AndrewPD writes:
A mollusc doesn't need a humans eye and the eye of a cat will never develop into a humans eye.
What an organism
needs does not affect how the species evolves. Evolution can only work with what is already there, through slight modifications (mutations) of structural or developmental genes and subsequent selective pressures.
Molluscs are an interesting group. There are many different types of eyes within Phylum Mollusca ranging all the way from no light sensitive organs, to the simple eyes of Snails and Slugs, the the complex eyes of Squids and Octopuses (has a pupil, lens, and retina, similar to vertebrate eyes).
No one is suggesting that cat eyes would develop into human eyes, but in the future cat descendent's eyes may be different due to new mutations that may spread through a population.
AndrewPD writes:
But to get from a simpler eye to the human eye you would surely need numerous beneficial mutations and at no stage from simpler eye to extremely complex eye could the mutation create a disadvantage.
It is true that there have been numerous beneficial mutations between simpler ancestral eyes and modern human eyes.
Interestingly, we know that mutations that cause a disadvantage do not necessarily interrupt the development of a more complex eye. Only if the disadvantage keeps the organism from surviving and reproducing will those mutations be removed from the gene pool. In the human population, for example, there are numerous mutations that cause impaired vision and that are passed on to offspring. It is even possible that disadvantageous mutations may confer an advantage to descendants in the future in a different environment.
AndrewPD writes:
If you see a professional Tom Cruise look-alike you don't assume he's directly related to Tom just someone randomly with a striking resemblance.
But we would know that he is the same species as Tom, and we could even surmise (if his resemblance is natural) that some of the genes controlling development of features are similar to Tom's.
What if Eleanor Roosevelt had wings? -- Monty Python
You can't build a Time Machine without Weird Optics -- S. Valley