Bolder-dash's very own little thread

Of course it's difficult to show fossil of animals with damaging mutations that make it hard for them to survive. Consider the case of the animals whose jaws were well positioned to give them better chewing and/or better hearnig, compared with your hypothetical animal with a disarticulated jaw who can't chew. The one with the disarticulated jaw would probably die quite young, being unable to feed effectively, and never reproduce. Each time such a mutation arose, it would leave only one animal behind to show for it, as it would never get passed on.The animals with more efficient jaws and/or better hearing, though, are going to be the successful animals. These are going to leave many descendants, so they'll be much easier to find in the fossil record than those with damaing congenital abnormalities. That's natural selection right there.

But you weren’t asking for neutral mutations. You were asking for examples of debilitating mutations like ones that reshape the skull to obstruct breathing, or disarticulate the jaw so the animal can’t bite. I’m not sure how you expect people to point to a fossil and say, ‘that was a neutral mutation’. Most mutations wouldn’t result in any significant morphological change at all, so there wouldn’t be any sign of them in fossils.Even those that do result in changes usually won’t leave a fossil record — how could we see a change of colour of an alteration in the shape of soft tissue (except in very rare cases)? The only thing we’re likely to spot in most cases are changes to the structure of bones or shells, and significant changes in this department are unlikely to be neutral.And how can we identify what features of any particular fossil are the result of mutations. We can find a Cretaceous theropod, and note all the differences in structure from early theropods — these are the records of mutations that had accumulated over the previous 150 million years. What else would you expect to see that’s not there?

What you seem to be asking for is something in modern animals that is the ancestor of a new structure in its descendants. But this is asking us to know the future. I could point to the Papillio butterfly, which has light-sensitive patches on its genitalia, and declare that these are the precursors of a second set of eyes in the Neopapillian buttermonsters the will one day occupy the southern coasts of Neopangaea. They will use these eyes to scan the holes in which they lay eggs for predators, whilst still being able to keep an eye out for any predators outside.Of course, I'd be making it all up. None of us can see the future, so none of us know which features are going to be adapted and in which ways. We can just speculate wildly.It almost seems like what you're asking is 'where is the variation in modern organisms for natural selection to act upon?' and if this is what you're getting at then all I can say is 'open your eyes!' Lots of species are highly variable - just look at humans! We range in adult height from less than 5' to well over 6'. We have skin colours varying from a deep brown that's almost black to all white - and everything in between. We have different blood groups, and different digestive enzymes (I retain the ability to digest lactose into adulthood - most people don't). We have different proportions, different shaped faces and different patterns of vellus hair. Human populations have all sorts of traits that differ from the norm - such as the enlarged labia minor in Khoisan women,