Theoretically, the most fit individuals in a population will survive longest and reproduce the most, however chance plays a role in who survives, reproduces, and whose offspring survive to reproduce as well. Genetic drift occurs as a result of chance events causing changes in the allele frequency of a population. It doesn't favor the most fit individuals, but occurs at random. Mutations can contribute to genetic drift, however, genetic drift is a more specific answer and more relevant to the question at hand.
Genetic drift is the random process of alleles being passed from parents to offspring. Increasing genetic diversity in a population requires introducing a greater number of alleles, which can only occur through mutations or addition of unrelated members to the population. Genetic drift only affects how already-existing alleles are passed down.
If an allele has a high frequency at baseline, the chance of it being passed down to subsequent generations is higher than alleles of a lower frequency. Conversely, a low-frequency allele can eventually disappear from the population if none of the few parents who possess that allele happen to pass it onto their offspring.
It plays a much larger role in determining the genetic makeup of populations than natural selection. Genetic drift describes the random selection of alleles that are passed from one generation to the next due to independent assortment in gametogenesis. Genetic drift cannot create new alleles, so it cannot increase genetic diversity the number of alleles in a population. It can, however, decrease genetic diversity if an allele of a low frequency is not passed down to subsequent generations due to pure chance.
There is no hard and fast rule for whether genetic drift or natural selection have had a greater effect on shaping populations. Both have greatly shaped the populations present on Earth today, but their relative importance varies between species and has also varied over time. The conditions of Hardy-Weinberg equilibrium require that both natural selection and genetic drift be negligible.
If genetic drift is occurring, then the population cannot be in Hardy-Weinberg equilibrium. The bottleneck effect occurs when a population's size is reduced for at least one generation.
Undergoing a bottleneck can greatly reduce the genetic variation in a population, leaving it more susceptible to extinction if it is unable to adapt to climactic changes or changes in resource availablility. Small populations tend to have less genetic variation to begin with. Introducing a bottleneck effect further reduces variation and population size, amplifying the effect of genetic drift.
This leaves them susceptible to changes in the environment that they may not be capable of adapting to due to limited differences among individuals. An ecologist observes a population of snakes on an island for one month every year. After the eleventh month, he sees that the snake population has been decimated, and decides to wait for the snakes to repopulate before coming back for further observation.
When he returns five years later, he finds a very homogenous looking population of snakes. What is the name of the effect he observed? A bottleneck effect is the term used to describe the loss of genetic variation that occurs after outside forces destroy most of a population. The few individuals left to reproduce pass their traits on to all of their offspring, which then may thrive without the competition of a large population.
Eventually, there may be a large, very genetically similar population based on the traits of the few original survivors. The founder effect describes the low genetic variation of a population derived from a small group of individuals in a new geographic location. Genetic drift is the random change of allele frequency in a population. A decrease in genetic variety due to a small number of individuals from a larger population establishing a new population. The bottleneck effect describes the phenomenon when a population has a sudden reduction in the gene pool due to natural environmental events, natural disasters, disease, or human involvement.
This reduction in the gene pool will likely cause a bias that did not exist in the original population. For example, suppose a population of birds has a small number with a mutation making them unable to fly. If a disease reaches this population that kills all birds when they reach an altitude above 50m, then the gene pool of the population will suddenly shift to favor the flightless birds.
The bottleneck effect, after a long time, could potentially lead to speciation, but this is not a defining factor of the effect. Introducing a new species can increase the pressures of natural selection, but does not directly relate to the bottleneck effect. A decrease in genetic variety due to a small number of individuals from a larger population establishing a new population more aptly describes the founder effect.
Human activities, such as deforestation and over-fishing, make up the majority of bottleneck events. The bottleneck effect describes the sudden, sharp decrease in the size of a population.
After a bottleneck event, a population could either recover or go extinct depending on the fitness of the individuals remaining in the population. Depending on the type of event that created the bottleneck, it is possible that the surviving members are the most fit, but this is not always the case.
The new smaller population likely has less genetic diversity, which typically makes successful adaption more difficult and less likely, but if the surviving members of the population are highly fit, their ability to adapt may not be hindered. While man-made events certainly are a source of bottleneck effects in the world today, there are still natural bottleneck events and no concrete evidence to say that man-made bottleneck events are more frequent or have more of an effect on genetic drift than natural events.
Loss of genetic variety due to a small number of individuals from a larger population establishing a new population. The founder effect describes the phenomenon when a smaller group that originally came from a part of a larger population forms their own population. This new population will likely have a biased gene pool that will not be identical to the parent population. For example, if a certain species of bird gains a mutation such that some members are capable of flying farther, these birds may eventually separate to a different location and form their own unique population with a higher predominance of the "sustained flight" mutation than the original population.
Population bottlenecks increase genetic drift. They also increase inbreeding due to the reduced pool of possible mates. What is the difference between genetic drift, founder effect, and bottleneck effect? Judy O. Apr 4, Explanation: Genetic drift is more precisely termed allelic drift. Related questions How does genetics relate to meiosis? How does genetics use the principles of probability? What are common mistakes students make with genetics?
Thus, the rate of genetic drift is not really proportional to census population size N c. Rather, it's proportional to something more abstract — specifically, the effective population size N e. In an ideal population of sexually reproducing individuals , N e will equal N c. An "ideal" population has the following characteristics, and most deviations will decrease the effective population size :. Essentially, anything that increases the variance among individuals in reproductive success above sampling variance will reduce N e the size of an ideal population that experiences genetic drift at the rate of the population in question.
For example, consider the effect of unequal numbers of mating males and females. In an ideal population, all males and all females would have an equal chance of mating. However, in situations in which one sex outnumbers the other, an individual's chance to mate is now affected by its sex, even if all individuals within each sex have an equal chance to mate. Figure 4 shows the relationship between N e and N f in a population of 1, mating individuals. In an ideal population, all individuals have an equal opportunity to pass on their genes.
In real life, however, this is rarely the case, and N e is particularly sensitive to unequal numbers of males and females in the population.
One way to think about the relationship between N e and genetic drift is to consider the time required for the fixation of one allele or the other if we assume selective neutrality. Therefore, fixation time scales with N e. This time is maximized when p equals 0.
Perhaps this is intuitive, but because intuition can sometimes be misleading, it's good that a formal mathematical treatment confirms our suspicions!
This would be 13, generations for a population with N e equal to 5, However, if p is 0. Moreover, if p is 0. Another way to think about drift is to consider the rate at which variation is lost.
As in the previous example, this depends on N e and the starting value of p. Here, we define " heterozygosity " H as the proportion of individuals who are heterozygous 2 pq under Hardy-Weinberg assumptions. If H 0 is the initial heterozygosity of the population, then the heterozygosity after t generations H t can be calculated using the following equation:.
Effective population size is also sensitive to changes in census population size over time. In a discrete generation model, N e is calculated as the harmonic mean of the population sizes at each generation i. In this equation, N i is population size at generation i, and k is the number of generations.
Note that the harmonic mean is always lower than the arithmetic mean often considerably lower , and it is especially sensitive to the lowest values of N i. This has special relevance to two related scenarios: a population bottleneck and a founder event. In the case of a population bottleneck, population size is substantially reduced for some period of time.
In the case of a founder event, a small sample of a larger population becomes geographically isolated. In either case, the population size is dramatically reduced, at least temporarily. The effects of this reduction on genetic variation depend on both the size of the population during the reduction phase and the duration of the reduction phase.
Let's place this idea in context. Based on molecular population theory, the implication is that humans have an effective population size in the order of tens of thousands of individuals. However, we know that our census population size is currently well over 6 billion! Even though the human population has exploded, our standing genetic variation largely reflects a much smaller past population size.
Remember, harmonic means are especially sensitive to the smallest values, so our N e still mainly reflects the much lower past population size. In fact, it is almost certain to do so for as long as humans survive as a species. Barring the possibility of moving to another planet, the expected eventual destruction of Earth by the Sun does not allow enough time for us to recover a value of N e close to our current census size.
This concept is relevant to conservation as well. A species that loses genetic variation to drift e. In fact, even if the census size of the population can be increased perhaps through captive breeding efforts , the genetic variation may continue to decrease, because N e still reflects the recent bottleneck.
Kimura, M. The average number of generations until fixation of a mutant gene in a population. Genetics 61 , — Quantitative Genetics: Growing Transgenic Tomatoes. Adaptation and Phenotypic Variance. Dimorphisms and Threshold Traits.
0コメント