Reverend Malthus and Evolution

"In science the credit goes to the man who convinces the world, not the man to whom the idea first occurs."

Sir Francis Darwin, Eugenics Review, April 1914 

http://www.quotationspage.com/quotes/Sir_Francis_Darwin/

Introduction

Malthus never wrote specifically about evolution, and never proposed any theory of natural selection. Indeed, Elliot Sober (1984) noted that what Malthus wrote:

"...concerned the regulation of population numbers, not the frequency of characteristics found in a population. In fact, Malthus thought his law prevents populations from changing much."

Malthus would have read of Aristotle's scala naturae, with its ladder of life and fixity of species. This view had held sway for over 2,000 years, right up to the works of his Malthusian convert and contemporary Archdeacon William Paley (1748-1805) who wrote his Natural Theology in 1802 on the Argument To Design as proof of the existence of God. 

Malthus stated, in the Summary View at the conclusion of his great work "An Essay on The Principle Of Population" (published 1798), that his own Principle Of Population was proof of the existence of God.

Darwin's response to Paley's designer God, and the fixity of species, came in 1859 with Origin Of Species.

Malthus' Influence On Darwin and Wallace

Most books on evolution briefly note that Darwin credits Malthus' essay as critical to his theory of Natural Selection. From Darwin's "Autobiography":

"In October 1838, fifteen months after I had begun my systematic inquiry, I happened to read for amusement Malthus on Population, and being prepared to appreciate the struggle for existence which everywhere goes on, from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The result would be the formation of a new species."

Some books on evolution also quote Wallace, co-founder of modern evolutionary theory. This quote noted by Professor Frey (1970) in his introduction to the Penguin Classics edition of Malthus' "An Essay On The Principle Of Population" is typical:

"...the most important book I read was Malthus' Principle Of Population...It was the first work I had yet read treating any of the problems of philosophical biology, and its main principles remained with me as a permanent possession, and twenty years later gave me the sought-after clue to the effective agent in the evolution of organic species."

In Francis Darwin's publication of his father's autobiography and letters (Dover edition, 1958) I came across a rarely quoted letter from Alfred Wallace to a Professor A. Newton, Dec. 3rd, 1887. This passage immediately caught my attention:

"The most interesting coincidence in the matter, I think, is, that I as well as Darwin, was led to the theory itself through Malthus - in my case it was his elaborate account of the action of "preventative checks" in keeping down the population of savage races to a tolerably fixed but scanty number. This had strongly impressed me, and it suddenly flashed upon me that all animals are necessarily thus kept down - "the struggle for existence" - while variations, on which I was thinking, must necessarily often be beneficial, and would then cause those varieties to increase while injurious variations diminished....I was lying on my bed (no hammocks in the East) in the hot fit of intermittent fever, when the idea suddenly came to me. I thought it almost all out before the fit was over, and the moment I got up began to write it down, and I believe finished the first draft the next day."

Despite this "most interesting coincidence", modern evolutionists are still apt to play down Malthus' unintended contribution to evolutionary theory.

Population Thinking

As noted by evolutionist Ernst Mayr (2001), one of those most generous in giving Malthus the nod, Darwin initiated "an entirely new way of thinking":

"Darwin's new way of thinking, being based on the study of populations, is now referred to as population thinking....Population thinking is one of the most important concepts in biology: It is the foundation of modern evolutionary theory..."

Later Mayr examines Darwin's explanatory model of Natural Selection, which is based on 5 facts and 3 inferences. What's interesting is the influence of Malthus on Facts 1 and 3:

"Fact 1: Every population has such a high fertility that its size would increase exponentially if not constrained. (Source: Paley and Malthus)...

...Fact 3: The resources available to every species are limited. (Source: observation, reinforced by Malthus)

Inference 1: There is intense competition (struggle for existence) among the members of a species. (Source: Malthus)..."

With regards to Fact 1, William Paley's Natural Theology (originally published in 1802, 4 years after Malthus' essay) is simply repeating Malthus' own argument, so I'm not convinced of Mayr's assertion Paley was a source of this fact along with Malthus:

"The order of generation proceeds by something like a geometrical progression. The increase of provision, under circumstances even the most advantageous, can only assume the form of an arithmetic series. Whence it follows, that the population will always overtake the provision, will pass beyond the line of plenty, and will continue to increase till checked by the difficulty of procuring subsistence (Note: See a statement of this subject, in a late treatise upon population.)"

Paley's note appears be a reference to Malthus' essay. Indeed, in the introduction to the Penguin Classics edition of Malthus' essay, Professor Flew (1970) states:

"Among his first distinguished converts Malthus was proud to number both William Paley and the younger Pitt."

However, I guess the main thing to note with regards to Fact 1 is that Darwin was influenced by Paley (who appears to have been influenced by Malthus), and by Malthus directly, so the source of this fact is basically Malthus alone. 

Thus, it was Malthus' original thinking on population which was pivotal to the creation of modern evolutionary theory. Not Paley, not Wallace, and not Darwin.

Intraspecies Competition

Also noted by Sober (1984) is the influence of Malthus on Darwin's thinking is the shift from interspecies competition to intraspecies competition:

"...the Malthusian paradigm pictures competition between organisms of the same species as an important force. Attention was shifted from the struggle between the lion and the lamb to that between the lamb and the lamb."

Of course, the species that Malthus focussed on was man. As Darwin put it (Ridley, 1997) in his notebook:

"Malthus on man should be studied;"

Refer Human Replicators - An Exponentialist View for more on that topic.

Differential Replication

Many evolutionists stress the importance of differential replication (though they may not use that exact phrase). Here Dawkins (1976) compares biology with physics:

"The laws of physics are supposed to be true all over the accessible universe. Are there any principles of biology that are likely to have similar universal validity?"

and asks:

"...will there still be any general principle that is true of all life?" 

Dawkins states that he doesn't know, but is prepared to put his money on the following fundamental principle:

"This is the law that all life evolves by the differential survival of replicating entities."

Even a cursory look through Malthus' essay, most especially from the second edition on, should convince anyone that Malthus was intrigued by this very point, especially in how it related to discrete populations of man. In A Summary View (1830), Malthus makes his case for the differential replication of human populations plain:

"It may be safely asserted, therefore, that population, when unchecked, increases in a geometrical progression of such nature as to double itself every twenty-five years. This statement, of course, refers to the general result, and not to each intermediate step of the progress. Practically, it would sometimes be slower, and sometimes faster."

And here in 1798 Malthus (well before Darwin or Wallace) extends the law of differential survival that Dawkins seeks to plants and animals:

"The greatest check to the increase of plants and animals, we know from experience, is the want of room and nourishment; and this experience would direct us to look for the greatest actual increase of them in those situations where room and nourishment were the most abundant."

How is this possible if Malthus believed in the fixity of species?

Malthusian Selection

The theory of Natural Selection introduces the principle of differential reproduction, which is typically explained as entirely the result of genetic variation within a species, including the effect of the genotype on the phenotype.

Mayr  (2001) again:

"Owing to unequal survival and reproductive success of its individuals, there is a continuing genetic turnover in each population as a result of chance and natural selection."

However, this statement discusses the genetic make-up of a population. Chance is acknowledged to play a part, but how much of a part?  Mayr (2001):

"Much of the differential survival and reproduction in a population are not the result of selection, but rather of chance.

This statement is the clincher. Differential survival and reproduction are largely due to chance. Yet, if populations are growing and shrinking at differential rates, then some are being selected for and some are being selected against. So, Mayr does not see chance as a selective force, only "selection" (meaning natural selection).

In discussing Herbert Spencer's famous phrase "the survival of the fittest", Mayr (2001) explains what it means to be fit:

"To be fit means to possess certain properties that increase the probability of survival."

Elsewhere in his book, Mayr makes it clear that it is random genetic variation caused through mutation and sexual recombination which makes non-random Natural Selection possible. 

Sober (1984, pp.194-195) doesn't think much of Malthusianism when it comes to selection:

"Malthusianism is not a proper starting point for the theory of natural selection for reasons made abundantly clear by Fisher (1930, pp.46-47)."

Sober (1984) notes the following quote from Fisher:

"There is something like a relic of creationist philosophy in arguing from the observation, let us say, that a cod spawns a million eggs, that therefore its offspring are subject to Natural Selection; and it has the disadvantage of excluding fecundity from the class of characteristics of which we may attempt to appreciate the aptitude... The historical fact that both Darwin and Wallace were led through reading Malthus's essay on population to appreciate the efficacy of selection, thought extremely instructive as to the philosophy of their age, should no longer confuse the consequences of that principle with its foundations."

Later Sober (1984) concludes:

"Malthusianism survives in contemporary selection theory, but not because Malthus identified natural tendencies of population and food that guarantee that the grim reaper of mortality selection will always be with us."

The exponentialist view is that Natural Selection, Artificial Selection, Malthusian Selection and Unnatural Selection all contribute to the rate of growth for a population and thus each influences differential replication. Malthusian Selection is an Exponentialist hypothesis which explains how environment (including Mayr's chance) and non-instinctive behaviour affect the rate of growth for a population and thus influences differential replication.

Malthusian Growth Model

Perhaps the key contribution made by Malthus to population thinking and the law of differential survival, Malthus again, A Summary View (1830):

"The immediate cause of the increase of population is the excess of the births above deaths; and the rate of increase, or the period of doubling, depends upon the proportion which the excess of the births above the deaths bears to the population."

To complete the picture, one must also state the complimentary law (I have turned Malthus' phrase on its head):

"The immediate cause of the decrease of population is the excess of the deaths above births; and the rate of decrease, or the period of halving, depends upon the proportion which the excess of the deaths above the births bears to the population."

Together these statements form the basis of the Malthusian Growth Model, an exponential law of population growth (Turchin, 2000). All exponential growth is inherently compound growth by nature, whether you use a constant rate or variable rates. However, as demonstrated in The Scales Of 70, the real-world population growth is based on variable rates of compound interest. Given the universal belief that exponential growth is defined by a constant rate of compound interest, I have given the name Couttsian Growth Model to the population growth we see in the real world, which is based on Couttsian Growth and Couttsian Shrinkage. Thus I have reached the entirely opposite view of Sober and Fisher, and declare that Malthusianism is precisely the right point for the study of Natural Selection. To that I would add Artificial Selection, Unnatural Selection and Malthusian Selection.

Natural Selection Versus Malthusian Selection

Consider the pace of Natural Selection, and all the classical evolutionary processes. They are generally agreed to be slow, though they speed up significantly with short-lived creatures such as insects or microbes. Now consider the rate of population doubling of all species. Compared to the pace of evolution, population numbers change dramatically in short fashion. 

Take Mankind, as Malthus did. We are capable of doubling a population every 25 years. But just how quickly do we evolve? So, if Malthusianism is "a consequence, not a foundation, of the theory of natural selection." (Sober, 1984, quoting Fisher again), just what it is that drives the differential replication of the human species?

In examining Darwin's thought processes, Zimmer (2001) considers the question from the point of view of a founding population destined to evolve into a new species:

"How could an incipient species become a full-blown one? Here Darwin brought Malthus into the argument. Even a slow-breeding species like a human or a condor can double its numbers in 20 or 30 years, easily overrunning the planet in a few millennia."

Whereas when Zimmer (2001) considers Natural Selection, he has this to say:

"If natural selection worked on a variety long enough, it would turn it into a new species of its own. After a thousand generations, a single species of bird made of two varieties might end up as two distinct species."

So if Natural Selection is the pretender to the throne of differential replication in explaining survival of the fittest, I'd say that the evolutionists have their work cut out for them. To assist them in their efforts, I offer the Exponentialist view (with a very generous nod to Malthus). 

Conclusion

What they will find is that Malthus revealed the basic mathematical growth model which forces Natural Selection upon all populations. They will see that, together with Artificial Selection / Unnatural Selection, Natural Selection works through the differential replication of populations (both intraspecies and interspecies). Small founding populations, during their slow evolution of millennia, can actually increase in numbers very rapidly - again, the Malthusian Growth Model can be used to model such growth (which lead directly to the Couttsian Growth Model, a scientific law of population growth).

It can also be proven that Malthus had already extended his Principle Of Population to all populations of all species, and that he considered the effect of the environment and behaviour upon the differential growth of populations (through what I call Malthusian Selection). 

Not bad for someone who wasn't even writing about evolution.

References

What Evolution Is - Ernst Mayr (2001)

Evolution The Triumph Of An Idea - Carl Zimmer (2001)

Evolution - Mark Ridley (1997)

The Selfish Gene - Richard Dawkins (1976, 1989). Permission to reproduce text from this book was granted by the Oxford University Press.

River Out Of Eden - Richard Dawkins (1995)

The Nature Of Selection - Elliot Sober (1984) 

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Last modified: 05 June, 2009