Species

Galapagos Finch

 

 

 

 

 

 

 

Species

Definition

While the The Origin of Species stands as one of the most well-known books in the world and is arguably the pivotal work in establishing the field of evolutionary biology, Darwin never defines the main subject of the book – “species.” Neither in  text nor in the“GLOSSARY OF THE PRINCIPAL SCIENTIFIC TERMS” is the term “”species” defined. Ironically, Darwin was of the opinion that the term was not definable:

“It is all-important to remember that naturalists have no golden rule by which to distinguish species.”

    and again,

“[There] is no possible test but individual opinion to determine which of them shall be considered as species.”

This problem is now known as “The Species Problem“. According to Massimo Pigliucci, evolutionary scientist and organizer of the largest convening of leading evolutionary scientists at the Altenberg Summit,

“First, the species problem is not primarily an empirical one, but it is rather fraught with philosophical questions that require – but cannot be settled by – empirical evidence.

Species is the most critical word in the book – used a total of 1,926 times. Species is used more than “natural” and “selection,” 764 and 563 times, respectively.

No Small Problem

Darwin thoroughly explains his problem with the term species:

  • “Finally, varieties cannot be distinguished from species–except, first, by the discovery of intermediate linking forms; and, secondly, by a certain indefinite amount of difference between them; for two forms, if differing very little, are generally ranked as varieties, notwithstanding that they cannot be closely connected; but the amount of difference considered necessary to give to any two forms the rank of species cannot be defined.
  • “There is no more reason to think that species have been specially endowed with various degrees of sterility to prevent their crossing and blending in nature.”
  • “… of the term species. No one definition has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. Generally the term includes the unknown element of a distinct act of creation.”
  • “Hence, in determining whether a form should be ranked as a species or a variety, the opinion of naturalists having sound judgment and wide experience seems the only guide to follow.”
  • “…it is not pretended that we have any sure criterion by which species and varieties can be discriminated.”
  • “There is no possible test but individual opinion to determine which of them shall be considered as species and which as varieties.”
  • “It is all-important to remember that naturalists have no golden rule by which to distinguish species and varieties”
  • “Many years ago, when comparing, and seeing others compare, the birds from the closely neighbouring islands of the Galapagos Archipelago, one with another, and with those from the American mainland, I was much struck how entirely vague and arbitrary is the distinction between species and varieties.”
  • “Nevertheless, no certain criterion can possibly be given by which variable forms, local forms, sub species and representative species can be recognised.”
  • “If a variety were to flourish so as to exceed in numbers the parent species, it would then rank as the species, and the species as the variety; or it might come to supplant and exterminate the parent species; or both might co-exist, and both rank as independent species. But we shall hereafter return to this subject.”
  • “Finally, varieties cannot be distinguished from species–except, first, by the discovery of intermediate linking forms; and, secondly, by a certain indefinite amount of difference between them; … but the amount of difference considered necessary to give to any two forms the rank of species cannot be defined.”
  • “From these remarks it will be seen that I look at the term species as one arbitrarily given, for the sake of convenience, to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, for convenience sake.”
  • “Nevertheless according to my view, varieties are species in the process of formation, or are, as I have called them, incipient species.
  • “Again, it may be asked, how is it that varieties, which I have called incipient species…?”
  • “Again … How do those groups of species, which constitute what are called distinct genera and which differ from each other more than do the species of the same genus, arise?”
  • “… the amount of variation in the individuals of the same species is so great that it is no exaggeration to state that the varieties of the same species differ more from each other in the characters derived from these important organs, than do the species belonging to other distinct genera.”
  • “It is immaterial for us whether a multitude of doubtful forms be called species or sub-species or varieties.”
  • “The view commonly entertained by naturalists is that species, when intercrossed, have been specially endowed with sterility, in order to prevent their confusion. This view certainly seems at first highly probable, for species living together could hardly have been kept distinct had they been capable of freely crossing.”
  • “We must, therefore, either give up the belief of the universal sterility of species when crossed; or we must look at this sterility in animals, not as an indelible characteristic, but as one capable of being removed by domestication.”
  • “Again, both with plants and animals, there is the clearest evidence that a cross between individuals of the same species, which differ to a certain extent, gives vigour and fertility to the offspring; and that close interbreeding continued during several generations between the nearest relations, if these be kept under the same conditions of life, almost always leads to decreased size, weakness, or sterility.”
  • “Naturalists, as we have seen, try to arrange the species, genera, and families in each class, on what is called the Natural System. But what is meant by this system? Some authors look at it merely as a scheme for arranging together those living … it seems to me that nothing is thus added to our knowledge.”
  • “As we have no written pedigrees, we are forced to trace community of descent by resemblances of any kind.”
  • “… there can be no doubt that embryonic, excluding larval characters, are of the highest value for classification, not only with animals but with plants.”
  • “Hereafter we shall be compelled to acknowledge that the only distinction between species and well-marked varieties is, that the latter are known, or believed to be connected at the present day by intermediate gradations, whereas species were formerly thus connected. … This may not be a cheering prospect; but we shall at least be freed from the vain search for the undiscovered and undiscoverable essence of the term species.”
  • “On the ordinary view of the independent creation of each being, we can only say that so it is; that it has pleased the Creator to construct all the animals and plants in each great class on a uniform plan; but this is not a scientific explanation.”
  • “Finally, the several classes of facts which have been considered in this chapter, seem to me to proclaim so plainly, that the innumerable species, genera and families, with which this world is peopled, are all descended, each within its own class or group, from common parents, and have all been modified in the course of descent, that I should without hesitation adopt this view, even if it were unsupported by other facts or arguments.”
  • “We have seen that there is no infallible criterion by which to distinguish species and well-marked varieties”
  • “But how, it may be asked, can any analogous principle apply in nature? I believe it can and does apply most efficiently (though it was a long time before I saw how), from the simple circumstance that the more diversified the descendants from any one species become in structure, constitution, and habits, by so much will they be better enabled to seize on many and widely diversified places in the polity of nature, and so be enabled to increase in numbers.”
  • “… the amount of variation in the individuals of the same species is so great that it is no exaggeration to state that the varieties of the same species differ more from each other in the characters derived from these important organs, than do the species belonging to other distinct genera.”
  • “We have seen that species at any one period are not indefinitely variable, and are not linked together by a multitude of intermediate gradations”

 

 

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Darwin, Then and Now is a journey through the most amazing story in the history of science - the history of evolution. The book encapsulates who Darwin was, what he said, and what scientists have discovered since the publication of The Origin of Species in 1859.

With over 1,000 references, Darwin Then and Now is a historical chronicle of the rise and fall of the once popular theory of biological evolution.

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