Principle of Systematic Biological & Evolutionary

Species concept By

Zoha Arshad

Species ConceptsDefining a Species is not as simple as

we might hope.There are many different Species

concepts.1. Typological species concept2. Nominalistic species concept3. Biological species concept4. Evolutionary species concept

Biological Species ConceptIt combines both Typological &

Nominalistic speceis concepts.According to Mayr:“Species are the groups of interbreeding natural

populations that are reproductively isolated from other such groups”.

It is the most widely used concept among Ecologists.

According to this concept species are; Ecological unit. ( Natural

populations) Genetic unit. (Interbreeding) Biological unit. Reproductive unit. ( Reproductively

isolated)

Reproductive isolating mechanisms:

Features that prevent mating outside the species.There are two types of reproductive isolating

mechanisms. Prezygotic Barriers.

Prevent mating between different species.

Postzygotic Barriers.A type of reproductive isolation that occurs after

members of two different species have mated and produced a hybrid offspring. Such hybrids are usually unable to reproduce.

Prezygotic mechanism:

It includes; Temporal isolation. Spatial isolation.

Mechanical isolation. Behavioral isolation.

Temporal isolation;Species have different breeding

seasons.Example:Western spotted skunks breed in fall

but the Eastern spotted skunks breed in late winters.

Western Eastern

Behavioral isolation;Signals to attract mates, elaborate

behaviors, courtship rituals differ between species.

Western and Eastern Meadowlark songs differ.

Twelve fiddler crab species inhabit a certain beach in Panama.

Males of each species have distinctive mating displays which include waving claws, elevating the body and moving around the burrow.

Mechanical isolation;

Physical or biological structures that prevent mating. For example difference in size or fit of genitalia may not allow mating. This can be found in certain snails, insects and plants.

The Bradybaena shown two different species of snails. The shells spiral in opposite direction, thus they are unable to mate with one another.

Spatial isolation;

It is also called geographic isolation. It is reproductive isolation of two or more populations of a species by distance or physical barriers.

Postzygotic mechanism; A type of reproductive isolation that

occurs after members of two different species have mated and produced a hybrid offspring. Such hybrids are usually unable to reproduce.

Example: A mule is produced when a horse and mare is mated.

Advantages;This concept suggests a research

program that will allow scientists to identify species using this concept.

It is the mark of a good theory E.g; A & B interbreed but neither with C because it is a different specie.

Disadvantages;Peterson (1958) suggested that the biological

species concept is faulty and should be replaced by a specie recognition concept. However, Coyne, Orr and Futuyma (1988) and Mayr (1988) showed that Peterson’s arguments are invalid, being laregely based on misunderstandings. The fact that difficulties sometimes arise when the biological species concept is applied to natural taxa does not mean that the concept is invalid. This has been shown by Simpson and Mayr.

Disadvantages/Problems; Insufficient information. Uniparental reproduction. Evolutionary intermediacy.

Acquisition of reproductive isolation without equivalent morphological change.

Acquisition of strong morphological difference without reproductive isolation.

The occasional breakdown of isolating mechanisms (Hybridization)

Evolutionary Species concept;

According to Simpson;“An evolutionary species is a lineage (an

ancestral-descendent sequence of populations) evolving separately from others and with its own unitary evolutionary role and tendencies.”

Historically, it is the most popular concept among paleontologists.

It is based on lineage, identity, tendencies and fates.

• Lineage (evolutionary tree of life)• Identity (biologically distinct identity)• Tendencies & Fate.

• Birth• Growth• Death

Species have an origin by Cladogenesis & undergo evolution by Anagenesis.

Anagenesis; “Species formation without branching of the

evolutionary line of descent.” It is also known as Phyletic transformation

and describes the process in which a specie gradually accumulating change, eventually becomes sufficiently distinct from its ancestral form that it may be labeled a new specie. With the passage of time, characters of species change and how species evolve from one another it does not explain.

Cladogenesis; “The formation of a new group of

organisms or higher taxon by evolutionary divergence from an ancestral form.”

It is a splitting event where present or current specie splits into two distinct species, forming a Clade.

References; Mayr, E and Ashlock, P.D, (Latest

edition). Principles of Systematic Zoology, McGraw-Hill Inc. New York.

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