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Biologists have stated that a species is a unit of evolution and takes an important role in conservation when determining which particular group requires our attention (and funding). In Latin, species stands for “kind” and according to the Oxford Dictionary, a species (abbr.: sp., spp.) is a group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding.

So how do you determine what is a species or not when “similar” and “unit” are your only indicators of speciation? In reality, speciation is still a work in process with many definitions illustrating how two species differ and have diverged from one species:

1) Morphological Species Concept (MSC)

MSC is a method that most biologists still use today to identify species, namely via the similarity of physical appearance. On an intuitive level this makes sense because how would you be able to even know in the wild what’s different if it didn’t look different first? However, looks aren’t everything and so the the disadvantage MSC are that it does not genetic similarity, evolutionary history, ecology or reproductive biology. Furthermore, groups with high phenotypic variability are split unnecessarily and groups that look superficially similar are lumped when they ought to stay separate.

2) Biological Species Concept (BSC)

BSC, also known as the Reproductive Isolation Species Concept, states that species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. In order words, species are species when there is gene flow between populations. Today, it is the most accepted species concept and the basis of the US Endangered Species Act (ESA). However, BSC does not take into account of genetics or history (phylogeny). And so if successful hybrids can be produced (i.e. the offspring are fertile and viable), the two species are grouped into one species. Furthermore, lab situations in determining whether two populations will interbreed or not present an unnatural setting, lumping the two populations into one species when perhaps they shouldn’t be (think of Cama, the offspring of a llama and camel. Another species or…?). Practically speaking, BSC cannot be applied to most organisms as well, including those without sexual reproduction and those that are now extinct. Lastly, successful hybridization may occur among species ecologically, geographically and structurally different, which does not necessarily indicate speciation. Think the Cama (Camel + Llama) or the Liger and her liliger cub.

3) Phylogenetic Species Concept (PSC)

PSC, also known as the Evolutionary Species Concept, characterizes a species as the smallest group of genetically distinctive individuals with a discrete evolutionary lineage. In other words, according to PSC a species is a group of organisms that all share a derived character state (including DNA) and that can not be subdivided into smaller groups. The advantages of using PS is using quantitative and testable (albeit expensive)
data to classify species, regardless of whether or not the species is asexual or extinct, and calling one species into several species based on unique derived characteristics. Unlike BSC, which tends to lump distinctive populations together (and so unless the entire species is under threat, specific populations may not receive conservation attention), it can give protection to distinctive populations under threat.

However, phylogenetics change with additional genetic data, which may divide species into groups based on characteristics with no clear biological relevance. Also, according to PSC, any unique genetic character defines a population as a species/ This may subdivide a species into many more species unnecessarily as species, who are able to reproduce with one another, may merge over time.

Below highlight the differences between BSC versus PSC:

Subspecies: If the two groups would interbreed if a external barrier were removed

  • BSC – 1 species
  • PSC – full species or subspecies

If groups cannot reproduce, both BSC and PSC consider separate species

  • BSC – ability to reproduce criterion
  • PSC – must have different derived traits preventing them from reproducing

If can reproduce and have distinctive, derived, trait:

  • BSC – 1 species
  • PSC – separate species

In general, here’s how to determine Species diversity:

Species diversity = Species Richness + Species Evenness

Species Richness = the number of species at a given place and time (weighs all species equally and doesn’t taken into account that species are usually not equally common).

Species Evenness = refers to how close in numbers each species in an environment are and quantifies how equal the abundances (i.e. the number of individuals in the community or the number of individuals in a given species) of the species are.

In general, the tropics are more species rich then temperate areas, though temperate areas have greater evenness (and thus, higher functional diversity). Consequently, if a disturbance occurs, the species in the tropics, unless the change favours a particular dominant species, will have lower resistance than the species experiencing the same stress in temperate regions.

The latitudinal diversity gradient (LDG) refers to the increase in biodiversity that occurs from the poles to the tropics. Put another way, in the present day localities at lower latitudes generally have more species than localities at higher latitudes. This may due to the fact that temperate areas are colder and drier than tropical regions, that the tropics are simply older and hence have more species accumulation and that speciation rates are higher in the tropics due to ecological niches and resources.

References

Chaves, Johel. “Concepts of Diversity.” CIEE Tropical Diversity Lecture. September 2014.