The random mating is one that happens when individuals choose to colleagues who want to mate. Non-random mating is that which occurs with the individuals that have a closer relationship.
Non-random mating causes a non-random distribution of alleles in an individual. If there are two alleles (A and a) in an individual with frequencies p and q, the frequency of the three possible genotypes (AA, Aa and aa) will be p², 2pq and q², respectively. This is known as the Hardy-Weinberg equilibrium.
The Hardy-Weinberg principle states that there are no significant changes in large populations of individuals, demonstrating genetic stability.
It anticipates what to expect when a population does not evolve and why dominant genotypes are not always more common than recessive ones.
For the Hardy-Weinberg principle to happen, it needs random mating to occur. In this way, every individual has the possibility of mating. This possibility is proportional to the frequencies found in the population.
Similarly, mutations cannot occur so that allele frequencies do not change. The population also needs to be large and isolated. And for this phenomenon to occur, it is necessary that natural selection does not exist
In a population that is in equilibrium, mating must be random. In non-random mating, individuals tend to choose mates more similar to themselves. Although this does not alter allele frequencies, less heterozygous individuals occur than in random mating.
For a deviation of the Hardy-Weinberg distribution to occur, the mating of the species must be selective. If we look at the example of humans, mating is selective but focusing on one race, since there is more probability of mating with someone closer.
If the mating is not random, the new generations of individuals will have less heterozygotes than other races than if they maintain the random mating.
So we can deduce that if the new generations of individuals of a species have less heterozygous in their DNA, it may be because it is a species that uses selective mating.
Most organisms have a limited dispersal capacity, so they will choose their mate from the local population. In many populations, matings with close members are more common than with more distant members of the population.
That is why neighbors tend to be more closely related. Mating with individuals of genetic similarities is known as inbreeding.
Homozygosity increases with each next generation of inbreeding. This happens in population groups such as plants where in many cases self-fertilization occurs.
Inbreeding is not always harmful, but there are cases that in some populations can cause inbreeding depression, where individuals are less apt than non-inbreeding.
But in non-random mating, the mate to breed with is chosen based on their phenotype. This makes phenotypic frequencies change and makes populations evolve.
Random and non-random mating example
It is very easy to understand through an example, one of non-random mating would be, for example, the crossing of dogs of the same breed to continue obtaining dogs with common characteristics.
And an example of random mating would be that of humans where they choose their mate.
Mutations
Many people believe that inbreeding can lead to mutations. However, this is not true, mutations can occur in both random and non-random matings.
Mutations are unpredictable changes in the DNA of the subject to be born. They are produced by errors in genetic information and their subsequent replication. Mutations are unavoidable and there is no way to prevent them, although most genes mutate with a small frequency.
If there were no mutations, the genetic variability that is key to natural selection would not be present.
Non-random mating occurs in animal species in which only a few males gain access to females, such as elephant seals, deer, and elk.
For evolution to continue in all species, there must be ways for genetic variability to increase. These mechanisms are mutations, natural selection, genetic drift, recombination, and gene flow.
The mechanisms that decrease genetic variety are natural selection and genetic drift. Natural selection makes those subjects with the best conditions survive, but through that genetic components of differentiation are lost. Genetic drift, as discussed above, occurs when populations of subjects reproduce with each other in non-random reproduction.
Mutations, recombination, and gene flow increase genetic variety in a population of individuals. As we discussed above, the genetic mutation can occur regardless of the type of reproduction, whether random or not.
The rest of the cases in which genetic variety can increase occur through random matings. Recombination occurs as if it were a deck of playing cards by joining two individuals to mate with totally different genes.
For example, in humans, each chromosome is duplicated, one inherited from the mother and the other from the father. When an organism produces gametes, the gametes obtain only one copy of each chromosome per cell.
The variation in gene flow can be influenced by mating with another organism that normally comes into play due to the immigration of one of the parents.
References
- SAHAGÚN-CASTELLANOS, Jaime. Determination of inbred sources of the ideal population under continuous sampling and random mating. Agrociencia, 2006, vol. 40, no 4, p. 471-482.
- LANDE, Russell. Quantitative genetic analysis of multivariate evolution, applied to brain: body size allometry. Evolution, 1979, p. 402-416.
- HALDANE, John Burdon Sanderson. Suggestions as to quantitative measurement of rates of evolution. Evolution, 1949, p. 51-56.
- KIRKPATRICK, Mark. Sexual selection and the evolution of female choice. Evolution, 1982, p. 1-12.
- FUTUYMA, Douglas J. Evolutionary Biology. SBG, 1992.
- COLLADO, Gonzalo. History of evolutionary thought. EVOLUTIONARY BIOLOGY, p. 31.
- COFRÉ, Hernán, et al. Explain life, or why we should all understand Evolutionary Theory. EVOLUTIONARY BIOLOGY, p. two.