Species are groups of actually or potentially interbreeding populations which are reproductively isolated from other such groups. The biological species concept has been prevalent in the evolutionary literature for the last several decades and is emphasized in many college-level biology courses. It is probably the species concept most familiar to biologists in diverse fields, such as conservation biology, forestry, fisheries, and wildlife management. Species defined by the biological species concept have also been championed as units of conservation. The species concept for most phycologists is based on the morphological characters and hence the term ‘species’ means morphospecies. On the other hand, for evolutionary biologists, the term means biological species that can be defined as a reproductive community of populations (reproductively isolated from others) that occupy a specific niche in Nature.

1. SPECIES CONCEPT
P MadhusudanaPatra
Research Scholar
Department of Zoology
University of Madras
(Presidency College)
Chennai-5

2. SPECIES CONCEPT
Cuvier in 1829 defined as ‘ The assemblage descended from one another or from common
parents and of those who resemble one another.
These taxonomic or naming systems are based on judgements about the degree of similarity
among organisms.
Species are the taxonomic units of biological classification, it is the smaller evolutionarily
independent unit.
Evolutionary independence occurs when mutation, selection, gene flow and drift operations
populations separately.
Evolution consists of changes in allele frequencies and species from a boundary for a spread of
alleles.
 TYPES OF SPECIES CONCEPT:
A) Morphological species
B) Biological species
C) Evolutionary species
D) Phylogenetic species

3. A) MORPHOLOGICAL SPECIES:
In traditional cultures, people name species based on morphological similarities
and differences.
In biology, careful analyses of morphological characters of adults and other life
history stages of multicellular organisms have been used to define species.
Based on uniting individuals who share more characters (features) with one another
then they do with any other organisms is called morphological species.
The morphological species concept is sometimes referred as morphospecies.
Morphospecies can be identified in species that are extinct or living and in species
that reproduce sexually or asexually.
Both adults anatomy and features of embryos provide importance classes of
information from which relationship between morphospecies can be determined.
Specialists in particular organisms like Entomologist, Ornithologist, developed
branching (Dichotomous) key based on morphological features as aid to species
identification.

4. DICHOTOMOUS KEY

5. Disadvantages:
When it is not applied carefully, species definition can become arbitrary and
idiosyncratic. In the worst-case scenario, species designations made by
different investigators are not comparable.
This concept can be difficult to apply in groups like bacteria, archaea, and
many fungi that are small and have few measurable morphological characters
to assess.
Fossil species that different in color or the anatomy of soft tissue cannot be
distinguished.
Similarity in morphology but were strongly divergent in traits like songs,
temperature or drought tolerance, habitat use or courtship displays. Whether
living or fossil, populations like these are called cryptic species.

6. B) BIOLOGICAL SPECIES
Biological species concept was propounded by Ernst Mayr in 1942 as “A biological
species are group of interbreeding natural populations that are reproductively
isolated from other such groups”.
Specifically, if population or organisms do not hybridize regularly in nature, or if
they fail to produce fertile offspring when they do, then they are reproductively
isolated and considered good species.
The greatest strength of the BSC is that reproductive isolation is a meaningful
criterion for identifying species because it confirms lack of gene flow. lack of gene
flow is the litmus test of evolutionary independence in organisms that reproduce
sexually.
Applying the biological species concept also allows taxonomist to separate into
different species groups that had been regarded as a single specie on the basis of
morphological and/or geographical criteria. Such pairs of species are often known
as sibling species.

7.  The two leafy-steamed sibling species in a genus of the phlox family,
the bird’s eye gilia, Gilia tricolor, and angel’s gilia, G.angelensis, are
an example from the plant kingdom.
 Although they are closely related species and co-exist in the same populations there is a strong reproductive
incompatibility barrier, which allows for minimal hybridization.

8.  Europian short-toed and common (Eurasian) tree creepers (Certhia
branchydactyla and c.familaris)- An examole ftom the animal kingdom.
 Differ morphologically only in the size of the bird toe and the patterning of the feathers on the
wing (Figure 2.6a,b) but their distribution, behavior and ecology are sufficient distinct to prevent
interbreeding

9. WHAT ABOUT BACTERIA AND ARCHAEA?
Many eukaryotes and in all bacteria and archaea, reproduction takes place asexually.
Thus there is no exchange of genetic material when bacteria and archaea reproduce.
When gene flow does occur between bacterial cells, it is limited to small segments of the
genome.
Gene flow in bacteria and archaea is also unidirectional, occurs in absence of
reproduction or may result in genetic recombination (i.e. creation of a chromosome with
a new combination of allele.)
Gene flow plays a relatively minor role on homogenizing allele frequencies among
bacteria populations.
In the organisms like bacteria and archaea the primary consequence of gene flow is the
certain cells acquire alleles- via one way flow from other cells with high fitness
advantages, such as sequences that confer antibiotic resistance or give the recipient cell
the ability to use a new type of sugar or other energy source.
Based on these observations, Lewrence and Ochman (1998) have proposed that acquiring
novel alleles through lateral gene transfer is the primary mechanism for speciation in
bacteria.

10. Cont..
 The logic is here that bacterial and archaeal species are best defined in an
ecological content, based on their availability to thrive in a particular
environment.
 This ability, in turn, is dependent in which genes are present in the genomes of
these organisms. Because the genes required for using particular sources of food
are often required through gene flow.
 The ecological view considered nicely with the idea that gene flow triggers
speciation in bacteria and archaea, instead of impending speciation as it does in
eukaryotes.

11. C) EVOLUTIONARY SPECIES:
Various specialists proposed an evolutionary species concept, which
considered the species as evolutionary entity.
Here species are defined by differences that are depend on their
“evolutionary” isolation rather than on their reproductive isolation.
American vertebrate paleontologist George Gaylord Simpson (1902-1984)
wrote in 1961, “ an evolutionary species is a lineage (an ancestor-
descendant sequences of populations) evolving separately from others and
with its own unitary evolutionary role and tendencies”

12. D) PHYLOGENETIC SPECIES:
 Also known as genealogical species.
 A phylogenetic species is defined by ancestor-descendant relationships
rather than by reproductive isolation.
 E.O. Wiley modified Simpsons evolutionary species definition to read; “A
species is a single lineage of ancestral descendant populations of organisms
which maintains its identify from other such lineages and which has its own
evolutionary tendencies and historical fact.”
 Under phylogenetic species concept, species are identified by estimating the
phylogeny of closely related populations and finding the smallest
monophyletic groups.

13. PHYLOGENENTIC SPECIES
 On a tree like this, species form the tips. For example, if the taxa labelled A-G in figure-1
represents population as opposed to genera, family, orders on the tree and represent
distinct species.
 In contract if population cannot be clearly distinguished in a phylogeny by unique, derived
characters, then they will form clusters like the populations designed B,E and G in figure-
1. The populations that make up these clusters would be considered part of the same
species.

14. APPLYING SPECIES CONCEPTS: Two case Histories.
I) DIVERSIFICATION IN MARINE COPEPODS:
Marine copepods are small crustaceans that are extremely abundant in the world’s ocean.
For example species Eutytemora affinis is only 1-2mm long but is the most important grazing animal in
many of the world’s estuaries (see below figure).
In many cases fish that spend their juvenile stage feeding on Eutytemora affinis and spend their adult lives
in the open ocean.
Altough E. affinis is found along the coasts of Asia, Europe, and North America. traditional analyses based
on the morphospecies concept had grouped all of the populations in to the same species.
To test this hypothesis, carols Eunmi Lee(2000) collected E. affinis from a wide array of locations
throughout the north-east biosphere.
To assess whether some of the 38 populations in her sample represent separate species under the
biological species concept, she tested individuals from several populations for the ability to mate and
produce fertile offspring.
To address the same question with the phylogenetic species concept, she sequenced two genes and used
similarities and differences in the base sequences observed in the 38 populations to estimate their
evolutionary relationships.
HER RESULT?
The phylogeny clearly

15. HER RESULT?
The phylogeny clearly showed that at least eight independent species exist, with
each occupying a distinctive geographic area (figure 3).
These results were supposed by the results of the mating tests, which showed that
population from different phylogenetic species are unable to produce fertile
offspring and are this reproductively isolated.
The take-home message of this study is that species diversity in copepods is likely to
be far grater than previously thought.
Employing more than one species concept can help biologists recognize diversity and
organize research on its consequences.

16. II) HOW MANY SPECIES OF ELEPHANT LIVE IN AFRICA?
Traditional the world’s elephants have been classified as two species; African (Loxodonta africina) and Asian (Elephas maximus).
This morphological analysis began to challenge this view, however, by pointing out that African elephants that live in forest habitats
versus savanan or grassland habitats have distinctive morphological features (features 4a).
Due to the populations that inhabit two habitat types don’t interact regularly, individuala rarely have a chance to interbreed. As a
result it has been difficult to assess whether forest and savannan elephants qualify as separate species under BSC.
To clarify the situation , Alfred Roca and coworkers (2001) applied the PSC.
They began by collecting tissue samples from 195 elephants in 21 populations throughout central and south Africa.
They then isolated DNA from the tissues, sequenced 4 genes from the each individual and used the resulting data to estimate which
populations were closely or more distantly related.
RESULT?
 The phylogeny that resulted from this analysis clearly showed that forest and savanna elephants qualify as distinct physiological
species (Figure4b).
 The group has proposed naming the forest elephants Loxodonta cyclotis and retaining the name L. africana for the savanna/
grassland populations.
 This result has a sense of urgency because many elephant populations in Africa are declining due to habitat loss and illegal hunting.
 Based on the above work, it is clear that conservation program should be focused on preserving both forest and savanna populations
as distinct entities.
REMEMBER: Reliable criteria for identifying species are essential for preserving biodiversity.

17. References:
 Scott Freeman and Jon C. Herron, 2016, Evolutionary Analysis.
 Brain K. Hall and Benedikt Hallgrinsson, 2014, Evolution.
 Pavan Kumar and Usha Mina, 2017, Life Sciences-I.
 Internet