how does natural selection produce adaptations in a speceis?

Answer 1

Organs of extreme perfection and complication.
To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree. When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vox populi, vox Dei, as every philosopher knows, cannot be trusted in science. Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be inherited, as is likewise certainly the case; and if such variations should be useful to any animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be considered as subversive of the theory.

How a nerve comes to be sensitive to light, hardly concerns us more than how life itself originated; but I may remark that, as some of the lowest organisms in which nerves cannot be detected, are capable of perceiving light, it does not seem impossible that certain sensitive elements in their sarcode should become aggregated and developed into nerves, endowed with this special sensibility.

In searching for the gradations through which an organ in any species has been perfected, we ought to look exclusively to its lineal progenitors; but this is scarcely ever possible, and we are forced to look to other species and genera of the same group, that is to the collateral descendants from the same parent-form, in order to see what gradations are possible, and for the chance of some gradations having been transmitted in an unaltered or little altered condition. But the state of the same organ in distinct classes may incidentally throw light on the steps by which it has been perfected.






The simplest organ which can be called an eye consists of an optic nerve, surrounded by pigment-cells and covered by translucent skin, but without any lens or other refractive body. We may, however, according to M. Jourdain, descend even a step lower and find aggregates of pigment-cells, apparently serving as organs of vision, without any nerves, and resting merely on sarcodic tissue. Eyes of the above simple nature are not capable of distinct vision, and serve only to distinguish light from darkness. In certain star-fishes, small depressions in the layer of pigment which surrounds the nerve are filled, as described by the author just quoted, with transparent gelatinous matter, projecting with a convex surface, like the cornea in the higher animals. He suggests that this serves not to form an image, but only to concentrate the luminous rays and render their perception more easy. In this concentration of the rays we gain the first and by far the most important step towards the formation of a true, picture-forming eye; for we have only to place the naked extremity of the optic nerve, which in some of the lower animals lies deeply buried in the body, and in some near the surface, at the right distance from the concentrating apparatus, and an image will be formed on it.

In the great class of the Articulata, we may start from an optic nerve simply coated with pigment, the latter sometimes forming a sort of pupil, but destitute of lens or other optical contrivance. With insects it is now known that the numerous facets on the cornea of their great compound eyes form true lenses, and that the cones include curiously modified nervous filaments. But these organs in the Articulata are so much diversified that Muller formerly made three main classes with seven subdivisions, besides a fourth main class of aggregated simple eyes.





When we reflect on these facts, here given much too briefly, with respect to the wide, diversified, and graduated range of structure in the eyes of the lower animals; and when we bear in mind how small the number of all living forms must be in comparison with those which have become extinct, the difficulty ceases to be very great in believing that natural selection may have converted the simple apparatus of an optic nerve, coated with pigment and invested by transparent membrane, into an optical instrument as perfect as is possessed by any member of the Articulata class.

He who will go thus far, ought not to hesitate to go one step further, if he finds on finishing this volume that large bodies of facts, otherwise inexplicable, can be explained by the theory of modification through natural selection; he ought to admit that a structure even as perfect as an eagle's eye might thus be formed, although in this case he does not know the transitional states. It has been objected that in order to modify the eye and still preserve it as a perfect instrument, many changes would have to be effected simultaneously, which, it is assumed, could not be done through natural selection; but as I have attempted to show in my work on the variation of domestic animals, it is not necessary to suppose that the modifications were all simultaneous, if they were extremely slight and gradual. Different kinds of modification would, also, serve for the same general purpose: as Mr. Wallace has remarked, "If a lens has too short or too long a focus, it may be amended either by an alteration of curvature, or an alteration of density; if the curvature be irregular, and the rays do not converge to a point, then any increased regularity of curvature will be an improvement. So the contraction of the iris and the muscular movements of the eye are neither of them essential to vision, but only improvements which might have been added and perfected at any stage of the construction of the instrument." Within the highest division of the animal kingdom, namely, the Vertebrata, we can start from an eye so simple, that it consists, as in the lancelet, of a little sack of transparent skin, furnished with a nerve and lined with pigment, but destitute of any other apparatus. In fishes and reptiles, as Owen has remarked, "The range of gradation of dioptric structures is very great." It is a significant fact that even in man, according to the high authority of Virchow, the beautiful crystalline lens is formed in the embryo by an accumulation of epidermic cells, lying in a sack-like fold of the skin; and the vitreous body is formed from embryonic subcutaneous tissue. To arrive, however, at a just conclusion regarding the formation of the eye, with all its marvellous yet not absolutely perfect characters, it is indispensable that the reason should conquer the imagination; but I have felt the difficulty far to keenly to be surprised at others hesitating to extend the principle of natural selection to so startling a length.

It is scarcely possible to avoid comparing the eye with a telescope. We know that this instrument has been perfected by the long-continued efforts of the highest human intellects; and we naturally infer that the eye has been formed by a somewhat analogous process. But may not this inference be presumptuous? Have we any right to assume that the Creator works by intellectual powers like those of man? If we must compare the eye to an optical instrument, we ought in imagination to take a thick layer of transparent tissue, with spaces filled with fluid, and with a nerve sensitive to light beneath, and then suppose every part of this layer to be continually changing slowly in density, so as to separate into layers of different densities and thicknesses, placed at different distances from each other, and with the surfaces of each layer slowly changing in form.

Further we must suppose that there is a power, represented by natural selection or the survival of the fittest, always intently watching each slight alteration in the transparent layers; and carefully preserving each which, under varied circumstances, in any way or degree, tends to produce a distincter image. We must suppose each new state of the instrument to be multiplied by the million; each to be preserved until a better is produced, and then the old ones to be all destroyed. In living bodies, variation will cause the slight alteration, generation will multiply them almost infinitely, and natural selection will pick out with unerring skill each improvement.

Let this process go on for millions of years; and during each year on millions of individuals of many kinds; and may we not believe that a living optical instrument might thus be formed as superior to one of glass, as the works of the Creator are to those of man?

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Answer 2

It selects the ones most adapted to the current situation. A good example is this, When the industrial revolution began, a species of moth that had adapted to the brown color of the trees in order to avoid detection now was faced with gray sooty trees that gave them away. Being the fast breeding insects that they are, some of the moths were bound to have a mutant allele (gene variation) that produced gray moths (The most hidden). Many brown moths died because of their browniness but some gray moths that, before would have been at a disadvantage, now were dominant. Over the next few generations, the brown was weeded out, and the gray was now the norm, the species had adapted. And just as a response to the guy whose answer is hidden due to low score, Darwin didnt deny evolution, thats just a story given by his crazy religious wife, all the other people who he knew refuted it.

Sorry if the explanation wasnt clear enough, I'm only 7th grade

Answer 3

Adaptations appear randomly in any species. Some are helpful, some are not. Adaptations which allow a particular animal to be more competative or better able to survive are more likely to pass on those genes (with the adaptation) to it's young and therefore pass on the adaptation. Adaptations that are not helpful or even harmful are more likely to die young and without passing on their genes. If the advantageous adaptation is enough of an advantage, it will eventually out-compete the others of it's species and be passing that adaptation on to more and more of it's kind and at some point all the members of that species will eventually have that adaptation.

Answer 4

Natural selection just selects the most fit individuals. Natural variations are produced by various kinds of mutations or changes in DNA that cause variation in species. There are all sorts of selection that are continuously involved in selecting the most fit organism. It could be the one with the largest horns that gets the most mates. That sort of trend will make males with the larger horns increase in number relative to the others.

Answer 5

i will use the solid previous peppered moths for occasion: version--there are alterations interior the species from one individual to the subsequent. (some moths are gray/white, some are black.) Inheritablity--the version is genetic and could as a result be exceeded directly to offspring. generating offspring will strengthen the prospect of seeing the comparable trait interior the subsequent technology. (Moths inherit their coloration from genes they have been given from their mum and dad.) Differential survivability--the trait impacts the moth's ability to outlive long sufficient to supply those offspring, or quickly impacts the moth's ability to supply offspring. (A white moth would be much less seen to predators on a white historic past (and as a result greater probable to outlive long sufficient to have offspring), mutually as a black moth would be much less seen to predators on a black historic past.) a pair hundred years in the past, peppered moths have been white--on uncommon activities you may see a black one. They stay and feed on timber that are white, so white moths combination in and black moths are greater seen and greater probable to be eaten previously they have offspring, so a peppered moth replaced into white many of the time. Then a production unit replaced into geared up that positioned black soot everywhere in the white timber. this transformation in enviroment made it much less probable that white moths could stay, and greater probable that black moths could stay. at last, over many generations, the survivablity earnings replaced into great sufficient for black moths that black grew to become the undemanding coloration for those moths, while it was once white. The species replaced over the years as alterations interior the atmosphere replaced which trait could have the final survivability. If the trait has not something to do with survivabilty (does not help you reside, entice a mate, or produce greater offspring) you will not see one version usual over yet another.

Answer 6

It doesn't; natural selection only selects the best adaptation(s) that currently exist in the population. "Best", in this cae, meaning 'whatever feature allows the holder to have more kids than those without it'.

Answer 7

It doesn't ...To believe in the delusion of evolution you must have a VERY good imagination.

Even Darwin admitted that...

'To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree.' - Charles Darwin...."Origin of Species"