For sexually reproducing species, progeny result from the fertilization of an egg by a sperm. Organisms that can interbreed and produce fertile offspring are members of the same species. Organisms that can’t interbreed and produce fertile offspring are separate species. Case closed.
* Asexual Reproduction Produces Clones *
However, there a many organisms that miss out on all the fun and instead reproduce asexually. They accomplish reproduction the same way that the cells of the human body divide to make more cells, as a person grows, develops, or heals from an injury. A cell makes a copy of its genetic material and then splits in two, each new cell with a complete copy of the genome. This method of reproduction results in clones, daughter cells that are exactly the same as the parent cell that generated them.
So if asexually reproducing organisms are clones, how is it that there are different species of them; and how are distinct species defined in organisms that do not reproduce sexually?
* How Are Species of Asexual Reproducers Defined? *
Genetic mutations are the raw material of genetic change, the differences between all living things ultimately started with mistakes when the genetic code was copied within a dividing cell. But asexually reproducing organisms are much trickier to classify into distinct species, since the definition, “Organisms that can breed and produce fertile offspring” can’t be used, since they don’t breed.
* Genetic Promiscuity of Bacteria *
Bacteria are a great example. These prokaryotic cells divide to reproduce, making clones, like all asexual reproducers. But bacteria are also genetically promiscuous. They share their genes with other bacteria in unusual ways, collectively known as horizontal gene transfer.
* Types of Horizontal Gene Transfer *
Sometimes one bacterium can hook up to another and transfer a few genes to the recipient bacteria, a process called conjugation. In other instances, bacteria can absorb naked DNA that they find floating in the environment, a process called transformation. Even more bizarre is transduction, in which a virus that infects a bacterium injects its host with the DNA obtained from a previously infected bacterium. These genetic exchanges can even occur between bacteria that are considered different species. Therefore, species of bacteria must be defined mainly through clues that microbiologist gather about there identity.
* Genomes and Metabolomes of Bacteria *
Often classification of bacteria genera or species of relies on the products of bacterial genomes, enzymes that confer distinctive capabilities. The metabolic products, such as enzymes, produced by an organisms’ genome is considered its metabolome.
* Key Metabolic Differences between Bacillus and Clostridium *
For example, some bacteria can tolerate and use oxygen. For other species of bacteria, oxygen is poisonous. Whether a bacterium can tolerate oxygen is based on its ability to produce two specific enzymes; catalase and superoxide dismutase (SOD). Without these enzymes, bacteria die in the presence of oxygen’s toxic waste. Species of the Gram-positive, endospore-producing genus Bacillus are obligate aerobes, meaning that they require oxygen, and are able to produce these two enzymes. Species of the Gram-positive, endospore-producing genus Clostridium are obligate anaerobes, unable to produce these enzymes. Clostridia die in the presence of oxygen.
* Sources *
Bauman, R. (2007). Microbiology with Diseases by Taxonomy. Pearson Benjamin Cummings.
Bauman, R. (2004). Microbiology. Pearson Benjamin Cummings.