Which type of selection tends to increase genetic variation?

Disruptive selection, also called diversifying selection, increases genetic variation. In this type of selection, extreme phenotypes are favored over intermediate ones and this often drives the formation of new species. Disruptive selection is seen in species such as lobsters where the males have multiple mating strategies depending on their size. Large, dominant alpha males get access to females using their strength, while on the other end of the size spectrum, small males are opportunistic and sneak in and mate with females. The medium-sized males fall in between—not large enough to fight for the right to mate and not small enough to sneak in. These males are selected against due to disruptive selection, driving the formation of more large and small male lobsters.

What are two main sources of genetic variation?

Two main sources of genetic variation are random mutations and genetic recombination. Mutations are changes in DNA coding that effect an organism’s physiology, behavior, and/or appearance. All mutations are random which means they just happen and are not in response to what an organism “needs” or what would be useful to it. Also, mutations in the DNA of somatic (body) cells don’t affect evolution like mutations to the DNA in sex cells. Overall, mutations are rare and most of them are harmful or do nothing. The main causes of mutations are chemicals, radiation, and mistakes that happen when DNA is copied during cell division.

Recombination, or the “shuffling” of genetic material creates variation during the process of crossing over which happens in meiosis (see the next question).

What is the relationship between meiosis and genetic variation?

When gametes are made during meiosis, a process called crossing over happens in prophase I. During this process, 4 sister chromatids (2 from each parent) line up parallel to each other in a formation called a tetrad. Male and female chromatids cross in certain areas, forming a structure called a chiasma. The chiasma then breaks apart, and the broken DNA segments from the maternal chromatids are joined into the DNA of the paternal chromatids and vice versa. More information on the relationship between meiosis and genetic variation is in the answer to the question below “How does sexual reproduction lead to genetic variation?”

Why is genetic variation important?

Without genetic variation, some of the basic mechanisms of evolution can’t operate and organisms will fail to adapt and eventually die. Genetic variation allows natural selection to occur, and this increases the ability of organisms to survive changes in their environment and ultimately reproduce. Having offspring means that adaptive changes in alleles will be carried on to the next generation.

How do mutations lead to genetic variation?

In sexually reproducing organisms, mutations in gametes can cause genetic variation. This is a rare occurrence and most often mutations are lethal or don’t have any effect. Mutations are random and don’t happen in response to environmental changes. Therefore, they are not adaptive, but they are important because they can bring out variations that are tested in the environment. If a mutation turns out to be advantageous for a species that variation is inherited by the offspring and helps ensure the survival of the population.

How does random fertilization add to the genetic variation?

Random fertilization is the concept that all gametes are created equal when it comes to their chances of being able to reproduce. Therefore, random fertilization doesn’t create new genetic variation, but it does give all gametes the same opportunity to pass on their DNA. This, in turn, preserves the variation that the gametes carry. If there was no random fertilization, this would mean that some sperm and eggs have better chances than others of fertilizing or being fertilized. Over generations, the gene versions with better chances are favored and the other variations die out. This would leave just one version of a gene or set of genes, decreasing the amount of genetic variation.

How does sexual reproduction lead to genetic variation?

Genetic recombination is the main way that sexual reproduction leads to genetic variation. Recombination happens in three ways: through the separation of genes during meiosis, the random coming together of genes during fertilization, and via the process of crossing over during meiosis.

Meiosis results in 4 daughter cells, each having half the number of chromosomes (haploid) as the parent cell and which are genetically distinct the parent cell. Also, during meiosis there is an exchange of genetic information between the maternal and paternal chromatids through the process of crossing over, creating more variation. The concept of random fertilization levels the playing field and gives all sex cells an equal chance of being successful in passing on their DNA, further increasing genetic variation.

References

  • Genetic Variation. (n.d.). In Wikipedia. Retrieved March 21, 2018 from https://en.wikipedia.org/wiki/Genetic_variation
  • Genetic Variation. (n.d.). In Understanding Evolution. Retrieved from https://evolution.berkeley.edu/evolibrary/article/evo_17
  • Population Genetics. (n.d.). In Lumen Learning, Boundless Biology. Retrieved from https://courses.lumenlearning.com/boundless-biology/chapter/population-genetics/