Genes exist in alternate versions, or alleles , that determine distinct traits that can be passed on from parents to offspring. Genetic variation is important to the processes of natural selection and biological evolution. The genetic variations that arise in a population happen by chance, but the process of natural selection does not. Natural selection is the result of the interactions between genetic variations in a population and the environment.
The environment determines which genetic variations are more favorable or better suited for survival. As organisms with these environmentally selected genes survive and reproduce, more favorable traits are passed on to the population as a whole.
Genetic variation occurs mainly through DNA mutation , gene flow movement of genes from one population to another , and sexual reproduction. Due to the fact that environments are unstable, populations that are genetically variable will be able to adapt to changing situations better than those that do not contain genetic variation.
Favorable genetic traits in a population are determined by the environment. Organisms that are better able to adapt to their environment survive to pass on their genes and favorable traits. Sexual selection is commonly seen in nature as animals tend to select mates that have traits that are favorable. As females mate more often with males considered to have more favorable traits, these genes occur more often in a population over time. A person's skin color , hair color, dimples, freckles, and blood type are all examples of genetic variations that can occur in a human population.
Examples of genetic variation in plants include the modified leaves of carnivorous plants and the development of flowers that resemble insects to lure plant pollinators. Gene variation in plants often occurs as the result of gene flow. Pollen is dispersed from one area to another by the wind or by pollinators over great distances.
Examples of genetic variation in animals include albinism, cheetahs with stripes, snakes that fly , animals that play dead , and animals that mimic leaves. These variations enable the animals to better adapt to conditions in their environments. Actively scan device characteristics for identification. Some species display geographic variation as well as variation within a population.
Geographic variation, or the distinctions in the genetic makeup of different populations, often occurs when populations are geographically separated by environmental barriers or when they are under selection pressures from a different environment.
One example of geographic variation are clines: graded changes in a character down a geographic axis. Gene duplication, mutation, or other processes can produce new genes and alleles and increase genetic variation.
New genetic variation can be created within generations in a population, so a population with rapid reproduction rates will probably have high genetic variation. However, existing genes can be arranged in new ways from chromosomal crossing over and recombination in sexual reproduction.
Overall, the main sources of genetic variation are the formation of new alleles, the altering of gene number or position, rapid reproduction, and sexual reproduction. Genetic drift is the change in allele frequencies of a population due to random chance events, such as natural disasters.
Genetic drift is the converse of natural selection. The theory of natural selection maintains that some individuals in a population have traits that enable to survive and produce more offspring, while other individuals have traits that are detrimental and may cause them to die before reproducing. Over successive generation, these selection pressures can change the gene pool and the traits within the population. For example, a big, powerful male gorilla will mate with more females than a small, weak male and therefore more of his genes will be passed on to the next generation.
His offspring may continue to dominate the troop and pass on their genes as well. Over time, the selection pressure will cause the allele frequencies in the gorilla population to shift toward large, strong males. Unlike natural selection, genetic drift describes the effect of chance on populations in the absence of positive or negative selection pressure. Through random sampling, or the survival or and reproduction of a random sample of individuals within a population, allele frequencies within a population may change.
Rather than a male gorilla producing more offspring because he is stronger, he may be the only male available when a female is ready to mate. His genes are passed on to future generation because of chance, not because he was the biggest or the strongest.
Genetic drift is the shift of alleles within a population due to chance events that cause random samples of the population to reproduce or not. Effect of genetic drift : Genetic drift in a population can lead to the elimination of an allele from that population by chance.
In this example, the brown coat color allele B is dominant over the white coat color allele b. In the first generation, the two alleles occur with equal frequency in the population, resulting in p and q values of. Only half of the individuals reproduce, resulting in a second generation with p and q values of. Only two individuals in the second generation reproduce and, by chance, these individuals are homozygous dominant for brown coat color.
As a result, in the third generation the recessive b allele is lost. Small populations are more susceptible to the forces of genetic drift. Large populations, on the other hand, are buffered against the effects of chance.
Genetic drift can also be magnified by natural events, such as a natural disaster that kills a large portion of the population at random. The bottleneck effect occurs when only a few individuals survive and reduces variation in the gene pool of a population.
The genetic structure of the survivors becomes the genetic structure of the entire population, which may be very different from the pre-disaster population. Effect of a bottleneck on a population : A chance event or catastrophe can reduce the genetic variability within a population. Another scenario in which populations might experience a strong influence of genetic drift is if some portion of the population leaves to start a new population in a new location or if a population gets divided by a physical barrier of some kind.
In this situation, it is improbable that those individuals are representative of the entire population, which results in the founder effect. The Founder Effect : The founder effect occurs when a portion of the population i.
The founder effect is believed to have been a key factor in the genetic history of the Afrikaner population of Dutch settlers in South Africa, as evidenced by mutations that are common in Afrikaners, but rare in most other populations. This was probably due to the fact that a higher-than-normal proportion of the founding colonists carried these mutations.
The Hardy—Weinberg principle states that within sufficiently large populations, the allele frequencies remain constant from one generation to the next unless the equilibrium is disturbed by migration, genetic mutation, or selection. Because the random sampling can remove, but not replace, an allele, and because random declines or increases in allele frequency influence expected allele distributions for the next generation, genetic drift drives a population towards genetic uniformity over time.
Once an allele becomes fixed, genetic drift for that allele comes to a halt, and the allele frequency cannot change unless a new allele is introduced in the population via mutation or gene flow. Thus even while genetic drift is a random, directionless process, it acts to eliminate genetic variation over time. Genetic drift over time : Ten simulations of random genetic drift of a single given allele with an initial frequency distribution 0.
In these simulations, alleles drift to loss or fixation frequency of 0. An important evolutionary force is gene flow: the flow of alleles in and out of a population due to the migration of individuals or gametes. While some populations are fairly stable, others experience more movement and fluctuation. Many plants, for example, send their pollen by wind, insects, or birds to pollinate other populations of the same species some distance away.
Genetic variation in a group of organisms enables some organisms to survive better than others in the environment in which they live. Organisms of even a small population can differ strikingly in terms of how well suited they are for life in a certain environment. An example would be moths of the same species with different color wings.
Moths with wings similar to the color of tree bark are better able to camouflage themselves than moths of a different color. As a result, the tree-colored moths are more likely to survive, reproduce, and pass on their genes.
This process is called natural selection , and it is the main force that drives evolution. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.
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