Classification of Living Things

Project Work

María Voegeli July 2000

There are about 10 to 100 million species in the world, but only 1 ¾ have been given scientific names. About ¾ of these are insects.

Almost 99% of all plant and animal species that have existed have already become extinctwith most leaving no fossils.

Before the advent of modern genetically based evolutionary studies, European biology consisted primarily of taxonomy. Most naturalists two centuries ago held the view that plants and animals had been created in their present form and that they have remaines unchanged. They didnt believe in the posibility of evolution.

In 1735, Linnaeus published an influential book entilted Systema Naturae – The Linnaean system of classification. This system uses two Latin name categories, genus and species. A genus is a higher level category that includes one or more species under it, this is why it is called binominal nomenclature.

Linnaeus also created higher, more inclusive classification categories (e.g. he placed all monkeys and apes along with humans into the order of Primates) His use of the word Primates (“first”) reflects the human centered world view of European science during the 18th century.

This static view of nature was overturned in science by the middle of the 19th century by Darwin. He accepted the idea of evolution, and proposed “natural selection” as the mechanism responsible for these changes. Crossbreeding resulting in new varieties of plants suggested to him that life forms could change somewhat.

Classifying living things is a way of reflecting evolutionary distances and relationships between organisms. The great diversity of life is a result of branching evolution or adaptive radiation.

When biologists discover an unknown organism, they begin their classification by looking for analogies, or anatomical structures that have ...

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Classifying living things is a way of reflecting evolutionary distances and relationships between organisms. The great diversity of life is a result of branching evolution or adaptive radiation.

When biologists discover an unknown organism, they begin their classification by looking for analogies, or anatomical structures that have the same function as those found on other species. The wings of a bird and a butterfly are analogous structures. Analogies do not necessarily result from common evolutionary descent.

If analogous structures are due to this sort of actual close genetic relationship, they are called homologies. Homologous structures are similar because they are inherited from a common ancestor that also had them.

There can also be nonhomologous structural similarities between species. Instead the similarities are due to independent acquisition in separate evolutionary lines. Such misleading similarities are called homoplasies. Homoplasies can result from parallelism (or parallel evolution). They can be similar evolutionary developments after divergence from a common ancestor. Homoplasies can also result when the common ancestor was even more distant in time, as in the case of convergence.

There is always a lively debate among researchers over defining new species because it is not obvious what are the most important traits. There are two schools of thought. The first defines new species based on minor differences between organisms. This is the “spliter” approach. The second tends to ignore minor differences and to emphasize major similarities. This “lumper” approach results in fewer species being defined.

Ideally, this dispute could be settled by breeding experiments. If two organisms can mate and produce fertile offspring, they are members of the same species.

The Linnaean system is a hierarchical system of classification with the highestcategory consisting of all living things. The lowest category consists of a single species.

There are three mammal variations which are the basis for dividing them into subclasses and infraclasses. The differences are in the reproductive systems.

Prototheria lay eggs like non-mammalian vertebrates. They feed their newborn with mammary gland secretions like all other mammals. The Prototheria are also referred to as being monotremes, which means that they have only one opening for excretion and reproduction. There are only three surviving rare species groups of Prototheria. The Australian platypus and 2 echidna (spiny anteater) species of Australia and New Guinea.

All other living mammalian species, including humans, are in the subclass Theria. These therian mammals apparently did not evolve from the Prototheria. The oldest infraclass of therian mammals is the Metatheria, or the marsupials.

Most mammal species, including humans, are in the infraclass Eutheria. These are pacental mammals. The mothers carry their young within the uterus before birth until an advanced stage is reached. This is made possible by the umbilical cord and placenta

Placental mammals have been extremely successful in out competing monotremes and marsupials for ecological niches.