Putting that BIOL1130 information to use I see.
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The question gets more interesting when you ask when one species becomes another species. Individuals of the first generation couldnât breed with individuals of the millionth generation after them and are, therefore, of a different species, even though they are their great-grandparents! And if you try to pinpoint which generation exactly marks the beginning of the new species, you will not find it.
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I have found it deeply amusing to actually use my high school and university schooling for something other than passing tests! And here I half-expected to leave my notes in their folders for a decade.
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That Wikipedia article that Mark shared is excellent. It goes down a lot of the rabbit holes with regard to the benefits and limitations of the species concept. All models are wrong; some models are useful.
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Some light? To make it darker: somebody says species donât even exist, all we have are SNARCs.
https://doi.org/10.3998/ptpbio.16039257.0010.001
(Note: I do not share their position).
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A species is a human classification to help us try to understand how nature works.
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A species is whatever the taxonomists say is a species, I just leave it to them, it is too confusing.
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A species is a group of closely related organisms that share the same evolutionary trajectory. At least thatâs something like one definition Iâve seen used. They interbreed or are capable of interbreeding except when theyâre unisexual and donât. And some times they can successfully interbreed with other closely related organisms of a different species. I know, thatâs probably not very helpful.
Species is both a human concept for categorizing life forms and a reality, although the reality of species is pretty messy and doesnât always fit into the neat compartments we like to create.
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We humans automatically group organisms into âkindsâ â organisms that look similar to each other and different from other âkinds.â â dog, cat, horse, zebra, etc. Our automatic âkindsâ are at various levels (bird, duck, Mallard) and can be kind of sloppy. The lowest categories are usually species, but not always. (Farmers precisely classify cattle by age, gender, and reproductive status.) In any one area, this informal classification can work very well. In fact, if the local people classify a group of organisms exhaustively, they usually name the same âkindsâ that scientific researchers do.
As scientists, we need to make our words precise. We need to define them. We need to make species neat, mutually exclusive categories. Here things get tricky because real life doesnât care about our need for neat words. We try to define what we mean by âspeciesâ and the definition may work for one group of organism but it fails for others.
Our automatic classification of âkindsâ involves looking alike. Linnaeus originally classified the male and female Mallard as different species. They certainly look different! (In the next edition he called them a single species.) Obviously, breeding together is important. Looking alike isnât enough.
A common definition is: A species is a group of organism that breed with each other (or potentially would, if they were in the same area) in nature, to produce fertile offspring. (Conversely, they donât interbreed with other species, or if they do the offspring are sterile.) This works pretty well for the animals that we work with most of the time. It fails miserably for some other organisms.
What if two slightly different âkindsâ donât breed with each other because they live in different areas? We canât apply our preferred definition. We have to make an arbitrary choice. (e.g. Scrub Jays, Rosy Finches)
What if a âkindâ consists of many different âsubkindsâ that look different, but where they meet each other they do interbreed freely? They can be called one species (Song Sparrow, Northern Flicker, Dark-eyed Junco, Yellow-rumped Warbler) or more (Luzuli/Indigo Buntings. Baltimore/Bullockâs Orioles, Carrion/Hooded Crows). What if there really do seem to be different âkindsâ with different ranges and habitats, but they frequently interbreed (Large white-headed Gulls, Willows).
What if they donât have sex? Or usually donât have sex? We see different âkindsâ but should we consider the âkindsâ to be different species or different clonal individuals within some larger group? How much variation should there be within a species? (Lots of species, each uniform in appearance, versus one larger species with variations?) Does it matter that the different âkindsâ can interbreed though they usually donât because they usually donât have sex at all? (The European Blackberry complex) What if the group includes âkindsâ that have sex only within their âkind,â âkindsâ that never have sex, and âkindsâ that set their own seeds without sex but also father the seeds of other âkindsâ? (Dandelions, which also have additional variations. Aargh!)
What if the organisms all appear to be one âkindâ but if we look closely we do find differences? (Empidonax flycatchers) What if the differences are really, really hard to see or seem smaller than the amount of variation within the âkindâ? (Fine-leaved Fescues) Right now weâre having a lot of arguments about this.
We expect all members of one species to be more closely related to each other than to other species. Thatâs obvious! What if they look very similar but examining their DNA shows that theyâre not so close after all, at least compared to other âkindsâ? (Downy and Hairy Woodpeckers) How much weight should we give to the idea that a species or genus âshouldâ also be a clade? (Clade = a group consisting of all and only each otherâs closest relatives.)
And of course there are species of hybrid origin. How do we classify them? Same genus as one or both of the parents? Different genus? And how does that mess up our idea that taxon = clade? (Look into the Wheatgrass Tribe of grasses!)
People who study birds, flowering plants, mosses, algae, etc. have different answer to the question of what âkindsâ should be called species. Generally we agree on the âkindsâ but not their rank. Especially, we argue about what should be called different species vs. variation within one species. We get an idea of what the âkindsâ are in our group, and after that we look for (or make up) a species definition that works for it. We try to be consistent within a group.
Example: When I finally felt like I understood something about the fescue grasses I studied and I had to put names on them, I dithered. Two were obviously sister species (each otherâs closest relatives). They interbred where they met each other, but they didnât do that often because of the mountain range between them. (one species!) But they looked slightly different, had differences in leaf structure, and as a result had somewhat different ecology. It was clear that European botanists studying these fescues would call them different species despite the small areas of overlap. (two species!) So I called them different species and thatâs the concept we use so far.
So what is a species? If a book has a long list of definitions, the last entry on the list is often â A species is what a competent taxonomist familiar with the group says it is. Not a definition that makes anyone happy! But sometimes itâs just real.
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Nice lesson. A big problem with any species concept/definition, as you point out, is that it doesnât work consistently across all taxa. Bacterial species canât be viewed the same as vertebrate animal species. Plants do all kinds of weird things, as do a few animals. A common group of lizards in my area of the SW US is the genus Aspidoscelis (whiptails) which contain diploid sexual species and diploid or polyploid unisexual hybrid species. They are more plant-like in what they can do reproductively than nearly all other vertebrates other than a few fishes and amphibians. The named species in that genus stretch the definition of what we think of as vertebrate species. Bottom line, the exceptions will scuttle almost any simplistic universal definition of species that is proposed.
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This is interesting in discussions of human evolution, where the divergence of geographically separated populations interacts with a human sense of identity. Frequently, you will see people arguing that if there was gene flow from Neanderthals to Homo sapiens, then Homo neanderthalensis canât have been a separate species. The same would then extend to Denisovans (where we know of a first-generation hybrid with Neanderthals) and so on, revealing the limitations of ignoring the time dimension.
The concept of a speciation continuum addresses situations like this. The link below should give full-text access.
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Imo the best contemporary definition of a species is the evolutionary species concept, which can be reconciled with the biological species concept. According to this thought process a species is an evolutionary lineage on an independant, isolated trajectory. This means that species can interbreed but still be separate species if conditions ensure that the lineages remain on isolated trajectories (the two lineages when treated as a whole will not merge, even though a few individuals may interbreed). This theory is pretty much generally accepted for sexually reproducing organisms, but aesexually reproducing organisms are far more difficult to categorize and really just require a different concept of âspeciesâ.
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Thank you for the reminder about the previous thread. I am going to move this discussion there since it is essentially the same topic. [EDIT: now done, moved here]
I would say that this gives more weight to the concept âspeciesâ than it can really carry. Those hominins vary. They interbreed, though probably not a lot. Some genes that originated in species A later become widespread in species B. All true. We need names to talk about them with. But then we argue about the reality of the names. Sigh.
Yes, I agree. Itâs more a question of people who are familiar with a high school-level definition of species but havenât read enough biology to understand when that definition isnât adequate.
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(This was incredibly helpful. Because you explained each step, in my mindâs eye I saw illustrations of what you were saying, as if I was watching a really well done instructional video. Farmers, cattle, like a movie reel! When you offered up Mallards, I immediately pictured the male and female, etc. They do look different, yes! When you named things I could not picture, like Fine-leaves Fescue, I simply looked them up. Thanks, so much @sedgequeen, what a tremendous post.)
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Yep, but having the entity named is certainly helpful. Maybe it wonât hold up in the long run as a full species and be relegated to a subspecies or variety. Or sunk into a junior synonym of an earlier named species. But a name is a handle for communication about individual forms, even when there is disagreement about its rank or validity.
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