Evolutionary Relationships

Humans have classified and organized biological organisms for thousands of years. Originally, primarily ordering objects necessary for survival. As human history progressed, so did the skill and detail in these classifications. In the fourth century B.C., Aristotle pioneered formal classifications, delineating plants and animals into different groups and then dividing these further based on their physical characteristics and traits like the habitats that they occupy. Later, in the mid 1700s, Linnaeus built on Aristotle's system. He called his highest level of grouping the kingdoms and, from there, divided the groups using synapomorphies, a defining physical feature which splits a branch. For example, if an animal possesses a backbone or a similar structure, it should be placed in the phylum chordata. If it doesn't, then there are many other phyla, which animals without backbones can be split into, including the arthropoda, a large group including insects. Linneaus continued splitting groups of organisms based on their synapomorphies at subsequent levels through the class, order, family, and genus, until reaching the final designation, generally, the species. We refer to Linnaeus' type of classification as cladistics, the classification of organisms based on differences in physical characteristics.

Today, scientists commonly construct trees called dendrograms to give visual representations of these splits and groups. This particular form of dendrogram, the cladogram, visualizes the cladistic relationships between the species so that the tips of the tree represent the species and the branches show how they're related to one another. For example, here the chimpanzee and bear are more closely related to one another and share more common characteristics than either of them do with the sunfish. The places where the branches meet are referred to as nodes and denote common ancestors for the species that follow. A second major dendrogram type is the phylogram. These are different from cladograms because the length of the branches between species varies, representing the degree of change between them. So the longer the branch, the more time has passed since the species diverged from their last common ancestor.

Dendrograms were constructed by simply analyzing the morphology of organisms. With the advent of modern technology, comparing DNA has also become a common way to build trees. DNA is made up of nucleotides associated with one of four different bases. Adenine, guanine, cytosine, or thymine. The order of these bases is the DNA code. This code is passed from parent to offspring. Consequently, if you look across a single species like humans, there is a very high degree of similarity in our genetic code, around 99.9%. We also share some of our DNA code with other species, like chimpanzees and mice, but the degree of overall similarity between our DNA and theirs is vastly different. This means that we can create trees, which group species based on the similarities or difference between their genetic codes. This field of analysis, combining statistics, mathematical modeling, and computer science, is referred to as bioinformatics. To compare DNA sequences, researchers often use a bioinformatics tool called the Basic Local Alignment Search Tool, or BLAST, which was created, and is maintained by, the National Center for Biotechnology Information.

In this laboratory, you will first create a cladogram of animals using morphological information, and then place a fossil species onto this cladogram based on its morphology. You will then use DNA sequences from several different modern-day relatives of the fossil and the BLAST database to verify your positioning of the fossil onto the tree.