This shows Darwinism in action at the micro-evolutionary scale to such a degree that no-one could really deny it I think.
This all happens within the 'Fish' Kind.
This example is taken from â€œThe Greatest Show on Earth by Richard Dawkins.
First, an anecdote from the book:
My colleague Dr John Endler, recently moved from North America to the University of Exeter, told me the following marvellous - well, also depressing - story. He was travelling on a domestic flight in the United States, and the passenger in the next seat made conversation by asking him what he did. Endler replied that he was a professor of biology, doing research on wild guppy populations in Trinidad. The man became increasingly interested in the research and asked many questions. Intrigued by the elegance of the theory that seemed to underlie the experiments, he asked Endler what that theory was, and who originated it. Only then did Dr Endler drop what he correctly guessed would be his bombshell: 'It's called Darwin's theory of evolution by natural selection!' The man's whole demeanour instantly changed. His face went red; abruptly, he turned away, refused to speak further and terminated what had hitherto been an amiable conversation. More than amiable, indeed: Dr Endler writes to me that the man had 'asked some excellent questions before this, indicating that he was enthusiastically and intellectually following the argument. This is really tragic.'
What did Endler do? He took some wild guppies Poecilia reticulata which in the wild have â€˜striking variationâ€™ in loal populations. Some were almost as bright as the aquarium versions, and he thought that they must have been s*xually selected for. This is the important point, he tested this hypothesis! He didnâ€™t just assume it.
Others were more plain and matched the rocky bed of the tributaries that they lived in, and he thoiught they were camoflagued to hide from predators, but again, he wanted to test this hypothesis.
He took some of these guppies back to the lab and put them in tanks with various bottoms, such as sand and gravel and different levels of predation from fish such as pike cichlid, Crenicichla alta, which is its natural predator.
He also had some put in with the Killifish ( Rivulus hartii) which is a weak predator, and with different layers on the bottom.
In another set of tanks were guppies with no predation, but the same layers along the bottom.
He saw the species change in 6 months (in spot number, size and the colour depending on whether the selection was the Cichlid, the Killifish or the female), and he then took some of these back and put them into a tributary with only killifish and checked on them two years later and they were already more brightly coloured.
This is only a quick summary of the detail. I couldnâ€™t do it justice, but nine years later, another researcher found the descendants of Endlerâ€™s introduced guppies and they were as brightly coloured as ever.