Biology 14.2 How Biologists Classify Organisms

How Biologists Classify Organisms

What is a species?

Have you ever wondered how scientists tell one species from another. Many species look identical but have invisible differences that set them apart.

In 1942, biologist Ernst Mayr proposed the biological species concept.

Mayr defined a biological species as a group of natural populations that are interbreeding or that could interbreed, and that are reproductively isolated from other similar groups.

The biological species concept defines a species as members of populations that actually or potentially interbreed in nature, not according to similarity of appearance. Although appearance is helpful in identifying species, it does not define species.Appearance isn’t everythingOrganisms may appear to be alike and be different species. For example, Western meadowlarks (Sturnella neglecta) and Eastern meadowlarks (Sturnella magna) look almost identical to one another, yet do not interbreed with each other—thus, they are separate species according to this definition.

What is a species?

Reproductive isolation occurs when a barrier separates two or more groups of organisms and prevents them from interbreeding.

In nature, however, reproductive barriers between sexually reproducing species are not always complete.

Sometimes individuals of different species interbreed and produce offspring called hybrids.

For example; wolves and dogs are separate species in the genus Canis, but interbreeding happens and produces fertile offspring that are hybrids.

wolf

husky

Husky-wolfhybrid

Evaluating the Biological Species Concept

The biological species concept works well for most members of the kingdom Animalia, in which strong barriers to interbreeding exist.

For example, Asian elephants and African elephants do not interbreed in nature.

But the biological species concept fails to describe species that reproduce asexually; such as all species of bacteria and some species of protists, fungi, plants, and even some animals.

Evaluating the Biological Species Concept

With many groups of organisms, there are no barriers to interbreeding between the species.

Many species of plants, some mammals, and many fishes are able to form fertile hybrids with one or a few closely related species.

In practice, modern biologists recognize species by studying an organisms features.

Liger (lion-tiger hybrid)

Evaluating the Biological Species Concept

The number of actual species in the world is much larger than what is now known. Large number of species are still being discovered.

Only about 1.5 million species have been described to date. Scientists estimate that 5 million to 10 million more species may live in the tropics alone.

Since than, no more than 500,000 tropical species have been named; it is clear that our knowledge of Earth’s diversity of species is limited.

Phylogeny

Linnaeus’s classification system was based on his observations that organisms have different degrees of similarity. A tiger resembles a gorilla more closely than it resembles a fish.

According to Darwin’s view, organisms that are more similar to one another than they are to other organisms have descended from a more recent common ancestor.

Therefore, classification based on similarities should reflect an organism’s phylogeny; it’s evolutionary history.

Phylogeny of certain dinosaurs

Concurrent Evolution

Inferring evolutionary connections from similarities, however, can be misleading.

Not all features that are similar are from a common ancestor. Consider the wings of a bird and the wings of an insect. Both enable flight but the structures of the two kids of wings are radically different.

Fossil evidence indicates that they evolved independently from one another.

Concurrent Evolution

Through a process called concurrent evolution, similarities evolve in organisms not closely related to one another, often because the organisms live in similar environments.

Similarities that arise through concurrent evolution are called analogous characters.

Cladistics

Most biologists today analyze evolutionary relationships using cladistics.

Cladistics is a method of analysis that reconstructs phylogenies (evolutionary histories) by inferring relationships based on shared characteristics.

Inferring means to make an educated guess based on examining evidence

Cladistics

Cladistics can be used to hypothesize the sequence in which different groups of organisms evolved.

For example, cladistics can be used to hypothesize the evolutionary development of winged birds.

To do this, cladistics focuses on the nature of the characteristics in different groups of organisms. It looks at the traits that a group has in common and what traits are different.

Ancestral Characters

With regard to two or more groups, a character is defined as an ancestral character if it evolved in a common ancestor of two or more later groups in an evolutionary chain.

For example, four limbs may be an ancestral characteristic that developed long ago when the first organisms came from the sea to walk on land.

When looking at humans and apes, a hand with a thumb is an ancestral character that developed long ago when both shared a common ancestor.

Derived Characters

When considering the relationship between a bird and a mammal, a backbone is an ancestral character.

Having feathers, however, is a derived character.

A derived character evolved in an ancestor of one group but not the other.

Feathers evolved in an ancestor of birds that was not also ancestral to mammals.

Derived characters

Cladistics is based on the principal that shared derived characters provide evidence that two groups are closely related.

Shared ancestral characters however, are not always closely related.

How can this be?

Ancestral Characters For an example; let us compare

lizards, dogs, and whales.

Lizards and dogs have limbs but a whale has no limbs.

Having limbs does NOT provide evidence that dogs and lizards are more closely related. Recall that whales are descended from ancestors that did have limbs.

Therefore, the presence of limbs is a shared ancestral character of all three.

However, dogs and whales have mammary glands; a shared derived character not found in lizards. This provides evidence that dogs and whales share a more recent common ancestor than either shares with a lizard.

Cladograms A biologist using cladistics

constructs a branching diagram called a cladogram.

A cladogram shows the evolutionary relationships among groups of organisms.

Organisms that share derived characters are grouped together on the cladogram.

As groups evolve, new derived characters appear on the cladogram that were not present before.

Cladogram of dinosaur development

Cladogram of ape-human development

Cladograms

Cladograms have strengths and weaknesses.

The strength of a cladogram is it’s objectivity. If a computer is fed the data, it will make the same cladogram each time.

The disadvantage of cladistics is that the degree of difference (how different something is) is not considered. Cladistic analysis simply indicates whether a character is present or not

Cladogram of dinosaur development

Cladogram of ape-human development

Cladograms

Cladistic analysis does not take into account variations in the “strength” of a character; such as the size or location of a fin or the effectiveness of a lung.

Each character is treated equally.

Thus, in a cladogram of vertebrate evolution seen at right, birds are grouped with dinosaurs and reptiles.

This reflects their ancestry; birds evolved from dinosaurs.

The immense evolutionary impact of the development of feathers is ignored by the cladogram.

Cladograms

Evolutionary success often depends on the high-impact developments; such as the development of feathers and flight.

Some modern cladistic studies therefore attempt to weigh the evolutionary importance of the characters being studied.

In evolutionary systematics, taxonomists give varying degrees of importance to characters and thus produce a more subjective analysis of evolutionary relationships.

Cladograms

In this type of analysis, evolutionary relationships are displayed in a branching diagram called a phylogenic tree.

In a phylogenic tree, evolutionary systematics places birds in an entirely separate class from reptiles., giving more importance to characteristics like feathers that made flight possible.

A phylogenic tree and a cladogram are similar in that each represents a hypothesis of evolutionary history, which must be inferred because it can not be observed.

Phylogenic tree

cladogram