Genetic Drift - Bottleneck effect

Hi there
I have some questions about genetic drift.

  • Regarding the bottleneck effect is it possible for the the bottleneck affect occur through migration or is it only possible through events such as natural disaster etc.?

  • So the way genetic drift has been defined to us it that it is completely random and purely by chance. However if a wildfire for example occurs and takes out some of a population, induvials that have alleles that allow them to have more resistance to heat damage have a greater chance of surviving and reproducing creating a bottleneck effect.
    In this example to me it seems like natural selection is at play so I am confused how genetic drift specifically the bottleneck effect can be purely random when some of the population affected by the event (that causes the reduction in population) may have some resistance or advantages which gives them more change of surviving and making it to the surviving population?

Very good question… :saluting_face: I can see why you would think that way. Although it look like they are similar to each other but the process in how it happens are different to each other.

Natural selection occurs naturally due to more favorable traits or genotypes and are morelikely to survive and repoduce and also pass on to the next generation.
As Charles Darwin (1859) argued in On the Origin of Species , if the following conditions are met, natural selection must occur:

  1. There is variation among individuals within a population in some trait.
  2. This variation is heritable (i.e., there is a genetic basis to the variation, such that offspring tend to resemble their parents in this trait).
  3. Variation in this trait is associated with variation in fitness (the average net reproduction of individuals with a given genotype relative to that of individuals with other genotypes).

Whereas Genetic Drift happens purely by chance. The way I understand Genetic- Drift is that it happened by default hence why a bottleneck effect is created or formed. The variations of the alleles within the population left if a wildfire occur will be very low, therefore the allele frequency will be very low as well and we won’t say that natural selection occur in this small population. Let’s use the example you gave. If the alleles that allow them to have more resistance to heat damage are the ones who are most likely to survive within that small population. The allele frequencies of heat resistance will continue to change over time in this population due to chance events such as gametes transmission when repoducing — this means, the population is undergoing genetic drift. While undergoing genetic drift, an allele was produced that is more fit to survive will cause the bottleneck effect hence why I prefer to say that it happens by default… :laughing:
I hope this help or shed some light about the difference between natural selection and genetic drift.