In lawn mowing!). Your neighbor wants to breed

In biology, a species is one of the basic units of biological classification. It most commonly refers to a group of physically similar organisms that can exchange genetic information and produce fertile offspring. Learn more in this lesson.

Biology and Classification

Imagine you’re out with friends and joking about that new guy at work.

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‘It’s like he’s a different species!’ you say. But what does that actually mean? What is a species? How do scientists tell the difference between all these different species on Earth? Well, this is a topic of active debate among biologists, but we have all agreed upon one thing: the idea of a species is fundamental to the study of biology.Biology is concerned with studying the life around us, and to make that easier, biologists like to group things together. These groups tend to consist of organisms (living things) that look or act alike. This allows us to study different plants, animals, and microbes based on what they have in common with each other. Did you ever play that game 20 Questions, and the first question asked is: ‘Animal, vegetable, or mineral?’ The answer to that question helped in the game, and it helps in science. Looking at living things in groups makes it easier to discover how different organisms work.

To efficiently put organisms in different groups, scientists developed a system of classifying these organisms. This system takes all the organisms on Earth and puts them in groups based on body form, genetic similarity, body chemistry, development, and behavior.The study of organisms in this way is called systematics.

Systematics starts with very large groups, based on very broad criteria, called domains. There are only three domains, and all life as we know it fits into one of these three groups. The groups get smaller and more specific, and the organisms in each group get more and more alike until we reach the smallest group, the species. Scientists estimate there are 30 million species of living organisms on Earth.

What Is a Species?

But how do we recognize a species?This is a harder question to answer than you think. The most common definition of a species is: a group of organisms capable of interbreeding and producing fertile offspring.

This definition works well, for the most part.Let’s look at an example. You buy a new, beautiful purebred show poodle and plan to take her on the dog show circuit. How fun! One evening, the neighbor’s golden retriever-lab mix gets into the yard, and a few months later, you have an adorable basket of poodle-golden retriever-lab puppies up for adoption. Those puppies grow up and have puppies of their own. What do we know about the poodle and golden retriever from this experiment? We know they are the same species. Even though you may have spent thousands of dollars on a purebred show dog, that dog is still able to mate and produce fertile offspring with the neighbor’s mutt.

According to the biological species concept, this means they are in the same species group.Let’s look at another scenario. Let’s say you have a pet donkey in your yard (hey, it cuts down on lawn mowing!). Your neighbor wants to breed your donkey with his horse, so he can have a nice, hardworking mule for his farm.

When that mule is born, you both try and breed it with others just like it, but nothing happens. No more baby mules! Why not? This is because donkeys and horses are different species. While they can mate and produce offspring, those offspring are not fertile. The mules cannot breed with each other to make any more baby mules. According to the biological species concept, this means the horse and the donkey are in different species groups.

Reproduction and Types of Mating

According to this definition, all we need to do to determine the species of something is have it mate.

While in many cases this definition is adequate, it can cause issues. Think about it…the idea of animals that are able to interbreed and produce viable offspring isn’t too hard to imagine, is it? The same thing with plants. But what about bacteria? Or viruses? Or some insects? These organisms commonly reproduce asexually, without mating, so they don’t need to breed. How do we determine what organism belongs where, when they don’t bother breeding? This is known as the species problem, and scientists have come up with various species definitions to try and solve this problem.Scientists are constantly trying out new definitions for species, especially now that we know a lot more about DNA. However, they haven’t yet come up with a good definition that encompasses all known life on earth.

The result? Different scientists define species slightly differently. While this can cause confusion, it allows scientists to effectively research different animals, plants, and bacteria. For the lay person, however, the basic idea that a species is a group of organisms that can mate and produce fertile offspring works just fine.

Lesson Summary

A species is the most basic unit of biological classification in systematics. It’s a complex idea but is most commonly defined as organisms that can mate with each other and produce fertile offspring that can breed with each other. While this definition doesn’t cover all living things on Earth, it is useful for the commonly studied animals.


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