The Action of Natural Selection

Illustrations of the action of Natural Selection

How does natural selection act?

A wolf preys on various animals by craft, strength, and fleetness.

Assume that:

• the fleetest prey, such as a deer had from any change in the country increased in numbers, or
• ther prey had decreased in numbers, during that season when the wolf is hardest pressed for food.

The swiftest and slimmest wolves would have the best chance of surviving, and so be preserved or selected.

• This is as long as they retained strength to master their prey at this or at some other period of the year, when they might be compelled to prey on other animals.

Man can improve the fleetness of his greyhounds:

• by careful and methodical selection, or
• by that unconscious selection which results from each man trying to keep the best dogs without any thought of modifying the breed.

Even without any change in the proportional numbers of the animals on which our wolf preyed, a cub might be born with an innate tendency to pursue certain kinds of prey.

We often observe great differences in the natural tendencies of our domestic animals; one cat, for instance, taking to catch rats, another mice.

According to Mr. St. John:

• one cat brings home winged game
• another cat brings hares or rabbits
• another brings woodcocks or snipes by hunting on marshy ground.

The tendency to catch rats rather than mice is inherited.

If any slight innate change of habit or of structure benefited an individual wolf, it would have the best chance of surviving and of leaving offspring.

• Some of its young would probably inherit the same habits or structure.
• The repetition of this process forms a new variety which would either supplant or coexist with the parent-form of wolf.

Or the wolves inhabiting a mountainous district, and those frequenting the lowlands, would naturally be forced to hunt different prey.

• From the continued preservation of the individuals best fitted for the two sites, two varieties might slowly be formed.

These varieties would cross and blend where they met.

According to Mr. Pierce, there are 2 varieties of the wolf inhabiting the Catskill Mountains in the US.

• one with a light greyhound-like form, which pursues deer
• the other more bulky, with shorter legs, which attacks the shepherd’s flocks.

Certain plants excrete a little sweet nectar to eliminate something injurious from their sap.

This is effected by glands at the base of the stipules in some Leguminosae, and at the back of the leaf of the common laurel.

This is greedily sought by insects.

Assume that nectar is excreted by the inner bases of the petals of a flower.

In this case insects in seeking the nectar would get dusted with pollen. would transport the pollen from one flower to the stigma of another flower.

The flowers of two distinct individuals of the same species would thus get crossed; and the act of crossing, we have good reason to believe (as will hereafter be more fully alluded to), would produce very vigorous seedlings, which consequently would have the best chance of flourishing and surviving. Some of these seedlings would probably inherit the nectar- excreting power. Those individual flowers which had the largest glands or nectaries, and which excreted most nectar, would be oftenest visited by insects, and would be oftenest crossed; and so in the long-run would gain the upper hand.

Those flowers, also, which had their stamens and pistils placed, in relation to the size and habits of the particular insects which visited them, so as to favour in any degree the transportal of their pollen from flower to flower, would likewise be favoured or selected. We might have taken the case of insects visiting flowers for the sake of collecting pollen instead of nectar; and as pollen is formed for the sole object of fertilisation, its destruction appears a simple loss to the plant; yet if a little pollen were carried, at first occasionally and then habitually, by the pollen-devouring insects from flower to flower, and a cross thus effected, although nine-tenths of the pollen were destroyed, it might still be a great gain to the plant; and those individuals which produced more and more pollen, and had larger and larger anthers, would be selected.

When our plant, by this process of the continued preservation or natural selection of more and more attractive flowers, had been rendered highly attractive to insects, they would, unintentionally on their part, regularly carry pollen from flower to flower; and that they can most effectually do this, I could easily show by many striking instances. I will give only one–not as a very striking case, but as likewise illustrating one step in the separation of the sexes of plants, presently to be alluded to.

Some holly-trees bear only male flowers, which have four stamens producing rather a small quantity of pollen, and a rudimentary pistil; other holly-trees bear only female flowers; these have a full-sized pistil, and four stamens with shrivelled anthers, in which not a grain of pollen can be detected. Having found a female tree exactly sixty yards from a male tree, I put the stigmas of twenty flowers, taken from different branches, under the microscope, and on all, without exception, there were pollen-grains, and on some a profusion of pollen.

As the wind had set for several days from the female to the male tree, the pollen could not thus have been carried. The weather had been cold and boisterous, and therefore not favourable to bees, nevertheless every female flower which I examined had been effectually fertilised by the bees, accidentally dusted with pollen, having flown from tree to tree in search of nectar. But to return to our imaginary case: as soon as the plant had been rendered so highly attractive to insects that pollen was regularly carried from flower to flower, another process might commence.

No naturalist doubts the advantage of what has been called the ‘physiological division of labour;’ hence we may believe that it would be advantageous to a plant to produce stamens alone in one flower or on one whole plant, and pistils alone in another flower or on another plant. In plants under culture and placed under new conditions of life, sometimes the male organs and sometimes the female organs become more or less impotent; now if we suppose this to occur in ever so slight a degree under nature, then as pollen is already carried regularly from flower to flower, and as a more complete separation of the sexes of our plant would be advantageous on the principle of the division of labour, individuals with this tendency more and more increased, would be continually favoured or selected, until at last a complete separation of the sexes would be effected.

Nectar-feeding insects

The plant of which we have been slowly increasing the nectar by continued selection, to be a common plant; and that certain insects depended in main part on its nectar for food.

I could give many facts, showing how anxious bees are to save time; for instance, their habit of cutting holes and sucking the nectar at the bases of certain flowers, which they can, with a very little more trouble, enter by the mouth. Bearing such facts in mind, I can see no reason to doubt that an accidental deviation in the size and form of the body, or in the curvature and length of the proboscis, &c., far too slight to be appreciated by us, might profit a bee or other insect, so that an individual so characterised would be able to obtain its food more quickly, and so have a better chance of living and leaving descendants.

Its descendants would probably inherit a tendency to a similar slight deviation of structure. The tubes of the corollas of the common red and incarnate clovers (Trifolium pratense and incarnatum) do not on a hasty glance appear to differ in length; yet the hive-bee can easily suck the nectar out of the incarnate clover, but not out of the common red clover, which is visited by humble-bees alone; so that whole fields of the red clover offer in vain an abundant supply of precious nectar to the hive-bee. Thus it might be a great advantage to the hive-bee to have a slightly longer or differently constructed proboscis. On the other hand, I have found by experiment that the fertility of clover greatly depends on bees visiting and moving parts of the corolla, so as to push the pollen on to thestigmatic surface.

Hence, if bumble-bees were to become rare in any country, it might be a great advantage to the red clover to have a shorter or more deeply divided tube to its corolla, so that the hive-bee could visit its flowers.

Thus I can understand how a flower and a bee might slowly become, either simultaneously or one after the other, modified and adapted in the most perfect manner to each other, by the continued preservation of individuals presenting mutual and slightly favourable deviations of structure.

I am well aware that this doctrine of natural selection, exemplified in the above imaginary instances, is open to the same objections which were at first urged against Sir Charles Lyell’s noble views on ’the modern changes of the earth, as illustrative of geology;’ but we now very seldom hear the action, for instance, of the coast-waves, called a trifling and insignificant cause, when applied to the excavation of gigantic valleys or to the formation of the longest lines of inland cliffs.

Natural selection can act only by the preservation and accumulation of infinitesimally small inherited modifications, each profitable to the preserved being; and as modern geology has almost banished such views as the excavation of a great valley by a single diluvial wave, so will natural selection, if it be a true principle, banish the belief of the continued creation of new organic beings, or of any great and sudden modification in their structure.