Darwins on hybridization, variation, instinct

Darwin on

Hybridization

On the theory of natural selection the case is

especially important, in as much as the sterility of

hybrids could not possibly be of any

advantage to them and of successive profitable degrees of sterility.

It is certain, on the other hand, that the sterility of various species when

crossed is so differ in degree and graduates away so insensibly, and on the other hand, that the fertility of pure species is so easily affected by various circumstances that for all practical purposes it is most difficult to say, where perfect fertility ends

and sterility begins.

Speciation is the reaction of isolated population to the niches they can mange in thus, we have varieties

suited to their various places all over the world in a few

hundred years.

Speciation can result in fitness peaks. Where a population becomes so adapted to a

particular environment that it cannot re-adapt to any other, having lost those members of its population in generations past who carried the genetic variations which would allow

them to adapt.

A careful reading of Darwin's writings other than the Origin

shows that he did come to believe that hybridization is a significant source of variation

and of new forms of life.

But at the same time it suggests that when the Origin was first

published he had not yet come to believe hybridization had a

significant role in breeding and that in the Origin he

downplayed any significance he did see.

One suspects also that, having had so much success

with the initial editions of the Origin, he might have

hesitated to incorporate new information and opinions on hybrids in later editions that

would have contradicted views he had expressed in the

first edition.

Certainly, he made conflicting statements

regarding the significance of hybridization, just as he did regarding the meaning of

species, and with respect to the importance of siltation.

Thus, in the first chapter of the Origin (1859), Darwin

strongly expresses the opinion that new forms of life rarely arise through

hybridization:

When in any country several domestic breeds have once been

established, their occasional intercrossing, with the aid of

selection, has, no doubt, largely aided in the formation of new sub-breeds; but the importance of the crossing of varieties has, I believe, been greatly exaggerated, both in

regard to animals and to those plants which are propagated by

seed.

In plants which are temporarily propagated by cuttings & budding

the importance of the crossing both of distinct species and of varieties is immense; for the cultivator here

quite disregards the extreme variability both of hybrids and

mongrels, and the frequent sterility of hybrids; but the cases of plants

not propagated by seed are of little importance to us, for their

endurance is only temporary.

He discounts hybridization, but only to the extent of dismissing the feasibility of producing a breed intermediate between two parent forms. He does,

however, say the parents can be modified by hybridization.

There he says ..

“A doctrine which originated with Pallas, has been largely accepted by modern naturalists; namely,

that most of our domestic animals have descended from two or more aboriginal species, since

commingled by intercrossing. On this view, the aboriginal species must either at first have

produced quite fertile hybrids, or the hybrids must have become in subsequent generations quite

fertile under domestication. This latter alternative seems to me the most probable, and I am inclined

to believe in its truth”

In a subsequent letter to Lyell (January 1865),

Darwin continued to avow his belief that only the accumulation of minor

variation was significant:

"The more I work, the more I feel convinced that it is by the

accumulation of such extremely slight variations that new species

arise."

And yet, it seems Darwin did in fact come to attribute more significance to hybridization.

In a letter to Huxley dated December 22, 1866, at the time that he was completing work on Variation of Animals and Plants under Domestication (1868), Darwin confides

"Now that I have worked up domestic animals, I am convinced of the truth of the Pallasian view of loss of sterility under domestication, and this seems to me to explain much."

Darwin’s on Variation

DARWIN ON VARIATION

Variation, or what we might call mutation, is the raw material on which natural selection acts.

Charles Darwin demonstrated that variation was common in many species but he did not know the cause. It wasn't until fifty years after the

publication of Origin of Species that geneticists began to understand that mutations were random and spontaneous.

He believed that variations arose as a result of the conditions of life and that some variations were due to

the use or disuse of organs.

Laws of Variation

Here's how Darwin thought of variation

“HAVE hitherto sometimes spoken as if the variations so common and multiform in organic beings under domestication, and in a lesser degree in those in a state of nature had been due to chance. This, of course, is a wholly incorrect expression, but it serves to acknowledge plainly our ignorance of the cause of each particular variation. Some authors believe it to be as much the function of the reproductive system to produce individual differences, or very slight deviations of structure, as to make the child like its parents. But the much greater variability, as well as the greater frequency of monstrosities, under domestication or cultivation, than under nature, leads me to believe that deviations of structure are in some way due to the nature of the conditions of life, to which the parents and their more remote ancestors have been exposed during several generation”

What is the biological nature of "variation"?

……. How does it arise?

1.  There's lots of variation (even more than between related wild species).

2.  Darwin suggests that the source of variation (i.e., mutation) is in

"reproductive elements (i.e., germ cells) prior to conception“

3.  Variation is "random" (i.e., no "inherent trends")

4.  Variation is heritable

5.  Variation in domestic varieties is different than in wild

populations. 

How did different varieties originate?.....  Why pigeons?

….Darwin uses pigeons as a model to study Variation and its origin

How did different breeds originate? 

2 alternative hypotheses:

a.  Many origins (from many aboriginal species), or

b.  Single origin (1 species, the rock pigeon, Columba livia).  If true, this would suggest that several divergent varieties could originate from a single ancestral variety. 

How does Darwin extrapolate from this conclusion to suggest that different species arise from a common ancestral species? 

…. use his arguments about the kind of variation that exists in domestic varieties along with his next arguments about varieties being incipient species.

Variation in nature

 A.  It is very difficult to differentiate species and varieties (especially when you become an expert about a particular group)B.  Metaphor:  This lack of delineation suggests a series, which suggests a "passage"C.  Terms "species" and "variety" are "arbitrary"D.  Darwin tests the hypothesis that varieties are "incipient species" (species arise from varieties, and varieties are thus potential species)  

1.  Prediction of cladogenetic Evolution if this hypothesis is correct:  Where many species have arisen (e.g., in larger genera), there should be many "incipient species"2.  Prediction of Transformism:  The number of varieties should be unrelated to the number of species in a genus (e.g., the number of varieties could be the same, regardless of which genus a species came from)3.  Prediction of Separate Creation:  No particular pattern is predicted

One reason that Darwin's ideas have endured is their simplicity.

The theory of evolution by natural selection has just three essential

parts:

When individuals in a population reproduce, the new generation must resemble their parents.

The resemblance between generations must be close, but not perfect, so that each generation includes new variations in characteristics.

There must be a link between some of these new variations and the chances that an individual will be better able to survive and reproduce.

The variations, and their effects, can be very small. Repeat the cycle thousands of times, and the results can be dramatic.

In summary: all you need for evolution is inheritance, variation, and selection.

In The Origin of Species Darwin laid out an abundance of evidence for evolution. But there were gaps in the story. One, which

is still not easily understood, was the origin of life. Another was that he had no convincing ideas about how variations in

characteristics were passed down the generations.

More recent science has supplied some details about the mechanism of

inheritance. Every creature can be defined by the information in its genes.

These are messages written in the sequence of chemical letters in

the DNA molecule.

Darwinon

Instinct

DARWIN’S CONCEPTION OF MOTIVATIONAL

INSTINCT

Darwin used the word Instinct in a number of different way:

-to refer to what impels a bird to breed; -to a disposition, such as courage or

obstinacy in a dog; -to selectively bred patterns of behavior

such as the tumbling movements of tumbler pigeons;

-to feelings such as sympathy in people; -and to stereotype actions such as those

employed by honeybees when constructing the cells of a honeycomb.

The instincts of animals under the point of view whether it is

possible that they could have been acquired through the means

indicated on our theory, or whether, even if the simpler ones could have been thus acquired,

others are so complex and wonderful that they must have

been specially endowed, and thus overthrow the theory.

Bearing in mind the facts given on the acquirement, through the

selection of self-originating tricks or modification of instinct, or

through training and habit, aided in some slight degree by imitation, of hereditary actions and dispositions in our domesticated animals; and

their parallelism (subject to having less time) to the instincts of animals in a state of nature:

Bearing in mind that in a state of

nature instincts do certainly vary in

some slight degree:

Bearing in mind how very generally we find in allied but distinct animals a gradation

in the more complex instincts, which show that it

is at least possible that a complex instinct might have been acquired by successive

steps; and

Which moreover generally indicate, according to our theory, the actual steps by which the instinct has been acquired, in as much as we suppose allied instincts to have branched off at different stages of descent from a common ancestor, and therefore to have retained, more or less unaltered, the instincts of the several lineal ancestral forms of any one species:

Bearing all this in mind, together with the certainty that instincts are as important to an animal as their generally correlated structures, and that in the struggle for life under changing conditions, slight modifications of instinct could hardly fail occasionally to be profitable to individuals, I can see no overwhelming difficulty on our theory.

Even in the most marvelous instinct known, that of the cells of the Hive-bee, we have seen how a simple

instinctive action may lead to results which fill the

mind with astonishment.

Moreover it seems to me that the very general fact

of the gradation of complexity of instincts within the limits of the

same group of animals; and

Likewise the fact of two allied species, placed in two distant

parts of the world and surrounded by wholly different conditions of life, still having very much in common in their

instincts, supports our theory of descent; for they are explained by it: whereas if we look at each instinct as specially endowed, we

can only say that it is so.

The imperfections and mistakes of instinct on our theory cease to be

surprising: indeed it would be wonderful that far more numerous

and flagrant cases could not be detected, if it were not that a species which has failed to become modified and so far perfected in its instincts

that it could continue struggling with the co-inhabitants of the same

region, would simply add one more to the myriads which have become

extinct.

It may not be logical, but to my imagination, it is far more satisfactory to look at the young cuckoo ejecting its

foster-brothers, ants making slaves, the larva of the Ichneumidae feeding

within the live bodies of their prey, cats playing with mice, otters and

cormorants with living fish, not as instincts specially given by the Creator, but as very small parts of one general law leading to the advancement of all organic bodies—Multiply, Vary, let the strongest Live and the weakest Die.

The End….

Prepared By:

Bas, Mary Joy G.Perez, Aurea Mara Z.Rodabia, Monette C.