Genetic variation

Genetic variation is the variety of DNA of members of a species or population. When coupled with natural selection, genetic variation makes evolution possible. Genetic variation is necessary for the long-term survival of a species. Variations occur through mutation, gene flow, and sex shuffling. However, only a very small percentage of variations are passed on to future generations. Even among those passed on, only a small percentage contribute to the evolution of a species.

Overview

Evolution is how species change over time. The process of evolution occurs as a result of genetic variation and natural selection. Genetic variation involves changes to the DNA of a single organism. Such changes cause an organism to express new physical or mental traits. Natural selection is the process by which a population decides which traits to pass on to the next generation. According to natural selection, organisms that are better suited to their environment survive longer than those that are less suited to their environment. Organisms that live longer tend to reproduce more than their shorter-lived peers do. If new traits are passed on to the organisms' offspring and those offspring continue to reproduce more than organisms that lack the new traits, the entire population will eventually exhibit the new traits.

How Genetic Variation Occurs

Genetic variation primarily occurs through three methods: mutations, gene flow, and sex shuffling. Mutations are random changes in the DNA of an organism. They occur when cells copy DNA strands imperfectly or when chemicals or radiation damage DNA. Mutations can cause changes in the behavior and physiology of the organism. Because of their random nature, mutations are not necessarily beneficial. They can even cause the death of their recipients. To contribute to the evolution of the species, a mutation must occur in reproductive cells and must alter something that makes selective breeding prefer that trait to the original trait.

Gene flow, also called migration, occurs when two separate populations of the same species interbreed. Because the populations have been separate for several generations, they probably have been selected for different traits. Examples of gene flow include pollen blowing to a new patch of flowers and humans migrating to different countries.

Lastly, variation occurs through sex shuffling. When organisms breed, genes from both parents combine in random ways. Individuals get some genes from each parent and express these combinations differently. This is why multiple siblings have unique DNA despite coming from the same parents. These new expressions of existing genes can create beneficial gene combinations. For example, children may be faster or smarter than their individual parents. Due to its random nature, however, sex shuffling also has the potential to break up beneficial gene combinations.

Impact of Genetic Variation

A large amount of genetic variation is beneficial to a species. It allows the species to better adapt to new environments and helps the species defend itself against disease and similar mass-extinction events. When a new disease infects a population with high genetic variation, some of the population will die off. A few, however, will probably have genes that resist the disease. As the disease continues to infect the population, the organisms with resistant genes will survive longer and reproduce more than those that lack them. This continues for several generations, and eventually most members of the population will have inherited genes that make them resistant to the disease.

Conversely, in a population with low genetic variation, the odds of any single organism being resistant to a new disease are much lower. As a result, the chance that a new disease could wipe out so many individuals that the population dies out entirely increases.

Causes of Low Gene Variation

A bottleneck is one of the most common ways a population loses genetic variation. In a bottleneck, almost all members of a population are removed from the breeding pool. A bottleneck can happen for several reasons. One is habitat fragmentation, which occurs when a small subset of a population is suddenly isolated from the rest of its species. Habitat fragmentation has become increasingly common as humans spread into new environments and destroy the natural habitats found there. Another potential cause of bottlenecks is near extinction. Near extinction occurs when the population of a species or population is suddenly reduced by such a large percentage that most of its genetic variations are lost.

Many scientists think humans went through a bottleneck about seventy-five thousand years ago. They theorize that a massive volcanic eruption began a chain reaction that reduced the human population to between one thousand and ten thousand breeding pairs. This explains why humans have less genetic variability than most other species.

A modern example of a dramatic bottleneck is the Florida panther. In the nineteenth and early twentieth centuries, settlers hunted and killed the large carnivore to protect themselves and their livestock. Additionally, human settlement destroyed more than 95 percent of the panthers' traditional hunting and breeding territories. By 1950, the panthers were found only in a small section of Florida. Moreover, humans had driven several other species with which the panthers traditionally had mated to complete extinction. The panthers' numbers dwindled so much that experts thought the species was extinct for five years before discovering an elderly female in the wild.

By the 1990s, the height of the panthers' bottleneck, the total number of surviving panthers was between thirty and fifty. These animals were forced to breed with close relatives to reproduce at all. Consequently, they continued to pass on detrimental genetic traits, including poor sperm quality, heart defects, increased parasite susceptibility, and several aesthetic defects. Because so few panthers remained, they did not have enough genetic variability to select against these traits, which damaged the species as a whole.

Scientists eventually released several members of a related panther species from Texas into the population, which successfully increased genetic variation. The new panthers in the breeding pool, along with the genetic variation they brought, drastically increased kitten survival rates. The Florida panther remains critically endangered, however.

Bibliography

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Starr, Barry. "Explosive Hypothesis about Humans' Lack of Genetic Diversity." Science.kqed.org. KQED Inc. Web. 17 Dec. 2014. http://science.kqed.org/quest/2008/03/17/explosive-hypothesis-about-humans-lack-of-genetic-diversity/

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