If any organism that interbreed with another and reproduced fertile offspring is defined as the same species. Then why do some scienist still regard some organisms that can interbreed as different species?
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"If any organism that interbreed with another and reproduced fertile offspring is defined as the same species."
That's an approximation with known exceptions, rather than a strict definition. To use a familiar example, wolves and domestic dogs are usually (but not always) assigned to separate species. However, wolves and domestic dogs can and do interbreed, and can produce viable offspring which, in turn, may also give rise to viable offspring. Nevertheless, across the generations, there's an increasing likelihood (not the certainty) of the next generation being sterile.
Many supposed rules and definitions have exceptions.
"Then why do some scienist still regard some organisms that can interbreed as different species?"
The above "definition" is actually a rule of thumb that doesn't absolutely apply in all cases. Nature is messier than human attempts at classification systems.
Update
"I'm pretty confident that there are no organisms that can produce fertile offspring, but are still considered different species."
Wolves and dogs, lions and tigers, cabbages and turnips all come readily to mind. (The latter case gave rise to the vegetable known as the swede,) Horses and donkeys also occassionally produce fertile mules, and Darwin mentions cases among species of ducks in his little book from 1859.
In some cases, hybridization is a route to speciation.
That's an approximation with known exceptions, rather than a strict definition. To use a familiar example, wolves and domestic dogs are usually (but not always) assigned to separate species. However, wolves and domestic dogs can and do interbreed, and can produce viable offspring which, in turn, may also give rise to viable offspring. Nevertheless, across the generations, there's an increasing likelihood (not the certainty) of the next generation being sterile.
Many supposed rules and definitions have exceptions.
"Then why do some scienist still regard some organisms that can interbreed as different species?"
The above "definition" is actually a rule of thumb that doesn't absolutely apply in all cases. Nature is messier than human attempts at classification systems.
Update
"I'm pretty confident that there are no organisms that can produce fertile offspring, but are still considered different species."
Wolves and dogs, lions and tigers, cabbages and turnips all come readily to mind. (The latter case gave rise to the vegetable known as the swede,) Horses and donkeys also occassionally produce fertile mules, and Darwin mentions cases among species of ducks in his little book from 1859.
In some cases, hybridization is a route to speciation.
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I'm pretty confident that there are no organisms that can produce fertile offspring, but are still considered different species. Do you have an example of what you're talking about?
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Cus way more often than not they don't due to some inherent characteristic.