On "transitional forms"

The strongest evidence these day that Darwin's theory of evolution by natural selection is an inescapable reality comes from molecular biology and genetics, not paleontology. However, creationists are fond of criticizing evolution for lacking what they call "transitional forms". I say it's what "they" call it, because "transitional form" isn't a term you will hear evolutionary biologists using, unless perhaps they are responding to creationists and trying to help them understand by using the same language.

If we want to trace a species' evolutionary lineage, there are many intermediate species. For example, if we want to trace the evolution of the modern whale from its land-dwelling relative Pakicetus which lived 50 million years ago, there are many, many species filling the gap between modern whales and Pakicetids – creatures that we can see gradually developing features (or losing them) to adapt to aquatic life.

But in my conversations with creationists, I get the sense that when they say "transitional form", they're talking about something entirely different. I could be wrong in my characterization here, but this is the impression I get: It seems like creationists imagine a species – say, a wooly mammoth; one day, a wooly mammoth gives birth to a mutant freak of a thing, some sort of half-mammoth, half-elephant monstrosity, and that thing somehow reproduces and presto, we have elephants. But since there are no fossils of half-wooly mammoth/half-elephant freaks, evolution must be wrong. Ray Comfort and Kirk Cameron's infamous "Crockoduck" was what they imagined as a "transitional form" – a half duck, half crocodile freak of nature that would somehow bridge an evolutionary gap between crocodiles and ducks.

So how does evolution actually work? Imagine you have a population of a species, say population A. Over time, this population becomes split into two geographically isolated populations, so now you have two populations: A and B. Populations A and B are living in different environments; there are different foods, different quantities of food, different means to acquire food, different weather conditions, and different predators. In each population, natural selection means that the individuals whose features are best adapted to the environment are the ones that will survive and reproduce. Over very long periods of time, very small changes are selected through each population as it adapts to changing environmental pressures. These small changes add up, becoming cumulative, and eventually populations A and B are barely recognizable as the homogeneous population they once were; the only traces they leave are vestigial genes and structures that give clues to their evolutionary heritage.

Intermediate forms and cetacean evolution

"Transitional form" is a kind of nonsensical concept, because evolution holds that all species change over time in adaptation to their environment. In essence, it's correct to say that all species ever are "transitional". We can, however trace the lineage of two species – say, between modern whales and Pakicetus – and find many intermediate species that start off looking a lot like Pakicetus and end up looking more and more like modern whales, yet still retaining features of both distant species.

It's important to remember that evolution proceeds according to an ordered hierarchy, which is illustrated by the phylogenetic tree:

Evolution has certain hierarchical limitations;  Pakicetids could not evolve into a fish – they could only evolve into an aquatic mammal like a dolphin or whale. Fish are evolving along a separate branche of the phylogenetic tree. This is why the "crockoduck" is a ludicrous criticism of evolution. Crocodiles and ducks are both modern animals. Neither is a descendant of the other. Like all life, they do share a common ancestor; but in this case, they both branched off long, long ago – pretty far down on the phylogenetic tree.

So, let's look at the evolution of whales. When we look at modern whales, we find a number of reasons to think that they evolved from land-dwelling mammals. For one, they're... mammals. They don't have gills and can't breathe under water; they have to come up to the surface for air every so often. The bones in their flippers are the same as the forelimbs in land-dwelling mammals, including human hands. Even their spines move vertically, like that of humans, rather than horizontally like fish. Further, they have vestigial structures – such as the vestigial pelvis – and vestigial genes, including the genes for making legs.

So we find the fossilized remains of Pakicetids. I'll quote the almighty Wikipedia:
They have been linked to whales by their ears: the structure of the auditory bulla is formed from the ectotympanic bone only. The shape of the ear region in Pakicetus is highly unusual and only resembles the skulls of whales. The feature is diagnostic for cetaceans and is found in no other species
In other words, we find these peculiar structures in this species that it shares with whales, but not, say, leopards or humans. These animals are thought to have evaded predators by hiding underwater. Thus, those that could stay underwater longer would be more likely to survive and reproduce.

The intermediate species we find bear increasing resemblance to modern whales while retaining features of their Pakicetus relatives. Ambulocetus, for example, has shorter and more flipper-like limbs, and was amphibious. The availability of food in water, as well as the ability to avoid predators, would mean that the Ambulocetids that were able to stay under water longer would survive and reproduce. Gradually, natural selection favored certain traits – slightly more flipper-like limbs, better swimming ability, and the ability to stay under water longer.

To give you an idea of just how slowly evolution takes place, take a look at Ambulocetus on the left and compare it to a slightly more recent relative, Rodhocetus on the right:

These animals are not that different; Rodhocetus has slightly shorter and more flipper-like limbs, and the nostril is located higher on the head. In this drawing, at least, it has more blubber as well. Based on the available food and predators, natural selection favored small changes that were better adapted to more time in the water. See the pattern yet? Very, very small novel changes add up, becoming cumulative. The time between these very similar animals? Roughly 4 million years. Evolution moves very, very slowly.

Evolve this

I'm not going to bother detailing every single intermediate species between Pakicetus and whales; there are numerous online references detailing the evolution of cetaceans, not to mention plenty of good old-fashioned books. But we would find something very specific: we would find that over the 50 million or so years that Pakicetids evolved into modern whales, the animals would start out looking very much like Pakicetids. Over tens of millions of years, each species along the way looks ever so slightly less like Pakicetids and more like whales – limbs become more flipper-like, the hind legs become shorter and less functional until they disappear altogether, nostrils move higher up the head, the ear becomes better adapted for hearing underwater, etc. etc.

At no point does one species give birth to some half-whale monstrosity. Very small changes are favored by natural selection and become cumulative over geological time scales. Along the way, each population of species is fully functional and fully adapted to the environment. Individuals who aren't quite as well adapted die, while the ones that are adapted reproduce, passing on those very minuscule changes from one generation to the next. Evolution does not occur in a single bound; like a long journey, it's a series of innumerable small steps that alone may seem insignificant, but over millions of years add up to produce the astounding changes we observe.


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