The Classification, Genetics & Evolution of Organisms

Instructor: Meredith Mikell
Understanding the reason for the high biodiversity of organisms on Earth requires a solid grasp of genetics, evolution, and the system by which we classify them. Here we will explore these concepts, and finish with a brief quiz.

So Much Life

So far, there is one very distinct characteristic of planet Earth that separates it from all other known planets: it is covered in life! Not just a lot of individual living things, but also an extremely high biodiversity of living things. There are currently around 8.7 million known species on Earth, a number that changes as species go extinct, as new species are discovered, and with improvements in our methods of differentiating between organisms.


We humans find it very useful to categorize things. It helps us to form a cognitive framework of understanding complexity, even if that framework is imperfect. We are not always 100% correct in our assessments of how to categorize, but in the world of science, it is an ever-improving process. The field of taxonomy, or classification of living things, seeks to best categorize the vast biodiversity of life on Earth, both existing and extinct.

All life is categorized by the following scale, from the most broad category to the most specific (humans are used as an example here):

Domain - Eukarya, Archaea, and Bacteria are the three cell types that define all life. This is the most broad category. Humans are within Eukarya.

Kingdom - Animal, Plant, Fungi, and Protist are all examples of kingdoms within the Eukarya domain. Humans are members of the Animal kingdom.

Phylum - Chordata, for example, is the phylum within the Animal Kingdom to which we belong.

Class - Mammalia is the class in which humans belong, which also includes all other mammals.

Order - Next you'll find us in Primates, which includes other great apes and monkeys.

Family - Then Hominidae, which includes humans and great apes.

Genus - Our genus is Homo, which currently includes only humans, but used to include other species.

Species - And finally our species is Sapiens, which only includes humans. Species is the most specific category.

The classification system.

Living organisms can generally be classified based by two main parameters: their DNA and their reproductive behavior. These were not always the criteria for classification, however. Early biologists and taxonomists initially classified based simply upon the physical appearance of an organism: those who looked alike were considered to be the same species. There are obvious problems with this method; take, for example, peacocks. Males are brilliantly colored and decorated, while females are brown and comparatively dull. This distinction is known as sexual dimorphism and is very common in the animal kingdom. Based on looks alone, one would classify a male peacock as a different species than the female, yet they are simply different sexes.

Male and female peacocks are a great example of why classification of species should not be based on looks alone.

Likewise, there are many organisms who may look the same, yet are not closely related to one another. A walking stick is a type of insect camouflaged to look exactly like a twig. There is a species of grasshopper that looks exactly like the walking stick insect, yet we know them to be very genetically distinct.

So if we can't go by looks alone, on what basis do we classify?


It wasn't until the early 20th century that we had any real idea of what DNA is, much less a gene, much less the significance of genetics in classification. We now understand that DNA is the blueprint molecule containing an organism's genetic code. The field of genetics involves the study of genes, segments of DNA that code for specific traits, behaviors, or other characteristics of organisms.

We know that genes are inherited, which means they're passed along from parent to offspring. We also know that genes don't necessarily manifest themselves even when present. With respect to classification, we know that members of the same species have nearly identical DNA, even if there is subtle variety in the genes; for example, genes for different hair color mean that humans could have brown, black, blonde, or red hair, yet are all still members of the same species. The larger the difference in DNA, the less closely-related two different species are from one another. By studying genetics and DNA, we can get the best idea of how species are related to each other, and therefore how to classify them. The genetic code of a species is not fixed, however. It changes over time! This is why we also need to factor in evolution.


Simply stated, evolution is genetic change over time. Not all individuals are well suited for different conditions in their environments. Some are inherently going to be better at surviving and reproducing than others because they have superior traits, which are coded for by their genes.

Lets go back to the walking stick insect and the walking stick-like grasshopper, for example. This likeness, even though they are genetically distinct species, is an evolutionary behavior called mimicry in which one species appears to mimic another for a survival advantage.

A walking stick insect.
walking stick insect

A walking stick grasshopper.
walking stick grasshopper

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