Blogging the Origin: Chapter III: Struggle for Existence

Alright, this is a nice short and sweet chapter that goes one step further in making Darwin's case. Let's recall that Darwin is attempting to provide an answer to the question, where do species come from? While several potential answers to this question already existed, they lacked a mechanistic component that could provide explanatory power.

So far we have established the variation within a population of organisms obviously exists in domesticated animals (think of all the variants of dogs and apples). We have also established that variation exists in the wild, although we often overlook it. If anyone has doubts about the existence of variation within populations, please leave your point or issue in the comments and I'll try to address it.

Now Darwin is laying out another, albeit distinct, fact. Namely that populations increase to the point where there is competition between individuals within and without the population. First, he lays out the idea of a 'struggle for existence' incorporating both within a population and between populations in one beautiful paragraph:

I should premise that I use this term in a large and metaphorical sense, including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny. Two canine animals, in a time of dearth, may be truly said to struggle with each other which shall get food and live. But a plant on the edge of a desert is said to struggle for life against the drought, though more properly it should be said to be dependent on the moisture. A plant which annually produces a thousand seeds, of which only one of an average comes to maturity, may be more truly said to struggle with the plants of the same and other kinds which already clothe the ground. The mistletoe is dependent on the apple and a few other trees, but can only in a far-fetched sense be said to struggle with these trees, for, if too many of these parasites grow on the same tree, it languishes and dies. But several seedling mistletoes, growing close together on the same branch, may more truly be said to struggle with each other. As the mistletoe is disseminated by birds, its existence depends on them; and it may metaphorically be said to struggle with other fruit-bearing plants, in tempting the birds to devour and thus disseminate its seeds. In these several senses, which pass into each other, I use for convenience sake the general term of Struggle for Existence.

Purves et al., Life: The Science of Biology, 4th Edition

It is trivial to think of examples, add your favorites to the comments. However, I want to add a popular figure that I think shows a between group 'struggle for existence' remarkably well. The old moose-wolf population change over time. I think it is important to consider the competition between wolves when the moose population is low (~1977) or between moose when their population is high (~1972 or 1987).

S. pneumoniae strains
growing in MS medium

It is obvious that all organisms 'struggle for existence', now Darwin folds in the concept of geometric growth. The point here is that organisms tend to increase in population at a high rate. For example, here is the growth of bacteria in culture. You can clearly see the rapid increase in population size (the vertical y-axis labelled OD620) over time (the horizontal x-axis in minutes). This kind of growth is termed exponential growth (geometric growth in Darwin's day).

A generic version of exponential growth looks like this.  The 
real data with S. pneumoniae shown above only has the A, B, and C aspects of the growth curve on it (we'll come back to the D aspect). So in a normal bacterial growth curve there is an A (lag) phase, a B (logarithmic) phase, a C (stationary) phase, and a D (death) phase. This also appears to be the case with other organisms including mammals (see the same phases for both moose and wolves above). Hell let's look at the human population:

There's a clear A and B phase. Where's C and D?

Alright, so life is tough (aka struggle for existence) and organisms tend to grow without end. However, Darwin points out the obvious. Nature puts constrictions on the ability of an organism to grow without end. In fact, the vast majority of most offspring don't make it, and by make it, I mean make it until able to reproduce. There are limitations on food or other resources (including space, try finding affordable housing in any urban center without competing with rats or roaches). There are limitations based on climate, mosquitoes will thrive this year from the lack of a winter in the US, mosquitoes probably won't be doing as well in Europe for the same reason. There are limitations by predation, already the chickadees and junckos have easily taken out several thousands seeds in my backyard this year or just look at the moose-wolf graph above. And don't even get me started on parasites:

Perhaps Paraguay offers the most curious instance of this; for here neither cattle nor horses nor dogs have ever run wild, though they swarm southward and northward in a feral state; and Azara and Rengger have shown that this is caused by the greater number in Paraguay of a certain fly, which lays its eggs in the navels of these animals when first born. The increase of these flies, numerous as they are, must be habitually checked by some means, probably by other parasitic insects. Hence, if certain insectivorous birds were to decrease in Paraguay, the parasitic insects would probably increase; and this would lessen the number of the navel-frequenting flies--then cattle and horses would become feral, and this would certainly greatly alter (as indeed I have observed in parts of South America) the vegetation: this again would largely affect the insects; and this, as we have just seen in Staffordshire, the insectivorous birds, and so onwards in ever-increasing circles of complexity.

From this one example, we see an obvious check on the growth of a population, but also the interconnectedness of many distinct species and how disrupting one (to benefit another) can have dramatic consequences. You can insert your favorite invasive species here as an example.

Shortly after the above statement, Darwin eloquently states:

Nevertheless, so profound is our ignorance, and so high our presumption, that we marvel when we hear of the extinction of an organic being; and as we do not see the cause, we invoke cataclysms to desolate the world, or invent laws on the duration of the forms of life!

I cannot help but wonder what critique or 'reviewer' this was addressed to.

The chapter finishes by coming back to where we began. There is an obvious struggle between and among individuals of the same and distinct species. Furthermore, there exists variation in essentially all traits between individuals of the same species. From these two well established facts, the deduction that the variation within a species can serve to benefit or hinder the ability of an individual in its struggle for existence. I'll let Darwin express it himself:

This ought to convince us of our ignorance on the mutual relations of all organic beings; a conviction as necessary, as it is difficult to acquire. All that we can do is to keep steadily in mind that each organic being is striving to increase in a geometrical ratio; that each, at some period of its life, during some season of the year, during each generation, or at intervals, has to struggle for life and to suffer great destruction. 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.

Please add your own thoughts and comments below and get cracking on chapter 4, it's great but a longer one.  I also want to thank RBH and CMB for reading and letting me know these posts matter at least a little.