Field of Science

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:

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:
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:
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:
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.


LE said...

If I may make a couple of technical suggestions. Add a small paragraph in italics for all the articles in this series explaining what you are doing. New readers like myself can then understand what is going on without having to dig.

Also, it might be worth insuring that the tags at the bottom show all the articles in the series. I clicked on one that was missing Chapter 2. You could even add a final section with direct links to the other articles in the series at the bottom.

Caleb said...

The human population curve scares the shit out of me....