Populations are the Key to Evolution

 

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aption id=”attachment_76″ align=”alignleft” width=”232″]Statue of Charles Darwin at the Natural History Musuem in London (Matt Buck) Statue of Charles Darwin at the Natural History Museum in London (Matt Buck)[/caption]

 

Evolution by natural selection as first elucidated by Charles Darwin and explained thoroughly in his book On the Origin of Species in 1859 describes how current life is the result of small slight changes in each generation of living things and that these tiny changes can either allow an individual to survive or not in its own natural environment but what is it about the organisms that change which would allow natural selection to act on? As was first realized implicitly by Darwin and made clear by German American biologist Ernst Mayer( and one of my intellectual heroes) , who was one of the founders of the Modern Synthesis or the combination of Darwinism and genetics which made the science of evolutionary biology legitimate, it is populations of living things whether they are microbe, fungi, plant, and animal and it is rather the adoption of a new way of thinking which was the most profound in the history of science and by understanding the true meaning of population thinking as is called by Mayr can one really understand the full meaning of evolution by natural selection which allows one to also appreciate Darwin’s radical genius.

 

What is population thinking? According to Mayr, population thinking puts emphasis on the variation of the individual beings in a population and through sexual reproduction mainly through meiosis or the formation of gametes which have half the genotype of a diploid organism or full genotype. The process of meiosis is what produces gametes but with a different combination of genes inherited through the mother and father and it is the same process that generates possible novel combinations which when expressed in the fully developed organism, is then subjected to the filter of natural selection whether or not these novel features or phenotypes of the organism will either survive or perish.

 

Why is that important? Without variation, there can be nothing at all for natural selection to act on and recognizing the fact that two individuals in a population are never alike, however slight the variation is, is what makes the difference between survival and death and if the organism survives in its own native habitat, then through the gradual expanse of time, the population will change slowly but inevitably into a distinct species if the environment changes whether it is change in temperature, the presence of a virus, or a new predator, populations and therefore species will eventually evolve into new forms.

 

The recognition of the reality of populations was the biggest breakthrough made by Darwin and population thinking was first realized by Darwin. This was the most profound kind of thinking which completely differs from a different kind of thinking, also implicit in the Western world called typology or the belief that everything living and non living are the manifestation of unchangeable “essences” that is everything in our world is a reflection of perfect objects. That believe can be traced back to work of the ancient Greeks, notably Plato and his belief of a  perfect realm. Later that kind of thinking was incorporated into medieval thinking which said that God as creator of the world, is the ultimate in perfection and created all objects including living things in their current forms. This thinking persisted until the Renaissance when scientists were beginning to question the Biblical doctrine as a result of discoveries of living things in other continents and up until the Enlightenment, belief in a static world was slowly beginning to lose credibility as more and more naturalists began to study the natural world with a more objective eye.

 

Around the early 1800’s, a French naturalist Jean Baptiste Chevalier de Lamarck, was the first scientist to take seriously the idea that all life forms are the descendants of previous life forms which change through the ages but according to Lamarck every living thing has an inner need to be “perfect” and strive towards perfection by changing their behavior and through time that kind of behavior is inherited into new forms. A classic example is the giraffe and the reason for its long neck is that the ancestors of giraffes were short necked and with a need to eat tree leaves, the short necked giraffe then left offspring with slightly higher necks and so on until the present form.

 

We know now and after Darwin, that behavior like that can never really be inherited that is your kids do not inherit a tattoo if you have one, and Lamarck was operating within the mindset of typology evolution would proceeds through changes of perfect forms from one form to another and plus typology is more like a problem of metaphysics than science the former concerned with truths that can neither be proven wrong nor correct while the latter is about observation and experimentation. Also in the British Isles, there was a belief known as natural theology in which it was thought that by examination of adaptations of each individuals, one can prove the existence of God, and this was articulated by British theologian William Paley, who used the analogy of the pocket watch; if you find a fully working pocket watch, and examined its contents inside, you are lead to think that all the gears and cogwheels could not have been the result of blind chance and this proves the existence of a Creator who would design a watch with an intention in mind.

 

It is interesting to note that Charles Darwin, as a undergraduate student was schooled in that belief of natural theology and even considered becoming a clergymen ( Imagine what would have happened if he actually he did.) but his real interest was the natural world and his chance to study much of the natural world was granted when he went aboard the HMS Beagle which was to study the coastline of South American and since he had the skills as a naturalist, he was suited for task. Remember, Darwin before the South American voyage, was convinced about natural theology and one of the things about natural theology is to study the living world. After his five year voyage, notably his study of the wildlife of the Galapagos islands, he begin to seriously question what he learned and was asking “why” questions, which in biology are called the ultimate questions (I’ll talk about these kind of questions in a later blog post). He was not satisfied about cataloguing specimens but he wanted to know why some species are unique to the Galapagoes islands but less so on the mainland, why some species of animals are alive and others went extinct. To truly answer these questions, it was not enough to find additional specimens, rather to fully answer the ultimate questions, one had to adopt a new way of thinking and that kind of thinking is population thinking.

Santa Cruz Island, which is part of the Galapagos Islands and it is where Darwin studied the wildlife and began asking ultimate questions (David Berkowitz)

Santa Cruz Island, which is part of the Galapagos Islands and it is where Darwin studied the wildlife and began asking ultimate questions (David Berkowitz)

 

After the Beagle voyage, Darwin become convinced that all of life are the descendants of a single simple ancestor, whatever it may be, and through vast expanses of geological time, species evolve into different species. In addition it was Darwin who designed “the tree of life” as if each species are at the tip of each branch of a tree. It was a novel approach by Darwin but to be plausible there had to be a mechanism for how species can change into different forms. It was where Darwin adopted population thinking (which wasn’t called that in Darwin’s time) and it came from a rather unlikely source, a treatise on human population by another theologian, Thomas Malthus.   According to Malthus, human populations tend to grow faster on average more than the natural resources which can support the population and to slow down the populations, things like disease, poverty, and war acts to slow down growing populations.  If what is true for growing populations of humans then the same applies to all living things. Every life form has the potential to grow rapidly or exponentially even with slow breeding organisms like elephants. The fact that no population of life form has complete dominance on the planet suggests that other factors act to slow down populations such as limited habitat size, limited amounts of food and so on. Many organisms produce a surplus of organisms but it also a fact that few survive. What of those surviving offspring and why did they survive?

Monarch butterflies resting on a plant. Since these are individuals in a population, no two are alike and whatever traits these butterflies possese , will make a difference in their survival. (LOSTIR)

Monarch butterflies resting on a plant. Since these are individuals in a population, no two are alike and whatever traits these butterflies possese , will make a difference in their survival. (LOSTIR)

 

It is as reasoned carefully by Darwin that the reason for survival is that each offspring may possesse certain traits, now called phenotypes and it is the possession of beneficial phenotypes such as being unpalatable to hungry carnivores, an ability to digest certain foods, etc which makes the difference in survival and it is these variations as stressed by Darwin which allows organisms to adapt mainly as hypothesized by Darwin and later confirmed, as a consequence of reproduction. These traits if they appear in the next generation will be inherited. No two individuals are alike and accepting that fact as Darwin did, he was lead to infer what is known as “struggle for existence” and that is competition for food, water, and mates between individuals of the same population and that is part of the mechanism called natural selection since it is nature which selects those individuals that have the ability to survive and it is Darwin, and independently another British naturalist Alfred Russel Wallace who came up with the same mechanism.

 

To summarize, as long as one accepted typology, it is not possible to satisfactorily answer the ultimate questions of why there are living things and why some are adapted and some are not. By treating the reality of the variation of the individual, Darwin was able to answer the ultimate questions thus putting the science of biology on a secure foundation which has been successful up to this day.

 

References:

Darwin, C.  (1859) On the Origin of Species By Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life.  London, England: Murray London

 

Mayr, E. (1969). The Biological Meaning of Species Biological Journal Linnaen Society pgs. 311-320

 

Mayr, E. (1970)Populations, Species, and Evolution Cambridge, MA: Harvard Belknap Press

 

Mayr, E. (1997) This is Biology Cambridge, MA: Harvard University Press

 

Mayr, E. ( 2001)What Evolution Is . New York, NY: Basic Books

 

Quammen, D. Was Darwin Wrong? National Geographic  (2004 November) 3-35

 

 

Photos:

IMGP8061 http://www.flicker.com/photos/mattbuck007/14525365548/ Matt Buck   CC BY-SA 2.0

 

Santa Cruz Dragon Hill-Galapagos Islands-Ecuador http://www.flicker.com/photos/David Berkowitz/4871237580/David Berkowitz  CC BY 2.0

 

_PHO7637http://www.flicker.com/photos/lost-ir/15044388379/ LOSTIR CC BY 2.0

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