Comment by pegasus | Hacker News Reader

Comment by pegasus | Hacker News Reader

Parent Full thread

> Seems to me, that we divide other animals based on some of the most minor of phenotypic expressions

It might seem like that to you, but you'd be wrong. Taxonomy prioritizes genetic distance and reproductive isolation over superficial visual traits that humans happen to find striking. While phenotypic variations like skin color or facial structure are highly visible, they represent a microscopic fraction of the overall genome and do not indicate the deep divergence required to define a new species.

And from a genetic standpoint, Homo sapiens is remarkably homogeneous. Two humans from opposite sides of the planet are generally more genetically similar to each other than two chimpanzees from the same patch of forest. Traits like skin color (an adaptation for UV protection) or nose shape (an adaptation for humidity/temperature) are rapid evolutionary adjustments. They change quickly on an evolutionary timescale without requiring a fundamental split in the species' lineage.

In contrast to other animals, because humans never stopped breeding with one another, we never had the chance to "drift" far enough apart to become different species. Geographic distance in humans has historically acted as a filter, not a wall.

So there's your answer. Because of this unique genetic homogeneity (and not because of some imagined woke censorship), speaking of human subspecies would be scientifically mistaken.

I’ve heard this argument but it sounds factually incorrect to me.

Take for example: Icterus gularis [1] vs Icterus galbula [2]

Are you really going to tell me that:

1. They’d refuse to have sex with each other or could not procreate

And:

2. Someone bothered to check if they’re sufficiently distinct genetically?

I suspect these species were deemed “distinct” by early naturalists like Carl Linnaeus or Charles Darwin neither of whom even knew what a gene was.

And to my eye these birds seem a lot closer than an Aboriginal Australian man is to a Norwegian man.

[1] https://en.wikipedia.org/wiki/Altamira_oriole

[2] https://en.wikipedia.org/wiki/Baltimore_oriole

1. They might not, but that's not decisive for reproductive isolation. Basically, those two populations have diverged enough that we know, from studying speciation in general, that they are on separate evolutionary trajectories never to join again. That's the decisive factor - they are two distinct branches of the tree of life. That's not the case with any two human populations. Human history shows that we are different from all other animals in this sense: no matter what obstacle – weather geographic or otherwise – separates us from each other, we ultimately find a way to overcome it.

2. You might have picked a bad example with these two species, since they appear to be surprisingly far apart genetically, a lot of their common appearance being explained by convergent evolution not by shared ancestry.

PS: A lot of this stuff is counterintuitive and understandably perplexing, but scientists have worked hard to get to the bottom of things and deserve a bit more credit for it. You base a lot of your arguments on suspicions and gut feeling. I recommend measuring those misgivings against the freely available AI chat apps, it will help you get a grip on both the depth and complexity our scientific understanding of this domain. Ask it for sources, go check those sources, ask deeper question, push back as much as you need. Here's my interaction with Claude on these questions:

https://claude.ai/share/bea11195-731b-4301-a2f5-fb669961a60e

> Taxonomy prioritizes genetic distance [..] Two humans from opposite sides of the planet are generally more genetically similar to each other than two chimpanzees from the same patch of forest.

Do you have a source? I've tried looking in the past, but couldn't find good "genetic distance" metrics that could be compared between humans and other species.

The metric is called fixation index

AI is great at these kind of searches. Here's what Claude found:

https://claude.ai/share/bea11195-731b-4301-a2f5-fb669961a60e

And Gemini:

https://gemini.google.com/share/bfe9c3d5a2de

TLDR: it checks out.

The Gemini answer first cites Kaessmann, Wiebe, and Pääbo (1999), which explicitly says it sampled from all 3 at the time recognized major subspecies of chimpanzee (and found it 4x more genetically diverse than humans), not the same patch of forest.

Then it cites Goldberg and Ruvolo (1997), which uses the frankly hilarious "more variation between than within groups" metric. Why hilarious? Because it looks at single genes, while most traits are polygenic. When you look at multiple genes, even with only 2-3 dimensions to display the results (the data has thousands of dimensions), populations can be clearly distinguished [1]. What is the value in such a useless metric? And even then, the paper doesn't state something so extreme - quite the opposite. Direct quote from the paper:

Eastern chimpanzees are not, however, the genetic equivalents of humans. Mean, modal and maximum levels of nucleotide difference are actually slightly lower in eastern chimpanzees than in humans. The last common maternal ancestor of eastern chimpanzees may therefore be even younger than the last common maternal ancestor of all humans.

In fact, even a cursory reading of Gemini's answer shows it to be inconsistent - it states: "In contrast, chimpanzees (Pan troglodytes) exhibit a nucleotide diversity that is often two to four times higher than that of humans, depending on the genomic regions analyzed."

2-4 higher diversity, but there are 4-5 recognized chimpanzee subspecies [2]! Far from "two chimpanzees (implied from the same subspecies) living side-by-side are more different than the two most different humans", it puts humans right on the edge, or slightly past it, of meriting at least one subspecies of our own.

The last study it cites, Fontserè et al. (2022), barely mentions human genetic variation, and doesn't actually provide any quantitative comparisons between it and that of chimpanzees.

Finally, I didn't actually get what I asked for. Nowhere in those articles, or the AI answer, is there anything equivalent to "the genetic distance between Eastern and Western Chimpanzees is X, while the distance between a Norwegian and a Pygmy is Y."

So no, it doesn't actually check out, if you apply minimum scrutiny.

The sibling answer claims the fixation index [F_st, 3] is a measure of genetic distance, but that's not exactly true. E.g. it can't be used to show that dogs are closely related to wolves, less closely to cats, and even less closely to salmon - the F_st for all those comparisons, save perhaps for wolves, would be simply 1. Still, I took your advice, and asked AI (Gemini). I asked:

What is the genetic distance between eastern and western chimpanzees, and how does it compare to the genetic distance between a Spaniard and a Han Chinese?

To summarize its answer (can't share the chat when not logged in, feel free to verify), it claimed the fixation index is used for this purpose, and gave the following numbers:

  Western vs. Eastern Chimpanzees F_st = ~0.32
  Central vs. Eastern Chimpanzees F_st = ~0.09
  Spaniard vs. Han Chinese       F_st = ~0.11 – 0.15

The values for the human comparison are more or less in line with [3], but I couldn't find a source for the chimpanzee numbers after a very brief search. I've already spent far too much time debunking a casual AI slop answer.

[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC10113208/

[2] https://en.wikipedia.org/wiki/Chimpanzee#Subspecies_and_popu...

[3] https://en.wikipedia.org/wiki/Fixation_index

Interesting, thanks for putting in the worthwhile effort to debunk that assertion. Clearly some hyperbolical claims have made their way into common wisdom and from there into model training data, which is unfortunate and feeds the conspiracy theories.

Because those claims go way beyond what is needed to support the current scientific viewpoint debated here. It's still true that homo sapiens is a startlingly recent species and that the visual characteristics which are so apparent to us (as a mainly visual species) depend on much more superficial genetic changes than one would imagine.

The bottom line, as I see it, is that there is good reason to apply a different standard for assigning (sub-)species status to a given population when we're talking about humans vs. other animals. If we think of a species as a branch of the evolutionary tree (i.e. a separate evolutionary trajectory), in the case of other animals, geographic isolation will, with overwhelming probability, lead to divergence over a long enough time. Human history shows that this is not the case for us humans. Whatever obstacle has divided us in the past, we managed to overcome it and mix our genes again.

Let's take the North Sentinelese people (possibly the most genetically isolated human population extant). It is believed that they were isolated from the main branch of humanity about 50kya. That's obviously a blink of an eye in terms of evolution, but maybe if we would be talking about chimps, scientists would have designated them as a subspecies. Probably not, but let's pretend that's the case. Should we then do the same? Taxonomically sanction that split and consign them to their own branch of the tree? It seems historically misguided, but also morally wrong. Like shutting the door on them. I guess this latter aspect is what's bothering some, but in my opinion it says more about them, than about science in general.

Reply on news.ycombinator.com