Identifying Species

What is a species?

The simplest answer to this question is that a species is a “type” of organism. We all recognize that there is a “boundary” that separates dogs from chimpanzees and chimpanzees from human beings, for example. For centuries, scientists distinguished one species from another by observable differences in external and internal features. Because different species can look nearly identical, and even members of the same species can look very different, behavior became an important clue. The ability to reproduce together was a key behavior used to determine species boundaries. This still generally holds true for organisms that reproduce sexually, although crossbreeding between species does occur in the natural world. This classification method is becoming outdated, however, as advances in DNA sequencing occur.

How is DNA sequencing influencing how species are identified?

Model of DNA

The genome of a species is considered to be unique to that species. The genome exists as segments of DNA in an organism’s cells — the 46 chromosomes in human cells, for example. Comparisons of segments of DNA that represent genes reveal how similar two organisms are. Scientists now use differences in DNA sequences as a more precise way of distinguishing among species and to propose their evolutionary relationships. At the simplest level, the degree of difference implies the degree of divergence from a common ancestor and thus allows the investigator to infer the relatedness among species. The assumption is that as two species diverge from one, their genomes will become more and more different. Using this assumption with DNA sequencing, chimpanzees and humans are inferred to be more closely related to each other than either are to dogs because their DNA is more similar.

The analysis of DNA sequencing still involves debate and varying practices. Differences in the DNA between separate species of mammals can be slight: Humans, for example, share 99.6% of their DNA with chimpanzees. For bacteria, however, current practice requires a 3% or greater DNA difference to constitute a separate species. It is easy to imagine how the diversity of bacteria could be easily underestimated by this practice — and bacteria are probably the most diverse group of organisms on the planet.

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