Eξέλιξη & φυσική επιλογή

Eξέλιξη & φυσική επιλογή

Φυσική Επιλογή Can it, then, be thought improbable... that... variations useful in some way to each being in the great and complex battle of life, should sometimes occur in the course of thousands of generations? If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favorable variations and the rejection of injurious variations, I call Natural Selection. (1859, 80 81)

Φυσική Επιλογή 1. φυσική vs τεχνητή επιλογή -> η ΦΕ δεν διαθέτει νου ( the survival of the fittest -> Origin, 5th ed) 2. η εξέλιξη των πληθυσμών μέσω ΦΕ εξαρτάται από ενδεχομενικά χαρακτηριστικά του περιβάλλοντος -> διαφορά με Lamarck -> όχι εγγενής τάση προς πολυπλοκότητα -> λόγω των αρχικών συνθηκών, η πολυπλοκότητα αναμένεται ότι θα αυξηθεί 3. Η ΦΣ δρα σε τυχαία ποικιλομορφία

Εικόνα α'ό το βιβλίο του αρβίνου The Variation of Animals and Plants Under Domestication

Σ'ίνοι των Γκαλα'άγκος -α'ό το βιβλίο του αρβίνου Journal of researches into the natural history and geology of the countries visited during the voyage of H.M.S. Beagle round the world, under the Command of Capt. Fitz Roy, R.N. (1845).

Let an architect be compelled to build an edifice with uncut stones, fallen from a precipice. The shape of each fragment may be called accidental; yet the shape of each has been determined by the force of gravity, the nature of the rock, and the slope of the precipice, events and circumstances all of which depend on natural laws; but there is no relation between these laws and the purpose for which each fragment is used by the builder. In the same manner the variations of each creature are determined by fixed and immutable laws; but these bear no relation to the living structure which is slowly built up through the power of natural selection, whether this be natural or artificial selection. (Darwin 1868, 236)

Φυσική Επιλογή 4. η ΦΕ δρα στα άτομα (ατομική επιλογή) -> τα χαρακτηριστικά επιλέγονται επειδή βοηθούν τους οργανισμούς που τα διαθέτουν να επιβιώσουν και να αναπαραχθούν -> δε δρα για το καλό του είδους, του οικοσυστήματος κλπ -> επιλογή ομάδας για εξήγηση αλτρουιστικών χαρακτηριστικών

Φυσική Επιλογή 5. σταδιακότητα 6. η ΦΣ ως το κύριο αλλά όχι το αποκλειστικό αίτιο της εξέλιξης (π.χ. use and disuse, ancestral influence, correlation of characters άλλα αίτια για Δαρβίνο)

Τα κριτήρια του Lewontin 1. There is variation in morphological, physiological, and behavioral traits among members of a species (the principle of variation). 2. The variation is in part heritable, so that individuals resemble their relations more than they resemble unrelated individuals and, in particular, offspring resemble their parents (the principle of heredity). 3. Different variants leave different numbers of offspring either in immediate or remote generations (the principle of differential fitness). [A]ll three conditions are necessary as well as sufficient conditions for evolution by natural selection.... Any trait for which the three principles apply may be expected to evolve. (1985, p. 76)

Φυσική Επιλογή In every species on earth, variation continually arises. Some of these new traits tend to be inherited across generations, and some inherited traits are beneficial to survival and reproduction while others are not. In many cases, the traits beneficial to survival and reproduction become more common, while less useful traits are lost. This leads to ongoing change in the features of organisms in all species. vs Ιf there is variation in a population, reproduction is asexual and offspring are exact copies of their mothers, everyone lives for the same length of time and reproduces at once, no one enters the population from outside or leaves, and individuals with some traits reproduce more than others, then the population will change. (PB 33) -> replicator dynamics -> idealisation

Fitness (αρμοστικότητα) Principle of Natural Selection: For all reproducing entities x and y, all environments E, and all generations n: if x is fitter than y in environment E at generation n, then probably there is some future generation n, after which x has more descendants than y. -> realised fitness vs fitness as propensity (Rosenberg & Kaplan 2005) [ propensity: a tendency or disposition that can be described in terms of probabilities ] -> fitness (as propensity): expected number of offspring

Εξηγήσεις καταγωγής vs εξηγήσεις κατανομής in order to be selected, a change must first have been produced (de Vries 1909) [Natural selection] is only a sieve, and not a force of nature, not a direct cause of improvement.... [W]ith the single steps of evolution it has nothing to do. Only after the step has been taken, the sieve acts, eliminating the unfit. (1906, pp. 6 7)

Το επιχείρημα του Δαρβίνου υπέρ της θεωρίας της εξέλιξης μέσω φυσικής επιλογής

From: Darwin, The Variation of Animals and Plants under Domestication, Introduction In scientific investigations it is permitted to invent any hypothesis, and if it explains various large and independent classes of facts it rises to the rank of a well-grounded theory. The principle of natural selection may be looked at as a mere hypothesis, but rendered in some degree probable by what we positively know of the variability of organic beings in a state of nature,-by what we positively know of the struggle for existence, and the consequent almost inevitable preservation of favourable variations,-and from the analogical formation of domestic races.

The principle of natural selection may be looked at as a mere hypothesis Now this hypothesis may be tested by trying whether it explains several large and independent classes of facts; such as the geological succession of organic beings, their distribution in past and present times, and their mutual affinities and homologies. -> If the principle of natural selection does explain these and other large bodies of facts, it ought to be received. On the ordinary view of each species having been independently created, we gain no scientific explanation of any one of these facts. We can only say that it has so pleased the Creator to command that the past and present inhabitants of the world should appear in a certain order and in certain areas; that He has impressed on them the most extraordinary resemblances, and has classed them in groups subordinate to groups. But by such statements we gain no new knowledge; we do not connect together facts and laws; we explain nothing.

Απολιθώματα Ν. Αμερικής To exhume with one's own hands tho bones of extinct and gigantic quadrupeds brings the whole question of the succession of species vividly before one's mind; and I found in South America great pieces of tessellated armour exactly like, but on a magnificent scale, that covering the pigmy armadillo; I had found great teeth like those of the living sloth, and bones like those of the cavy.

Eίδη των Γκαλαπάγκος When I visited during the voyage of H.M.S. Beagle, the Galapagos Archipelago, situated in the Pacific Ocean about 500 miles from South America, I found myself surrounded by peculiar species of birds, reptiles, and plants, existing nowhere else in the world. -> μοιάζουν με είδη Ν. Αμερικής Yet they nearly all bore an American stamp. In the song of the mocking-thrush, in the harsh cry of the carrion-hawk, in the great candlestick-like opuntias, I clearly perceived the neighbourhood of America, though the islands were separated by so many miles of ocean from the mainland, and differed much in their geological constitution and climate. -> διαφέρουν από νησί σε νησί -> τα νησιά σχετικά πρόσφατα Still more surprising was the fact that most of the inhabitants of each separate island in this small archipelago were specifically different, though most closely related to each other. The archipelago, with its innumerable craters and bare streams of lava, appeared to be of recent origin; and this I fancied myself brought near to the very act of creation.

Mimus trifasciatus Mimus melanotis Mimus parvulus

-> εξέλιξη η πιο απλή εξήγηση I often asked myself how these many peculiar animals and plants had been produced: the simplest answer seemed to be that the inhabitants of the several islands had descended from each other, undergoing modification in the course of their descent; and that all the inhabitants of the archipelago were descended from those of the nearest land, namely America, whence colonists word naturally have been derived. These facts have as yet received no explanation on the theory of independent Creation; they cannot be grouped together under one point of view, but each has to be considered as an ultimate fact.

-> φυσική επιλογή ως αίτιο της εξέλιξης But it long remained to me an inexplicable problem how the necessary degree of modification could have been effected, and it would have thus remained for ever, had I not studied domestic productions, and thus acquired a just idea of the power of Selection. -> εξήγηση ομολογιών How inexplicable is the similar pattern of the hand of a man, the foot of a dog, the wing of a bat, the flipper of a seal, on the doctrine of independent acts of creation! how simply explained on the principle of the natural selection of successive slight variations in the diverging descendants from a single progenitor!