On reading The Origin of Species: Ch. 4 - Divergence of Character (2)

A lot is said in this section, and Darwin tries to put forth a solid basis for his reasoning that variation leads to speciation by giving a conceptual example.

He suggests that if we look at a given point in time, we may have eleven different species within a single genus, "large within its own country" - i.e. there are many different species and much variation within each species of this genus in a given area.

If we imagine that two of these species vary a lot, and produce, after many generations several varieties each, and these in turn produce multiple varieties, and so on up the ages. While at the same time most of the other species in that genus die off - possibly because the two which are varying a lot are doing so because they have some advantage, and start to occupy the niches that the other species were. And because they have this advantage, they perform much better than the other species that are not varying quite so much, and not producing offspring that can survive quite as well. But still these two species' offspring are also competing amongst themselves, and of course some of the varieties die off...

After many thousands of generations, we might have eight variants of the first species, and six of the second species, and perhaps one of the original 11 species manages to survive through that many generations, also, but relatively unchanged. From Darwin's diagram on pp. 160-1 we can see that from the one species (A), out of the eight variants that have made it through, one group of three is fairly closely related, another group of two is also closely related, and a further group of three is closely related. We can also see that the latter group of three is more distantly related, and hence more distinctly different to either of the first two groups.

A similar situation exists in the second species' (I's) progeny. We have two groups, each consisting of three very closely related varieties each.

We can also see that these two groups will be quite distinct from each other, for not only were the ancestors whose progeny they are somewhat distinct, they have also accumulated differences as the ages pass.

What's more, they are each somewhat closer to the species that descended from the other species, F, than they are to each other, by the fact that species A was more distinct from I than it was from F, and the accumulated variations are less likely to have brought them closer together than further apart.(*)

He then goes on to point out that a naturalist looking today at the varieties would most likely lump the three groups descended from A into three separate sub-genera or genera, and likewise those of the descendants of I....In other words, they are separate species.

(*) At least - I think this is what he is trying to say. I am not 100% convinced on this front, although I can see an argument for saying that the number of ways of diverging are greater than the number of ways of converging, thus statistically they are more likely to diverge than converge.

After reading this chapter, I can still see how a creationist might claim that Darwin has not proven that speciation does actually occur. My other thought is that he is relying on variation which he has stated stems from sexual reproduction on which natural selection can then work. If we start with a single single-celled organism produced by abiogenesis, where does the variation come from? I guess we are relying on imperfect copying creating sufficient distinct entities quickly enough that natural selection eventually has something to work on.

John Whitfield has a blog on the Origin of Species - a post per chapter as he goes.

In chapter 2's entry he covers some of the ground I did in one of my earlier posts - about how Darwin views the definition of species. I particularly noted his comment:

Here's some advice for anyone agonizing over creating a perfect, all-encompassing species concept: chill out. Pretty much any biological category you care to think of has fuzzy boundaries. Genomics is making the concept of the gene more problematic. Colonial, clonal and modular organisms, such as slime moulds, aspen, and the vast underground mycelia of some fungi, make the concept of the individual tricky to pin down. The giant mimivirus blurs the line between viruses and cellular life. There's been a long debate over whether viruses themselves should be classed as living things. And it's proved impossible to come up with a list of properties that unambiguously define life.

This does not mean that terms such as species, gene, individual or life are useless. In many or most cases it'll be obvious whether the thing or group of things you're looking at belongs in a particular category or not.

I shall post, asking for comments on thoughts about the utility of the terms...will at least enlighten me!

On reading The Origin of Species (ch 4)

Darwin postulates that there may be a 'law of nature' that "a cross with another individual is occasionally - perhaps at very long intervals - indispensable [to increasing 'vigour and fertility']" as I mentioned in my last post.

He goes on to talk about the methods by which flowers may be pollinated - pollination by bees and insects, either by them carrying pollen from the anthers of one flower to the stigma of another, or by triggering some mechanism that causes the flower to self-pollinate - for example by pushing its anthers against its own stigma as the bee or insect crawls into the flower to get at the nectar.

He then goes on to note that in many flowers, such as Lobelia fulgens:

...either the anthers burst before the stigma is ready for fertilisation, or the stigma is ready before the pollen of that flower is ready, so that these plants have in fact separated sexes, and must habitually be crossed. How strange are these facts! How strange that the pollen and stigmatic surface of the same flower, though placed so close together, as if for the very purpose of self-fertilisation, should in so many cases be butually useless to each other! How simply are these facts explained on the view of an occasional cross with a distinct individual being advantageous or indispensable!

He also says two things that seem somewhat at odds - and he hints at further coverage of the points later on. First off:

...if you bring on the same brush a plant's own pollen and pollen from another species, the former will have such a prepotent effect, that it will invariably and completely destroy, as has been shown by Gartner, any influence from the foreign pollen.

Then a few paragraphs later:

Yet the pistil of each cabbage-flower is surrounded not only by its own six stamens, but by those of the many other flowers on the same plant. How, then, comes it that such a vast number of the seedlings are mongrelised? I suspect that it must arise from the pollen of a distinct variety having a prepotent effect over a flower's own pollen...

So...it appears to me that the principle is acting one way in one case and in another way in a different case - almost as if nature knows which organisms are distinct species, and which are distinct varieties....

On reading The Origin of Species (ch 4)

...a cross between different varieties, or between individuals of the same variety but of another strain, gives vigour and fertility to the offspring; and on the other hand, that close interbreeding diminishes vigour and fertility...no organic being self-fertilises itself for an eternity of generations; but that a cross with another individual is occasionally - perhaps at very long intervals - indispensable.

So...sex is necessary for improved vigour in a species...and what happened before species that had sexual reproduction? Or indeed with species that do not reproduce sexually now? Do amoeba not just split? How is it that they continue to have "vigour" and "fertility"?


Further on Darwin mentions that he has not found one land animal that is perfectly hermaphroditic, and that although some marine animals are, that water currents can provide the means by which the occasional cross-fertilisation can take place. He mentions one species that he had initially had problems with - Cirripedes - but he found evidence that it, too, sometimes crosses.

I am still not sure about amoeba and the like - is there any chance by which they can cross-fertilise?

A final quote from Darwin on this:

Finally, then, we may conclude that in many organic beings, a cross between two individuals is an obvious necessity for each birth; in many others it occurs perhaps only at long intervals; but in none, as I suspect, can self-fertilisation go on for perpetuity.

On reading The Origin of Species (ch4)

...but in all cases natural selection will ensure that modifications consequent on other modifications at a different period of life, shall not be in the least degree injurious: for if they became so, they would cause the extinction of the species.

From other sources I know of the way that some genes can piggyback on the success of others just by being close to them on the genome(?). Now, I know that what Darwin said is generally correct, but I cannot see where he would have got that statistic without also noticing that there are also deleterious inherited effects.

Natural selection obviously selects for the benefit of the beneficial gene, and not for the piggybacker's deleterious effect. And should the deleterious effect be worse than the beneficial effect, presumably it would be selected against. And of course it is all relative. When the environment provides a niche that selects for a particular benefit the piggybacker's effects are perhaps neutral....until such time as the environmental forces start selecting over the remit of that sort of effect.

So what am I saying? Just that Darwin wasn't aware of genes and how his theory would get reworded once we understood more about how selection works on genes?

I'm not sure! I'm just dumping my thoughts onto a post; there is no direction to them (yet?). I would like at some point to summarise everything I've written into a post that is more comprehensive, and perhaps actually has a point to make. For now, bear with the ramblings of my mind.

On reading The Origin of Species (p 126)

The dependency of one organic being on another, as of a parasite on its prey, lies generally between beings remote in the scale of nature. This is often the case with those which may strictly be said to struggle with each other for existence [...] But the struggle almost invariably will be most severe between the individuals of the same species, for they frequent the same districts, require the same food, and are exposed to the same dangers.

This is the start of the central point of chapter 3 - The Struggle for Existence. He goes on to point out that the struggle between varieties will be almost as severe, and (I am guessing, now, as I have not read much further yet) so decreases as you move further away on the 'tree of life'.

The struggle for existence has previously in the chapter been demonstrated to be the major check on population; different species competing for the same resources, and particularly how different species which were previously deprived of a share of the resources available by a more successful organism, when that organism is removed, begin to thrive.

On reading The Origin of Species

p. 110

Where many species of a genus have been formed through variation, circumstances have been favourable for variation; and hence we might expect that the circumstances would generally be still favourable to variation. On the other hand, if we look at each species as a special act of creation, there is no apparent reason why more varieties should occur in a group haveing many species, than in one having few.

In other words, and turning the statement on its head somewhat, where the environment (and/or other factors) have allowed an organism to diversify and we find many different varieties, we are more likely to find a greater variety of allied species of the organism than in the opposite case where the environmental factors have not been favourable to the diversification.

Darwin goes on to make the case for this statement with real-world examples.

On reading The Origin of Species

It looks like Darwin was driving at exactly the idea I had had - presumably because I am actually understanding what he's saying!

From these remarks it will be seen tht I look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, and for mere convenience sake.

I still have to look up what the advantages of continuing to use this dichotomy (or, as Darwin points out, trichotomy - individual difference, variety, species) is. Convenience is an obvious candidate, as is custom - "we've always done it like this"

On reading The Origin of Species

Is the distinction between 'species' and 'variety' useful? Does it fuel the macro/micro duality?

If we formally regard all organisms as 'varieties', then perhaps the macro/micro misunderstanding would be cleared up.

Darwin notes on p. 105:

A wide distance between the homes of two doubtful forms leads many naturalists to rank both as distinct species; but what distance, it has been well asked, will suffice? if that between America and Europe is ample, will that between the Continent and the Azores, or Madeira, or the Canaries, or Ireland, be sufficient? It must be admitted that many forms, considered by other highly competent judges as varieties, have so perfectly the character of species that they are ranked by other highly-competent judges as good and true species. But to discuss whether they are rightly called species or varieties, before any definition of these terms has been generally accepted, is vainly to beat the air.

What are the advantages of the distinction? Of classifying organisms according to genera, phyla, etc? The question perhaps sounds rather vacuous - of course labelling has utility. It allows communication. But what if the act of labelling things is actually causing a perceived either/or duality where none actually exists? What are the alternatives?

On reading The Origin of Species

p103-4.

But cases of great difficulty, which I will not here enumerate, sometimesoccur in deciding whether or not to rank one form as a variety of another, even when they are closely connected by intermediate links; nor will the commonly-assumed hybrid nature of the intermediate links always remove the difficulty. In very many cases, however, one form is ranked as a variety of another, not because the intermediate links have actually been found, but because analogy leads the observer to suppose either that they do now somewhere exist, or may formerly have existed; and here a wide door for the entry of doubt and conjecture is opened.

Darwin seems to have hit the nail on the head, here - this is where much creationist attack appears to stem from. They see all the evidence, but then say "but you're just guessing at the intermediate forms". Of course, we can nowadays compare DNA and the like, which (again) Darwin would have been most glad to have had. However, he goes on, in an effort to allay the problem raised by his last sentence:

Hence, in determining whether a form should be ranked as a species or a variety, the opinion of naturalists having sound judgement and wide experience seems the only guide to follow. We must, however, in many cases, decide y a majority of naturalists, for few well-marked and well-known varieties can be named which have not been ranked as species by at least some competent judges.

So...let's have a vote? Perhaps before the advent of being able to use DNA to place an organism on the tree of life by comparing its sequences to others already catalogued, this was indeed the best way. I suppose, in a way, the current strategy is also a majority vote - the majority of biologists agree that this method works as well as, or, in fact, better than, a rather subjective appraisal by a committee of naturalists.

On reading "On the Origin of Species"

p. 99

I believe that the conditions of life, from their action on the reproductive system, are so far of the highest importance as causing variability. I do not believe that variability is an inherent and necessary contingency, under all circumstances, with all organic beings, as some authors have thought. The effects of variability are modified by various degrees of inheritance and of reversion. Variability is governed by many unknown laws, more especially by that of correlation of growth. Something may be attributed to the direct action of the conditions of life. Something must be attributed to use and disuse.

Some of this passage, the conclusion to the chapter on 'Variation under Domestication', confuses me. I get the impression that what I understand by variation is a more general meaning than that used in this passage.

I also do not know quite what he means by "correlation of growth".

I assume that by "conditions of life" he is talking about deaths by natural causes, and the like.

The comment on "use and disuse" sounds rather Lamarckian. I wonder if it is a throw-away comment, a nod to Lamarck's work.