The above post has barely anything to do with the question. 

Dear Harsimran,

Species that are members of the same genus share the same broad adaptive zone. An adaptive zone represents a general ecological lifestyle more basic than the narrower ecological niches characteristic of individual species. This ecological definition of genus can be an immense aid in interpreting, for example, fossil primates. Teeth are the most frequently preserved parts and they often can provide excellent general ecological inferences.  

You define your answer quite well, genera is some group of individual (species) with broad commonality. In taxonomic language, there should be a special Character (genitalia, plumage, shape, coloure pattern) that markedly different from other group of individual. Generic limit is a boundary that is anthropocentric, it will change according to taxonomist discreteness.  Where as in Ecology, virtually there is no genus concept but it is said that alike species share or interact with their niche (habitat niche, food niche, hyper volume) and then a concept grow on larger super species or Genus.

You said that genera are always monophyletic, it's not true. there are many example in insect world that some genera are polytyphyletic. That's way reticulated species and genus build up.     

I think you should say that ideally, a genus should be monophyletic, but genetic analysis has shown repeatedly in many many taxa that this is simply not true. For metazoans (and land plants), genera are more usually morphological groupings. 

I don't like the ecological concept of a genus. Nor do I like one based on a specific character. What happens when exceptions to these rules are discovered? 

Although I don't think biologists should spend too much time trying to define what a genus is or is not (debates about the nature of species have really hindered our progress of the study of speciation), I think that there may be a biological basis that one might use to define the term genera. 

One thing that we have learnt is that many closely related species (even the ones that remain distinct in sympatry) hybridize at some level, even if its low. Coyne and Orr, Mallet and others have argued that despite these low levels of gene flow, the process has advanced to the point where they should be considered species even if they don't fully adhear to a strict BSC. In this case, these taxa would clearly be part of the same genus. This definition falls apart if we find other closely related taxa that don't that hybridize. 

Interesting discussion.

Genus contain variety of species so the size of genera varies.

There are various definitions to define what species is, still we face problematic issues. We can address them hybrid zones where two different species come to contact so that exchange of genetic material takes place. Geneflow thus result decrease variation between the two groups. So geographic variation thus show substantial variation may leads to variation in local species form subspecies.

I believe certainly ecological view give some information about speciation. 

In a nutshell speciation must include the following

- Change of geneflow within or between population either by geographical isolation or dispersal or intrapopulation adaptations that leads to genetic changes.

- Divergence of individuals within populations by genetic drift or mutation or selection.

- Reproductive isolation develop gradually as a result of this divergence.

- This entire process with desirable consequences maintenance of genetic identity through selection against hybrids. 

A good paper to read about how 'real' genera might be, and following which concept, is "Concept versus Data in Delimitation of Plant Genera" by Humphries & Linder - available on JStor.

Hi!

Refer Evolution by Douglas Futuyma the following chapters you get clear information about speciation.

The following links too give sufficient infromation

http://www.globalchange.umich.edu/globalchange1/current/lectures/speciation/speciation.html

http://evolution.berkeley.edu/evolibrary/article/evo_40

Chapter 17 Outline

What Are Species?

Phylogenetic species concepts

The biological species concept

Domain and application of the biological species concept

Barriers to Gene Flow

Premating barriers

Postmating, prezygotic barriers

Postzygotic barriers

Multiple isolating barriers

How Species Are Diagnosed

Differences among Species

The Genetic Basis of Reproductive Isolation

Chromosome differences and postzygotic isolation

Genes affecting reproductive isolation

The significance of genetic studies of reproductive isolation

Genetic Divergence and Exchange

Ancestral variation and coalescence

Gene flow and hybridization

The fate of hybrid zones

Chapter 18 Outline

Modes of Speciation

Allopatric Speciation

Evidence for allopatric speciation

Mechanisms of vicariant allopatric speciation

Ecological selection and speciation

Sexual selection and speciation

Reinforcement of reproductive isolation

Peripatric speciation

Alternatives to Allopatric Speciation: Speciation with Gene Flow

Genomic studies of speciation with gene flow

Parapatric speciation

Sympatric speciation

Polyploidy and Recombinational Speciation

Polyploidy

Recombinational speciation

How Fast Is Speciation?

Consequences of Speciation

The above post has barely anything to do with the question. 

 @Sean Stankowski I am not expert like you. So thought people like me who follow the question should know right from the beginning so posted details about speciation though I posted my answer briefly. 

'Genus is the first part of the binomial nomenclature of species name for each species within the genus' as Harsimran asked to discuss about genus and ecological supplement.

I am grateful if you provide the details how it is not relevant.

Thanks.

I agree with Harsimran that a genus should be a monophyletic taxon (if we know it), but as for all Linnean categories, there are no objective limits for a "genus". Even when you have a nice cladogram, it is still completely arbitrary at which node the genus "begins" or where any higher taxon "begins". The decision will be different, depending entirely on the author (if he is a "splinter" or a "lumper" :-)). The category "genus" has only some practical importance to indicate a closely related group of species with well recognizable character(s). The problem is even worse for the higher categories which should be rejected all together (because arbitrary). I also think the category "subspecies" is rather useless. Unfortunately, the attempt of the "phylogenetic nomenclature", to stabilize (at least) the names of species, failed...

Hi!

I posted the utmost basic things like what a species is, how they develop. It should be known before to interpret about dendrogram/cladogram. So things are clear. I know all are experts not laymen except me.

Go through the following. In fact already explained by Ernest.

As per biological taxonomy monophyletic group is if all organisms in that group are known to have developed from a common ancestral form, and all descendants of that form are included in the group.

Eg. all organisms in the genus Homo are known to have come from the same ancestral form in the family Hominidae, and no other descendants are known. Thus the genus Homo is monophyletic. On the other hand, it were discovered that Homo habilis had developed from a different ancestor than Homo sapiens, and this ancestor was not included in the genus, then the genus would be polyphyletic. Since biologists by and large prefer groups to be monophyletic, in this case they would likely either split the genus or broaden it to include the additional forms.

Ref. Encyclopedia.

The concept of genus is not clearly formed. There is room for a lot of conjecture on this. This could be one of them: There is recognition of the fact that there are selective sweeps operating on natural populations. This reduces continuous variation to discrete ones by funneling large variation into bins that make up ranks. From a functional point of view, these selective sweeps are probably driven by available niches. This probably results in a collection of individuals belonging to genus probably define a ecological niche.