Natural selection is the process where heritable traits that make it more likely for an organism to survive long enough to reproduce become more common over successive generations of a population. It is a key mechanism of evolution. The natural variation within a population of animals, plants, bacteria, etc. means that some individuals will survive better than others in their current environment.
Fitness The concept of fitness is central to natural selection. Broadly, individuals which are more "fit" have better potential for survival, as in the well-known phrase survival of the fittest. Modern evolutionary theory defines fitness not by how long an organism lives, but by how successful it is at reproducing. If an organism lives half as long as others of its species, but has twice as many offspring surviving to adulthood, its genes will become more common in the adult population of the next generation. This is known as differential reproduction. 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.
Types of selection: 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.
Natural selection occurs at every life stage of an individual. An individual organism must survive until adulthood before it can reproduce, and selection of those that reach this stage is called viability selection.
In many species, adults must compete with each other for mates via sexual selection, and success in this competition determines who will parent the next generation.
When individuals can reproduce more than once, a longer survival in the reproductive phase increases the number of offspring, called survival selection.
The fecundity of both females and males (for example, giant sperm in certain species of Drosophila) can be limited via fecundity selection. The viability of produced gametes can differ, while intragenomic conflicts such as meiotic drive between the haploid gametes can result in gametic or genic selection.
The union of some combinations of eggs and sperm might be more compatible than others; this is termed compatibility selection.
It is also useful to distinguish between ecological selection and the narrower term sexual selection. Ecological selection covers any mechanism of selection as a result of the environment (including relatives, e.g. kin selection, competition, and infanticide), while sexual selection refers specifically to competition for mates. Sexual selection can be intrasexual, as in cases of competition among individuals of the same sex in a population, or intersexual, as in cases where one sex controls reproductive access by choosing among a population of available mates. Most commonly, intrasexual selection involves male-male competition and intersexual selection involves female choice of suitable males, due to the generally greater investment of resources for a female than a male in a single offspring organism.
No hay comentarios:
Publicar un comentario