QUOTE(Pandora2173 @ May 21 2007, 06:09 PM) [snapback]1686505[/snapback]
This may not be an original line of thought, but it just came up in my head.
What if evolution is accidental? What if a creature was born with a mutation that, instead of hindering it's survival, actually boosted it's chances. Since it didn't die, it passed it on to it's offspring. Eventually the only ones left would all have that "defect". Would that not explain why there are no "transitional" fossils?
"Why do gaps exist? (or seem to exist)
Ideally, of course, we would like to know each lineage right down to the species level, and have detailed species-to-species transitions linking every species in the lineage. But in practice, we get an uneven mix of the two, with only a few species-to-species transitions, and occasionally long time breaks in the lineage. Many laypeople even have the (incorrect) impression that the situation is even worse, and that there are no known transitions at all. Why are there still gaps? And why do many people think that there are even more gaps than there really are?
Stratigraphic gaps
The first and most major reason for gaps is "stratigraphic discontinuities", meaning that fossil-bearing strata are not at all continuous. There are often large time breaks from one stratum to the next, and there are even some times for which no fossil strata have been found. For instance, the Aalenian (mid-Jurassic) has shown no known tetrapod fossils anywhere in the world, and other stratigraphic stages in the Carboniferous, Jurassic, and Cretaceous have produced only a few mangled tetrapods. Most other strata have produced at least one fossil from between 50% and 100% of the vertebrate families that we know had already arisen by then (Benton, 1989) -- so the vertebrate record at the family level is only about 75% complete, and much less complete at the genus or species level. (One study estimated that we may have fossils from as little as 3% of the species that existed in the Eocene!) This, obviously, is the major reason for a break in a general lineage. To further complicate the picture, certain types of animals tend not to get fossilized -- terrestrial animals, small animals, fragile animals, and forest-dwellers are worst. And finally, fossils from very early times just don't survive the passage of eons very well, what with all the folding, crushing, and melting that goes on. Due to these facts of life and death, there will always be some major breaks in the fossil record.
Species-to-species transitions are even harder to document. To demonstrate anything about how a species arose, whether it arose gradually or suddenly, you need exceptionally complete strata, with many dead animals buried under constant, rapid sedimentation. This is rare for terrestrial animals. Even the famous Clark's Fork (Wyoming) site, known for its fine Eocene mammal transitions, only has about one fossil per lineage about every 27,000 years. Luckily, this is enough to record most episodes of evolutionary change (provided that they occurred at Clark's Fork Basin and not somewhere else), though it misses the most rapid evolutionary bursts. In general, in order to document transitions between species, you specimens separated by only tens of thousands of years (e.g. every 20,000-80,000 years). If you have only one specimen for hundreds of thousands of years (e.g. every 500,000 years), you can usually determine the order of species, but not the transitions between species. If you have a specimen every million years, you can get the order of genera, but not which species were involved. And so on. These are rough estimates (from Gingerich, 1976, 1980) but should give an idea of the completeness required.
Note that fossils separated by more than about a hundred thousand years cannot show anything about how a species arose. Think about it: there could have been a smooth transition, or the species could have appeared suddenly, but either way, if there aren't enough fossils, we can't tell which way it happened."
From:http://www.talkorigins.org/faqs/faq-transitional/part1a.html#gaps
An interesting example of a species that evolved in the sea then came onto the land then returned to the sea is the whale:
"Call it an unfinished story, but with a plot that's a grabber. It's the tale of an ancient land mammal making its way back to the sea, becoming the forerunner of today's whales. In doing so, it lost its legs, and all of its vital systems became adapted to a marine existence -- the reverse of what happened millions of years previously, when the first animals crawled out of the sea onto land.
Some details remain fuzzy and under investigation. But we know for certain that this back-to-the-water evolution did occur, thanks to a profusion of intermediate fossils that have been uncovered over the past two decades.
In 1978, paleontologist Phil Gingerich discovered a 52-million-year-old skull in Pakistan that resembled fossils of creodonts -- wolf-sized carnivores that lived between 60 and 37 million years ago, in the early Eocene epoch. But the skull also had characteristics in common with the Archaeocetes, the oldest known whales. The new bones, dubbed Pakicetus, proved to have key features that were transitional between terrestrial mammals and the earliest true whales. One of the most interesting was the ear region of the skull. In whales, it is extensively modified for directional hearing underwater. In Pakicetus, the ear region is intermediate between that of terrestrial and fully aquatic animals.
Another, slightly more recent form, called Ambulocetus, was an amphibious animal. Its forelimbs were equipped with fingers and small hooves. The hind feet of Ambulocetus, however, were clearly adapted for swimming. Functional analysis of its skeleton shows that it could get around effectively on land and could swim by pushing back with its hind feet and undulating its tail, as otters do today.
Rhodocetus shows evidence of an increasingly marine lifestyle. Its neck vertebrae are shorter, giving it a less flexible, more stable neck -- an adaptation for swimming also seen in other aquatic animals such as sea cows, and in an extreme form in modern whales. The ear region of its skull is more specialized for underwater hearing. And its legs are disengaged from its pelvis, symbolizing the severance of the connection to land locomotion.
By 40 million years ago, Basilosaurus -- clearly an animal fully adapted to an aquatic environment -- was swimming the ancient seas, propelled by its sturdy flippers and long, flexible body. Yet Basilosaurus still retained small, weak hind legs -- baggage from its evolutionary past -- even though it could not walk on land.
None of these animals is necessarily a direct ancestor of the whales we know today; they may be side branches of the family tree. But the important thing is that each fossil whale shares new, whale-like features with the whales we know today, and in the fossil record, we can observe the gradual accumulation of these aquatic adaptations in the lineage that led to modern whales.
As evolutionary biologist Neil Shubin points out, "In one sense, evolution didn't invent anything new with whales. It was just tinkering with land mammals. It's using the old to make the new."
From:
http://www.pbs.org/wgbh/evolution/library/03/4/l_034_05.html