A common criticism of evolutionary theory is the apparent lack of transitional fossils throughout the fossil record. If evolution were true, argue opponents, why do we not find more fossils that "link" different species together? Isn't evolution a dubious theory until we find more of these "missing links"?
While scientists have identified the fossils for over 250,000 different species, we must realize that this is but a tiny fraction of the number of species that have existed on earth (less than 1% in fact). The fossil record, after all, is based on fossils, and fossilization is a rather rare occurrence. When they die, organisms tend to decay rather than petrify. Only when an organism dies in the right place at the right time can fossilization even have a chance of happening. For certain species -- such as sharks whose skeletons are cartilage rather than bone -- the chance of fossilization becomes slimmer still.
Furthermore, when we do find fossils, they are statistically far more likely to come from large, unchanging populations of creatures. In contrast, transitional species are far more likely to occur in smaller populations. Thus, the rarity of fossilization, compounded with the smaller populations of transitional species, means finding transitional fossils on a regular basis is highly unlikely.
In his book Coming to Peace With Science, Darrel Falk compares our current fossil record to a collection of 250,000 random aerial snapshots of America. While these pictures may offer some interesting information, they certainly would not provide a complete picture of the country. There would be only a 1 in 4 chance that even one photo contained the image of a person. The chances of capturing a major landmark -- like the Capitol or White House -- would be smaller still. Similarly, the chances that the fossil record would consistently find transitional fossils are not very high.
Regardless, the fossil record has provided us with glimpses into our evolutionary past through the few transitional species it has found. While it certainly does not provide a complete picture, combined with other findings such as genetic evidence, the fossil record makes a strong case for the validity of evolutionary theory.