To me, "causal factor" suggests the role of mutation rather than selection,
whereas your further questions below suggest that you are thinking of the
role of selection in favoring these mutations. Perhaps miscommunication is
part of our problem.
>You wrote, "Natural selection can give a partial account for transitional
>that they are better adapted than the ancestor for the niche they are
>transitioning into. The variation must arise by mutation if it is going to
>persist (unless the organism can transmit learned behavior). The fact that
>a particular transitional form survives and goes on to evolve into the new
>form might be largely attributed to natural selection."
>My comment: This is not evidence, Dave. Take a real case from paleontology.
>The modern blue whale is supposed to have evolved from some original
>mammalian land ancestor. This is not a simple transition to another little
>niche, as you know. This transition involves a long series of incipient
>forms. The ancestor was presumably well adapted to its terrestrial
>environment. Would the first mutation toward an eventual aquatic blue whale
>been adaptive for the ancestor? It had to lose its well-adapted paws or
>hooves for initially poorly adapted flippers. What niches did the incipient
>forms fill? How did natural selection, which by definition only enhances
>immediate adaptation, carry the incipient forms through so many inadaptive
Within the past decade, several transitional forms between fully
terrestrial and fully marine whales have been found. I do not remember all
of the details, but the intermediates are functional.
The initial stage was along the general lines of an otter or other
semi-aquatic animal (though much bigger than otters)-a more elongate body
form. (Webbed feet would be likely, too, but unless someone finds good
footprints the fossil record won't show any trace of that, and trying to
prove that the footprint was from ancestral whales would be difficult.)
The environment appears to have been rivers and, later, shallow coastal
water. The oldest fossil whales were toothed, feeding on fish and other
aquatic animals. Adaptations for improved swimming ability are obviously
beneficial in this setting. The large aquatic reptiles went extinct a few
million years before this, so there was not much competition. As long as
it could catch fish, even something with limited swimming abilities could
survive (grizzly bears do well, for example). However, slight improvements
in swimming ability (streamlining shape, ability to close ears and nose,
reduced fur, etc.) would give them an edge relative to their closest
competitors-others of their own kind. Better swimming would enable them to
either catch fish more effectively in the rivers or let them reach areas
such as the open ocean that were not accessible to others. Those features
which are fossilizable do show a gradual development towards the modern
condition. Also, other aquatic mammals do OK with intermediate levels of
adaptation (sea lions still have small external ears, some ability to get
around on land using front and hind flippers, and birth on land, yet they
Icthyosaurs would have been a better choice-their ancestry remains
uncertain as far as I know.
>Alternatively, isn't it just as rational to posit some unseen _directing_
>force at work that drove the land-to-sea transition in spite of the passage
>through many inadptive stages? Doesn't it seem to you that a long-range goal
>was involved in guiding the whole passage?
I am not certain that the intermediates were inadaptive. I do see a
long-range goal, but only from either hindsight or theological
considerations (as opposed to something inherent in ancestral whales).