Evolutionary classification

Biological classification organises organisms into groups that reflect how they are related through evolution. Organisms placed together are expected to share a common ancestor: an ancestral organism or population from which they descended. Classification therefore aims to represent evolutionary history, rather than simply sort organisms by appearance.

Evolutionary relationships within groups

Classification places smaller groups within larger groups. This nested arrangement represents different degrees of evolutionary relationship. Organisms in the same smaller group share a more recent common ancestor with one another than they do with organisms outside that group. A larger group includes several smaller groups whose members share a more distant common ancestor.

For example, imagine a large group containing two smaller groups. The members of each smaller group share a relatively recent common ancestor. Members of the two different groups are still related, but their shared ancestor lived further back in evolutionary history. Placing both smaller groups inside the larger group represents that older connection.

Evolutionary classification and appearance

Similarities in appearance can help identify relationships, but appearance alone is not the basis of evolutionary classification. Organisms may look similar without sharing a recent common ancestor, while related organisms may develop different appearances over time. The central question is therefore how recently organisms shared a common ancestor, not simply how alike they look.

DNA sequence evidence for classification

Scientists can use similarities and differences in DNA to group organisms according to how closely they are related. DNA stores inherited information in its base sequence: the order of bases along the DNA molecule.

Offspring inherit DNA from their parents, but changes to base sequences can occur and be passed on. Over many generations, these changes can accumulate. DNA therefore provides evidence of an organism’s ancestry.

DNA similarity and common ancestry

A common ancestor is an ancestral organism from which two or more organisms or groups descended. When groups diverge from a common ancestor, they initially share inherited DNA sequences. After divergence, changes can accumulate independently in each group.

Organisms that share a more recent common ancestor usually have more similar DNA base sequences. Less time has passed since their lineages separated, so fewer differences have had time to build up. Organisms whose common ancestor lived further in the past usually show more differences because changes have accumulated over a longer period.

Using DNA comparisons to form groups

Scientists compare corresponding sections of DNA from different organisms and identify where the base sequences match or differ. If two organisms have more similar sequences than either has to a third organism, this supports the conclusion that the first two share a more recent common ancestor and are more closely related.

These comparisons provide evidence for classification that reflects evolutionary relationships: organisms with more recent shared ancestry are placed together in more closely related groups. Greater DNA similarity usually indicates closer relatedness, rather than simply showing that organisms look alike.

Classification by shared features

Organisms can be organised into groups by comparing their features, such as their body covering, number of legs or leaf shape. Placing organisms into groups according to features they share is called classification.

A shared feature is a characteristic that different organisms have in common. It provides a reason for placing those organisms together. Differences also matter: they help distinguish organisms in one group from those in another. Classification therefore depends on identifying both similarities and differences, rather than considering each organism separately.

Using features to form groups

Suppose several animals are being compared by their number of legs. Animals with six legs can be placed together, while animals with eight legs can form a different group. Within each group, the animals share the feature being used. Between the groups, that feature differs.

The animals in one group do not have to be identical. Two six-legged animals might differ in colour or size, but they still belong together when number of legs is the basis for grouping. The important point is to make clear which shared feature is being used and compare organisms consistently using that feature.

Species and fertile offspring

A species is a group of organisms that can reproduce with one another to produce fertile offspring. Fertile offspring can themselves reproduce, so the criterion involves more than producing young: those young must also be capable of having offspring of their own.

To apply this definition, consider two generations. First, organisms reproduce and produce offspring. Then, those offspring must be able to reproduce too. If the offspring are infertile, meaning they cannot reproduce, producing them does not meet the criterion for belonging to the same species.

Visible similarities alone do not establish that organisms belong to the same species. Organisms may look alike without being able to produce fertile offspring together, while members of the same species may differ in appearance. The defining distinction is the ability to produce fertile offspring, not simply how similar the organisms look.

Binomial naming of species

Each species has a scientific name made of two parts. This is called a binomial name. The name can be used internationally, so people can identify the same species even when they speak different languages.

The first part names the genus, a group that can contain more than one species. The second part identifies the species within that genus. Both parts are needed to give the full species name.

For example, the scientific name for humans is Homo sapiens. Homo is the genus, and sapiens identifies the species within that genus. The genus name starts with a capital letter; the second part starts with a lowercase letter. Both parts are written in italics.

Dichotomous keys

A dichotomous key identifies an organism through a sequence of choices. At each step, the user chooses between two contrasting descriptions of observable features. The matching description directs the user either to another pair of choices or to the organism’s identification.

Using a dichotomous key

The following key distinguishes four organisms: an adult butterfly, a spider, a snail and an earthworm.

1. Legs present → go to 2.
   Legs absent → go to 3.
2. Wings present → adult butterfly.
   Wings absent → spider.
3. External shell present → snail.
   External shell absent → earthworm.

To identify a spider, start at step 1. It has legs, so go to step 2. It has no wings, so the key identifies it as a spider. The key distinguishes only the organisms it was designed to identify; another organism with legs and no wings would also follow that route.

Constructing a dichotomous key

Choose features that distinguish the organisms and can be identified reliably. For each pair of choices, write two contrasting descriptions so that an organism fits one alternative, not both.

Arrange the choices so that each one narrows the remaining possibilities. In the key above, the first pair separates organisms with legs from those without legs. Each group is then divided again using another feature. Continue until each route leads to an identification.