YES or NO... ?

This would explain the potential changes in the frequency of phenotypic changes (due to genetic drift) in animals / finches / tortoises that Darwin noticed on or in the Galapagos islands vs. similar changes (or a lack thereof) in their main land relatives.

Remember, Darwin sailed around the world, saw a great deal of flora and fauna, but it was the Galapagos islands that provide the important location for his idea of natural selection.

Really, all Darwin observed were phenotypic "variations of different shared phenotypes" 

Question: Are there any constraints to evolutionary changes?

Answer: Yes.

At this stage, however, the scientific method and our understanding of these "constraints" upon evolutionary change can get bogged down in details.......

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Remember, implicit within this understanding of Evolution is that Evolution works on populations of "individuals" -each contributing a slightly different genotypic "signature" to the population's gene pool.

How did we get such a diverse gene pool?

Unfortunately, that question we will have to leave for another day -once we have dealt with Genetics and Mendal and his findings.

For now, I would like to ask how do we now adapt Darwin's evolutionary principles and begin to determine that there are sufficient differences between populations with "similar, but different, genetic makeup", and assign to each a different name,.....

....In essence how do we determine and then justify the term SPECIES that Darwin uses in his book, The Origin of Species, ?

Darwin and his contempories used appearance(s) (phenotype[s]) of animals/plants to define differences......often to distinguish one species from another, or (at the very least) recognize members of the same species present in different geographical locations.

   

Over almost 300 years ago, Carolus Linnaeus -following on from John Ray (> 350 yrs ago)- created the system of naming organisms that we still use today. even so, neither of these men were the first to do so, officially (remember Aristotle).

Linnaeus classified over 4,000 organisms based on their physical characteristics and appearance, using a morphological concept of species, and his best-known accomplishment is creating a hierarchical system of organization for living organisms, using, what is called a binomial nomenclature. K P C O F G S

How many kingdoms are there....? (A) 3, (B) 4, (C) 5, (D) 6, (E) 8

"Species are groups of interbreeding (or potentially interbreeding) natural populations which are reproductively isolated from other such groups."

Problems....

One cannot apply the BSC definition to a considerable number of asexual populations. 

Application of the BSC toward self-pollenating plants is (by its own definition) somewhat limiting.

Moreover, it is often very difficult to apply (practically) anyway.  It requires breeding tests to verify the separation, but this is rarely used.  Indeed, were breeding tests to be used to define such criterea, natural environments would potentially play an overiding role. 

e.g. Do Homo erectus, Homo neanderthalensis and Homo sapiens represent the same or different species? This question is unresolvable using the BSC definition, especially when (as we now suspect) Neanderthals and Homo sapiens may have interbred.

Other Definition of Species include: The Phenetic (or Morphological) Species Concept -Cronquist (1988)- who proposed an alternative to the BSC that he called a "renewed practical species definition", where he very clearly defines species as

"... the smallest groups that are consistently and persistently distinct and distinguishable by ordinary means."

As you can see, determining if two populations are actually different "species"a can be quite a difficult task, especially as the very process of speciation is often a gradual process.

In a more current definition of the term "species", the concept of genetic integration has been coined, and used to differentiate between SOME types of species.

Genetic integration works on the premise that if individuals within a population mate with one another, but not with individuals of other populations, they are considered to be an "independent evolutionary unit", and can safely be called a Species.

But just how do/did New Species Arise?

Speciation is the process by which one "species" splits into two.

We have already observed some possible mechanism of a potential speciation....have we not?...in the "black-bellied seed cracking finches of West Africa. But not all such evolutionary changes result in new species. The critical process in the formation of new species is the segregation within the gene pool of the ancestral species into two separate gene pools. Consequently, such speciation is often facilitated by an interruption of gene flow between or among different populations.

Allopatric speciation requires total genetic "reproductive" isolation.....or, when two or more parts of a single population become divided by a geographic barrier, alternatively known as geographic speciation.

Allopatric Speciation is thought to be the dominant form of speciation among most groups of organisms.

Even so, Darwin's seed-eating and bud-eating finches of the Galapagos archipelago demonstrate the importance of geographic isolation as a distinction that (along with population size, perhaps) can enhance the process of speciation.

However, the effectiveness of any barrier -geographical or otherwise- in preventing gene flow depends on the size and mobility of the species in question.

eg. an impenetrable physical barrier for a terrestrial snail may be not be as remarkable a hurdle for a butterfly.

Indirect evidence that most speciation among animals is allopatric is provided by patterns of species distribution.

These findings can lead to a tantalizingly plausible (but ultimately false) "aboslute" inferences......... if that geographic barriers always give rise to speciation....all organisms that are separated by a geographical boundary become species......long-distance romances don't work?

Unfortunately, it is not true...long-distance relationships can work. 

Geographic isolation does not necessarily lead to reproductive incompatibility, as evidenced by the European and American sycamores.

Apparently, 20 million years of separation isn't such a long time if neither partner has to change too much.

Consequently, "physical" barriers need not be the only part of the "species" equation. 

Anything that begins to separate the gene pool of the ancestral species will suffice.

Speciation, in the absence of any geographical barrier -or the subdivision of a gene pool when members of the daughter species are not geographically separated, but the two species become distinct as a function of inherent genetic polymorphism- is called...

Sympatric speciation.

Any example of a FUTURE sympatric speciation even spring to mind??

(A) American Sycamores?

(B) West African Finches?

(C) Long necked Giant Toitoise in the Galapagos Islands?

(D) Birds and Bats?

(E) Red winged Blackbirds?

Additionally:

Fruit flies have speciated sympatrically in New York State for more than a hundred years.
These fruit flies originally courted, mated, and deposited eggs only on hawthorn fruits.
About 150 years ago, large commercial apple orchards were planted in New York.
Some fruit flies began to lay their eggs on the apple trees, perhaps by mistake.
Consequently, their offspring sought out apple trees as adults and, therefore, mated with other fruit flies of similar heritage.

Today there are two sympatric species of picture-winged fruit flies in New York.