Year 1 · Classification · Section 3.2

Similarities and relationships

Similarities

Organisms are classified using similarities, including body structures and DNA. Organisms with many relevant shared characteristics are often more closely related.

Members of the same genus generally share more specific features than organisms belonging only to the same kingdom. A lion and a tiger belong to the same genus and resemble one another much more than a starfish and a bird of prey, which share the animal kingdom.

Species

For many sexually reproducing organisms, members of the same species can reproduce with one another under natural conditions and produce fertile offspring.

Similar appearance does not prove that two organisms belong to the same species. African and Asian elephants, for example, belong to different species and genera despite their similarities. The species definition based on reproduction has limitations, particularly for organisms that reproduce asexually.

Breeds

A species can contain different breeds. Breeds may look very different, as a miniature dachshund and a Great Dane do. Nevertheless, they belong to the same species, whose members can produce fertile offspring. Breeds are variations within a species, not separate species.

Evolution

Individuals within a species show variation, such as light and dark shells in snails. If inherited differences affect survival and reproduction, some characteristics become more common through natural selection.

Across generations, populations change. Over long periods, new species may arise and existing species may become extinct. This process is evolution. Living species share common ancestors.

Relatedness

Species sharing a common ancestor are related. The more recent their common ancestor, the more closely related they are.

A tiger is more closely related to a lion than to a monkey because tigers and lions share a more recent common ancestor. DNA comparisons also help reveal these relationships.