Genetic drift is the establishment of certain alleles due to random sampling of the gene pool. Dobzhansky defined genetic drift as "random fluctuations in gene frequencies in effectively small populations". Genetic drift refers to the net decrease in genetic variability and heterozygosity over time. In stable populations, genetic drift causes genetic variation to decrease significantly more quickly than mutation can add new variation. Genetic Drift is a stocastic or random genetic process.
Analogy with marbles in a jar
The process of genetic drift can be illustrated using 15 marbles in a jar to represent a population of 15 organisms. Consider this jar of marbles as the starting population. One third of the marbles in the jar are red, one-third of the marbles are yellow and one third of the marbles are blue corresponding to three different alleles of gene in the population.
In each new generation the organisms reproduce at random. To represent this reproduction, randomly select a marble from the original bottle and settle a new marble with the same color as the organism's "father" on the bottle containing the new generation. (The marble remains selected in the original bottle). Repeat this process until you get 15 new marbles in the second bottle. The second jar contains then a second generation of "children", consisting of 15 balls of various colors. Unless the second jar contains exactly five balls of each color (red, yellow and blue), a random change in allele frequencies occurred.
Repeat this process a number of times, randomly reproducing each generation of marbles to form the next. The number of reds, yellow and blue marbles chosen every generation fluctuates: sometimes more red, sometimes more yellow, sometimes more blue. This fluctuation is genetic drift - a change in frequency of alleles of the population resulting from random variation in the distribution of alleles from one generation to the next.
It is even possible that no marbles of a particular color are chosen, which means that they have no offspring. In this example, if there are no red balls selected, the jar representing the new generation contains only blue and yellow marbles. If this happens, the red allele in the population was permanently lost. If the yellow marbles are subsequently lost, the remaining blue alleles will become fixed: all future generations will be entirely blue. In small populations, fixation can occur in only a few generations. The rate of change in gene frequency by genetic drift depends on population size and the random sampling are more important in small populations. The smaller the population size, the greater are random variations in gene frequencies, and also, less effective become weak selection pressures.
Heterozygosity and homozygosity
Heterozygosity is the degree to which an individual or population has different genes at the same locus. For example, an individual with two genes for blue eyes is homozygous, while an individual with one gene for blue eyes and one gene for brown eyes is heterozygous.
Heterozygosity is important for the survival of a population, because heterozygous populations exhibit a great deal more variation. For a simple example, if both parents have one blue-eyed gene and one brown-eyed gene, then on average:
- 1/4 of the children will have two blue eyed genes;
- 1/2 of the children will have one brown and one blue eyes;
- 1/4 of the chidlren will have tow brown-eyed genes.
However, if both parents have two genes for brown eyes, then all their children will have all brown eyes, and there will be no variation.
While eye-color is not a characteristic relevent to survival, many others are. For example, light skin is better adapted to areas with less sunshine, because it produces more Vitamin D with less light, and there is less need to protect the skin from sun damage. Dark skin is better adapted to areas with more sunshine, because it provides more protection from the sun, and there is less need for vitamin D production.
A population that is heterozygous for skin color, therefore, will vary a great deal in skin tone, thus able to adapt to different environments in which different skin colors are appropriate. A heterozygous population can spread to either northern or southern climes, and has the native genetic diversity to grow and thrive there.
However, just as with the eye color described above, a population that is homozygous for skin color will not vary much. Thus, those with light skin will be at a disadvantage in southern climes because of sun damage, and the populations will be unable to adapt. Similarly, those with dark skin will be at a disadvantage in northern climes because they will not get enough vitamin D from the sun, and the population will be unable to adapt.
Thus, heterozygosity is an extremely important element for the survival of a species. The less heterozygosity, the less able a population is to adapt. The less able a population is to adapt, the more vulnerable it is to changing conditions. The more vulnerable it is, the more likely it is to die out.
Genetic drift: the nemesis of heterozygosity
Genetic drift is the process by which populations are stripped of their heterozygosity over time.
Causes of genetic drift
In sexual reproduction, both parents have two of each chromosome. Eggs and sperm, however, contain only one of each chromosome, and the chromosome is a combination of the genes on both of its donors chromosomes. When the egg and sperm unite, therefore, the new zygote has two chromosomes, containing a combination of genes on both chromosomes different than both its parents.
However, because the parents had a total of four chromosomes and the child has only two, it receives only half of their genetic information. The other half dies with the parents.
Consider the following example. Imagine that at a particular locus, the father and mother have the following genes: Father: A on one chromosome, C on other. Mother: B on one chromosome, D on the other. --- When the child is born, let's say he receives the B gene from his mother, and the C gene from his father. This he has two chromosomes, containing B and C.
In this case, he has not received the A or D genes. If the parents have only one child before they die, the A and D genes are permanently lost to their line.
The inevitable result is that if parents have only one child, then half of their genetic variability is lost to their children. The population started with 4 different genes, and ended with only two.
The more children a couple have, the less significant the effect. If a couple has two children, then both on average, children will receive half the parents' total information, and half the children's genetic information will overlap, meaning that on average, only approximately 25% of the parent's genetic variation will be lost.
Less children = more genetic drift = more loss of heterozygosity; More children = less genetic drift = more conservation of heterozygosity;
Population growth and genetic drift
As noted above, the more children a couple has, the more heterozygosity is preserved. In a stable population, however, with no population growth, on average each couple will have two children that survive to propogate further. This results in a significant degree of genetic drift.
This loss is masked under ordinary circumstances because each child has children with another member of the species, which contributes genes of her own. However, if that child is also one of only two children, then half of her parents' heterozygosity was lost. Thus, the third generation may receive from her mother some of the genes that her father did not receive from his parents. However, the net result is the same: 8 chromosomes of information from 4 grandparents were available to the second generation, but only 4 chromosomes of information from 2 parents is available to the third generation. Thus, heterozygosity is reduced in each generation.
This phenomenon occurs in other individuals of a population and this can, occasionally, cause a gene to be extinct within that population. In other words, on each new generation there is a probability of one or more genes can not spread to the next generation. The smaller the population, the greater the chance of genetic drift.
Genetic drift and mutation
Mutation rates are generally well under one mutation in several generations. Rates of advantageous mutations are even lower, if they occur at all. In a stable population, however, the rate of genetic drift is significantly higher than this. The result is that mutations are weeded out of the population more quickly than they are introduced. Stable populations do not become more diverse. They become less diverse.
Effect of genetic drift
In the long-run, genetic drift and natural selection cause populations to be very specialized and adapted to their particular environment, but exhibiting so little variation as to be unable to adapt to other environments.
White Europeans, for instance, have lost the capacity for black skin tone, and Black Africans have lost the capacity for white skin tone. They are both well-adapted to their respective environments, but both ill-adapted to the other, and both incapable of quickly adapting to the other environment.
Thus, the net effect of genetic drift is that populations become adapted to a particular environment, but unable to adapt to others.
And as the population becomes more isolated in its environment, genetic drift increases continually, until the population is almost totally homozygous: well-adapted to one environment, but unreceptive to change.
- The evolutionary claim that populations increase in diversity over time is demonstrably false. Populations decrease in diversity over time, because genetic drift strips variability from populations more quickly than mutation can add it;
- The trend of decreasing heterozygosity shows that life in the past exhibited greater heterozygosity than life today, a fact consistent with Creationism;
- The development of the human races can be explained in terms of rapid genetic drift due to population isolation immediately after the flood. As populations spread over the planet and became isolated with each other, they lost heterozygosity that would permit them the traits of other populations. Thus, racial characteristics are a consequence of inbreeding rather than progressive evolution. This deevolution due to inbreeding is consistent with the rapidly decreasing lifespans immediately after the Great flood.
- Speciation within the created kinds can be explained by the same mechanism. Speciation occurs not by the increase in population diversity, but in the loss of heterozygosity in isolated populations. Speciation is a means of creating diversity among types of living beings, but macroevolution is much more than diversity, because it requires the addition of new genetic information.
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