Powerpoint: Feb 11

“Chaos is reality, order is illusion” – what every taxonomist knows, but hates to admit.

Learning objectives

• Use binomial nomenclature to identify organisms
• List the kingdom, phylum (sub-phylum), class, order, family, genus and species
• Identify the characteristics of a good classification system
• Identify the criteria (evolutionary relationships) that are used to classify organisms
• Explain why we use these criteria vs. others, in other words, why don’t we use criteria like morphology and habitat?

Taxonomy is the branch of science concerned with the description, identification, naming and classification of organisms.

The purpose of taxonomy is to produce a system that helps scientists and people to better organize, understand and make sense of the living world. Having such a system also allows scientists to have a common language to communicate with each other. In order to produce such a system, the ideal system must have a few characteristics:

1. Specific – the system should be as specific as possible. For example, talking to another scientist about Canis familiaris is far more specific than saying “that adorable, furry, four legged animal smaller than a wolf that lives with humans”.
2. Clear and objective as possible – the criteria for grouping organisms should be based on criteria that are as objective and clear-cut as possible. For example, we may have a group for flying animals and furry land animals, both of which a bat might fall into. Or, we may have a group for “worm-like” creatures and “insects”, which a caterpillar and butterfly would fall into, even though these are of the same species.
3. Descriptive – a good classification system should also be descriptive. Based on what group the organism falls into, I should have a good idea the “type” of organism I am thinking of. For example, if we are talking about “flying” creatures, I may immediately envision wings and feathers.

Early efforts to classify tried to incorporate these three characteristics by describing the organisms in as much detail as possible. For example, the Hoary plantain (Plantago media) was named:

“Plantain with pubescent ovate-lanceolate leaves, a cylindric spike and a terete scape”

And this was not the longest of names; some names reached twenty Latin words or more! You can imagine just how cumbersome it was to have organisms with such lengthy names. Not only that, with new organisms being discovered all the time, organism names will have to be even more detailed to differentiate them!

Carolus Linnaeus (1707-78), a Swedish taxonomist, knew right away that this system was definitely not going to work at this rate. To circumvent the impending problems, he invented the system we use today, known as binomial nomenclature and taxa.

Instead of giving every organism a very lengthy descriptive name, each organism was grouped into seven hierarchial groups:

Kingdom

Phylum (sub-phylum)

Class

Order

Family

Genus

Species

The organism’s names were reduced to two words: genus and species. According to convention, the groups are always italicized, with the genus capitalized, followed by the species name:

Homo sapiens

This new system is much less cumbersome. It is specific since all organisms have a unique name and grouping. It is also descriptive, since each of the levels says something about the organism’s characteristics. For example, the cat:

Kingdom Animalia – multicellular, mobile organisms

Phlyum Chordata – has a hollow nerve cord on the back (dorsal side) of the animal

Class Mammalia – has fur and mammary glands and gives birth to live young

Order Carnivora – organisms that mainly feed on meat

Family Felidae – cat like creatures, including lions, tigers, panthers, etc.

Genus Felis – small cat like creatures, including wild cats and domesticated cats

Species Catus – family cat

Finally, the system is made as clear and objective as possible by using evolutionary relationships as the basis for classification. Although during Linnaeus’ time, the theory of evolution was not to come for another 100 years, Linnaeus was able to infer that some organisms were more related than others. For example, even though a butterfly and a bird both have wings, the two organisms are clearly different.

Today, we infer evolutionary relationships based on three factors:

1. Homologous structures – “parts of different organisms, often quite dissimilar that developed from the same ancestral parts”. Homologous structures are a product of adaptive radiation, or divergent evolution. An example of this is the forearm of bats, birds, reptiles and mammals. The bones in the arm are derived from a common ancestor.
   1. Note: it can be tricky to differentiate homologous structures from analogous structures, which are products of convergent evolution. For example, the fins of a shark and dolphin
2. Biochemical relationships – are chemicals that are vital to living organisms. Many of the chemical substances, the ones that are not ingested are produced from the DNA. Therefore, a difference in these chemicals suggests a difference in the DNA of the organism. And since organisms that are more distantly related tend to have DNA codes that are more different (from mutations that have accumulated over time), we can infer how closely related organisms are based on the differences in the chemicals.
3. Embryological Relationships – the same applies to embryological relationships. Just like similarities and differences can be accumulated over time, so can differences in embryological development. In the early stages of fish, turtles, chick, pigs, mouse and human development, the embryos look very similar which suggests that similar genes are being expressed. But as more genes are being expressed, more and more differences begin to surface. This suggests that all these organisms once shared a common ancestor.

Tree of life explorer (find your closest common ancestor to an organism here!): https://www.evogeneao.com/explore/tree-of-life-explorer