Breed usually refers to domesticated animals, not wild animals. There are breeds of dogs, cows, and sheep, rather than lions, tigers, and bears. A breed is intentionally created by humans – selective breeding – and a species are created by nature.
Species is also a larger category than “breed.” Dogs are a species, and there are many breeds within this species. The opposite is never true – a breed is never split into different species.
What is a species?
A species is a group of very similar plants, animals, or bacteria that can reproduce with each other. If two animals are of different species, it is difficult or impossible for them to reproduce with each other.
A horse and a donkey can make a mule – but a mule is sterile, so they can barely interbreed. Biologists would not call two animals that can easily interbreed different species. A bluejay is a species.
Species appear when an animal slowly splits into two species. If two populations of crabs are isolated from each other for a long time, they will slowly become more different until they can no longer reproduce. This is known as speciation.
Scientists also define species by similarity. If they are physically, genetically, and behaviourally similar, they may count as the same species.
What is a breed?
A group of animals that is different from another group of animals because of natural selection is usually not called a breed. The term subspecies is used for small variations in wild animals. Breeds are subspecies created by humans.
Humans create a breed by emphasizing certain genes. People have bred cattle to be docile for thousands of years. All livestock animals are bred to produce enough meat or milk. A poodle is a breed.
Breeds of dogs were created by breeding the largest dogs, the smallest dogs, the more intelligent, aggressive, or harmless dogs. Breeds of dogs are very different, but all dogs are still considered the same species.
Animals of different breeds can usually also reproduce with each other. A different breed is only physically and behaviorally different. The DNA of two breeds is less different than the DNA of two species.
People do not usually define different variations within each breed. Scientists do define different subspecies, however. Most animals within the same breed look similar; this is not the same for species.
Other differences between the two
Plants and bacteria have species, not breeds. People do not usually refer to breeds of plants and never to breeds of microorganisms. Everything alive is divided into species; only some animals are divided into breeds.
Even between two dogs that look very different and are of very different sizes, there isn’t much genetic variation. Breeds of dogs differ in specific genes that make them look very different.
A horse and a zebra may seem more similar to each other than two very different breeds of dogs do. However, if you look at the DNA, the horse and the zebra are actually more different. All dogs came from wolves, and all of them have a lot of genes in common.
What is the difference between a species and a subspecies?
While scientists mostly agree on how species should be defined, they have more trouble with subspecies. Definitions of subspecies can get fairly subjective and uncertain. Biologists do their best to keep these classifications scientific, even though it is harder than with larger groups.
So we see that breed and species are related, and a breed is a type of domesticated animal.