Topic:

“Biological and Evolutionary species concept

One of the most elementary urges of humankind is to identify things and name them. Even the most primitive peoples have names for kinds of birds, fishes, flowers and trees. If only individual organisms existed and the diversity of nature was continuous, it would be difficult to sort them into groups and distinguish "kinds". Actually, at least in sexually reproducing higher organisms, the diversity of nature is discontinuous and consists in any local fauna of the more or less well defined kinds of animals we call species.

Defining a Species is not as simple as we might hope. There are many different “species concepts”.

Biological species concepts:An entirely new species concept began to emerge after 1750. It is argued by statements made by Buffon in his writings, Merrem, Voigt, Walsh and many other naturalists and taxonomists of the nineteenth century. K.Jordan (1905) was the first to clearly formulate the concept in all its consequences. It combines elements of typological and the nominalistic concepts by stating that species have independent reality and are typified by the statistics of populations of individuals.

According to Mayr;

“Species are the groups of interbreeding natural populations that are reproductively isolated from other such groups”.

As a result, the members of a species form a Reproductive community. The individuals of a species of animals recognize each other. So species are Reproductive unit. The species are also an Ecological unit which, regardless of the individuals that constitute it, interacts as a unit with other species with which it shares its environment. The species finally is a Genetic unit consisting of a large intercommunicating gene pool whereas the individual organism is merely a temporary vessel holding a small portion of the contents of the gene pool for a short period of time.

On the basis of definition, species are reproductively isolated, they process isolating mechanisms that isolate them.

Reproductive isolating mechanisms:

The features that prevent mating outside the species involve reproductive isolating mechanisms. There are two mechanisms of reproductive isolation.

1. Pre-zygotic Barriers 2. Post-zygotic Barriers.

Pre-zygotic Barriers:

These barriers prevent mating between different species. It includes Spatial, Behavioral, mechanical and Temporal isolation.

Temporal Isolation:

A mechanism, that prevents species from mating because they breed at different times. Different animals breed at different times. There are five different species of Frogs in South America but have different breeding seasons.

Example; Western spotted Skunks breeds in the fall but Eastern species breeds in late winter.

Mechanical Isolation:

There is a physical or biological structure that prevents mating. For example differences in the size or fit of genitalia may not allow mating or may be internal conditions of female does not accept the male sperm. This isolation can be found in certain snails, insects and plants.

Example: Two different species of snails Bradybaena. The shells spiral in opposite directions, thus they are unable to meet with one another.

Behavioral Isolation:

In all species male attracts the female. Each bird has its own peculiar song to attract its female. Postures or spot may also be important. Signals to attract mates, elaborate behaviors, courtship rituals differ between species.

Example: Eastern & Western species of Meadowlark songs differ.

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.

Post-zygotic 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.

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

Advantages:

This concept has its primary significance with respect to sympatric (occur in the same place) and synchronic (occur at the same time) populations and these are precisely the situations where the application of the concept poses the fewest difficulties.

Biological species concept suggests a research program that will allow scientists to identify species using this concept. It is the mark of a good theory. This makes it quite clear that the word species in biology is a rational term. A is a species in relation to B and C because it is reproductively isolated from them. So species can be defined as the individuals that can interbreed or have potential to interbreed.

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 largely 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.

Insufficient information.

We must have knowledge that two individuals can interbreed with one another or not, but it is not easy. Sometimes we have to place them in labs to observe breeding and sometimes they do not breed. It becomes suitable to define species on the basis of Morphology.

Uni-parental Reproduction.

This concept is applicable on bi-parental reproduction. This concept is not applicable on uni-parental reproduction. Sexually reproducing organisms are low in number. Fishes, reptiles reproduce both sexually and asexually. Aphids breed asexually when environmental conditions are suitable and wings are not produced but during unfavorable conditions, they breed sexually and produce wings. So species can not be defined on the basis of sexual reproduction.

Evolutionary intermediacy.

During evolution this is difficult to guess that the two species were two subspecies or two different species. Some species show more resemblance at one period or stage like 80% but become too different at other stage so it is hard to decide. In species, category name is binomial but when subspecies is added then it will become trinomial. All subspecies are trinomial. When there is no complete distinction among species or individuals to keep them in separate species, no distinctions in morphology or in reproduction then a decision is difficult when the acquisition of morphological distinctions is not closely correlated with the acquisition of reproductive isolation. The various difficulties for the taxonomists that may result from evolutionary intermediacy can be summarized as follows.

1. Acquisition of reproductive isolation without equivalent morphological change:

Reproductively isolated species without morphological difference are called “sibling species.”

2. Acquisition of strong morphological difference without reproductive isolation:

A number of genera of animals and plants are known in which morphologically very different populations interbreed at random wherever they come in contact. The typological solution of calling every morphologically distinct population a species is clearly inappropriate in such situations. Conversely, there are genera in which the isolating mechanisms between any two species break down occasionally. To consider such species con specific would be to go to the opposite extreme. No generalized solution is possible in cases where morphological divergence and the acquisition of reproductive isolation do not coincide. The only recommendation to the specialist is to delimit species in such a way that they form biologically meaningful natural entities. The difficulty posed by the rapid morphological divergence of populations that lack reproductive isolation is well illustrated by West Indian snail genus Cerion.

3. The occasional breakdown of isolating mechanisms (Hybridization):

Reproductively isolation may break down occasionally even between good species. Usually this leads only to the production of occasional hybrids which being either sterile or of lowered viability do not cause any taxonomic difficulty. More rarely there is a complete local breakdown of isolation that results in the production of an extensive hybrid swarm and more or less complete introgression. Hybrid individuals are sometimes described as species before their hybrid nature is discovered. Such names lose their validity as soon as the hybridism is established. Only populations are recognized as taxa , and hybrid individuals are not populations.

Evolutionary species concept:Some authors, particularly paleontologists, are not satisfied with the biological species concept because of its strict applicability only to the non-dimensional situation. They prefer a definition which as Simpson has said, involves evolutionary criteria. Simpson therefore proposed the following definition:

“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 used extensively in comparative biology and phylogenetic systematics.

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

Lineage:

An evolutionary lineage is a series of organisms, populations, cells or genes connected by a continuous line of descent from ancestor to descendant. They are subsets of the evolutionary tree of life. They are often determined by the techniques of molecular systematics.

Identity:

A biologically distinct entity is known as identity. It includes concepts associated with biological species. The biological species concept is the broadest general case of the evolutionary species concept because a biological species is an evolutionary species at a particular point in time.

Tendencies and fate:

Fate depends upon birth, growth and death or extinction. Species have an origin by Cladogenesis. Species undergo evolution by Anagenesis. And disappear by extinction when a lineage is terminated. In actual species have not risen 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.

Wiley (1981) attempted to improve the evolutionary species definition ‘Maintenance of identity is constructed as reproductive isolation’ but his definition is that of a specie taxon, not of the species category. Neither Simpson, Wiley, nor Henning has solved the problem how to deal with the relationship of descendant populations in a single lineage. Simpson arbitrarily divides the lineage into species by inferring reproductive isolation from the degree of morphological difference. By contrast, Wiley believes that “no presumed separate, single, evolutionary lineage may be subdivided into a series of ancestral and descendant species” a proposal that led Simpson to comment that one “could start with man and run back to a protist still in the species Homo sapiens.”

Finally, Henning arbitrarily terminated every evolutionary species when a daughter species branched off the parental lineage, ignoring the fact a peripatric speciation event usually leaves the parental species unchanged.

References:

Mayr, E and Ashlock, P.D, (Latest edition). Principles of Systematic Zoology, McGraw-Hill Inc. New York.

http://study.com/academy/lesson/temporal-isolation-example-definition-quiz.html