Classification Systems

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.

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:

1. Animals - Eukaryotic, multicellular organisms which can be split into vertebrates and invertebrates.

2. Plants - Eukaryotic, multicellular organisms which contain the photosynthetic pigment chlorophyll.

3. Prokaryotes (Monera) - Unicellular organisms which do not contain membrane-bound organelles.

4. Protists- Small eukaryotic organisms which are primarily found in aquatic environments.

5. 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!

Phyla can be further broken down into classes.

The classes are then broken down into orders.

These orders are then divided into families.

Families are subsequently divided into different genera.

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.

Here are some examples of genera and the species within them (combining the two produces the binomial classification for the organism:

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.

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.

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.

Invertebrates

Invertebrates are animals without backbones and are ectotherms (meaning they are cold-blooded).

Invertebrates warm their bodies by absorbing heat from the surroundings, as they do not possess the internal mechanisms to regulate their body temperature.

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.