Are two members of the same species necessarily more closely related to each other than they are to a member of another species?
Yes. That’s what a species is – a member of a shared gene pool.
Got it in one.
So they all share an INDIVIDUAL ancestor of that species?
Technically the definition of a species is that its members are so closely related to each other genetically that they can produce fertile offspring. So by definition they have to be more closely related to each other than they are to members of a different species.
Probably half-way between species and non-species are what are called “ring species”. In this case you have minor variations in species members so that members who are close to each other (geographically) can breed and produce fertile offspring, but once the species members get far enough away from each other they can no longer breed, despite being the same “species”. In other words, if you have populations A, B, C, and D of a ring species, A and B can breed, B and C can breed, and C and D can breed, but A and D can’t.
If you go back far enough, everything goes back to a common ancenstor.
Or two of them. Depends if they’re parthenogenic lizards.
Go back far enough and it’s probably some unicellular prokaryote.
Well, strictly speaking…
It is actually possible, sometimes, for members of two different (but close) species to mate and produce offspring, and it’s occasionally even possible for them to produce fertile offspring. The usual definition of “species”, nowadays, just says that this doesn’t happen in the wild, but it can still happen in extreme circumstances. So you could end up with an individual whose ancestry is mostly one species, but with a trace of some other species, and therefore a few members of that other species to which they’re closely related.
I would say that this is generally true for the most part. However, I believe that Mother Nature does not find herself constrained to follow our human-imposed definitions. Take, for example, ring species. The Larus gulls of the Arctic are a classic example. Larus - Wikipedia Do you think it might be possible for one gull on one extreme edge of its species range is more closely related to the gulls of another species just past its species range than it is to gulls on the opposite end of its own species range? The problem with this question is that the gulls are a species continuum and there is no bright line where one species stops and another starts. Does anyone have any argument one way or another about this?
Take for example this girl whose parents were respectively a Denisovan father and a Neanderthal mother, announced about 6 years ago. Apart from her own parents she would be closer to some Denisovans than all Neaderthals (and vice versa).
Complicating the example in this case is that palaeontological species are not constructed in the same way as living species, so while we draw circles around different human ancestors as species, we are not able to apply the same definitional criteria as we would for say, living monkeys, birds or beetles so the circles may be capturing different characteristics than those connected to genetic relatedness.
But, as the article points out, ‘Discovery suggests that distinct ancient human species may have mingled and interbred happily’, so that’s nice and lovely and I’m sure there was nothing horrible or coercive involved.
Just want to highlight this sentence.
Humans like to think about things in neat category boxes with tidy boundaries. We have an instinctive bias to classify everything like this.
The natural world, however, is under no obligation to conform to this narrow kind of thinking.
Everyone in the thread is clearly aware of this, but I thought it was worth explicitly calling out, for the benefit of the OP, that this principle must be kept in mind when contemplating these answers.
That’s a great question and has concentrated, in my mind, my own question which I would like to rephrase in the following way;
Say you have a group of siblings within a population. Obviously their descendants, at least for quite a few generations, would be more closely related to each other than to a lot of other members of the population, especially, probably, the ones geographically further away.
But say the population splits into two with some of the siblings ending up in one of the new populations and the other siblings ending up in the other new population, could the descendants of the siblings in one of the populations still be more closely related to the descendants of the siblings in the other population than to at least some of the other members of their own population at least for a few generations after speciation?
What speciation?
If the populations split, they will still be the same species for quite a few generations and closely related family members (these are essentially 2nd and 3rd and 4th cousins) will of course be more closely related to cousins than general members of either population.
You are basically describing what happens to immigrant populations right now - some members of family a hundred years ago moving to a different country and neither side of the family contacts the other. A genetic test will still show they are more closely related to their 2nd or 3rd generation removed family members back in the old country than some random stranger. But they’re all still the same species (human, in this case).
By the time speciation occurs much, much, much further down the line, except in the case of insane inbreeding, there will have rather thorough mixing of the genetic pools on either side of the split. It’s not like each individual person in a population is going to generate their own separate descendant species.
Thank you!
Could two species of completely different lineages, through random mutation of their own independent lines, not co-breeding, eventually converge to become more similar to another species? Not just morphologically (like wings evolving independently several times) but even genetically?
e.g. Could species A and B, who initially share 70% of their genes and live in completely different parts of the world and never meet, but facing similar environmental constraints, eventually evolve similar adaptations and end up sharing 72% of the same genes?
In that case, would they have become more genetically “similar” even if they’re not directly related?
Well, not exactly the same genes, but convergent evolution does occur where different species can eventually develop similar traits. Convergent evolution where the same gene is somehow independently evolved doesn’t really happen. Except in laboratories, where artificially introducing genes can be tested.
One example:
I don’t think I understand your question, unless you are speaking of convergent evolution, in which unrelated species become similar to each other phenotypically (e.g. in observable characteristics), because they occupy a similar niche, just widely separated geographically.
This is, for example, why hyraxes from Africa look and live much like marmots from North America, although marmots are related to ground squirrels, and hyraxes to manatees and elephants.
But genetically they remain just as distant as ground squirrels and elephants. Genes in combination possess vast powers of phenotypic expression while remaining relatively static.
In theory, genetic mutation could occur to the extent that entire separate species become extremely similar genetically. But in practice, it’s about the same as monkeys writing Hamlet given sufficient time. Actually I think the monkeys would get there first.
Sorry for the poor phrasing. That’s all I meant to ask 
And you both answered it… possible in theory, just by sheer dumb luck, but highly improbable in reality vs the relatively common convergence of phenotypes.
I suppose even if that happens, we could still differentiate lineage from e.g. mitochondrial DNA and certain sequences of other genes being retained in closer ancestors?
Edit: I guess what I’m asking, ultimately, is “what is the likelihood of a genetic fingerprint ‘collision’ – sharing accidental similarity – between two species that share no recent ancestors (who presumably independently evolved the same genes purely by chance)”
Probably not through random mutations, but environmental conditions could force something similar. One example that isn’t different species but does refer to different populations of the same species is lighter skin. All light-skinned humans from northern Europe have a particular defect in one of the genes that controls melanin production (we still have the gene for dark skin, it’s just defective, so technically white people are just defective black people). Lighter skinned people from Asia have different genetic changes and don’t have the same genetic defect that white Europeans have. What this means is that light skin evolved completely separately in two different human populations. There is some debate about why skin color lightened in both populations, but the most popular theory at the moment is that basically the weather in those areas sucks and lighter skin allows humans to produce more vitamin D which makes them healthier in those crappy conditions.
It’s important to note however that having the same traits (light skin in this case) did not result in having the same genes.
70-ish percent is probably way too low for two species to interbreed. Humans and chimpanzees share something like 98 percent, and no one is really sure if we can interbreed or not (rumors of humanzees notwithstanding). Humans and mice share about 80 percent IIRC. 70 percent gets you in the realm of humans vs. worms or slugs.
Very interesting! It reminds me of the Artur C Clarke micro story SPOILER
Alien ships come to earth and send a message. We are actually a kind of leper colony. Our ancestors suffered from a harmless, but horrifically disfiguring disease. The last line is “If any of you are still white, we can cure you.”
Also, this thread in general and your post in particular have been very educational.