Natural selection is the selection process in which favorable genetic traits will more generally exist in the offspring of a certain population of breeding organisms, while unfavorable genetic traits will reduce the probability of existence in their offspring.
natural selection acts on the phenotype (that is, the manifestation characteristics of biological machines), so that individuals with favorable phenotype in organisms are more likely to survive and reproduce than those with unfavorable phenotype. If these phenotypes are hereditary, the genotypes associated with favorable phenotypes will increase their frequency in the next generation. After a period of time, this process can produce adaptability, which makes the organism adapt to a specific niche, and thus may eventually produce new species.
The theory of natural selection is a cornerstone of modern biology. Charles Darwin's landmark book "Origin of Species" published in 1859 describes the term natural selection by analogy (that is, the process in which animals with favorable traits are favored by breeders and selected for systematic reproduction). At the beginning of the development of natural selection theory, there was a lack of reasonable genetic theory, because when Darwin wrote this book, although Gregor Mendel, the father of modern genetics, was a contemporary scholar of Darwin, because Mendel's work was unknown until the beginning of the 2th century, Darwin still knew nothing about modern genetics at that time. Darwin's traditional theory of evolution, together with later discovered molecular genetics, is collectively called modern comprehensive theory (neo-Darwinism). Although there are other molecular evolution mechanisms (such as neutral mutation theory proposed by Motoo Kimura, which explains the important reasons for genetic diversity), natural selection is still the most important explanation theory to adapt to evolution theory.
general principles
natural selection acts on the phenotype of organisms, that is, external characteristics. The phenotype of an organism is determined by its gene composition (genotype) and its living environment. Usually, natural selection acts on a specific trait of an individual, and these special traits can be defined by the terms phenotype and genotype in a narrow sense.
when different organisms in the same population have different versions of a certain trait, any version of the gene is called an allele. This genetic variation is the cause of phenotypic characteristics: certain combinations of genes determine the color of human eyes, such as the phenotype that produces blue eyes. On the other hand, when all the organisms in a population have the same allele for a certain trait and this state is basically stable for a certain period of time, the population can be said to have fixed alleles.
some traits are controlled by a single gene, but most traits are affected by the interaction of multiple genes. The variation of one of the genes that determine a trait will only have a small impact on the final phenotype of the organism. All these genes * * * work together to produce a unity of possible phenotypic values.
Nomenclature and Idiom
The term "natural selection" has slightly different definitions in different contexts. Simply put, "natural selection" is mainly used to define the selection that acts on some genetic traits, but sometimes it refers to the successful reproductive differences of each phenotype regardless of whether the phenotype is hereditary or not. Natural selection is "blind" in the sense that successful reproduction at the individual level is the result of phenotype rather than whether this phenotype is hereditary or not. According to Darwin's main use of the term "natural selection", this term is generally used to refer to the result and mechanism of blind selection. Sometimes it is helpful for us to clearly distinguish the mechanism and effect of choice. When this distinction is important, scientists will define natural selection as "those mechanisms that lead to the selection of reproductive individuals", regardless of whether this selection is hereditary or not. This is sometimes called "phenotypic natural selection". Those traits that lead to higher reproductive success rate of organisms will be selected, while those that reduce success will be eliminated. The selection of a certain trait will also lead to the selection of other traits related to this trait but not directly affecting adaptability. This phenomenon may be caused by gene pleiotropy or gene linkage.
fitness
the concept of fitness is the central topic of natural selection. As the well-known saying "survival of the fittest" states, although fitness can sometimes be popularly understood as a characteristic of improving the survival rate of individuals, modern time-based theory defines fitness from the perspective of individual reproduction. The basis of this theory is that if an organism has only half the life span of other species, but the survival rate of its adult offspring is twice that of other species, the genes of this organism will be more common in the adult population of the next generation. This is also known as heterogeneous regeneration. Although natural selection acts on individuals, its average effect on all individuals with a particular genotype is consistent with the fitness of that genotype. Generally speaking, genotypes with extremely low fitness will make their carriers produce few or no offspring. This kind of situation includes many cases of human genetic disorder, such as cystic fibrosis. Patients with sickle cell anemia will have lower fitness compared with the general population, but because this disease can produce immunity to malaria, this disease has higher fitness value for susceptible people with higher malaria infection rate. Generally speaking, the fitness of an organism changes with the change of its allele fitness. Because fitness is an average, for some unrelated reasons, favorable mutations may also occur in an individual who died as a minor. Similarly, unfavorable mutations may occur in individuals who survive to adulthood for some unrelated reasons.
selection type
natural selection can act on any phenotypic trait and any component of the living environment (such as mating, individuals of the same species, members of the same species, etc.) can produce selection pressure. However, this does not mean that natural selection is always directional and will produce adaptive evolution. In addition, natural selection will often maintain the status quo by eliminating unsuitable variants.
The selected unit can be an individual organism or another level of organism structure, such as genes, cells, and relatives. The argument about whether natural selection acts on groups or species to produce adaptability that will benefit larger and unrelated groups is still controversial. Natural selection at different levels, such as genes, will enhance the fitness of the gene, but at the same time it will reduce the fitness of the individual carrying the gene. This process is called intragenomic conflict. Generally speaking, the comprehensive effect of all the selection pressures at all levels will determine the overall fitness of an individual, thus producing the final result of natural selection.
natural selection occurs in every life stage of an individual. Every single organism must survive into adulthood before it can reproduce. The selection of adult organisms is called viability selection. In many species, adult organisms must compete for mating, that is, the so-called sexual selection process, and the ultimate winner of sexual selection will get the right to give birth to the next generation. When an individual can reproduce many times, a longer survival and reproduction period increases the number of offspring, which is called survival selection. Male or female fertility (such as the giant sperm of a certain fruit fly) can be limited by fertility selection. When intra-genomic conflicts, such as meiosis drive between haploid gametes, can lead to gamete or gene selection, the viability of gametes is different. Finally, the combination of some eggs and sperm may be more compatible than that of others, which is also called compatible selection.
sexual selection
when mechanically distinguishing ecological selection from narrow selection, the term sexual selection will be very helpful to us. Ecological selection covers all environments (including relatives, such as parental selection; And the selection mechanism caused by the same kind of individuals, such as competition and infanticide, and sexual selection refers to the competition of the same kind of individuals for mating. Sexual selection can occur within the same sex, such as competition between individuals of the same sex in a biological group. In addition, sexual selection can also occur between different sexes, for example, one gender controls reproductive rights by choosing existing mating objects in a certain group. Because more research investment is put into female species than male species in a single offspring organism, generally speaking, intra-sex selection involves competition between males, while inter-sex selection includes the opportunity for female to choose suitable males. However, some species will show the reversal of gender roles, that is, males are in more selected positions in mating selection. Although this kind of behavior has been found in some amphibians and birds, the most widely known examples of this kind of behavior are some fish in the family Polypodiaceae. The unique characteristics of one sex of a particular species can be explained by the mating choice of the other sex, such as the bright feathers of some males. Similarly, attacks between members of the same sex can sometimes be associated with some very distinctive features, such as stag horn, which is often used in competitions with other stags. More generally speaking, intra-sex selection is often associated with sexual differences, including differences in body size between males and females in the same species.