Species

A species is the main way scientists group living things. It is the largest group where any two members can have babies together and those babies will be healthy. Scientists also look at body shapes, behavior, or DNA to decide if something is a new species.

There are millions of different species on Earth, but only some have been found and named by scientists. Each species has a special two-part name. For example, the snake called Boa constrictor has Boa as the first part, which tells us the group it belongs to, and constrictor as the second part, which tells us the exact species.

The idea of a species has changed over time. Long ago, people thought species never changed. Now we know that species can change and split into new ones over many years. This happens because of things like changes in their genes, moving to new places, and facing different challenges. This helps scientists learn about life on our planet and how to protect it.

Definition

Scientists have tried to define what a species is for a long time. They first looked at things like size, color, and shape. Later, they learned that animals of the same species can usually have babies with each other, but not with animals from other species.

Today, scientists use many ways to decide if animals are the same species. They study DNA, look at how animals behave, and see where they live. Some scientists watch to see if animals can find each other as friends or partners. Others compare DNA to see if animals are different enough to be separate species. These different ways help us learn how to sort and group living things.

The species problem

It is hard to give one definition of a species that works for all living things. This debate is called the species problem. People have known about this problem since 1859, when Darwin wrote in On the Origin of Species:

I was much struck how entirely vague and arbitrary is the distinction between species and varieties.

He also said:

No one definition has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species.

It is easier to define species for most multi-celled organisms, but there are some tricky situations:

• When organisms reproduce asexually, like single-celled organisms such as bacteria and other prokaryotes, and parthenogenetic or apomictic multi-celled organisms. We use DNA barcoding and phylogenetics to help in these cases.
• When scientists do not know if two groups of organisms can breed together. This is true for all extinct life in palaeontology, because we cannot do breeding experiments.
• When hybridisation allows gene flow between species.
• In ring species, where nearby groups can breed together, but groups far apart cannot.

Finding species is also made hard when studies of molecules and looks do not agree. Horizontal gene transfer (HGT) can also make it hard to define a species.

Some groups of organisms have very little genetic change and are called microspecies. These often group together into species aggregates. For example, the dandelion Taraxacum officinale and the blackberry Rubus fruticosus each have many microspecies.

When two groups can make healthy hybrids, this makes it hard to say they are different species. For example, the carrion crow Corvus corone and the hooded crow Corvus cornix look different and are separate species, but they can breed together where they live close.

A ring species is a line of nearby groups that can breed with each other, but the two groups at the ends are too different to breed. This makes it hard for ideas about species that need breeding to work. But ring species are very rare.

Main article: Hybrid (biology)

Main article: Ring species

Taxonomy and naming

The names we use for animals and plants can be confusing. For example, the word "cat" can mean a pet or a group of animals. To avoid confusion, scientists use special names with two parts: the group name and the specific name. These scientific names are used all over the world so everyone understands the same thing.

When scientists find a new type of plant or animal, they give it a special name and describe it well. This description is kept in a special collection for others to study. Sometimes, scientists are not sure which group an animal belongs to and use short codes like "sp." to show this. There are also special numbers to help identify species quickly in databases.

Change

Species can change over time. They might evolve into new species, share genes with other species, or even disappear completely.

Speciation

Main article: Speciation

Speciation is when groups of the same species slowly change and become separate species. This often happens when groups are separated by distance and can no longer mix their genes. Over time, they change in different ways and can no longer produce offspring together.

Exchange of genes between species

Main article: Horizontal gene transfer

Sometimes, species can share genes with each other. This can happen through mixing or through viruses that move genes between species. Even bacteria can share small pieces of DNA with other bacteria.

Extinction

Further information: Extinction and Extinction event

A species becomes extinct when the very last one of them is gone. Scientists believe that most species that ever lived are now extinct. Some disappeared during big events called mass extinctions, which happened at the ends of several ancient time periods.

Practical implications

Biologists and conservationists need to correctly identify different kinds of living things for their work. If it's hard to tell which organisms belong to the same group, it can make research results unreliable. For example, counting how many of a certain kind of plant or animal live in an ecosystem can become confusing.

Some rules to protect wildlife help prevent species from disappearing. However, mixing between two similar kinds of animals — one protected and one not — can create problems. For example, in North America, the protected northern spotted owl sometimes mixes with the California spotted owl and the barred owl, leading to debates among lawmakers, landowners, and those who protect nature. Some scientists also argue that simply counting species may not be the best way to measure biodiversity.

History

Classical forms

Main article: Aristotle's biology

Aristotle used words to describe groups of living things. He talked about kinds, like birds or fish, and specific types within those kinds, like cranes or sparrows. He thought these kinds were fixed and unchanging.

Fixed species

Main article: Great chain of being

Later, scientists tried to organize living things into groups. An English scientist named John Ray said that plants or animals from the same parent belong to the same species. In the 1700s, Carl Linnaeus created a system to classify organisms based on their features. He believed species were fixed but could change a little through mixing.

Mutability

By the 1800s, scientists realized species could change over time. Jean-Baptiste Lamarck suggested that species could transform. Then, Charles Darwin and Alfred Russel Wallace introduced the idea of evolution, explaining how new species form through natural selection. Darwin saw species as useful labels for groups of similar individuals.