Neutral, slightly deleterious and moderately favorable alleles all have nearly equal chances to spread in diploid populations - as the neutral theory of population genetics has definitely shown (Kimura, 1983; ReMine, 1993; see already Fisher, 1958). The neutral theory "contends that at the molecular level the majority of evolutionary changes and much of the variability within species are caused neither by positive selection of advantageous alleles nor by balancing selection, but by random genetic drift of mutant alleles that are selectively neutral or nearly so" (Li, 1997, p. 55). Hence, the net result of larger numbers of gene mutations can mean overall degeneration of a species instead of upward evolution. Moreover, the costs of the many substitutions necessary for neo-Darwinian evolution to function successfully in large populations can quickly surpass the adaptive possibilities of a species (see the discussions of by Dobzhansky et al., 1977; and especially ReMine, 1993.)
The concept of natural selection predates the understanding of genetics, the mechanism of heredity for all known life forms. In modern terms, selection acts on an organism's phenotype, or observable characteristics, but it is the organism's genetic make-up or genotype that is inherited. The phenotype is the result of the genotype and the environment in which the organism lives (see Genotype-phenotype distinction).
NATURAL SELECTION AS METAPHYSICS AND AS SCIENCE
This is the link between natural selection and genetics, as described in the modern evolutionary synthesis. Although a complete theory of evolution also requires an account of how genetic variation arises in the first place (such as by mutation and sexual reproduction) and includes other evolutionary mechanisms (such as genetic drift and gene flow), natural selection appears to be the most important mechanism for creating complex adaptations in nature.
NATURAL SELECTION - Intelligent design
If the traits that give these individuals a reproductive advantage are also heritable, that is, passed from parent to child, then there will be a slightly higher proportion of fast rabbits or efficient algae in the next generation. This is known as differential reproduction. Even if the reproductive advantage is very slight, over many generations any heritable advantage will become dominant in the population. In this way the natural environment of an organism "selects" for traits that confer a reproductive advantage, causing gradual changes or evolution of life. This effect was first described and named by Charles Darwin.
ActionBioscience - promoting bioscience literacy
Though natural selection acts on individuals, the effects of chance mean that fitness can only really be defined "on average" for the individuals within a population. The fitness of a particular genotype corresponds to the average effect on all individuals with that genotype. Very low-fitness genotypes cause their bearers to have few or no offspring on average; examples include many human genetic disorders like cystic fibrosis.
001 - Natural Selection — bozemanscience
Natural selection can act on any heritable phenotypic trait, and selective pressure can be produced by any aspect of the environment, including sexual selection and competition with members of the same or other species. However, this does not imply that natural selection is always directional and results in adaptive evolution; natural selection often results in the maintenance of the status quo by eliminating less fit variants.
Gardening & Landscape Design
The unit of selection can be the individual or it can be another level within the hierarchy of biological organisation, such as genes, cells, and kin groups. There is still debate about whether natural selection acts at the level of groups or species to produce adaptations that benefit a larger, non-kin group. Likewise, there is debate as to whether selection at the molecular level prior to gene mutations and fertilization of the zygote should be ascribed to conventional natural selection because traditionally natural selection is an environmental and exterior force that acts upon a phenotype typically after birth. Some science journalists distinguish gene selection from natural selection by informally referencing selection of mutations as "pre-selection."