don't know how to put it simply, but there are dozens of potential definitions of a "species" using criteria from morphology to habitat to strict genetics and they all fail in one place or another, probably because there's no such thing as a "species" in nature.
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in example, defining species through morphological (form/structure) traits has been outmoded for at least a century, as convergent evolution is everywhere. Things often look the same despite being entirely unrelated, and sometimes closely-related things look almost nothing alike
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sexual compatibility as a limiting border for a "species" doesn't work because it implies regular speciation events where a creature is born incapable of inbreeding up the familial line. Plus, a whole lot of organisms reproduce asexually.
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The natural world is composed of transition, and a strict, static, universal and functional definition of a species (a definition that requires every creature be one thing or the other) would be so overwhelmingly specific as to be functionally useless.
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Biology's solution to the species problem is mostly to say "look, you all know what we're talking about and we have better things to study" which is reasonable and necessary. The species problem becomes more a philosophical issue than a scientific one.
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See, back in high school they taught me Lamarck as a kinda goofball who thought giraffes had long necks because their parents stretched a bunch, then Darwin looked at turtles and finches and saw it weren't true. All of which is wrong.
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Lamarck's real error was in the common-wisdom assumption of evolution as a directional, ever-upward escalator with humanity at the top. He explained the continued presence of lower lifeforms as a constant unseen arising from the primordial slime.
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Darwin's genius was in recognizing this escalator as an illusion. Upwards, “creative” pressure is not a guarantee—things can evolve towards simpler forms or stay the same indefinitely if conditions favor those outcomes.
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The way Lamarck is kept around as a silly foil is kinda ridiculous. If he was a big idiot who couldn't intelligently justify his ideas to the intellectual community then Darwin wouldn't have been challenged to expand on them. “Take my name from me and make it a meme.”
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Scientist myth-making always seems to do this to its own detriment, to record Galileos and Newtons and Darwins as iconoclasts separate from their culture, not than participants. And it's just not like that.
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The great scientists are always part of a continuum, building off preexisting ideas, participating in a cultural discussion. If Copernicus' arguments had landed only on deaf ears, heliocentrism would have disappeared. But plenty of others were ready to hear him out.
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Anyhow, better high schools than mine taught August Weismann, a Darwin follower, as the disprover of acquired characteristic heredity when he cut the tails off mice and their grandchildren still had tails, but this is still wrong. I think Scientists have always hated mice.
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By his own admission Weismann's experiments proved nothing, convinced nobody, as Lamarck made claims on use and disuse, not inheritance of mutilated limbs. Weismann's work was writing, where he deduced the mechanism of DNA more than half a century before Crick, Franklin & Watson
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It doesn't diminish Newton to recognize to that Einstein essentially proved his equations wrong for large parts of the universe, and it shouldn't diminish Darwin to recognize that his theory required refining. Maybe people are scared of being misinterpreted as creationists?
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Darwin could never fully abandon inheritance of acquired characteristics, and as we learn more about epigenetics it appears he was somewhat ironically right for the wrong-ish reasons. We do pass down all kinds of things that don't appear directly connected to our DNA.
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But he did make a mistake in elevating the Organism as the exclusive unit upon which selection acts. Turns out wrong because the biological hierarchy (Macromolecule|Cell|Tissue|Organ|OrganSystem|Organism|Population|Community|Ecosystem) suffers from its own Species Problem.
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Said twice: the species problem also applies hierarchically and I love it. It's another spot where biology overlaps with philosophy/epistemology/the meaning of language.
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If the minimum definition of an organism is “behaves like life”, then some computer programs or a single RNA strand could be considered organisms.
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If an "organism" is a self-sufficient being with its own unique DNA, then you are more of a colony than an organism, as you require the workings of your mitochondria and gut bacteria to survive, all of which contain their own genetic code.
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If you want to consider your body as the walking vat that holds your brain, then you start to consider yourself an organ or an organ system. I'd recommend you don't do this, but that's a different discussion.
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If the families of mitochondria living within your cells are not organisms themselves but support structures for a larger organism, then perhaps you yourself are simply a support structure for a larger “Humanity” organism.
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The categories within the hierarchy are as (needfully) reductive as the concept of a species. Definitional barriers are so easily crossed as to be functionally nonexistent, and if nature does not recognize an organism, how could natural selection operate exclusively upon it?
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Modern evolutionary theory reinvigorates multilevel selection, a difficult concept Darwin once had to dispense with in order to move the theory forward. It's a fascinating history, and if you want to read more in depth I'd suggest Stephen Jay Gould and Thomas Kuhn
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in summary, biology always verges on soft science, but economics, social science and psychology are never sciences at all since they depend on non-falsifiable assumptions, and Kim Cattrall is 100% in the right while SJP can burn in the scorching winds of Jahannam.
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