Darwin’s ‘Origin’ (ch.7-8): The evolution of instinct and the effects of hybridity

Every day this week I will be publishing excerpts from my distilled version of On the Origin of Species. If you never have, or never will read it, hopefully this will give you a sense of what an amazing book it is.

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Chapter 7: Instinct

Instincts and their development under Natural Selection deserve special focus. By ‘instincts’, I mean a range of behaviours that we intuitively understand to require no practice or knowledge, that are innate. To illustrate how powerful instincts can be, let us look at the cocoon-making of moth caterpillars. Many caterpillars follow a six-stage process to build their metamorphic hideaways. If they are interrupted at the third stage by an inconsiderate breeder, and are placed into a construction in the fifth stage of building, they still try to build the third, fourth and fifth stages.

Silk moth lifecycle engraving

Such instincts are just as vital for animals’ own survival as physical attributes. If we can detect any variation in instincts between members of the same species, then I suggest that instincts may be acted on by natural selection in exactly the same way as physical traits, and small changes accrued over generations to give wonderful complexity. Examples of varying instincts abound in nature, such as the relative tameness of Norwegian magpies compared to the extremely wary and heavily-persecuted English stock.

Magpie engraving

We can easily illustrate how instincts might develop in the wild under natural selection. The parasitic egg-laying of the cuckoo, the bane of so many small birds, is thought to result from the long intervals between the production of each cuckoo egg. Caring for a brood of chicks spanning every stage of development would be impossible, laying eggs in other birds nests might give cuckoo chicks excellent foster parents, better care than from overwrought cuckoo mothers. The instinct to coopt the nursing services of other birds would therefore spread.

Above all others, perhaps, I regard the hive-building instinct of the honey bee to be a wonder in Nature. Even supplied with all the tools imaginable, I very much doubt a group of human workmen would be capable of producing such perfectly aligned sets of hexagonal cells as worker bees achieve purely through instinct. The bees’ hexagonal design is exquisitely elegant, allowing the maximum number of perfectly-tessellating cells to be built with minimal wax.

Honeybee comb engraving

If you compare the different types of cells: the primitive tubes of bumblebees; the round-celled combs of less primitive Melipona bees; or the double-layered combs of crisply hexagonal cells built by honey bees, it is possible to imagine how small changes in instincts could lead to transitions from one to the other. Were Melipona bees to build planar walls rather than rounded, or build back-to-back cells, their combs might easily turn out to be more like those of honey bees.

My own experiments have given me some insight into how this apparently miraculous building ability relies on simple instincts. Briefly, bees can control the distance at which they work from one another, the rate at which they build and excavate, and use their own bodies as measuring tools, resulting in regular structures of ingenious economy.

Though this prodigious workmanship may seem of no benefit to individual sterile workers, the benefit to the colony of such economical cells would be vast. Workers with the instincts for cell building may not pass them on directly, but the queens of efficient colonies would be likely to produce more new colonies, and to pass on these beneficial building instincts. A better example of the simple yet powerful action of Natural Selection on every aspect of organisms, I cannot imagine.


‘to my imagination it is far more satisfactory to look at such instincts as the young cuckoo ejecting its foster-brothers, – ants making slaves, – the larvae of ichneumonidae feeding within the live bodies of caterpillars, – not as specially endowed or created instincts, but as small consequences of one general law, leading to the advancement of all organic beings…’


Chapter 8: Hybridism

It is easy to take for granted that distinct species generally keep themselves to themselves where breeding is concerned. There are two obstacles to interbreeding. Firstly, the individuals of two separate species cannot easily produce any offspring at all. Secondly, if hybrid offspring are produced, these usually have non-functional reproductive organs, and there the matter ends.

Sometimes the barriers to interbreeding are immediately obvious: the tube from a pollen grain on a flower simply cannot reach the ovule, or a male animal has not the wherewithal to mate with a female. In most cases, though, we do not know.

Two points of contention regarding hybridism are of particular interest. The first is that most naturalists assert that the sterility between species must be divinely endowed to prevent the organic world blending into an undifferentiated chaos. I cannot subscribe to this view, but will show how sterility is important for my theory in a very different way.

In contrast different varieties of the same species generally seem able to breed. Many naturalists use this difference to divide the otherwise ambiguous categories of ‘varieties’ and ‘species’. I believe, however, there is no clear-cut distinction. 

Laws of sterility: intentional or incidental?

Looking closely at when sterility occurs demonstrates that it cannot be a divinely endowed quality. The copious observations I have collected from the best breeders suggest that the ease with which species cross and the fertility of the hybrids produced is fantastically varied.

Crinium botanical plate

For example, in the case of Crinium plants, flowers are not only possible to fertilise with the pollen of closely-related species, but are far more fecund than when fertilized with their own pollen. Yet, in other cases from the same group of species, the pollen of other species yields only a tiny proportion of fertilized seed. Equally puzzling is the often great difference between the two directions of a reciprocal cross. For example, a stallion is much easier to breed with a female donkey than a male donkey with a mare, though both produce infertile offspring.

Crossing varieties: a defining characteristic?

Now on to our second point of contention, whether the ability to interbreed might be the elusive defining characteristic between the categories of varieties and species. The distinction does not hold. For example, very similar varieties of pigeons, certainly descended from one ancestral rock dove, can be impossible to cross. Neither external similarity nor shared descent, determine how easy it is to interbreed two groups.

Therefore, the fertility of crosses cannot be used as a definitive characteristic between species and varieties. As varieties gradually become species, species gradually become genera, so I suggest that there is no clearly defined point at which free interbreeding between varieties becomes an impossible cross between species.


‘there is no more reason to think that species have been specially endowed with various degrees of sterility to prevent them crossing and blending in nature, than to think that trees have been specifically endowed with various and somewhat analogous degrees of difficulty of being grafted together in order to prevent them becoming inarched in our forests’


Previous:

Darwin’s Origin: Introduction

Darwin’s Origin, chapters 1-3: variation and struggle

Darwin’s Origin, chapters 4-6: natural selection and some difficulties

Next:

Darwin’s Origin, chapters 9-10: the geological record

Darwin’s Origin, chapters 11-12: geographies

Darwin’s Origin, chapter 13 and conclusion

 

Tamach rose

© Natalie Lawrence 2018
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