This is not correct. At no point can the offspring in a single generation be differnet enough to be called a different species.
I’m not saying we should call it a different species but if we’re saying species Y is the direct descendant of species X, then, we can imagine a dividing line, and the line must always begin with an egg because eggs are different from their parents but adults are not different from the egg they started off as.
In reality things change very slowley over a large amount of time and there a no clear transition points.
But that line can only ever be imaginary. There was never a proto-chicken that birthed a chicken. All chickens were birthed by chickens, all proto-chickens birthed proto-chickens.
We can make an imaginary line, but if went looking for it we would never find it.
Nonsense. All you do is say “Here is what defines a chicken” and then, advancing backwards through the generations, point to the first creature who didn’t meet that criteria. That’s your “proto-chicken” and everything after it is a “chicken.” Yes, the last proto-chicken and first chicken would be considered the same species as one another if we were building a taxonomy of species today, but we’re not; this is a historical exercise.
The definition may be based on some scientific criteria that’s specific to a point-in-time and may be somewhat arbitrary as a result, but it’s not “imaginary.”
The last chicken and first proto-chicken wouldn’t be 1 generation apart. The changes are so small that it takes thousands of generations for anything even close to beginning speciation to occur. If we literally did what you said, we would go backwards forever and when we got to something that looked completely unlike a chicken we’d be “shit, we have to go the other way around and check again, all the animals around this one look exactly like it, for thousands of generations”.
The point is that we aren’t comparing the generations to one another; we’re comparing them to modern day chickens. And we aren’t comparing by checking to see if a given creature “looks completely unlike a chicken.” That’s not how we differentiate species today, so why would we use it here?
As one example, one common and fairly simple definition of a species is a collection of individuals that can breed with one another and produce healthy offspring (“healthy,” in this instance also meaning that they must be able to produce offspring of their own). Obviously this doesn’t apply to bacteria or other things that reproduce asexually, but for our purposes, it could be sufficient. So you take this and turn it into a test: “Can this creature breed with modern-day chickens and produce healthy offspring?”
Now, even that simple question may involve qualifications in order to allow a binary answer. For example, maybe modern day chickens can breed with only 30% of other modern day chickens (of the opposite gender) and that number steadily decreases as we move back in time. The threshold for species differentiation here is going to be arbitrary.
That specific question is a bad choice in this instance, since chickens are descended from red junglefowl and can breed with them. In fact, they’re sometimes considered to be of the same species - for our purposes, we want to know when we first had a chicken - red junglefowl don’t qualify. As such, with chickens specifically it likely makes more sense to make the distinguishing criteria something that would differentiate a chicken from a red junglefowl, like “Is it domesticated?” That even gives us a good place to start looking - current understanding is that all modern chicken owe their origins to a single domestication event in Southeast Asia, roughly 8,000 years ago. Another option would be basing it off the DNA similarity to modern-day chickens (red junglefowl have 71-79% of the same DNA as modern chickens), e.g., once the DNA is no longer at least 80.000% the same, it’s no longer a chicken.
And you’re not limited to a single question, so long as the outcome of the test is binary.
Regardless of the specific test, at some point, the answer will change from “Yes, it is technically a chicken” to “No, it is technically not a chicken.”
I’m not saying we should call it a different species but if we’re saying species Y is the direct descendant of species X, then, we can imagine a dividing line, and the line must always begin with an egg because eggs are different from their parents but adults are not different from the egg they started off as.
Isn’t that obvious?
But that line can only ever be imaginary. There was never a proto-chicken that birthed a chicken. All chickens were birthed by chickens, all proto-chickens birthed proto-chickens.
We can make an imaginary line, but if went looking for it we would never find it.
Nonsense. All you do is say “Here is what defines a chicken” and then, advancing backwards through the generations, point to the first creature who didn’t meet that criteria. That’s your “proto-chicken” and everything after it is a “chicken.” Yes, the last proto-chicken and first chicken would be considered the same species as one another if we were building a taxonomy of species today, but we’re not; this is a historical exercise.
The definition may be based on some scientific criteria that’s specific to a point-in-time and may be somewhat arbitrary as a result, but it’s not “imaginary.”
The last chicken and first proto-chicken wouldn’t be 1 generation apart. The changes are so small that it takes thousands of generations for anything even close to beginning speciation to occur. If we literally did what you said, we would go backwards forever and when we got to something that looked completely unlike a chicken we’d be “shit, we have to go the other way around and check again, all the animals around this one look exactly like it, for thousands of generations”.
The point is that we aren’t comparing the generations to one another; we’re comparing them to modern day chickens. And we aren’t comparing by checking to see if a given creature “looks completely unlike a chicken.” That’s not how we differentiate species today, so why would we use it here?
As one example, one common and fairly simple definition of a species is a collection of individuals that can breed with one another and produce healthy offspring (“healthy,” in this instance also meaning that they must be able to produce offspring of their own). Obviously this doesn’t apply to bacteria or other things that reproduce asexually, but for our purposes, it could be sufficient. So you take this and turn it into a test: “Can this creature breed with modern-day chickens and produce healthy offspring?”
Now, even that simple question may involve qualifications in order to allow a binary answer. For example, maybe modern day chickens can breed with only 30% of other modern day chickens (of the opposite gender) and that number steadily decreases as we move back in time. The threshold for species differentiation here is going to be arbitrary.
That specific question is a bad choice in this instance, since chickens are descended from red junglefowl and can breed with them. In fact, they’re sometimes considered to be of the same species - for our purposes, we want to know when we first had a chicken - red junglefowl don’t qualify. As such, with chickens specifically it likely makes more sense to make the distinguishing criteria something that would differentiate a chicken from a red junglefowl, like “Is it domesticated?” That even gives us a good place to start looking - current understanding is that all modern chicken owe their origins to a single domestication event in Southeast Asia, roughly 8,000 years ago. Another option would be basing it off the DNA similarity to modern-day chickens (red junglefowl have 71-79% of the same DNA as modern chickens), e.g., once the DNA is no longer at least 80.000% the same, it’s no longer a chicken.
And you’re not limited to a single question, so long as the outcome of the test is binary.
Regardless of the specific test, at some point, the answer will change from “Yes, it is technically a chicken” to “No, it is technically not a chicken.”