While there seems to be lots of evidence that Darwin's theory of evolution by natural selection occurs on the scale of microbes, how much evidence is there that it occurs on a "macro" scale, meaning evolution of species? From the initial publication of Darwin's On the Origin of Species in 1859 until today, those who have been skeptical of Darwin say there isn't evidence of macro-evolution, meaning on the scale of species; and in particular, there hasn't been evidence of "transitional species."
What is meant by "transitional"? As an example, the theory suggests that at some point, life forms made a transition between the seas and land. What evidence, then, is there for such transitional species? Opponents of Darwin like to point out that he, himself, acknowledged that if evidence of such transitional species couldn't be found, the theory would fall apart. The answer is, yes, there is evidence, just not a lot. The interesting thing is, biologists and other scientists say two things: 1) there is evidence of transitional species; and 2) it should not be surprising there is very little evidence. Let's consider why this is the case.
Most anyone who has ever been to a science museum has seen fossils. Scientists over past 200 years have uncovered and catalogued at least 250,000 different species of fossils. That sounds like a lot, until you consider that just today, there are at least ten million different species alive on Earth. That doesn't take into consideration all of the species that have gone extinct over time. Darrell Falk, a well respected biologist as well as committed evangelical Christian, estimates that only one percent of all species that have ever lived on Earth have fossil remains.
An obvious question to ask is, given all of the research that has been done, why are there only fossil remains for about one percent of all species? Falk, in his excellent book called Coming to Peace with Science notes the following: "As wonderful as fossils are for exploring the history of life, fossilization is an exceedingly unlikely event. Only a minuscule fraction of organisms happened to be at the right place at the right time to be preserved in perpetuity. Usually when an organism dies, it decays, leaving hard parts that are eventually subject to microbial decomposition, scavenging or chemical destruction. In order for it to be converted into fossil form, most often the body has to be suddenly buried in a rapid accumulation of sediment. The overwhelming majority of organisms, especially land organisms, would never be in the right time and the right place to be preserved forever in this manner."
One percent is a small percentage, but in certain cases, that's actually a pretty large percentage. Take polling of voters in an election. For example, the USA has about 200 million people eligible to vote in a Presidential election. Organizations such as Gallup routinely sample about one or two thousand likely voters and are able to predict the result with plus or minus three percentage points. A sample of 2,000 out of 200 million is far, far less than one percent, so what's the problem? The answer is that those conducting the voter poll know the entire population, so they can select a representative sample of voters to poll. Paleontologists can't do that with fossils because they don't have any data on the missing 99% of species that have never been fossilized.
Darrell Falk offers another very interesting analogy. Imagine one wants to get an idea of what life in the USA is like. In order to do this, imagine that someone launched a drone with a camera. The drone can take as many pictures as it wants, with two conditions: 1) each picture can be of an area of only 1,000 square feet (that's a square with a length of about 32 feet (10 meters) on each side; and 2) the pictures need to be taken randomly. Think of each 1,000 square foot picture as analogous to a fossil.
What kind of results would the drone's camera record? For the drone to record everything in one square mile (approximately 2.6 square kilometers), it would have to take 27,878 pictures, each being of a 1,000 square foot area. That's just one square mile! The USA's total land mass is 3.797 million square miles! So how many pictures would the drone have to take of the country to get a representative idea of what life might be like in the USA? Falk notes that if the drone took 250,000 random pictures, there would only be a 25% chance that at least one of the pictures actually had a human in it! Based upon my calculations, if the drone took one billion random pictures, it would only cover about one percent of the land mass of the country.
But there's even more to it than that. Falk notes that transitional species are most likely to occur in tiny populations. To demonstrate this idea, he envisions two populations of the same species of bird. On the mainland there are 100,000 birds while on the island, there are only 100 of the birds. Now envision that on the island, a genetic mutation occurs that creates a bird with a much longer beak than usual. That means one out of one hundred birds will have this characteristic. Now envision that the mutation makes the bird better able to hunt food. The bird has a greater probability of surviving and reproducing. Now, also envision another genetic change occurs that causes females of the species to prefer males with longer beaks. Over time, a greater and greater percentage of the population will have long beaks. Given that the population of birds on the island was small to begin, the birds will become inbred. Dog breeders are certainly familiar with this. Most likely, even after a relatively short period of time, virtually all of the bird on the island will possess long beaks.
On the mainland, however, if the same genetic mutations occurred, it would spread far more slowly in the population of 100,000 birds. The more likely place for the mutation to spread is in the small, isolated population. Falk continues: "But how does that relate to the scarcity of transitional forms in the fossil record? The main point is that where species change their characteristics, they tend to do so in small, isolated populations. And remember that fossilization is an exceedingly unlikely event. Thus, when fossilization does happen, there is a much higher probability that it will happen somewhere in the larger landmass." Moreover, Falk notes that, "Similarly, if the transitional forms are present for only a relatively brief period of time as compared to the larger stable populations, then this further lowers the probability of being able to "catch" a transitional form in the fossil record … This is not surprising; it is exactly what geneticists would predict."
And yet, despite the low probability of finding transitional fossils, they have in fact been found. An excellent example, called Tiktaalik, is cited in my book, The Unexpected Perspective. Tiktaalik (see an artist's representation above) was discovered near Ellesmere Island in northern Canada. It's name is an Inuit word for "big freshwater fish." The Tiktaalik was about the size of a crocodile, with scales like a fish and fins like limbs and an elbow joint that could push the animal off the ground. One of the members of the discovery team said, "It's like a fish that can do push ups." Tiktaalik most likely lived in the Devonian period near the Equator, in an environment similar to the Mississippi delta. After 400 million years of continental drift, the fossils ended up in the Canadian north. Darrell Falk cites other examples of transitional fossils in Coming to Peace with Science.
While comparatively few transitional fossils have been found, it is important to note that they have been found, thus providing additional evidence to back up Darwin, evidence of evolution by natural selection on the scale of species.