Our planet is inhabited by a multitude of extraordinary organisms, many of which are still being discovered. Developing a deeper understanding of the evolutionary relationships between these organisms is essential for scientists attempting to comprehend past Speciation events and predicting future developments in the Earth’s biome.
Classification in Biology is the arrangement of living organisms into groups based on their characteristics and evolutionary relationships between species. The similarities and differences between organisms allow us to categorise them. Since ancient times scientists have been attempting to classify organisms into groups. This was initially carried out by comparing the superficial characteristics of each organism, but with modern advances in molecular technology and a better understanding of evolutionary mechanisms, it became easier for scientists to decipher the phylogenetic relationships between species.
Biological classification system types
The first scientist to classify organisms was Aristotle. He observed simple morphological characteristics and used these to classify organisms as Plants, shrubs, trees and herbs. He classified Animals into two groups: one with red Blood Cells (Enaima) and one without red Blood Cells (Anaima). In modern times, more complicated and distinctive classification systems have been developed. Biological classification systems can be artificial or natural.
Artificial classification involves classifying organisms based solely on observable characteristics.
Natural classification concerns the comparison of the molecules inside organisms (e.g. DNA sequences). The more similar the molecules are, the closer the two organisms are related.
The Linnaean system
Early improvements to the classification of organisms were made by Carl Linnaeus in the 1700s. In the Linnaean system, living organisms are divided into groups based on their observable characteristics and the anatomical features they exhibit. Organisms are initially divided into kingdoms and then into phylum, class, order, family, genus and species.
There are five main kingdoms:
Animals - Eukaryotic, multicellular organisms which can be split into vertebrates and invertebrates.
Plants - Eukaryotic, multicellular organisms which contain the photosynthetic pigment chlorophyll.
Prokaryotes (Monera) - Unicellular organisms which do not contain membrane-bound organelles.
Protists- Small eukaryotic organisms which are primarily found in aquatic environments.
Fungi - Eukaryotic, multicellular organisms which possess cell walls made of chitin.
There are two main types of organisms: eukaryotic and prokaryotic. They differ in the type of cells that make up their body! Eukaryotic organisms have Eukaryotic Cells, and prokaryotic organisms have prokaryotic cells. The main difference between these cells is that Eukaryotic Cells have a nucleus that contains most of their DNA, while prokaryotic cells have no such organelle!
Read our article Eukaryotes and Prokaryotes to learn more!
Take a look at some examples of known phylum:
Protozoa - unicellular animals such as amoeba, monocystis and malarial parasites.
Cnidaria - marine organisms which are radially symmetrical like jellyfish, corals and hydra.
Platyhelminthes - flat, bilaterally symmetrical worm-like creatures (e.g. tapeworms, flukes, flatworms).
Arthropods - bilaterally symmetrical, segmented animals with an outer skeleton made of chitin (e.g. crabs)
Molluscs - Mainly marine animals whose bodies are enclosed in a calcareous outer shell (e.g. octopuses).
Chordata - Animals possessing backbones (e.g. cheetah, elephant, badger).
Phyla can be further broken down into classes.
For example, the phylum of Chordata can be divided into Mammals, birds, fish, Reptiles and amphibians.
The classes are then broken down into orders.
For example, Mammals are split into groups such as carnivores, herbivores, omnivores and primates.
These orders are then divided into families.
For instance, in the order Carnivora, there are Ursidae (bears), Felidae (cats) and Canidae (dogs).
Families are subsequently divided into different genera.
For instance, humans, who are a part of the Primate order and Hominidae family, have the genus ‘homo’.
Species is the final classification, and for modern-day humans, this is ‘sapiens’.
The binomial system
Organisms are now identified by the binomial system, in which they are denoted by their respective genus followed by their species from the Linnaean system. These are both Latin words and are standardised internationally so that there is no confusion between scientists who speak different languages. Species are groups of organisms that are similar enough to reproduce and generate fertile offspring.
For example, the binomial nomenclature for dogs is Canis familiaris. The binomial name of a wolf would be Canis lupus showing that wolves and dogs are in the same genus and therefore have stemmed from the same evolutionary line.
Here are some examples of genera and the species within them (combining the two produces the binomial classification for the organism:
Genus | Species |
Equus- large hoofed breeds | Horses- ferus Zebras- zebra Donkeys- africanus |
Canis- canine dog-like breeds | Dogs- familiaris Wolves- lupus Dingo- lupus dingo |
Homo- human-like species | Sapiens Erectus Neathanderlensis |
Rosa- different types of rose | Red climbing rose- santana Shrub rose- flutterbye Arctic rose- acicularis |
The three-domain system of organism classification
With modern developments in research and analysis of organisms, there is now a greater understanding of these organisms’ internal structures and biochemical processes. Advances in technology such as higher-resolution Microscopes and DNA sequencing technology have allowed scientists to study organisms with greater detail, allowing them to distinguish more similarities and differences between them.
German scientist Carl Woese proposed the three-domain system at the start of the 1990s. This system consisted of all organisms being split into three large groups before being divided into smaller groups of kingdom, class, order etc. These three groups are Archaea, Bacteria and Eukaryota. Using modern techniques such as RNA sequence analysis, Woese deduced that many species initially grouped into the same divisions in the Linnaean system are not as closely related as first thought.
Archaea: a group of single-celled organisms which do not contain a true nucleus. They are primitive Bacteria and are primarily found in extreme locations.
Archaea were initially classified as archaebacteria, but after extensive research, it was discovered that they exhibit some characteristics that you would expect from a eukaryote, such as having three RNA polymerases (e.g. crenarchaeota, euryarchaeota, thermophiles).
Bacteria: a domain containing ‘true bacteria’. These are microscopic, unicellular organisms which are found in all environments. They do not contain a true nucleus and have no membrane protecting their DNA. They often have additional circular strands of DNA called plasmids.
Bacteria are split into two groups, namely gram-positive and Gram-Negative Bacteria. One way to distinguish between these groups is that gram-negative bacteria are surrounded by a lipid envelope, whereas gram-positive bacteria are not.
Eukaryota: This domain encompasses a wide range of organisms, containing a true nucleus and membrane-bound organelles. This includes animals, plants, Fungi and protists.
The common ancestor of all known life is called LUCA (Last Universal Common Ancestor), a single-celled organism resembling a bacterium. Archaea and Bacteria were both direct descendants of Luca, and then Eukarya emerged from these domains around 1.5 billion years ago. LUCA possessed an extremely complex Genome, but as Genes move around so much in genetic mutation, this genome has been completely overwritten by now.
What is Cladistics?
Cladistics are another method of classification by which organisms are grouped by considering their shared traits. This method was first pioneered by Willi Hennig and was referred to as ‘phylogenetic systematics’. He constructed evolutionary lines by studying morphological and molecular data from various organisms. This is often done by rigorous DNA and RNA analysis of these organisms.
Taxonomy Definition
Taxonomy is the science of identifying different organisms by classifying them into distinct groups depending on their morphological characteristics and biochemical processes and then giving them a scientific name. There are 8 different hierarchical categories in Taxonomy: domain, kingdom, phylum, class, order, family, genus and species.
Why is the classification of organisms important?
Classification of organisms is of paramount importance to scientists as it allows them to easily study a wide variety of organisms without confusion. The binomial and Linnaean classification systems are also internationally consistent, so they can be used universally by scientists speaking different languages.
Classification allows scientists to deduce evolutionary and ancestral relationships between organisms as species which stem from the same phylogenetic groups are more closely related. The fact that organisms can be classified helps measure changes in Biodiversity in Populations, which is necessary knowledge when attempting to conserve these Populations.
Classification system examples: evolutionary trees
Evolutionary trees are diagrams which show the ancestral relationships between various organisms. The joining of the various branches represents where two or more species share a common ancestor. This point of splitting of the branches shows where speciation has occurred, meaning that new species distinct from the previous species have evolved. To produce these evolutionary trees, scientists use data from DNA and RNA analysis, differences and similarities between observable morphological characteristics, and Fossil Record data.
Speciation is the phenomenon resulting in the appearance of a new species. It usually results when two groups of organisms of the same population have become isolated for so long that they can no longer mate to produce fertile offspring and are now classified as different species.
Read our article on Speciation to learn more about these important events!
Below are three examples of important types of organisms!
Vertebrates
Vertebrates are animals classified under the phylum Vertebrata. Vertebrates all own a skeleton made of bone or cartilage, which consists of a spinal column and braincase.
This group of animals includes mammals, fish, birds, Reptiles and amphibians.
Here are some important facts about vertebrates:
Vertebrates make up just 3% of all animal species.
Vertebrates can be warm or cold-blooded. Mammals and birds are warm-blooded and have various mechanisms to ensure their internal temperature is not affected by their surroundings. Fish, reptiles and amphibians are cold-blooded, so their internal temperatures vary depending on their environment.
The spinal cord is surrounded by the vertebral column, providing structural support for the animals.
Vertebrates are the only animals which have an adaptive Immune System.
Invertebrates
Invertebrates are animals without backbones and are ectotherms (meaning they are cold-blooded).
Examples include spiders, worms, crabs and insects.
Invertebrates warm their bodies by absorbing heat from the surroundings, as they do not possess the internal mechanisms to regulate their body temperature.
Here are some important points about invertebrates:
There are 8 different phylum classifications of invertebrates: Porifera, Nematoda, Annelida, Echinodermata, Cnidaria, Platyhelminthes, Mollusca, and Arthropoda.
There is an abundance of marine invertebrates. These include corals, sponges, jellyfish and molluscs.
Invertebrates can range in size from microscopic insects to giant deep sea-dwelling squid.
Flowering plants
Flowering plants can be classified based on their presence or absence of leaves, stems, roots, production of seeds, or flowering. The most common way to classify plants is into flowering and non-flowering plants, Angiosperms and Gymnosperms, respectively. The two largest groups of flowering plants are monocotyledons and eudicotyledons.
Monocotyledons include bamboos, bananas, tulips and palms. They flower in multiples of three, have parallel veins in their leaves, and their roots can extend from any part of the plant that comes into contact with the soil.
Eudicotyledons include oaks, roses, cacti and blueberries. Plants in this group often have well-defined petals and sepals and include both herb and woody species.
Classification Systems - Key takeaways
- Classification is the arrangement of organisms into groups based on their morphological characteristics and evolutionary relationships.
- Observations of the characteristics of existent animals and fossils can help scientists to discern which organisms are more closely related and when they may have diverged to another species.
- Modern advances in genomic analysis have allowed scientists to be more accurate when distinguishing organisms.
- The three-domain system (these being Archaea, Eukaryota, and Bacteria) proposed by Carl Woese is now universally accepted.
Explanations 
Textbooks 
Exams 
Magazine 
