Nearly 2 million kinds of animals have been identified by scientists, and thousands more are discovered every year. The real total may be 10 times this number, which means that the task of locating and identifying all the world’s animals will never be complete. To make sense of this bewildering diversity, biologists use classification – a system that works like a gigantic filing system of all the past and present life on our planet. In classification, the entire living world is divided into groups, known as taxa. Each type of animal is given a unique scientific name and is placed with its closest relatives, reflecting the path that evolution is thought to have followed.
PRINCIPLES OF CLASSIFICATION
Scientific classification dates back over 300 years, to the work of John Ray (1627-1705). He classified plants and animals, using names that described features such as leaf shape. He also realized the importance of species, an idea that is still key to classification today. In the 18th century, another botanist – Carl Linnaeus (1707-1778) – laid the foundations of modern classification by devising classification hierarchies and 2-part scientific names. Like Ray, Linnaeus worked long before the discovery of evolution. He saw species as fixed, with unique shapes and lifestyles that were part of a divine plan.
At first sight, Linnaeus’s binomial names may seem cumbersome, even though they are far more concise than the ones that were used before. They have 2 immense advantages. Unlike common names, they are unique to a particular species and they act as signposts, showing exactly where a species fits into life as a whole. The study of the classification, known as taxonomy, has rules for the way scientific names are devised and used. Each species has a genus name and a specific name by which it can be identified.
The tiger, for example, is Panthera tigris, showing that its closest relatives are other members of the genus Panthera – the lion, the leopard, and the jaguar. The wild cat, on the other hand, belongs to the genus Felis, which includes most of the world’s smaller cat’s. At a glance, this shows that there are differences between tigers and wild cats, in the way they have evolved as well as in their size.
In Linnaean classification, the species level is the basic starting point. Species are organized into groups of increasing size, starting with genera and groups of increasing size, starting with genera and working upwards through families, orders, classes, and phyla, and then into kingdoms and domains, which are the largest groups of all. These groups work like flexible folders, and even equivalent levels can vary enormously in size. The cat family, for example, contains 37 species, while the aardvark family contains just one. At the other extreme, in the insect world, the weevil family currently contains 55,000 species, and there are probably many more. Not all animals fit neatly into groups that Linnaeus originally devised and so, over 2 centuries of study, taxonomists have created a wide array of intermediate levels. These include superfamilies, infraclasses, suborders, and subphyla, each fitting into the hierarchy at different points. At a much finer level, many species are divided into variants called subspecies — the milksnake, for example, has about 24, each with its own distinctive markings and geographic range.
These intermediate levels do not mean that Linnaeus’s system does not work. Rather. it simply reflects the fact that classification levels are convenient labels, rather than things that have a physical existence of their own. The only category that really exists is the species, and even this level can be difficult to define. Traditionally, a species is defined as a group of living things that breed exclusively with their own kind to produce fertile young. This works well enough with many animals but does not always hold true.
Identifying which group an animal belongs to is vital to the classification process, and anatomical is vital to the classification process, and anatomical features play a key part. They can be very useful in tracing the path of evolution, often showing how body parts, such as limbs or teeth, have been developed and modified for different ways of life. The limbs of 4-legged vertebrates are one of the best-known examples of this kind of evolutionary improvization. The basic limb pattern, dating back over 300 million years, is built around 3 main sets of bones: a single bone near the body, 2 bones further out, and 5 sets of smaller bones at the limb’s furthest point.
As 4-legged vertebrates spread from the water to the land and air, their limbs became very different in shape and size, and in the way they worked, however, the underlying pattern of bones was retained, making it an identifying feature that remains to this day. As this pattern is unlikely to have evolved more than once, it provides important evidence that these animals evolved from a shared ancestor.
Taxonomy is rarely straightforward, because evolution provides a mix of useful and confusing clues. Useful ones include the underlying patterns of bones segments, and microscopic features. such as the way a single cell divides when a new animal starts to take shape. They also include vestigial organs that have lost their original function over time, and become smaller as a result. Among them are the hind limb bones of some snakes that play no part in movement, but indicate a 4-legged ancestry. Similarity does not always mean genetic relationship. Convergence happens when unrelated species evolve similar adaptations to similar ways of life – such as the wings of flying bats and birds. Convergence can sometimes obscure genetic relationships – especially within families or genera.
But when examined in more detail, fundamental biological differences can reveal the true pattern of the evolutionary tree and how species have evolved from separate branches. In extreme cases, convergence can make the relationships between organisms extremely difficult to unravel. A record of the past is built into every living thing in the form of DNA. Since the 1990s, technological advances in both computing and DNA extraction have helped to explain convergence and many other questions. By studying the similarities and differences in genetic material, relationships between different organisms can be established regardless of their appearance.
Ever year, the world’s taxonomists examine thousands of new species that have been collected or photographed in many different parts of the world. Most of them are invertebrates, particularly from the sea and the rain forests, but there are also reptiles, birds, and mammals, often from remote areas of the tropics. Each new species is formally identified, named, and described as part of the classification process.
Since classification first began, taxonomists have identified and cataloged species from all over the world. Today, that process continues. Most large, land-dwelling animals are known, but huge numbers of invertebrates, particularly in the deep ocean, have yet to be discovered and described.
Victorian zoologists – including famous ones such as Charles Darwin (1809-1882) — thought little about trapping new species, or shooting them out of the trees. In today’s more enlightened times, technology plays a role instead. Automatic cameras photograph animals on rainforest floors, and deep in the oceans where there is no natural light. Camera traps helped confirm that the Annamite striped rabbit, on the Laos-Vietnam border, was a genuinely new species.
New species can also be found in another way by re-examining ones that are already known. Sometimes, this shows that 2 species are actually one, but it can also have the opposite effect. Two species may look similar, but their anatomy, behavior, and genes may show that this similarity is just skin-deep. When this happens, the species is split into 2, and one of them renamed. There are examples of new species being “discovered” in this way.
For example, the African elephant is now recognized as being different from the African savanna elephant whereas formerly they were thought to be one. The giraffe was long thought to be a single species but has now been split into 4 separate species. “Splitting” is particularly common among forest primates, accounting for a considerable growth in the number of species, even though their numbers are often in serious decline.