Natural selection

Evolution by means of natural selection is the process by which genetic mutations that enhance reproduction become and remain, more common in successive generations of a population. It has often been called a "self-evident" mechanism because it necessarily follows from three simple facts: Heritable variation exists within populations of organisms. Organisms produce more progeny than can survive. These offspring vary in their ability to survive and reproduce. These conditions produce competition between organisms for survival and reproduction. Consequently, organisms with traits that give them an advantage over their competitors pass these advantageous traits on, while traits that do not confer an advantage are not passed on to the next generation.[93] The central concept of natural selection is the evolutionary fitness of an organism.[94] Fitness is measured by an organism's ability to survive and reproduce, which determines the size of its genetic contribution to the next generation.[94] However, fitness is not the same as the total number of offspring: instead fitness is indicated by the proportion of subsequent generations that carry an organism's genes.[95] For example, if an organism could survive well and reproduce rapidly, but its offspring were all too small and weak to survive, this organism would make little genetic contribution to future generations and would thus have low fitness.[94] If an allele increases fitness more than the other alleles of that gene, then with each generation this allele will become more common within the population. These traits are said to

e "selected for". Examples of traits that can increase fitness are enhanced survival and increased fecundity. Conversely, the lower fitness caused by having a less beneficial or deleterious allele results in this allele becoming rarer they are "selected against".[96] Importantly, the fitness of an allele is not a fixed characteristic; if the environment changes, previously neutral or harmful traits may become beneficial and previously beneficial traits become harmful.[54] However, even if the direction of selection does reverse in this way, traits that were lost in the past may not re-evolve in an identical form (see Dollo's law).[97][98] A chart showing three types of selection. 1. Disruptive selection 2. Stabilizing selection 3. Directional selection Natural selection within a population for a trait that can vary across a range of values, such as height, can be categorised into three different types. The first is directional selection, which is a shift in the average value of a trait over time for example, organisms slowly getting taller.[99] Secondly, disruptive selection is selection for extreme trait values and often results in two different values becoming most common, with selection against the average value. This would be when either short or tall organisms had an advantage, but not those of medium height. Finally, in stabilizing selection there is selection against extreme trait values on both ends, which causes a decrease in variance around the average value and less diversity.[93][100] This would, for example, cause organisms to slowly become all the same height.