DNA barcoding used to test food

DNA barcoding used to test food (NTNU Vitenskapsmuseet [CC BY], Wikimedia Commons)

The Classification of Life: From Linnaeus to DNA Barcoding

Krysta Levac

Summary

Learn about two taxonomy systems that scientists use to classify the life around us.

People have always tried to classify living things. But Earth is home to between 10 million and one trillion different species. That’s a lot of things to identify and name! 

Biologists who classify organisms are called taxonomists. Taxonomy involves three steps. First, you identify a living thing. Then, you name it. Finally, you classify it in relation to other living things. 

For a long time, taxonomy was based on morphology. That means it was all about physical attributes like size, shape, colour and body structure. People have managed to describe about two million species this way. But that means most of the planet’s species still need to be named and classified.

New technologies like DNA barcoding are helping scientists catch up. This is the second big change in taxonomy. The first one happened about 250 years ago. Together, they provide the keys to understanding Earth’s biodiversity

The Birth of Modern Taxonomy

Carl Linnaeus (1707-1778) was a famous Swedish scientist. His work totally changed taxonomy. Scientists still use the Linnaean taxonomic system today.

Title page of Species plantarum
Title page of Species plantarum (1753) by Carl Linnaeus (Carl Linnaeus [Public domain] via Wikimedia Commons).

Biological nomenclature is a fancy way of saying “how you name living things.” Before Linnaeus, people classified organisms using long strings of Latin words. Consider the European honeybee’s full scientific name. It was Apis pubescens, thorace subgriseo, abdomine fusco, pedibus posticus glabis, untrinque margine ciliatus. That doesn’t exactly roll off the tongue!

Linnaeus simplified things by using just two names for each species. One name refers to the genus. The other one refers to the species. This is called the binomial naming system. 

For example, humans are called Homo sapiens. Homo is our genus. Sapiens is our species. This system gave the European honeybee a much simpler scientific name: Apis mellifera

Did you know?

The oldest standard scientific names for plants and animals come from two books by Linnaeus. He published Systema Naturae in 1735 and Species Plantarum in 1753.

Linnaeus borrowed the terms “genus” and “species” from Aristotle. Have you heard of him? He was a Greek philosopher and zoologist who lived about 2 400 years ago. Aristotle put animals with similar attributes into broad groups. He called these groups genera (the plural form of genus). Then, he defined species within each genus. 

Did you know?

Aristotle classified animals based on whether they or not had red blood. This broadly matches the modern categories of vertebrates and invertebrates.

Aristotle's classification of animals
Aristotle's classification of animals (©2019 Let’s Talk Science).

Linnaeus also changed how scientists classify organisms. These changes were especially important for plants. Linnaeus continued to group plants based on shared physical attributes. But he stopped looking at the entire plant. Instead, he focused on the anatomy of the reproductive system. This made plant taxonomy much simpler.

Linnaeus divided plants and animals into broad kingdoms. He then subdivided them into phyla, classes, orders, families, genera and species. Sound familiar? Generations of biologists have followed this system. Scientists still use it today.

The Linnaean classification of an Asian elephant
The Linnaean classification of an Asian elephant (Source: blueringmedia via iStockphoto).

What are the limitations of the Linnaean taxonomic system?

The Linnaean system isn’t perfect. 

To begin with, there’s the problem of damaged or incomplete specimens. Sometimes scientist only have a tiny bit of a plant or animal. That means they can’t identify it based on morphology.

Some species go through different stages of development. They can look very different at each stage. Think of a caterpillar that goes through metamorphosis. The adult butterfly belongs to the same species (Danaus plexippus). But its morphology is completely different.

Monarch caterpillar and adult
Monarch butterfly caterpillar on the left and adult Monarch butterfly on the right (Sources: Pseudopanax [Public domain] via Wikimedia Commons and Richiebits [Public domain] via Wikimedia Commons).

 

Finally, there’s the problem of cryptic species. These are organisms that look very similar but are actually different species. Northern and Southern Leopard frogs are good examples of cryptic species.

Northern Leopard Frog and Southern Leopard Frog
Northern Leopard Frog (Rana pipiens) on the left and Southern Leopard Frog (Rana sphenocephala) on the right (Sources: Douglas Wilhelm Harder [CC BY-SA 3.0] via Wikimedia Commons and USGS [Public domain] via Wikimedia Commons).

 

These problems make it harder to identify species using Linnaean taxonomy. Even expert taxonomists have their limits. There are only so many species one person can identify! 

DNA Barcoding Shakes Things Up

Paul Hebert is a professor at the University of Guelph. In 2003, he introduced DNA barcoding. This technique is based on DNA sequencing. 

DNA barcodes are like Universal Product Codes (UPCs). UPCs use a series of black bars to create unique tags for consumer products. Professor Hebert found a way to create tags based on an organism’s DNA. Each species has a different barcode. These barcodes reflect genetic differences at the molecular level.

DNA is like a recipe book for living things. The recipes are written with molecules of Adenine (A), Cytosine (C), Thymine (T) and Guanine (G). These bases bond with each other to create base pairs. Base pairs form strands of DNA. And these strands create the famous double helix structure of DNA. 

DNA structure showing the parts of DNA strands
DNA structure showing the parts of DNA strands (Let’s Talk Science using an image by ttsz via iStockphoto).

In some genes, the DNA sequence is the same for all members of the same species. But it’s different for members of different species. So a DNA barcode is simply the sequence of molecules in a specific stretch of DNA. It identifies a species at the molecular level.       

DNA barcoding improves on Linnaean taxonomy. It overcomes some pitfalls you face when trying to identify a species based on morphology. DNA is a very tough molecule. You can recover it from damaged, degraded or even incomplete specimens. A species might go through different stages of development. But its genetic code never changes. And cryptic species will always have differences in their DNA. Their barcodes will be different no matter how much they look alike! 

How is DNA Barcoding used?

In 2008, scientists established the International Barcode of Life (iBOL) consortium. It is a research alliance between groups in 25 different countries.

The University of Guelph isn’t just the birthplace of DNA barcoding. It’s also home to the Biodiversity Research Institute of Ontario. This group develops barcoding tools and analyzes specimens. It also manages a public collection of barcodes called the Barcode of Life Database (BOLD). So far, BOLD has collected DNA barcodes for more than 600 000 named species.

DNA barcoding has widespread applications. 

Since 2016, students across Canada have collected fish samples at grocery stores to help identify food fraud

DNA barcoding also supports conservation efforts. For example, it’s being used to find predators of the Colorado potato beetle. This beetle destroys potato, tomato and pepper crops. Scientists are barcoding the stomach contents of different insects. They want to see which ones eat the most potato beetles. The insects can then be used to control the beetle population. 

DNA barcoding can even help catch poachers. It can be used to analyze the remains of an animal. Even a little piece of meat will do. Authorities look to see if the DNA barcode matches a protected animal. If it does, they’ll know that the remains came from a poached animal. 

Linnaean taxonomy and DNA barcoding are both important. They’re more than just stamp collecting for scientists. They give us a better understanding of the diversity of life on Earth. 

Many of Earth’s ecosystems are threatened by climate change, pollution and development. So it is extremely important to know what creatures share the planet with us. We also need to understand how these species are distributed and how they interact. Protecting biodiversity would be a huge victory for science. It would certainly make Linnaeus proud!

 

Learn More

Public Data Portal BOLD Systems -- The public data portal of the Barcode of Life Database (BOLD) system that allows people to look up and find information about any of the species that are catalogues within the system.

Why is biodiversity so important? (2015) TED-Ed -- A video (4:18 min.) overview of biodiversity and how ecosystems and the organisms within them work together to support one another.

Monarch Butterfly Metamorphosis Time-Lapse (2014) FrontYardVideo -- Time-lapse video (2:42 min.) of the process of metamorphosis a monarch caterpillar undergoes to become a butterfly.

DNA Barcoding Pioneer Wins Global Research Prize (2018) University of Guelph -- News bulletin outlining that Paul Hebert, the “father of DNA barcoding,” has won a prestigious environmental science research prize that will help him to roll out a major biosurveillance project in Southern Ontario.

References

Chao, Z., Liao, J., Liang, Z., Huang, S., Zhang, L., & Li, J. (2014, December). Cytochrome C oxidase subunit I barcodes provide an efficient tool for Jinqian Baihua She (Bungarus parvus) authentication. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4239722/

Dimoff, A. (2018, January 26). DNA barcoding reveals widespread seafood fraud in Metro Vancouver. Retrieved from https://www.cbc.ca/news/canada/british-columbia/seafood-fraud-vancouver-1.4506831

Pearl, H. (2017, February 5). Conservationists See DNA Barcoding as Crucial Method for Combating Poaching. Retrieved from https://www.seeker.com/earth/conservation/conservationists-see-dna-barcoding-as-crucial-method-for-combating-wildlife-poaching

Rettner, R. (2017, December 7). DNA: Definition, Structure & Discovery. Retrieved from https://www.livescience.com/37247-dna.html

Saint Louis University. (n.d.). Biological Nomenclature. Retrieved from http://bio.slu.edu/mayden/systematics/bsc420520lect2.html

Study.com. (n.d.). What are some limitations of the Linnaean classification system? Retrieved from https://study.com/academy/answer/what-are-some-limitations-of-the-linnaean-classification-system.html

University of California, Berkeley. (2000, July 7). Carl Linnaeus. Retrieved from https://ucmp.berkeley.edu/history/linnaeus.html

University of Chicago. (2017, August 30). A new estimate of biodiversity on Earth. Retrieved from https://phys.org/news/2017-08-biodiversity-earth.html

Krysta Levac

After an undergraduate degree at the University of Guelph, I earned a PhD in nutritional biochemistry from Cornell University in 2001. I spent 7 years as a post-doctoral fellow and research associate in stem cell biology at Robarts Research Institute at Western University in London, ON. I currently enjoy science writing, Let's Talk Science outreach, and volunteering at my son's school. I love sharing my passion for science with others, especially children and youth. I am also a bookworm, a yogi, a quilter, a Lego builder and an occasional "ninja spy" with my son.