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The Evolutionary Relationships of Organisms

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

In this lesson, we will discuss what evolutionary relationships mean, how we describe them, and how we determine them using morphological and molecular data. We will learn how the molecular clock determines our relationships by tracing random mutations.

The Evolutionary Timescale

What kind of clock tells time, not in seconds, but in millions of years?

A molecular clock!

A molecular clock tells time on an evolutionary timescale. Changes in the molecular sequences of our DNA, mutations, may be random, but they take place at a relatively predictable rate. Thus, we can estimate how long ago two species branched off from each other by counting how many differences there are in their gene sequences.

How to Determine Relatedness

When we sketch out relatedness between organisms, we describe the result as a phylogenetic tree.

Phylogenetic tree
Phylogenetic tree

How do we determine a phylogenetic tree? One good example of using molecular data to construct a phylogenetic tree is with cytochrome c. Cytochrome c is used in cellular respiration, so it is present in many diverse organisms, from humans down to many single-celled organisms. You would expect that the more closely related an organism is to humans, the more their cytochrome c gene would look alike. Indeed, this is the case. Chimpanzees and humans have exactly the same sequence for cytochrome c. There is one difference between the cytochrome c gene of rhesus monkeys and that of humans. There are over 10 differences between the cytochrome c gene of a pig or a duck and that of a human; over 30 for a moth; and 44 differences between that of a yeast and a human.

You and your siblings share common ancestors - your parents. You and your cousins share common ancestors - your grandparents. If you take it back far enough, all life on earth at one time shared a common ancestor. We call it the Last Universal Common Ancestor, often abbreviated LUCA, and we don't know much about it. We used to think that LUCA was likely very simple, with no internal structures. Recently, scientists have realized that even the simplest cells do have at least one organelle (organ-like structure), a storage site for complex phosphate molecules.

Scientists can now determine relatedness by comparing the frequency of nucleic acid bases (the G + C content), perform nucleic acid hybridization to determine the degree to which DNA sequences are the same, perform nucleic acid sequencing, or perform other advanced molecular techniques. But in the past, the only way we had to determine relatedness between organisms was morphology, or the observable structure of the organisms. Very often molecular techniques will give us the same results as morphological techniques, but occasionally there are surprises.

The Linnaean System

One of the most important advancements in classifying living things came from an eighteenth-century gardener. Swedish scientist Carolus Linnaeus developed a system of classifying living things based on their shared similarities. Linnaeus was a medical doctor and professor whose true love was gardening. He loved naming plants, and his greatest contribution was to develop a system by which we can discuss the interrelatedness of plants and other organisms.

Though refined techniques, such as genetic analysis, have shown some of Linnaeus's classifications to be incorrect, we still use a system of classification that is very similar to his. In order from the largest to the smallest group of organisms are: Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species. (Domain was added much later, in 1977.) It is important to note that these classifications, while useful, are categories designed by humans, not categories that exist in nature. We can use biological techniques to show relatedness, but there is no test to 'prove' whether two organisms are in the same family or the same order. Occasionally, systematicists (scientists that study the classification of organisms) find it useful to use other terms, like tribe (a category between family and genus) or superfamily (a category between order and family).

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