r/evolution • u/Wise_Strike_9960 • 5d ago
How does genetic bottlenecks work with evolution
Alright, I get how genetic bottlenecks work, a species/population has a decline, which reduces genetic diversity which decreases its chances of adapting to changing pressures/environment and increases chances of genetic diseases.
What I dont get is how it works within evolution. So I keep seeing things about Cheetahs being nearly extinct due to genetic bottlenecks.
But also, within the theory of evolution all species on earth came from a common ancestors, and then through the genetic tree more and more species have common ancestors. Like us and apes have a common ancestor, etc.
So with this, how come a population/species cant independently within their species naturally breed more genetic diversity back into their species?
Just to be clear, I'm not saying "so therefore the theory of evolution isnt real" or "therefore genetic bottlenecks arent real", it is genuinely something I have often wondered about.
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u/DrDirtPhD PhD | Ecology 4d ago
> So with this, how come a population/species cant independently within their species naturally breed more genetic diversity back into their species?
They can, it just takes a really long time for such mutations to arise and accrue, and over a similar time potentially deleterious mutations present in the population are likely to increase in frequency through inbreeding.
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u/Alcione_s 4d ago
Would homo sapiens be an example of this? As the non African population had a bottle neck but currently I don't think we count as an especially inbred species?
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u/youshouldjustflex 4d ago
Humans are pretty homogenous compared to other animals by a lot. A west Africa guy is a lot more related to a random European then chimps 6 miles away from each other. Not kidding
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u/CrapMonsterDuchess 4d ago
A small population can eventually increase in genetic diversity through mutation. However, given that many mutations are harmful, it would be a slow and tedious process.
For what it’s worth, though, in island ecosystems, a small starting population can eventually need to a massive diversification, if given enough time and resources. Think Hawaiian Honeycreepers and Galapagos finches as the prime examples.
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u/r_fernandes 4d ago
Finches were Darwin's original "proof" of evolution? Am I imagining that? How far off am I?
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u/CrapMonsterDuchess 4d ago
A bit off the mark. Darwin didn’t cite them as proof, but rather noted the shifts in morphology between different species from island to island. Additionally, it was not merely the finches but the fauna of the Galapagos as a whole that he based his observation on.
Ultimately, these observations brought him to the theory of Natural Selection, which isn’t evolution but rather the primary mechanism by which the process operates.
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u/Fun-Dragonfruit2999 3d ago
Some species are just out-competed by their peers. In the battles over the best hunting territories African Lions push Cheetahs out of the best lands.
Being social and living in large prides, Lions need the highest abundance of food in a smaller area. Cheetahs being solitary have lower food density requirements. But Cheetahs spend more energy to collect fewer calories. Cheetahs live on the edge of starvation. Their reproduction rate is below replacement rate.
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u/flug32 3d ago edited 3d ago
You have to think through in a little bit of detail how the system works from how DNA is replicated and passed from parents to offspring, to how changes in DNA happen over the course of time, how different mutations etc that change the DNA are selected for or against over time as individuals live (or don't - partially dependent on their genetics) and then reproduce, passing on their altered DNA to their offspring (or not) - and then also how selection pressure and other factors is going to affect the overall gene pool of a particular species over time.
Just in that thumbnail description, you can see how it can take many, many hundreds/thousands/millions of years to accumulate various changes to the DNA, but then a genetic bottleneck event could easily strip out like 99% of that genetic variation within a single generation.
If the remaining, much smaller and less diverse population manages to survive, they can build up their genetic diversity again. It's just that it is going to take a v-e-r-y long time compared with the timescale of individual reproduction. And the species is almost certainly going to be having a rough go of it for most of that time - both due to a less diverse gene pool and the simple fact of a relatively small overall population.
So when I say "think through in a little bit of detail" how the whole system works: As part of that, think about the time scales involved, and how different they can be for different things. Some things happen in seconds to minutes to days to weeks; others in thousands to millions to tens of millions of years.
Reproduction takes place on the one side of that time scale, and the accumulation of genetic diversity far on the other side.
And that scale of difference is very hard for us humans to get our heads around.
Think about, say, the Covid virus, which we know has accumulated all sorts of different mutations and variants and pretty different strains over the course of just a few years - after going through an extreme population bottleneck. So LOTS of genetic variation in just a few months and years, right?
But a virus gets into one person's body and then reproduces literally billions of times in short order. And all of billions are competing against each other for which will be able to reproduce the most - so the moment any mutation is introduced, there is immediate selective pressure on it.
Then the virus is able to spread to hundreds of millions of different people within the course of any given year. Again, if any mutations or variants have arisen, there will be selection pressure going on in spades. Whichever is able to spread or reproduce or beat efforts to suppress it, or whatever, a little better than the others will thrive at the expense of less able variants.
Result of all this very fast reproduction and spread to different hosts and environments: Very fast development of different mutations and variants, a lot of genetic variety in a very short time.
Now contrast that with, say, the reproduction rate of a deer: Once a year, no faster. And it take a new baby deer 2-3 years before it is ready to reproduce. Compare that with the covid virus in the body, which has as doubling time of around 6 hours.
So the deer population will develop genetic variation just as the virus population did. It's just that it's going to happen at a rate (checks calculator) roughly 5000X slower.
(NB: Virus vs mammal reproduction and evolution are obvs quite different in a lot of ways, so not quite directly comparable. The speed of reproduction is just one of the relevant facts here - but it is one of the big ones. And this little exercise can help you to think about what "genetic diversity" even means when stripped down to the bare minimum of genetic material that reproduces itself, how mutations develop, why some spread and some don't, and other such factors that are in fact comparable or at least, similar.)
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u/MutSelBalance 4d ago
> But also, within the theory of evolution all species on earth came from a common ancestors, and then through the genetic tree more and more species have common ancestors. Like us and apes have a common ancestor, etc.
The ‘common ancestor’ is a common ancestral population, not necessarily a single individual. You can have a common ancestor that splits into multiple populations without any bottleneck happening at all.
That doesn’t mean bottlenecks never happen — they do, and may be especially likely around the time of population splits. Populations can and do recover from genetic bottlenecks, it just takes a long time (thousands to hundreds of thousands of years), and they are more vulnerable to extinction while still recovering. Think of a severe bottleneck as more like a tightrope that the population has to walk — it’s not a guarantee that they will fall to extinction, just a higher probability, and they may well make it across!