Phylogenetic taxonomies – Evidence of Common Descent

The theory of evolution predicts that all living things are related. You may have seen a diagram like this before:

Image

It is called a phylogenetic tree and it is similar to a family tree in that it shows how individuals are related to other individuals. One main difference is that family trees deal with individuals and phylogenetic trees deal with the level of relatedness between entire species.

darwinNotebook

The most famous phylogenetic tree is this one by Charles Darwin when he write about his famous theory natural selection. He postulated that different organisms could be related to each other and that their differences could be due to minor changes over many generations.

Although seemingly simple, phylogenetic trees produced from various sources constitute powerful evidence of common descent, the idea that groups of creatures share a common ancestor, and in fact, that all living creatures descend from a single common ancestor.

Even 130 years before Darwin’s seminal work, a man named Carl Linnaeus wrote a book called Systema Naturae where he set forth a taxonomy of known biological organisms on the basis of shared characteristics. Until this day, scientists give all living things two-part Linnean names which we know as “Latin names” eg. Homo sapiens, Canis domesticus.

Linnaeus based his system on the structural and anatomical similarities of the different organisms which he examined.

Neither Linnaeus nor Darwin has any knowledge about DNA, the genetic code and nucleaotides, yet, based on anatomy alone, they both came up with taxonomic trees which match modern phylogenetic trees surprisingly closely.

25-11-Cladogram-AL

Sometimes though, surface traits may not be enough to determine the inter-relatedness of a group of animals.

taxo_wrong

If we looked at several superficial traits of these 6 animals, we would come up with an unusual famly tree. Bats and birds can fly, sharks and dolphins can swim, but neither pairs of hose animals are closely related contrary to what you may think. Even the Bible in Leviticus 11:13-19 classifies bats in a group of animals which is often translated as “birds” (scientific classification was not a concern for the biblical writers, classification by superficial traits was much more important). So how do we know that the phlygenetic tree of those 6 animals looks more like the one below?

taxo_right

Well, we need to look at the underlying characteristics of those animals. If we look in this way, we will see that actually, dolphins share much more in common with mammals than with fish. Dolphins belong to a cascading group of animals: cetaceans (dolphins and whales), cetartiodactyla (dolphins, whales, pigs, hippos, giraffes, sheep and cattle), mammals (including humans, dogs, cats, rodents, etc) and synapsids (which include all mammals, and a group of extinct animals often called mammal-like reptiles, which form a fossil bridge between mammals and reptiles).

The artiodactyla have even numbers of toes and are ungulates (meaning hoofed animals). Their weight is borne about equally by the third and fourth toes, rather than mostly or entirely by the third as in odd-toed ungulates (perissodactyls), such as horses. A further distinguishing feature of the group is the shape of the astragalus (talus), a bone in the ankle joint, which has a double-pulley structure. This gives the foot greater flexibility. The DNA of the artiodactyla was recently sequences, showing that they have a close relation to the cetaceans (the dolphins and whales, which do not have hooves at all) to make a new combined clade called the cetartiodactyla.

Mammals have sweat glands, including those which became specialised to produce milk to feed their young, they have hair (dolphins have a few bristle-like hairs around the tip of their rostrum) they have a unique jaw joint, a unique middle ear arrangement (malleus, incus, stapes), they undergo tooth replacement and most specifically, the have prismatic enamel and occipital condyles… look them up.

Finally and most tellingly, sharks have gills, but dolphins have lungs, and have to come to the surface to breathe, and dolphins swim with an up-down movement and their tails are on the horizontal axis because they evolved from land mammals, whereas all other fish swim with a side-to-side movement and have tails on the vertical axis.

delfin

The synapsids have a temporal fenestra, which is an opening low in the skull roof behind each eye, which leaves a bony arch beneath each.

All these fetures tell us that although bats can fly and dolphins can swim, they are clearly not birds or fish respectively, they are mammals, and specific type of mammals at that. The “family tree” of the artiodactyla can be formulated using the collective data from anatomy, biogeography and genetics.

Cladogram_of_Cetacea_within_Artiodactyla

Cladogram of Cetacea within Artiodactyla

We are now in the genetic age where hundreds of animals’ genomes have been decoded and transcribed and we can see, nucleotide by nucleotide, which organisms have the most in common with each other.

The evidence of common ancestry is now so overwhelming, that only the hard-headed and unreasonable are still able to deny that we all share a common ancestry.

Phylogeny is much more complicated even that this. If you would like to know more, go to Kennesawn State University’s page on phylogeny, or check the other references which follow:

http://science.kennesaw.edu/~jdirnber/Bio2108/Lecture/LecPhylogeny/LecPhylogeny.html

http://evolution.berkeley.edu/evosite/evo101/IIBPhylogenies.shtml

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