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Cats and dogs are not the same species. Clearly they don’t look the same, but is that enough to say they are different species? Imagine that we compare a Beagle, a short and stout dog with spots, to a Komondor dog, also known as a ‘mop dog’, which has long cord-like hair that nearly touches the ground and looks like a walking mop. Based on appearances, they look as different as the cat and the dog did, and yet they both belong to the same species. Dogs actually are part of a sub-species called Canis lupus familiaris that are derived from the grey wolf Canis lupus.
Appearances are not everything. Another way that species are defined is by their ability to breed together - in other words, to produce offspring. We know that a cat and a dog cannot have babies together, so they must be separate species. A beagle and a komondor dog can have puppies together (although they might be strange looking). While the ability to breed together is a useful concept when trying to define a species, it can be limiting. If you wanted to determine that a crocodile and a flamingo were separate species by attempting to breed them, the crocodile is more likely to try and eat the flamingo than to breed with it! There are also many exceptions to the breeding rule; for example, a Great Dane cannot breed with a Chihuahua, even though they are both Canis lupus familiaris. On the contrary, a donkey and a female horse (both separate species, donkeys are Equus africanus asinus, horses are Equus ferus caballus) can breed together to make a mule. The mule is called a hybrid, because it is a blend of two species; however, it cannot breed to make more mules.
Scientists can now use DNA to help define species. In the 1950s, the structure of DNA was discovered, and scientists would soon learn that DNA was the way that organisms pass along information to their offspring. The structure of DNA is very complicated; it is made of billions of chemical subunits called nucleotides. There are only four major types of nucleotides – Adenine, Cytosine, Thymine and Guanine (abbreviated as A, C, T, and G). These four nucleotides are linked together into codons which are segments of DNA composed of three nucleotide base pairs. Codons define specific amino acids, like arginine, lysine and methionine. In turn, sequences of amino acids make genes that are the codes for different proteins that make up an organism. Genes are what ultimately produce the characteristics and features of an organism... genes encode the cat’s whiskers, the dog’s hair, and the crocodile’s appetite.
Most members of a species have very similar DNA. If you compared the DNA of yourself to your neighbor, the DNA sequence (the order of the nucleotides) would be about 99.9% the same. That means that only 0.1% (or about 3 million nucleotides) is different. This sounds like a lot, but remember, there are over 3 billion nucleotides in a human’s DNA. Some of these differences are called polymorphisms, and they can make a person more or less susceptible to diseases, or cause unique traits (like eye colour). If you compare human DNA to chimpanzee DNA, they are actually about 96% similar. The 4% that is different means that we don’t climb trees every day, and a chimpanzee cannot drive a car...we are a separate species. Other species differ even more when compared to humans; the fruit fly (Drosophila melanogaster) genome is only about 60% similar to the human genome.
Another way to measure differences between species is called DNA barcoding. A DNA barcode is a relatively short and simple section of DNA that helps define the species, a lot like the barcode on packages at a grocery store. Scientists all agreed to use one gene in particular for all animal species, called Cytochrome c oxidase subunit 1, or CO1 for short. They chose CO1 because it is a gene that is essential for life, and almost all organisms have it in their mitochondria (which make the energy needed to live!). The barcoding of plants is slightly different because it requires the use two different chloroplast genes because plants do not have any mitochondrial DNA.
The barcode method is a lot easier than trying to determine the sequence of the whole genome, which is currently very expensive and time consuming. To make a DNA barcode, the nucleotide sequence of CO1 is determined in the laboratory. Then, computer programs compare it to other sequences. If two organisms have less than 2% difference in their DNA barcode, than it is very likely that they are part of the same species. DNA barcoding is useful, but it does not work for some species, such as those in the Phylum Cnidaria (Jellyfish).
In summary, there are at least four ways to define a species. The first is by appearances, the second is by the ability to breed together, the third is by comparing their entire DNA structures, and fourth is by determining their DNA barcode. This works well for most organisms, except for some microorganisms such as types of bacteria, which are able to directly share genes with each other. Understanding more about how to define a species is especially important as we enter a future where more and more of the Earth’s species are becoming extinct.