I have just learned that dolphins evolved from a dog/cat like land mammal (Mesonix) that became ever-more water venturing. I understand and can visualise how the arms legs and tail slowly evolved into the fish-like ones of dolphins. The necessary transformation appears complete, with no benefit to further change of these limbs. So, I'm wondering if when you look at a seal, are they at a sort of intermediary stage, with the tail kind of like two little feet, which might 'improve' over time? Eitherway, when I read that the nostrils of Mesonix 'slowly moved' upward to the top of the head, I wonder why is there no sea-mammal that is only part way on this evolution, with a nostril between its eyes or something? (Is that even possible?) Or has evolution tended to happen in relatively short periods of accelerated rates relating to climate changes as opposed to arbitrary and consistant rates, and we are now in a 'dormant' period? Also, surely there are some evolved biological characteristics that are only beneficial once 100% arrived at (eg nostril on top of head) and any intermediary form/position has no benefit and so cant be assumed to have been a natural/obvious path for evolution to have followed, yet how else do we explain how the transformation occurred?
Every species in existence is "midway through its evolution"
This is because evolution is a never ending process. Technically, we humans are still evolving. Evolution only ends when the species dies out.
Evolution is a very long process spanning countless generations. In a way its also an incredibly democratic process with voting in the form of reproduction. The populace tends to vote for things that increase survival or looks good.
The main cycle of evolution involves mutation and selection.
Selection is in my opinion the most misunderstood part of it. In order for a select trait to gain adoption by a populace it has to push out inferior traits. This normally means inferior members dying off by various means.
Let me give a scenario that starts to tie into your question:
Imagine a dog like creature that can only hunt on land, we'll call them a scruffy. Say one day a scruffy is born with webbed feet which makes it able to swim better allowing it obtain food from water. Now say something kills off ALL of the scruffies land based food sources. The scruffy with webbed feet can obtain food and provide for his progeny while the normal scruffies die off from starvation. Flash forward a few generations and now say a scruffy is born with longer toes (more like fins) which makes him an even better swimmer. That scruffy can be a better provider to his family making him a more desirable mate. Flash forward MORE generations and the scruffies have become mostly aquatic since that's been their best food source.
One last distinction, it's considered a breed until one breed cannot successfully reproduce with another breed, that's when technically it becomes a new species (or so I've heard the argument). This is also the current great ambiguity with evolution because we don't know when or where this has happened for any species. Not to mention I read somewhere that Neanderthals did successfully copulate with humans which bares the question of whether they were a breed or a species (is a great Dane compared to Chihuahua a breed or species).
Also, surely there are some evolved biological characteristics that are only beneficial once 100% arrived at (eg nostril on top of head) and any intermediary form/position has no benefit and so cant be assumed to have been a natural/obvious path for evolution to have followed, yet how else do we explain how the transformation occurred?
Trait development is a very complex process. Things as arbitrary as sexual attraction play in. Mutation alone is incredibly random and can sometimes be dramatic. Something like a nostril on a snout could have migrated up the head over generations as a quality that was deemed sexually appealing. Or a mutation could have created a hole on the back that decoupled breathing from situational awareness and over time that hole was refined to be a more effective breathing apparatus.
NOTE: to all whom want to debate the breed vs species aspect. Lets not and just agree that it is an ongoing debate to be settled by doctoral thesis.
Clarification: In my opinion, as well as others, the biological classification system is not a very defined or very concrete with various cases breaking the system. This is especially the case with the category known as species where one begins or becomes another. The common, yet still fallible definition of a species, is members of group who are capable of producing reproductively viable offspring. There are many cases where this categorization breaks down or becomes blurred.
No, because that assumes evolution has some "end goal" which of course is not the case.
The statement "surely there are some evolved biological characteristics that are only beneficial once 100% arrived at..." is a fallacy. I would recommend checking out Dawkin's The Blind Watchmaker and/or Climbing Mount Improbable for some very nice elaboration on this exact topic.