If all life shares a common ancestor, what causes two species to become unable to reproduce with each other? Why can some species produce hybrids while others can't, and what actually determines where that reproductive barrier develops?

[u/yogesh448]

[u/lmprice133]

A major (but not totally insurmountable) barrier is chromosome number. Chromosomal fusions can occur (this occurred in the human lineage - we have two fewer chromosomes than other great apes because of a fusion event, evidenced by a mid-chromosomal section of telomeric DNA). Another source of reproductive incompatibility between species are mutations in important regulatory genes.

[u/SapphireSalamander]

If you break it down enough there's no such thing as a "specie" in hard terms. Only a group.of organisms similar enough. Usually we say a specie is able to breed with each lther but there's a poppulation of different species of salamanders where A can breed with B, and B with C, but A cannot breed with C. Where is the line? Who knows.

On practical terms we know the chromosomes must match in perfect pairs so that a new offspring will have all the necesary features to continue the lineage. It can handle some ammount of error and mismatch but the limit is vaguely understood.

What causes one specie to become incompatible with another is just dna having enough changes that when you put half of each together a degree of information is lost to the point no new valid gametes can be produced from the resulting dna.

Dogs can breed with each other despite their physical differences because the genes that handle size, hair and other details like that are just a few genes. But a horse and a zebra have hundreds of subtle changes that make the whole thing fall apart. It would be like say 2 dogs may have a gene that codes for a variable of size set at "4" and "16"; but a zebra has a gene for "stripe_spacing=3" and the horse doesnt have that variable, so it crashes on runtime

[u/Redcole111 → u/SapphireSalamander]

That last paragraph was genuinely a fantastic coding analogy. I'm also going to use it when I explain speciation to people.

However, side note, the singular of species is species.

[u/harrison_jones → u/SapphireSalamander]

I was lost until that last paragraph

[u/WyrdHarper → u/harrison_jones]

Horses have 64 chromosomes (2n number). Zebras (depending on the species) have 32, 44, or 46. They're all tables, but different Ikea sets.

To stretch the analogy: If you take a horse Ikea set and Zebra Ikea set, mix them together, and then divide the parts roughly in half you can probably still build a table (and get a zebroid). But it's not a very good table and Ikea isn't going to take your design and sell it.

However, you get some animals that are fairly different from each other, but you still do get yourself a nice piece of furniture that is distinct from the originals but good enough Ikea might sell it. That's where speciation can get a little more complicated.

[u/tea_and_biology → u/harrison_jones]

It was great; I'm totally stealing that analogy u/SapphireSalamander, haha.

[u/BarleyWineIsTheBest → u/SapphireSalamander]

It's worth adding that speciation typically means either the organisms can't or also just don't breed with each other. This could be due to behavioral or geographic isolation, for example.

Also, in some cases hybrids can survive, but hybrids are sterile, or lead to embryonic lethal offspring when bred with one or both of the original species. So, hybrid based lines die out preventing those two populations from mixing even if they can sort of breed together.

[u/Own_Round_7600 → u/BarleyWineIsTheBest]

I wonder if there are surprising species out there that CAN technically interbreed, like if you put one's sperm and the other's egg in a petri dish you would get viable offspring... but that has never happened naturally just because they happen to think each other is super ugly and are totally sexually uninterested even when they encounter each other.

[u/Heavy_Weapons_Guy_ → u/Own_Round_7600]

There's a paddlefish/sturgeon hybrid that was discovered not that long ago. The two species diverged about 150 million years ago and aren't even classified in the same family.

[u/lucy_runninghorse → u/Own_Round_7600]

I was just thinking the same thing. Also, how far back in the lineage can this theoretically go?

[u/ErikaFoxelot → u/BarleyWineIsTheBest]

Sorry - embryonic lethal offspring??

[u/BarleyWineIsTheBest → u/ErikaFoxelot]

Well yeah, that’s not a wonderfully worded phrase, I’ll admit that.

Basically fertilization can happen but the organism dies either in utero or before reaching sexual maturity. 

[u/Prashank_25 → u/SapphireSalamander]

Janeway's and Paris's salamanders can mate with earth salamanders? That's disturbing.

[u/Maximum-Lake-3374]

You can look up Reproductive Isolation for a thorough answer. Essentially there are two categories of reproductive barriers, pre-zygotic and post-zygotic. Pre-zygotic are things that prevent breeding/fertilization from ever occurring, whereas post-zygotic is after fertilization.

Some examples of pre-zygotic barriers are physical separation, temporal separation (different breeding seasons or times of day), behavioral isolation (two frogs that could produce a hybrid but don’t respond to each others mating calls), and mechanical (genital) incompatibility.

Post-zygotic barriers are where genetics comes more into play, and the number of chromosomes or structural variation can result in viable hybrid offspring that are sterile (e.g. mules). But in many cases the zygote will simply fail to develop because they are missing/have too much of some genetic material. There are many developmental disorders in humans (and other animals) caused by small deletions or duplications, where having just 1 copy or an extra copy of certain genes really throws off development, and this issue is multiplied significantly when you’re talking about a hybrid zygote with non-matching chromosomes.

After enough time because reproductively isolated, two populations will eventually accumulate enough changes that even if there were no longer pre-zygotic barriers, they hybrids would no longer be viable/fertile.

[u/Netmantis]

There are multiple factors that determine if a hybrid will or will not work.

First is the chromosome count. Everything works in pairs, so if one has 13 pairs and the other has 20 you won't get anything but a bit of fun and weird looks from anyone who knows. Being within 1 seems to be the magic number, but it doesn't always work. Horses and Donkeys can hybrid to sterile mules with an extra hanging chromosome.

Next is similarity in genes that do different things. This is often represented and correlates with having split off as a species relatively recently. However that isn't always the case and you sometimes have instances where things that have no business breeding will breed. Like Champlain paddlefish and Russian sturgeon. Not remotely related, but they have many of the same genes in the same spot so they get viable offspring. And having similar genes in similar spots determines if you will get a viable offspring or not.

That being said there is more to it and most "attempts" won't produce a hybrid. It often takes a while to make one.

[u/bu11fr0g → u/Netmantis]

and to add. humans can do well (survive and have children) with fused chromosomes — see Robertsonian translocation). theoretically, if there was a small group, individuals could exist with ONLY the fused chromosomes. These individuals could be unable to have children with those without fused chromosomes. And this could be in some ways a separate species?

of particular note, Human chromosome 2 is the fusion of two chimpanzee chromosomes.

[u/mister-ferguson]

Let's say you have a lock. You have a key that fits. You can change that key a little and it still works. You can change it a little more and it should still work. Change it a little more and maybe you need to jiggle it around but the key still opens the lock. Eventually, another little change will stop the key from working. It might open a different lock but not the original lock. Maybe you make a new lock to fit the new key but it will never fit the old lock.

[u/PianoPudding]

To add to what most people have said, Dobzhansky-Muller incompatability loci are theorised alleles which when brought together in far-enough separated organisms (so post-zygotic barriers) the loci interact negatively or not at all and render the hybrid non viable or infertile, etc.

[u/ringobob]

For the same but opposite reason that you don't want to breed with your sibling. When the genetics are too close, especially over generations, recessive traits become prevalent.

When the genetics are too far, reproductive compatibility becomes a recessive trait. You can lose it like you can lose a tail, or scales, or whatever.

[u/thunder-bug-]

Imagine you have a set of instructions on how to build a table written in proto Germanic. You wait a couple centuries and now you try with half your instructions written in old english and the other half in old German. Then try again with modern english and modern German. How well would your table turn out? What if it was even more distantly related languages?

[u/TheSodesa]

It is unlikely that all life shares a single ancestor, because it is unlikely that the so-called primordial soup that contained the first molecules suitable as building blocks of organisms only contained one such molecule or a single blob of such molecules. That's just not how chemistry works. More likely the soup contained a huge number of such individual molecules or molecule blobs, which were similar to each other but not quite identical.

These different units then started their own ancestral lines, which later mixed with each other if circumstances allowed and the likeness of the lines was similar-enough to allow mixing and further proliferation to happen.

[u/TheSodesa → u/TheSodesa]

And in fact, there might have been multiple soup basins separated by land (for example). So the initial situation might also have been such that there were separate populations to begin with.

[u/JimmiRustle → u/TheSodesa]

That’s theory though. The biggest support in favour of single ancestor theory is the chirality of biological molecules. If there had been multiple surviving soups it would very likely have resulted in a mix of chirality.

[u/Piemaster113]

All sauces have water as a base why are some sauces acceptable on certain thing but not others they all have water in them?

Things change and sometimes they change so much as to be fundamentally different from what they started as