"Let’s make a drawing of this idea. First, the idea of gradual constant change
(*) is shown schematically in the following figure:
The original ancestor species is represented by individuals of red color in the above figure. Each individual gives birth to another individual, represented by the next little black line under the colored strip. Each descendant differs from its parent (in color in this figure, but in physical traits in reality), but the difference is so tiny that we cannot notice it merely by looking at the parent and the child. However, looking at a sufficiently distant descendant (green color) over a long period of time we can see that there is a definite difference (red vs. green) between original ancestor and distant descendant.
But, as I said, this view is most likely wrong. The following figure seems to be a more accurate representation of reality:
The difference in the above figure is that the change is relatively abrupt. The word “relatively” in this context is important. The individuals shown in orange, yellow, and lime in the figure (labeled as “individuals making up transitional species”) might have lived for several thousand years. However, the entire colored strip from left (red) to right (green) is supposed to have lasted for
tens of millions of years, or more. Thus, the transitional species are represented by a very narrow piece of time on the strip. Therein lies the first reason of the fossil record appearing as having gaps. Because, suppose that you can take a few random snapshots within this time interval, represented by the black vertical lines on the following strip (assume each black line is a fossil):
As you see, the snapshots (fossils), being random and sparse, “missed” the transitional species (orange, yellow, and lime) entirely. In reality, the reader should imagine a much narrower interval of time for transitional species than the one shown above, and a sparser set of fossil snapshots.
(*) then
the transitional period would be only five thousandths of the entire length; it would take a width of only
about two pixels to represent it in the above figure. Imagine having to “hit” that kind of interval in your sample of fossils. It is in this sense that we say that species evolve to other species “abruptly”: not literally abruptly (not from parent to child, of course, as Lamarck thought), but so fast that when seen from a faraway perspective the transitional period almost vanishes, and the change appears as if it is abrupt."
According to Richard Cowen (in
History of Life) the majority of animal species living now are unlikely ever to leave fossils, since they are soft-bodied invertebrates such as worms and slugs. Consider that of the more than 30
phyla of living animals, two-thirds of these have never been found as fossils! For all the above reasons, transition from “red” to “green” might happen, as in the previous figures, but if none of the “yellow”, etc., transitional individuals fossilized, then the change went undocumented in the fossil record. No matter how hard we dig, a transitional individual will not be unearthed, because none is there to be found. In that case it is not the paleontologist’s ax, but nature itself that “missed” recording the transition...
(After looking at Fish->Amphibian fossils in the link)
Putting now the above-mentioned fossils onto the colored strip that was shown earlier, we obtain roughly the following diagram:
So we see that in the case of the fish-to-amphibian transition we were lucky enough to have found just the right kind of fossils that document precisely that transition. Naturally, the search is not over, so the collection keeps being enriched, decade after decade.
http://www.foundalis.com/bio/evo/evolution_and_gaps_in_fossil_record.htm