Biological Species Concept

Attempting to determine whether different animals are the same species by appearance (phenotype) has been used extensively over the years, but may not be reliable, due to the subtle variations that are displayed. Morphological traits may not always be useful in distinguishing species. Members of the same species may appear different because of environmental conditions. Morphology can vary with age and sex. Different species can appear identical.

The biological species concept relies on reproduction to define relatedness of species. Ernst Mayer is credited with developing the official definition of a species: "Species are groups of interbreeding natural populations that are reproductively isolated from other such groups."

As good as it is, this definition is troublesome for organisms that are non-sexually reproducing, for those known only from fossils, or when life histories have not been studied.

As long as gene flow continues individuals will remain members of a species even though they may be geographically distant.

Genetic divergence is the process whereby local units of a population become reproductively isolated from other units. When this happens, genetic drift, natural selection, and mutation are free to operate in each isolated population and thus lead to changes in gene frequencies.

Whether or not a physical barrier deters gene flow depends upon an organism's mode of dispersal or locomotion and the duration of time the organism can move. Sometimes it is easy to see how an animal that flies long distances would be able to take its genes from one place to another, but what about the slow-moving snail? Robert Selander studied Helix aspersa. He collected snails from a two-block area and analyzed the allele frequencies for five genes. He found that even snails confined to relatively small areas show genetic variation that could possibly lead to eventual speciation many years hence.

Speciation is the process by which species form when two genetically diverging species remain isolated from each other. Because genetic divergence is a gradual process, it is impossible to say precisely when species form.

REVIEW: In the biological species concept of Ernst Mayer, what aspect of a population is critical to determining a species?
Answer interbreeding capabilities