1. Modern Taxonomy is about identifying clades 

The branching diagram shown to the left is called a phylogenetic tree. Phylogeny is the study of evolutionary history and relationships among individuals or populations.

The species shown in this phylogenetic tree are among the most famous in biology. They’re Galapagos Finches, first identified by Charles Darwin in the early 1800s. The Galapagos are a group of volcanic islands in the Pacific Ocean,  about 1000 km (600 miles) west of Ecuador. It’s thought that a small population of birds from Ecuador colonized the islands, which then evolved over time into over a dozen species.

In a very compact format, phylogenetic trees accomplish two things.

• First, they tell the evolutionary history of a group of species. For example, a quick glance at the phylogenetic tree for the Galapagos Finches shows how one species (A) is the common ancestor of the entire group. A speciation event caused “A” to split into two major lineages or lines of descent. One lineage led to the Green Warbler finch. The other led to all the species that are descended from “B.”
• Second, phylogenetic trees show us the kind of nested, hierarchical groupings of species that we looked at in the last tutorial.

Identifying groups with a common ancestor is one of the most important moves in modern taxonomy. A clade is a group of organisms that includes a common ancestor and all of its descendants. A clade can be a group of species, a single species, or a population within a species.

Clades can be nested within clades. Knowing that, let’s do some analysis of this phylogenetic tree. Answer the question below.

To help you understand how clades work, I’ve redrawn the diagram below, and added shading so that the clades are easier to identify.

The clade that’s descended from species “A” is numbered as clade “1.” Because “A” is the common ancestor of all of the species shown in the diagram, clade 1 consists of all of the six species shown on the right, plus their common ancestor on the far left.

For any clade that consists of more than one species, the common ancestors are located at nodes. The nodes are labeled “A” through “E.” Because nodes are where speciation events occur, they’re also called branch points.

A speciation event at node “A” created two clades, “2” and “3.” A second speciation event at node “B” produced two more clades (“4” and “5.”). Another speciation event at “C” produced clades “6” and “7.” Speciations at nodes “D” and “E” produced additional clades, each consisting of a single species.

A big part of taxonomy involves identifying the clades that an organism belongs to. For example, the large ground finch is a member of 5 clades

• Its own species (the large ground finch)
• Clade 7
• Clade 5
• Clade 3, and
• Clade 1

Note that some clades are very specific, consisting of just one species. Others are more general. In the preceding example, the large ground finch clade is the most specific: it contains just one species. Clade 7 is slightly more general: it contains two species. Clade 3 contains 5 species.

Let’s connect this to the hierarchical classification categories developed by Linnaeus. If classification is done correctly, then each taxon, within each category, should be a clade. That’s what we see in the example to the left.

All of the foxes within the genus Vulpes are a clade. That clade excludes other members of the dog family (the Canidae) such as wolves. Canidae is a clade that contains all members of the dog family but excludes other carnivores. Similarly, the class Mammalia is a clade. All mammals, from kangaroos to whales, share a common ancestor. Other non-mammalian Chordates (such as fish) are not part of the clade.

Below, we’ll learn about how biologists identify clades. But before that, let’s consolidate what you read with some flashcards and a quiz.

2. Phylogeny and Phylogenetic Trees: Checking Understanding

2a. Flashcards

2b. Quiz

Links

1. Continue to Clades and Phylogenetic Trees, Part  2 (the next tutorial in this module)
2. Classification and Phylogenetic Trees (HS Level) Main Menu