Thursday, March 08, 2012

Struggle for Existence - Chapter 3 in the Origin of Species

Locust swarms involve intense competition for resources
At this point in our tour of Darwin’s Origin, Darwin has questioned the 19th-century view of species as distinct entities created by God, and varieties as deviations from the species-type.  Instead, Darwin has presented compelling evidence, in large part through domestic organisms, that varieties and species are not different in kind, but only in degree.  That is, varieties, when they become especially well-marked, are identified as species.  A species evolves into different varieties; those varieties, in turn, may evolve into new species.

Darwin has also argued that there is a considerable amount of individual-level variation that exists in wild populations, and it is this variation that is the fuel for what he calls ‘natural selection’, a metaphorical phrase that he took from the phrase ‘artificial selection’.  Artificial selection occurs when humans look at a population and choose the individuals that have the best features for breeding.  Natural selection, although there is no conscious choice occurring, is the equivalent process that occurs in the wild.  Survival of the fittest.

In chapter three, Darwin sets the scene for his next chapter on natural selection.  Here he argues that waste and death is the hallmark of nature and is essential for speciation, rather than direct miraculous intervention from a beneficent God.  Babies die, and they die in large quantities.  This is an essential truth, if we wish to understand evolution. 

‘The mere existence of individual variability and of some few well-marked varieties, though necessary as the foundation for the work, helps us but little in understanding how species arise in nature,’ says Darwin at the beginning of this chapter.  We are interested in explaining adaptation – how the woodpecker got is beak, how the diving beetle got its swimming legs.  The first step to understanding adaptation is to understand variation.  The second step is to understand the cruelty of nature.

In this chapter, Darwin drew heavily from the work of an Anglican by the name of Thomas Malthus.  In 1798 Malthus published An Essay on the Principle of Population, in which he argued that the human population was growing exponentially, whereas resources were growing linearly.  Unchecked population growth, therefore, would be met by intense competition for resources, and would inevitably result in human death due to famine and disease.   Darwin recognized that, in nature, a similar principle must apply.  Natural populations do not tend to grow, they remain relatively stable over time.  Yet, based on the number of offspring they produce, they should be growing exponentially.  He realized that there must be numerous checks on population growth.  The most important check, he thought, was competition for resources by members of the same species, followed by competition with other species, predation, disease, and environmental effects.

Darwin was writing to English folk who traversed their green countrysides and admired their beautiful surroundings, while ignoring the day-to-day struggle that occurred among organisms.  With very few large predators, people simply were not confronted with the life-and-death chaos of nature.  It was easy to remark on how well-adapted creatures were to their environment.  It was easy to forget that some of those adaptations involved hinged jaws in snakes so that bird eggs could be swallowed whole, or special mouthparts for sucking blood, or special proteins for invading host cells. 

‘We behold the face of nature bright with gladness, we often see superabundance of food; we do not see, or we forget, that the birds which are idly singing round us mostly live on insects or seeds, and are thus constantly destroying life; or we forget how largely those songsters, or their eggs, or their nestlings, are destroyed by birds and beasts of prey; we do not always bear in mind, that though food may be now superabundant, it is not so at all seasons of each recurring year.’

 ‘Nothing is easier than to admit in words the truth of the universal struggle for life, or more difficult – at least I have found it so – than constantly to bear this conclusion in mind.’

Darwin realized that, just because we find two live chicks in a bird’s nest, does not mean that that bird had only produced two eggs, and it definitely does not mean that those chicks would survive to adulthood. A back-of-the-envelope calculation proved to Darwin that there were nowhere near as many birds as there should be, given how many offspring they produce.  Even the slow-breeding elephant should be covering the face of the earth by now, if they had only three offspring during their ninety year lives. 

Invasive species provided particularly compelling cases.  Rabbits, when first introduced to Australia, quickly overwhelmed the Australian countryside.  Their powers of reproduction were so amazing that the population exploded.  Yet if we look to their homeland, we do not see them performing nearly so well.  Why?  Because in Australia more of their babies survive.  In Australia, they had plenty of resources and few predators.

If babies didn’t die, and in large quantities, every species would be overwhelming the planet.

Organisms produce far more young than can possibly survive.  Same with eggs, or pollen, or sperm.

Herring eggs exposed by the tide.  Many of these eggs will die of exposure or be consumed by predators.

The Brits were right to think that organisms were well-adapted to their environments, but one of those adaptations was ensuring that you produced enough offspring to at least replace yourself.  Sometimes that would involve releasing millions of fertilized eggs, just to make sure that two of your young survived to adulthood.  Indeed, this is the strategy employed by many fish.  Or, you could have few offspring, but dedicate your lives to rearing and protecting them.  This is done by many large mammals.

Why is so much death so important to Darwin?

‘Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring.  The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive.  I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to marks its relation to man’s power of selection.’

In other words, if you produce a lot of children, there is going to be a lot of variation within the population.  Those variants that are poorer competitors will perish young, or will not produce offspring, while those variants that are superior competitors will flourish, passing on their superior traits to their offspring.  In this manner nature can be said to ‘select’ those that are better-adapted for their environment.

This struggle for existence, suggested Darwin, could be used to explain everything from patterns of extinction to the present distribution of organisms.

There are lots of little gems in this chapter.  Darwin essentially defines a number of things that would come to dominate the field of ecology.  In the last half of the chapter, for instance, he ponders the complex interactions that occur when multiple species struggle for existence. Cats, says Darwin, are known to eat mice.  Mice, in turn, destroy the hives of humble-bees.  Humble-bees pollinate flowers.  It has been observed that flowers do quite well along the borders of towns, the reason being that those towns contain cats.  Cats, due to the complexity of struggles, influence flower production.

This was a remarkably advanced observation.

Although Darwin’s outlook on nature was perhaps bleak (although in his defense, he used the term ‘struggle’ metaphorically, and did include in this mutualism and cooperation), he closes the chapter by putting a positive spin on things.

‘When we reflect on this struggle, we may console ourselves with the full belief, that the war of nature is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, the healthy, and the happy survive and multiply.’


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chrislantz said...

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Anonymous said...

Good post on a good chapter. I think Malthus was a prophet.
I wish Darwin put more examples in his book--or that's what I wished when I read it.

Matthew said...

A prophet in the sense of being well ahead of his time in recognizing competition for resources, but certainly not in his application of this principle towards the poor!