Species

A species is the basic way scientists group living things. It is the largest group of organisms where any two members can produce healthy babies together, usually by having babies. Scientists also use other ways to decide if something is a new species, like looking at their body shapes, behavior, or even their DNA.

There are millions of different species on Earth, but only a part of them have been discovered and named by scientists. Each species gets 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. A long time ago, people thought species never changed. But we now 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 in their environment. This understanding helps scientists learn about life on our planet and how to protect it.

Definition

Biologists and taxonomists have made many attempts to define species, starting with looking at physical features and moving towards studying genetics. Early scientists like Linnaeus described species based on what they observed, focusing on physical characteristics. Later, scientists like Ernst Mayr emphasized the importance of reproductive isolation — meaning that species are groups that can only reproduce with each other and not with other groups.

Today, scientists use many different ways to define a species, including studying DNA, physical traits, behavior, and the environment. Some scientists look at whether organisms can recognize each other as mates, while others study genetic differences to determine if groups are separate species. These different methods help us understand the many ways species can be identified and classified.

The species problem

It is difficult to define a species in a way that applies to all organisms. This debate about species concepts is called the species problem. The problem was recognised even in 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 went on to write:

No one definition has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. Generally the term includes the unknown element of a distinct act of creation.

Many authors have argued that a simple textbook definition works well for most multi-celled organisms, but breaks down in several situations:

• When organisms reproduce asexually, as in single-celled organisms such as bacteria and other prokaryotes, and parthenogenetic or apomictic multi-celled organisms. DNA barcoding and phylogenetics are commonly used in these cases.
• When scientists do not know whether two morphologically similar groups of organisms are capable of interbreeding; this is the case with all extinct life-forms in palaeontology, as breeding experiments are not possible.
• When hybridisation permits substantial gene flow between species.
• In ring species, when members of adjacent populations in a widely continuous distribution range interbreed successfully but members of more distant populations do not.

Species identification is made difficult by discordance between molecular and morphological investigations. In addition, horizontal gene transfer (HGT) makes it difficult to define a species.

The species concept is further weakened by the existence of microspecies, groups of organisms with very little genetic variability, usually forming species aggregates. For example, the dandelion Taraxacum officinale and the blackberry Rubus fruticosus are aggregates with many microspecies.

Natural hybridisation presents a challenge to the concept of a reproductively isolated species, as fertile hybrids permit gene flow between two populations. For example, the carrion crow Corvus corone and the hooded crow Corvus cornix appear and are classified as separate species, yet they can hybridise where their geographical ranges overlap.

A ring species is a connected series of neighbouring populations, each of which can sexually interbreed with adjacent related populations, but for which there exist at least two "end" populations in the series, which are too distantly related to interbreed. Ring species thus present a difficulty for any species concept that relies on reproductive isolation. However, ring species are at best rare.

Main article: Hybrid (biology)

Main article: Ring species

Taxonomy and naming

The names we use for animals and plants can be confusing because everyday names like "cat" can mean many things. For example, "cat" might mean a house pet or a whole family of animals. To avoid this confusion, scientists use special names made of two parts: the group name and the specific name. These scientific names are used all over the world so everyone means the same thing.

When scientists discover a new type of plant or animal, they give it a special name and describe it in detail. This description is kept in a special collection so others can study it. Sometimes, scientists aren't sure exactly which species something belongs to and use short codes like "sp." to show this. There are also special numbers and codes to help identify species quickly in databases.

Change

Species can change over time. They might evolve into new species, share genes with other species, merge with them, 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, which makes it tricky to define what a bacterial species really is.

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. These events were caused by things like huge volcanic eruptions and big changes in climate.

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

In his work, Aristotle used words to describe groups of living things. He talked about kinds, like birds or fish, and specific forms within those kinds, like cranes or sparrows. He thought these kinds and forms 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.