Both adult, same gender, same habits.
I suppose “exactly the same” has to be qualified. As far as humans are used to looking at animals, we can’t notice any big difference from one species to the next.
Examples?
Lots of them. They are known as sibling species. Usually there are some subtle differences but to a casual glance they appear identical.
One well known pair are the Willow and Alder Flycatchers, formerly known as Traill’s Flycatcher. They can only be separated in the field by call.
Genetic work has recently been turning up more and more examples of this kind.
Emerald tree boas (Corallus caninus) and green tree pythons (* Morelia viridis*) look almost identical. They aren’t actually closely related at all.
Could these sibling species occur completely apart from each other, say, an ocean apart?
It’s theoretically possible. However, since they usually became evolutionary separate fairly recently they are usually close together geographically. The Willow and Alder Flycatchers overlap in range.
It’s becoming increasingly common for taxonomists to split widespread species into multiple species based on genetics. In this case, the species look very similar but may occur on different continents. An example would be the North American and Eurasian Beavers, which until recently were considered the same species.
Note that the example given by Maggie the Ocelot does not involve true sibling species. They only look similar coincidentally; as she notes they are not closely related. Another example of this kind would be the Eastern Meadowlarkof North America and the Yellow-throated Longclaw of Africa, which belong to entirely different families.
It was only recently determined that Savannah elephants and Forest elephants are separate species.
You can seriously drive yourself crazy trying to determine the difference between some ammonite fossils.
That’s a different kind of phenomenon than we’ve been talking about so far. That’s mimicry, in which an edible species evolves to look like a toxic one to avoid being eaten by predators. Also, toxic species can mimic one another so that they all benefit from having the same warning coloration.
In 2016, Western scrub jays were split into two species–Woodhouse’s (interior) and California (coastal). Before that, they were considered to be the same species, with more dull-colored individuals in the interior. I would only be able to tell them apart by range.
I don’t think it’s in the spirit of the question to talk about species that are only just barely separate, and which look the same because they split very recently, and haven’t yet had time to diverge very much. I mean, given that speciation occurs, that must happen, at least briefly, every time two species split.
And then there’s the opposite of the OP. Two individuals once thought to be different species, eventually discovered to be the same species, just different sexes. Ecletus roratus is an example.
My initial obvious reaction is, if course, did no one not notice that all of one bird were male and all examples of another bird were female? But I’m guessing low sample size and/or difficulty in sexing birds?
I’m not sure what the “spirit of the question” is. The point is that evolving genetic and morphological differences, while often going hand-in-hand, are two separate processes. Species can evolve genetic differences without becoming different in external appearance, and some of such species have been separate for millions of years. There are also many species in which different “morphs” that are very different in appearance occur in the same population but which interbreed freely. (This is even within the same sex.) An obvious example is the so-called “Black Panther,” a melanistic morph of the Leopard that occurs only in Asian populations.
It should also be mentioned that standards for recognizing species have changed over time. In the early 20th century most populations that differed in appearance from related ones were described as separate species. With the Neodarwinian synthesis and the development of the Biological Species Concept that took place 1930-1950 or so, most species that showed any hybridization were lumped as the same species. Now with genetic work the criterion for species designation has shifted again. If hybridization is restricted to a small area or is only occasional, two forms now are generally accepted as different species. Under these shifting concepts Baltimore and Bullock’s Orioles were described as separate species, lumped as Northern Oriole for a time, and have now been split again.
The Spiny echidna and the hedgehog look surprising alike, considering they are pretty far apart.
They are not sibling species, but they are pretty close to what the Op asked.
There are lots of fish examples. Lake trout are sometimes mistaken for Bull trout as are other char species to the point where education campaigns have been developed for anglers, lake whitefish and cisco (size differs though), and some strains of rainbow trout and cutthroat trout are pretty much identical in appearance.
But the OP didn’t specify “closely related.” Sibling species do qualify as a good answer to the OP, but I think Maggie’s example does too–if those two snakes have the “same habits,” which I don’t know.
ETA, the two beaver species seem like an even better answer, as do the trout species.
Not really. They are easily distinguishable by anything more than a casual glance.
This kind of mistake usually occurs when there are only a few specimens of each sex. It can also occur when juveniles look different, or there are different morphs. Once enough specimens are available the situation can usually be figured out. Also, collectors will make special efforts to collect mated pairs to establish species identity. Now genetic data can usually establish species identity regardless of sexual or age differences.
If you say so, but several cites say they resemble each other, including the cite I posted.