[ROLCAM]

What is genetic drift?

[J_9]

Here let me google that for you

[Alty]

Rol, what's with all the weird questions lately? All of the answers are easy to find on the internet.

Bored?

[asking]

The explanation at Wikipedia is not wrong, but rather technical and there's no big picture. I'm going to try for something that I hope will be more helpful.

Background: You need to know what an allele is first. Every gene is a position on the DNA, like a particular row of seats in a stadium. But different people can be sitting there, right? In the same way, the sequence of DNA (the gene) can come in different forms, called alleles. One person has one allele (version of the gene), someone else has a different allele. (Actually, we each have two of every gene, one from our mother and one from our father.) But a gene can have one version, two versions or even up to 30 different versions of the same gene.

Ok, so now that you know what an allele is...

Genetic drift is when the proportion of alleles in a group of animals or plants changes because of random events. It's sort of the opposite of natural selection, where the proportion of alleles changes because one allele results in more offspring than another allele. Basically, in genetic drift, allele A becomes more common because the individuals that have it just happen to leave more offspring. Maybe a volcano killed off everyone who had allele B because they were all eating berries by the volcano on the wrong day.

It's like if you toss a coin 1000 times, you get about 50% heads and 50% tails. But if you toss it 4 times you are less likely to get two each. For the same reason, genetic drift is a big deal in very small populations--say, 100 birds on a small island. It's unimportant in big populations--like the 320 million people of the United States. In big populations, you are unlikely to see an allele disappear or become rare because of genetic drift. If it became rare, it would most likely be because of selection.

Because genetic drift is important in small isolated populations, it's one reason (of several) why you see a lot of unique species on islands. You are probably always reading about rare species in Hawaii, the Galapagos, Madagascar, or wherever. They are rare because they evolved there and live nowhere else. So they are "endemic" to that island. If something happens to disrupt the ecosystem where they evolved (like human introduced rats or snakes) these endemic species have a high chance of going extinct. But the reason islands have so many of these rare endemic species is partly because of genetic drift. Also, because natural selection ALSO happens rapidly in small isolated populations.

[asking]

Thank you, ROLCAM.

[ROLCAM]

asking,

These are my views:-

Genetic drift is a random change in the frequencies of genes in populations. It is caused by the random way that egg and sperm cells receive some chromosomes from each parent as they form. Because these reproductive cells contain only half a set of chromosomes, only half of a parent's genes are present in an egg or sperm. If the parents produce a limited number of offspring, some of their genes may not be passed on.

Genetic drift does not enable species to evolve to adapt to their environment because it causes random changes in the frequencies of traits. Over time, however, genetic drift can gradually change the genetic makeup of a population.

[asking]

ROLCAM,
Yes, this is mostly correct. I would say that the distribution of chromosomes during egg and sperm formation ("gametogenesis") is not the whole story though.
There are other sources of random drift, including which particular egg and sperm end up becoming an individual, and of course what happens to that individual throughout its life. In addition, some drift occurs as a result of actual mutations. This is a subtle area.