Plant Sciences, Zoology & Evolution / Re: I have fish in my free-standing water tank - how is this possible?« on: 09/09/2018 01:04:11 »
The presence of identical freshwater fish species in different rivers has always intrigued me.The simplest model of an island(such as Great Britain) is a pyramid with a dry apex and water runing down the sides. The rivers are not connected at the watershed, and the salt sea at the base prevents freshwater fish from migrating between rivers at their mouths. But somewhere in the middle of the stream you will find the same native species (pike, perch, roach) in each river, supported and predated by pretty much the same ecosystem The probability of identical evolution of the top species in isolated biomes is negligible,and fish can neither fly nor walk across the land.Not all that long ago, a few tens of thousands of years, Great Britain used to be connected to the mainland of Europe, because the sea level was a lot lower due to massive glaciation. Lots of rivers that now individually lead to the sea used to join up before proceeding to the sea in what are now the English and Bristol channels, and also, the glaciers themselves did wacky things, like damming up stuff and changing the topography by grinding grooves in the land, and when they melted, huge freshwater flooding happened that could easily push fish around.
The least unlikely solution I have been offered is accidental transfer of pregnant fish or eggs, by birds, and I guess that might account for fish in a tropical water tank.