those existing before. If genes can be transmitted separately from genomes, perhaps the appropriate unit of evolutionary change could be the gene, and not the genome; yet, without a doubt, the genome does influence genetic function. Early plant and animal life may have been modified by a similar genetic combination process. Polyploidy (more than two sets of chromosomes per nucleus) is widespread in plants and is known to exist in animals (Levinton). Animal genetic variation, polymorphism, may be genotypic or phenotypic; nearly every trait has allelic variation (Levinton). Allozymes, enzymes that are one of a series of alternative gene products of a given locus, may exhibit protein polymorphisms (Levinton). Nearly every morphological trait in plants and animals has a genetic basis under polygenic (governed by the cumulative effects of many genes) control, including polymorphisms involving discrete (absolute—not additive) morphological traits (Levinton—chapter on Patterns of Morphological Change). In addition to the genetic architecture of both discrete (absolute) and continuous traits being polygenic (additive traits whose variations are measured with a scale), the genetic determination of the phenotype is a complex result of interactions among genes that perform different functions; the control comes from much of the genome and usually cannot be restricted to switch genes of major effect (Levinton—chapter on Patterns of Morphological Change). Developmental thresholds may be involved (Levinton). A correspondence between genetic correlations and functional relatedness of traits suggests that the genotype is a co-evolved unit, designed to serve the entire organism (Levinton—chapter on Developmental Evolution).

Revisiting an earlier discussion of development, Levinton notes: “If, as an organism grows larger, a defined sequence of morphological changes best suits it to function within the environment, then those stages might be added… and preserved as adaptive solutions to… series of changing environments. Adaptation, improved function of a phenotype, involves

the evolutionary change of the structure-function relationship (Levinton—

chapter on Patterns of Morphological Change). “Sudden environmental shifts may be the basis for adaptation and subsequent stasis; speciation, when it occurs, is more the effect of natural selection than the generator of variation” (Levinton). In this last statement, substituting environmental selection (a process) for “natural selection” seems to clarify communication; recall that Levinton previously defined natural selection as a change (a result) in gene frequencies in a population, relative to fitness.

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