This featured "Dinner with..." series builds on the classic thought experiment: "Which 5 historical figures would you invite to dinner, and how would you seat them?" While the field of astrobiology historically rests on many "shoulders of giants" --too many for one dinner party, the Astrobiology Magazine has selected some initial candidates for our dinner party, and then asks them to introduce their area of expertise in a brief question and answer format.
The answers are their own, as gleaned from some of their most famous, controversial, or seminal contributions to science. In many cases, the selection of commentary is driven by the curiousity to understand these great historical figures as one might imagine them as more modern characters, perhaps joining in on table talk or an informal interview.
Tonight's dinner introduces Charles Darwin, voyager on the ship, H.M.S. Beagle, headed towards South America, as he seeks to understand the origin of species.
Astrobiology Magazine [AM]: How did you first undertake your project?
Charles Darwin [CD]: When on board H.M.S. 'Beagle,' as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to me to throw some light on the origin of species--that mystery of mysteries.
Who can explain why one species ranges widely and is very numerous, and why another allied species has a narrow range and is rare? Yet these relations are of the highest importance, for they determine the present welfare, and, as I believe, the future success and modification of every inhabitant of this world.
AM: What have you found out so far?
CD: The view that each species has been independently created--is erroneous. I am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendants of some other and generally extinct species, in the same manner as the acknowledged varieties of any one species are the descendants of that species. Furthermore, I am convinced that Natural Selection has been the main but not exclusive means of modification.
AM: What do you mean, by natural selection?
CD: The theory of natural selection is grounded on the belief that each new variety, and ultimately each new species, is produced and maintained by having some advantage over those with which it comes into competition; and the consequent extinction of less-favoured forms almost inevitably follows.
AM: Nature as a competition? But what about a parasite--something that is very dependent and not competing?
CD: In the case of the misseltoe, which draws its nourishment from certain trees, which has seeds that must be transported by certain birds, and which has flowers with separate sexes absolutely requiring the agency of certain insects to bring pollen from one flower to the other, it is equally preposterous to account for the structure of this parasite, with its relations to several distinct organic beings, by the effects of external conditions, or of habit, or of the volition of the plant itself.
There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.
AM: So after studying the classic mammalian debates, which do you favor: "nature or nurture" as the more important to a species chances to succeed?
CD: No breeder doubts how strong is the tendency to inheritance: like produces like...
|Charles Darwin (1809-1882)
Perhaps the correct way of viewing the whole subject, would be, to look at the inheritance of every character whatever as the rule, and non-inheritance as the anomaly.
AM: Once you accept variability as a rule, the question of selection, relative fertility and survival comes up. So why did you choose to begin most of your theory of 'natural' selection with the list of challenges to doing 'unnatural' selection, either by breeders or domesticators?
CD: The principle of selection I find distinctly given in an ancient Chinese encyclopedia. Explicit rules are laid down by some of the Roman classical writers. From passages in Genesis, it is clear that the color of domestic animals was at that early period attended to.
When we compare the many breeds of dogs, each good for man in very different ways; when we compare the game birds, so pertinacious in battle, with other breeds so little quarrelsome, with 'everlasting layers' which never desire to sit, and with the bantam so small and elegant; when we compare the host of agricultural, culinary, orchard, and flower-garden races of plants, most useful to man at different seasons and for different purposes, or so beautiful in his eyes, we must, I think, look further than to mere variability.
We cannot suppose that all the breeds were suddenly produced as perfect and as useful as we now see them; indeed, in several cases, we know that this has not been their history. The key is man's power of accumulative selection: nature gives successive variations; man adds them up in certain directions useful to him. In this sense he may be said to make for himself useful breeds.
AM: So humans have given direction to an otherwise directionless evolution?
CD: The great power of this principle of selection is not hypothetical...By a similar process of selection, and by careful training, the whole body of English racehorses have come to surpass in fleetness and size the parent Arab stock. Breeders habitually speak of an animal's organisation as something quite plastic, which they can model almost as they please.
But it is very far from true that the principle is a modern discovery.
|Progression from land to sea, legs to fins, walking to swimming
AM: Such changes are that dramatic, modifying to a new species after only after a few generations?
CD: It has taken centuries or thousands of years to improve or modify most of our plants up to their present standard of usefulness to man. In a vast number of cases we cannot recognise, and therefore do not know, the wild parent-stocks of the plants which have been longest cultivated in our flower and kitchen gardens.
That most skilful breeder, Sir John Sebright, used to say, with respect to pigeons, that 'he would produce any given feather in three years, but it would take him six years to obtain head and beak.'
AM: You have experimented with pigeons yourself, correct?
CD: Believing that it is always best to study some special group, I have, after deliberation, taken up domestic pigeons. I have kept every breed which I could purchase or obtain
Pigeons can be mated for life, and this is a great convenience to the fancier, for thus many races may be kept true, though mingled in the same aviary; and this circumstance must have largely favoured the improvement and formation of new breeds. Pigeons, I may add, can be propagated in great numbers and at a very quick rate.
AM: Doesn't the breeder's art presume a diverse stock of choices to select? This doesn't seem to answer the question of how the diversity originates in the wild, as say the environmental pressures change?
CD: Man can hardly select, or only with much difficulty, any deviation of structure excepting such as is externally visible; and indeed he rarely cares for what is internal. He can never act by selection, excepting on variations which are first given to him in some slight degree by nature.
I think these views further explain what has sometimes been noticed--namely that we know nothing about the origin or history of any of our domestic breeds. But, in fact, a breed, like a dialect of a language, can hardly be said to have had a definite origin.
|Progression from fangs to teeth, low-brow to high
AM: So one would have to look for examples in transition, where the history of say an organ or function was undergoing changes that a life of observation or records would reveal? Can you give us another example?
CD: In some of the crabs the foot-stalk for the eye remains, though the eye is gone; the stand for the telescope is there, though the telescope with its glasses has been lost.
AM: So through use and disuse, advantage and disadvantage, all the many diverse species "evolve" towards a more robust variety? A kind of "ascent via family descent with modification" is sometimes referred to here, correct?
CD: Whatever the cause may be of each slight difference in the offspring from their parents--and a cause for each must exist--it is the steady accumulation, through natural selection, of such differences, when beneficial to the individual, that gives rise to all the more important modifications of structure, by which the innumerable beings on the face of this earth are enabled to struggle with each other, and the best adapted to survive.
AM: So one shouldn't be surprised by missing gaps in the fossil record, and apparent jumps in evolution?
CD: Extinction and natural selection will, as we have seen, go hand in hand.
|"Planetary biospheres are complex entities whose histories are fraught with contingency, accident, and luck." -David Grinspoon
Image Credit: NASA
Representative species often meet and interlock; and as the one becomes rarer and rarer, the other becomes more and more frequent, till the one replaces the other.
[But] why do we not now find closely-linking intermediate varieties? This difficulty for a long time quite confounded me.
It has been asked by the opponents of such views as I hold, how, for instance, a land carnivorous animal could have been converted into one with aquatic habits; for how could the animal in its transitional state have subsisted?
AM: What's your theory then?
CD: I think such difficulties have very little weight.
I can see no difficulty, more especially under changing conditions of life...by the accumulated effects of this process of natural selection, a perfect so-called flying squirrel was produced.
AM: A transitional species, the ground and tree dweller, converted in form to its better aerial life, correct?
Or the flying lemur.
AM: Do these same transition play from air to land?
CD: If about a dozen genera of birds had become extinct or were unknown, who would have ventured to have surmised that birds might have existed which used wings solely as flappers, like the logger-headed duck; as fins in the water and front legs on the land, like the penguin; as sails, like the ostrich; and functionally for no purpose, like the Apteryx. They serve, at least, to show what diversified means of transition are possible.
|Tube worms near deep sea vents are just one of the many life forms found in these complex vent communities.
Credit: University of Delaware Graduate College of Marine Studies
When we see any structure highly perfected for any particular habit, as the wings of a bird for flight, we should bear in mind that animals displaying early transitional grades of the structure will seldom continue to exist to the present day, for they will have been supplanted by the very process of perfection through natural selection.
AM: But I thought variation in itself made for highly adapted habits, and so do you think perfection as fewer, better species seems a biological risk in itself?
CD: He who believes that each being has been created as we now see it, must occasionally have felt surprise when he has met with an animal having habits and structure not at all in agreement.
Can a more striking instance of adaptation be given than that of a woodpecker for climbing trees and for seizing insects in the chinks of the bark? Yet in North America there are woodpeckers which feed largely on fruit, and others with elongated wings which chase insects on the wing.
It is difficult to tell, and immaterial for us, whether habits generally change first and structure afterwards; or whether slight modifications of structure lead to changed habits; both probably often change almost simultaneously.
AM: That seems like a naturalist view where there is considerable waste?
CD: He who believes in the struggle for existence and in the principle of natural selection, will acknowledge that every organic being is constantly endeavouring to increase in numbers; and that if any one being vary ever so little, either in habits or structure, and thus gain an advantage over some other inhabitant of the country, it will seize on the place of that inhabitant, however different it may be from its own place.
There should be woodpeckers where not a tree grows.