But what is a species?
Umm, problem. There are over 20 different definitions for a species. Some of these include:
-Biological definition (this is the one you must learn)
-Morphological definition
-Ecological definition
-Genetic definition
-Phylogenetic definition
Biological Species Concept
A group of similar organisms that are capable of sexually interbreeding to produce fertile offspring. Every species has a binomial name.
Homo sapiens
Pteropus natalis
Plebejus argus
The key thing here is that the offspring are fertile. Interspecific hybrids occur when closely related species breed together, but they are not fertile.
Limitations of the biological species concept
•It cannot be applied to extinct species because we cannot test whether individuals could interbreed.
•It does not work for asexual organisms such as bacteria since they do not reproduce through mating.
•It struggles with species that hybridise. Some distinct species can interbreed and produce fertile offspring, which blurs species boundaries.
•It is difficult to use for organisms with long generation times or those that are hard to observe mating, such as deep-sea species.
•It assumes populations are geographically together. When populations are isolated, we cannot always determine whether they would interbreed if brought into contact.
•It focuses only on reproductive isolation and ignores evolutionary relationships, genetics, morphology, behaviour and ecological roles.
Some other species definitions
Note that you are not expected to know these, but may be useful as a comparison with the biological species concept.
Morphological Species Concept
Differences in the form and structure of organisms is used to divide organisms into species.
However, there can often be lots of variation within groups, and it also needs to take into account sexual dimorphism. Sometimes scientist only have a tiny bit of a plant or animal. That means they can’t identify it based on morphology. Some species go through different stages of development. They can look very different at each stage. Think of a caterpillar that goes through metamorphosis. The adult butterfly belongs to the same species. But its morphology is completely different.
Or cryptic species, where organisms look the same, but are actually different species (Northern and Southern Leopard frogs).
Phylogenetic Species Concept
A species under the phylogenetic concept is the smallest group of organisms that share a common ancestor and have unique inherited traits that set them apart as a distinct evolutionary lineage.
Limitations include:
•Incomplete data: It relies heavily on full evolutionary histories, which are often unavailable, particularly for species with poor fossil records.
•Excessive splitting: Distinct genetic markers can lead to every small variation being treated as a separate species, even when this is not biologically meaningful.
•Rapidly evolving traits: It performs poorly for organisms whose traits change quickly or frequently.
Ecological Species Concept
Defines a species by the resources that the particular group of organisms uses, and depends on the specialised ecological niche the group occupies. However, niches tend to be assumed and are difficult to define completely. Not a particularly robust concept!
