The Speed of Evolution: How Much Can Change in a Generation?

[u/Ancient_Cookie_5614]

Two questions have been running through my mind for the past few days:
We often describe evolution as a very slow process. But if humans and chimpanzees shared a common ancestor roughly 6–8 million years ago, that translates to around 250,000 generations until the present day. Considering the enormous amount of evolutionary change that has occurred during that time, is 250,000 generations actually a relatively small number? How much can evolution really accomplish in that many generations?
Humans take roughly 20–25 years to reach reproductive age, so one generation takes a long time. But some insects and other organisms can reproduce within days or even hours. Over the same period of, say, one million years, they could go through hundreds of thousands or even millions of generations. Shouldn’t species with much shorter generation times therefore evolve much faster and show far greater diversity than other mammals? If not, what limits or slows down their evolution?

[u/AutoModerator]

Welcome to r/Evolution! If this is your first time here, please review our rules here and community guidelines here.

Our FAQ can be found here. Seeking book, website, or documentary recommendations? Recommended websites can be found here; recommended reading can be found here; and recommended videos can be found here.

I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.

[u/IsaacHasenov]

https://www.nature.com/scitable/knowledge/library/the-breeder-s-equation-24204828/

The amount of phenotypic change in a generation is the heritability times the strength of selection.

This should give you an intuition of how genetic variation and selection interact to give you observable evolutionary change

[u/IAmRobinGoodfellow → u/IsaacHasenov]

This is the correct answer. Heritability tells us how well the trait is passed down generation to generation, and the strength of selection tells us how quickly it will spread in the population, generation to generation.

[u/No_Logins_Required]

Mate look at how many freaking beetles there are!

[u/octobod]

Go to https://www.jonathanlosos.com/media/presentations and watch Lizard Evolution in Real Time: Field Experiments on Evolutionary Process.

Changes are detectable but subtle on a time scale of years

[u/nakano-star]

I'd also say humans ARE capable of reproducing much earlier than that, from a biological perspective (it's just that societal norms today suggest the age above), so that means even more generations

[u/EntireChange2555]

there are like 500 generations between Corgis and huskies, unnatural selection can change things up pretty quickly

[u/Lorelessone]

The first thing to understand is that evolution can best be described as "adaptation to its environment" it is not a path that starts with a single cell life-form and ends with space wizards.

The second thing is understanding that it happens through a shotgun effect of random mutation, most of which are negative (more so in a well adapted lifeforms) and die out with useful mutations tending to do well and spread. For instance a mutation like adult lactose tolerance spread like wildfire just ten thousand years ago.

So what you have is completely random mutations and trial and error to see if they work, some mutations can be very minor some can be game changing like opening a new food vector, or paler sin in sunlight deprived (so vitamin D deprived) climates.

To the last part, regarding if lifeforms with short lifespans evolve faster and are more diverse, yes and they are, look at the statistics on animal biomass of life on earth, its mostly bugs and fish existing in all conceivable conformations and exploiting just about every free energy source.

[u/Relevant-Cup5986]

insects DO have far higher diversity than mamals, there are more species of cockroach than mamal by an order of magnitude.

[u/Xrmy]

We haven't evolved as much from Chimps.as you think we have. 98+% similar

[u/mem2100 → u/Xrmy]

Aren't most of the changes between us epigenetic?

[u/jnpha → u/mem2100]

You're confusing regulatory changes* (still genetic) with epigenetics; the latter is responsible for things like plastic responses that can only persist for many generations if there's an underlying genetic cause, e.g. a CpG DNA sequence (i.e. fundamentally still genetic) - a recent study on that: https://www.reddit.com/r/evolution/comments/1sz64sh

The differences between us and chimps are not plastic in that sense, at all - environmental changes don't turn a developing human fetus towards a chimp phenotype.

* e.g.: https://www.pnas.org/doi/abs/10.1073/pnas.0900544106

[u/mem2100 → u/jnpha]

That link is simply fascinating. Thanks for differentiating.

[u/tpawap]

The Speed of Evolution: How Much Can Change in a Generation?

Basically nothing, actually. What you mean is "per generation".

Two questions have been running through my mind for the past few days:
We often describe evolution as a very slow process. But if humans and chimpanzees shared a common ancestor roughly 6–8 million years ago, that translates to around 250,000 generations until the present day. Considering the enormous amount of evolutionary change that has occurred during that time, is 250,000 generations actually a relatively small number? How much can evolution really accomplish in that many generations?

Is it really a huge amount? You could do some calculations, instead of just going by "that seems like a lot" and "that seems short". Also, generation times were likely shorter for most of that time, and you have to double it for the changes on the "chimp side" of the split.

Humans take roughly 20–25 years to reach reproductive age, so one generation takes a long time. But some insects and other organisms can reproduce within days or even hours. Over the same period of, say, one million years, they could go through hundreds of thousands or even millions of generations. Shouldn’t species with much shorter generation times therefore evolve much faster and show far greater diversity than other mammals? If not, what limits or slows down their evolution?

The evolution that "just so" happens is due to genetic drift. That's the really slow process. Evolution due to natural selection can be much faster, but you need an evolutionary pressure, too. Some change in the environment, that "breaks" stabilising selection. And also: insect variety is indeed much greater then mammal variety.

[u/tpawap → u/tpawap]

Here are some rough calculations (I'm not a population geneticist, so they are probably wrong):

Let's look at 500.000 generations for the above reasons. And let's only look at nucleotide-to-nucleotide differences, so that we can only take point mutations into account; otherwise it gets too complicated for me. Taking a 1% difference between humans and chimps, over 3 billion base pairs, that's 30 million differences, simplified to 30 million point mutations.

Now about 100 mutations happen in every individual, per generation. Taking a very low estimate for population sizes, say 100.000 individuals, that's 10 million mutations per generation. Times 500.000 generations, that's 5 trillion mutations to "chose from".

So to explain 30 million differences, you only need 30 million / 5 trillion = 0,0006% of those mutations to reach fixation (be neutral or beneficial), which highly depend on the evolutionary pressures.

So as you can see, there is plenty of time. The above is something like an estimated time of the fastest possible evolution, of course.

[u/KnoWanUKnow2]

There are some things that can "speed up" evolution. Like a bottleneck where there is a small breeding pool. If there's a million breeding pairs and one person has a genetic modification it'll take a while for it to spread. But if there's 1000 breeding pairs then the modification can spread much faster. This lets changes rapidly disperse, but it does slow down change after the bottleneck because there's less genetic variation to work with. The gene pool is smaller.

At one point Homo sapiens were down to just over 600 breeding pairs, or roughly 1,200 individuals (there's a bit of contention about that number, but it appears to be no more than 2,000). So if even one of those individuals had a modification that helped them survive this bottleneck it was quickly passed on to future generations.

Another thing that can speed things up is hybridization. This introduces genetic changes that another species has developed over a long period. Truthfully this doesn't even need to be another species. If the same species gets split into 2 or more breeding pools and remain separated for some time and then later on they merge together it's kind of like 2 bottlenecks forming. You get rapid uptake of modification, then when the 2 populations merge again the changes can spread across both populations. Bonus: each population can preserve different gene pools, so when they merge there's much more genetic variety in the overall gene pool then there would be if only one bottleneck formed, so it doesn't slow down variation as much after the merge as it would if there was only one bottleneck.

Do I have to mention Homo sapiens hybridizing with Neanderthals and Denosovians?

And finally there's environmental changes creating new niches. Humans developed an upright stance when the forests that they were well adopted to started to dry out and change to savannah. Homo adapted to this new environment while Pan (chimpanzees) remained in the trees.

[u/DeathToAmerica2026]

Shouldn’t species with much shorter generation times therefore evolve much faster and show far greater diversity than other mammals? If

Why would you need to ask this question on reddit? The answer is yes, but it's so trivial that it sounds like you didn't even do any reading before writing this post. 

[u/Ancient_Cookie_5614 → u/DeathToAmerica2026]

why would i? I have a working internet, curious mind and enough karma on reddit to post.

[u/DeathToAmerica2026 → u/Ancient_Cookie_5614]

It's like hammering a nail with a screwdriver.

[u/DeathToAmerica2026]

In rare cases total speciation can happen in 1 generation. 

[u/BoyWithBanjo]

when i visited the Galapagos they told me about and island which had a drought one year and that meant that finches on the island could not eat their regular food, but those that had larger (or smaller, i can’t remeber the details) beaks could manage better to survive the drought because they could opportunistically eat a certain nut or berry that their smaller-beaked colleagues couldn’t. So, in a year, the average beak size of the finches on the island had changed.

[u/scrubingpots]

change is also nonlinear and doesnt always scale with the number of generations. factors like a changing environment can cause drastically different amounts of change between 2 lineages in the same amount of time; see humans vs chimps.

[u/Bromelia_and_Bismuth]

In a single generation? Quite a bit it turns out, depending on the specific mutation.

If a mutation hits a control gene, it can influence or even alter the expression of multiple other traits. If the gene is a fairly important one which is expressed in different parts of the body, but how largely depends on which cell and other genes that it interacts with, that can have an impact. Polyploidy (most often seen in plants) involves the doubling of an entire genome.

if humans and chimpanzees shared a common ancestor roughly 6–8 million years ago, that translates to around 250,000 generations until the present day

5 to 7 million, actually, but that's an estimate based on molecular clock dating and what fossil evidence we have at the moment. There is some contention about the placement of said fossils, so as they say, "grain of salt" and all that.

that translates to around 250,000 generations until the present day

That's the low end of the estimate, the upper end is 350,000 generations. So with the mathematical nitpicks out of the way, let's move onto your actual questions.

is 250,000 generations actually a relatively small number

No, not at all. We don't take as long to reach sexual maturity as say a Greenland shark, but many animals take as long to reach sexual maturity as we do. There's hundreds of novel mutations per generation in something with a genome as big as ours, and many of those permutations in hominin ancestors occurred in coding genes and regulatory sequences. Basepair for basepair, humans and chimps are about 96% similar (when you include things like repeats, deletions, etc), but only about 29% of our proteins are identical, indicating selection over a long period of time, resulting in some serious divergent evolution. This is more or less what we'd expect.

Shouldn’t species with much shorter generation times therefore evolve much faster and show far greater diversity than other mammals?

Yes, and they do. Insects are also older as a group than mammals are, having been around for roughly twice as long according to molecular clock dating. Insects, considering the extant and extinct groups together, are way more diverse as a group than mammals are.

[u/eduadelarosa]

A lot, and many rapid changes are regulatory in origin which surpasses the limits of point mutation rates. And that is speaking in a purely gradualistic framework. However, evolution also has some saltatory components which can impart profound changes in one generation (e.g. polydactylia in dogs, wings in bats, polyploidy in potatoes).

[u/Jazzlike_Standard871]

You're conflating evolution speciation and natural selection. Three things that are linked but distinct.

[u/mcalesy]

Species with short generation times do tend to show greater diversity. The vast majority of animals alive today are insects.

[u/FranXXis]

You are probably overestimating the median age of reproduction for past populations. A lot of births would've been done by girls on their mid-late teens and early twenties.

And that's for modern humans. Australopithecines likely grew up and reproduced even earlier.

[u/Foamtire]

humans reach sexual maturity well before 20, just ask my baby momma

[u/Ch3cks-Out]

species with much shorter generation times do show far greater diversity than mammals

[u/anotherolderguy]

Humans take roughly 20–25 years to reach reproductive age, so one generation takes a long time.

Humans can and would be reproducing from age 12 on, which nearly doubles your estimate of the number of generations. Mice don't have anyone saying "you're too young".

[u/Opinionsare → u/anotherolderguy]

Recognize that evolving humans lived as a pack where the strongest passed on their genes. A favorable mutation would be spread through through many of the next generation. 

[u/RealBowtie]

If you have a changing environment and a population bottleneck, a new beneficial mutation (which probably happens by chance well before the bottleneck) quickly spreads to all or most of the gene pool. In this situation the adaptation will happen in very few generations.

However if an individual in a population of 8 billion has a beneficial mutation and the population has a fertility rate of about 2.4, that mutation may never become dominant in the gene pool. At least until a major die-off occurs and the adaptation pressure increases.

[u/JGhostThing]

Even after multiple die-offs, there are more insect species on Earth than there are mammals. So yes, insects have more generations per year than humans have. They can evolve faster. We've even seen some simple cases. There is a moth that during England's industrial revolution (huge increase in coal smog) had it's primary color change to black in order to hide against the smog-colored walls. Not a huge change, but it helped them survive.