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4.5.1. Genetic variability

Genetic variability

The genetic variability is the amount of genotypes different is in a given population. This variability is what makes all individuals different from each other, with different characteristics.

The clearest examples of genetic variability are domesticated species, in which humans have taken advantage of this variability to create races and varieties of fruits, horses, cows, dogs, and cats, for example.

The increase in genetic variability is mainly due to these processes:

Mutations

The mutations are changes occurring in the DNA of organisms. They can affect a nucleotide of DNA, a gene, or entire chromosomes. These changes cause other different proteins to be expressed and the individual has other characteristics.

Mutations are the cause that there are several alternatives for a genealleles, in the population.

The mutations occur at random (typically, although there are factors that can cause), and may have beneficial effects, neutral or negative.

For Darwin, mutations were the main source of genetic variability in the population. These changes in DNA are a gradual, slow and continuous process.

Genetic recombination

In sexual reproduction, the descendants are different from each other and from their parents. No new genetic varieties are produced, but new genotypes (combinations of genes) are produced.

During meiosis, gametes are produced, sex cells with half the chromosomes of somatic cells. In prophase I, the crossover of homologous chromosomes occurs. Later, when the homologous chromosomes are separated, they contain genetic information from both parents. The distribution occurs randomly, so that each gamete (haploid) contains fragments of paternal and maternal chromosomes. When fertilization arrives, the zygote will be the carrier of genetic information different from that of its parents.

Genetic drift

Genetic drift occurs when allelic ratios change in a population. It can occur when a group of individuals in a population are isolated from the rest and form a new population in which there may be a different proportion of alleles than the original.

The best known phenomena of genetic drift are:

  • Founding effect. It occurs when a small group is separated and isolated from the original population. Genetic variation in the new population is greatly reduced. The new population can be very different from the original, both genotypically and phenotypically.
  • Bottleneck effect. It occurs when only a few individuals survive after a population has suffered a high mortality event. As a consequence, genetic variability decreases.

Efecto cuello de botella

By Anjile (Own work) [Public domain], via Wikimedia Commons

Gene flow

Gene flow occurs when alleles of genes are transferred from one population to another.

When some individuals migrate from one population to another, the population that receives them increases its genetic variability, since if they have offspring they can contribute alleles that did not exist in the population and produce new phenotypes that can be beneficial.