1.3 - Species & Naming

How classification has changed

• Organisms were originally classified by similarities and differences in their observable internal and external features.
• As microscopes and other technology improved, scientists could compare organisms in far more detail.
• The modern aim of a classification system is to reflect the evolutionary relationships between organisms - grouping each organism with its closest relatives so the classification mirrors how the organisms are actually related by descent.

DNA as evidence for classification

• The most powerful evidence for evolutionary relationships now comes from DNA.
• Scientists can determine the sequence of bases in different organisms' DNA and compare them.
• The more similar the base sequences of a gene, the more closely related the organisms, and the more recent their shared ancestor.
• Organisms that share a recent common ancestor have more similar sequences than those whose common ancestor is far in the past.

Evolutionary trees

• Evolutionary relationships are often shown as an evolutionary tree, which links species and groups through their common ancestors.
• The more recent the common ancestor of two groups, the more closely related they are - and the more characteristics and DNA they share.
• Each point where a line splits in two represents a common ancestor dividing into two separate groups.
• For example, humans and mice share a recent common ancestor, so are closely related to each other. Both are only distantly related to elephants, whose shared ancestor with them lies much further back.

Species and scientific naming

• A species is the smallest group organisms are sorted into.
• It's a group of similar organisms that can reproduce with one another to give fertile offspring - offspring that can themselves go on to reproduce.
• This makes a species a natural unit: its members breed together, while members of different species generally don't produce fertile offspring.

The binomial system

• Because there are so many species, each is given its own scientific name using the binomial system - an internationally agreed, two-part naming system, mostly in Latin.
• The two parts are:
  1. Genus - The first part, shared with closely related organisms, giving information about ancestry
  2. Species - The second part, the species itself
• Example: humans are Homo sapiens - Homo is the genus and sapiens is the species.
• Writing conventions:
  • The whole name is written in italics
  • Only the first letter of the genus is capitalised

Why the system is used

• The point is consistency.
• The same organism is often given many different common or local names in different places and languages - over a hundred plant species are commonly called raspberries, for instance - which causes confusion.
• A single agreed scientific name means scientists anywhere in the world refer to a species by exactly the same name, so there's no doubt about which organism is meant.

Broad grouping and the five-kingdom scheme

• Living organisms vary enormously in size, features, and complexity - from tiny simple bacteria to large complex animals and plants.
• They can be sorted into groups according to the features they share.

Familiar broad groups

At the broadest, most familiar level, living things are placed into a few large descriptive groups:

• Plants - Divided into flowering plants (e.g. daisies and grasses) and non-flowering plants (e.g. ferns and mosses)
• Animals - Divided into vertebrates, which have a backbone (e.g. mammals and birds), and invertebrates, which have no backbone (e.g. insects)

The five kingdoms

• Above these familiar groups, one widely used scheme places every organism into one of five large groups called kingdoms.
• The five kingdoms are:
  1. Animal kingdom (Animalia)
  2. Plant kingdom (Plantae)
  3. Fungus kingdom (Fungi)
  4. Prokaryote kingdom (Prokaryotae) - single-celled organisms without a nucleus
  5. Protoctist kingdom (Protoctista)
• Each kingdom is subdivided into smaller and smaller groups whose members share more and more features, until the smallest groups each contain only a single type of organism.
• This is the top-level framework of the classification system: an organism is first assigned to a kingdom, then placed into the finer groups within it.