Species Chapter 19
Definition of Species
According to the Stanford encyclopedia “the nature of species is controversial in biology
and philosophy. Biologists disagree on the definition of the term ‘species’”
- species are the fundamental taxonomic units of biological classification.
- Biological species concept defines a species as a group of organisms that can
successfully interbreed and produce fertile offspring
- The phylogenetic species concept defines species as a group of organisms bound
by a unique ancestry.
- The ecological species concept defines a species as a group of organisms that
share a distinct ecological niche.
How can so many definitions of species be used in a biological context?
There are several problems with the biological species concept defined above. One
important problem is that although the definition can work for species that reproduce
sexually, it does not deal with species that reproduce asexually. Patterns of reproduction
can blur the definition of species.
- Androdioecous organisms exist as natural populations of functional males and
hermaphrodites but include no true females.
- Gynogenetic species have only females. (females must mate with other species in
order to obtain sperm that is required to reproduce)
- Hybridization is when two species interbreed and produce fertile offspring
One size does not fit all
Organisms are a product of evolution, which is a dynamic process that does not easily
accommodate rigid definitions.
- The biological species concept defines species in terms of population genetics and
evolutionary theory in a static world
The definition alludes to the genetic cohesiveness of species.
1. populations of the same species are said to experience gene flow that mixes their
genetic material and could be the “glue” holding a species together.
2. Emphasizes the genetic distinctness of each species.
The biological species concept could explain why individuals of a species generally look
alike. If phenotype reflects genotype, then members of the same gene pool should share
genetic traits that determine phenotype.
- Morphological species concept is based on the idea that all individuals of a
species share measurable traits that distinguish them from individuals of other
Gene flow: four examples
Distribution influences gene flow, and sparsely distributed species may experience less
gene flow than those with a continuous distribution. Species may depend on other species
for dispersal. Social behaviour can also limit gene flow within continuous populations.
Changes in habitat continuity affect gene flow, and a discontinuous habitat may inhibit
gene flow between populations. Species Chapter 19
Subspecies are local variants of a species.
Various patters of geographic variation have provided insight into the speciation
1. Ring species: adjacent populations exchange genetic material directly and gene
flow between distant populations occurs only through the intermediate
2. Clinical variation: clinical changes are smooth patterns of variation along a
- Clines occur when there is gene flow between adjacent populations that are
adapting to slightly different conditions
Reproductive isolating mechanism is a biological characteristic that prevents the gene
pools of two species from mixing even when they are sympatric (occupying the same
spaces at the same time)
- Prezygotic isolating mechanisms exert their efforts before th