Friday, May 27, 2011

The Big Questions of Evolutionary Biology - A Guide to 'Genetics and the Origin of Species'

Perhaps you don't believe in evolution.  Rob Bell, in Love Wins, asks people who say they don't believe in God, 'Which God?'  When they describe their understanding of the Christian God, Bell often discovers that he doesn't believe in that God either.  So if you don't believe in evolution, 'Which evolution?'  What is your understanding of evolution?  Because odds are, if your knowledge comes from Creationists, it is going to be filled with misunderstandings.  The best way to understand evolution is to delve into the writings of evolutionary researchers.  This is exactly what we will be doing over the next few weeks, as we explore Theodosius Dobzhansky's 1937 evolutionary classic, Genetics and the Origin of Species.  I will try to limit the scientific words that I use (or at least define the difficult ones), but I make no apologies.  If you want to know if evolution is true, you're going to need to learn some jargon.  Thankfully, Google dictionary exists to help us out!  If you're an evolutionist, you may also be surprised to read what evolutionary biology was like in the 1930s, and it may challenge some of your own beliefs as well.

Lets begin by pondering life.  There are over 4600 species of mammals currently living on this planet.  Mammals are considered a class in the Linnaean system of classifying organisms.  This system was originally developed by Linnaeus in the 1700s to categorize organisms based in their degree of similarity in body structure, but actually fit quite well with similarity in genetic structure (ie relatedness).  The highest category is the domain, of which there are three: Archaea, Bacteria and Eukarya.  Eukarya is the domain we are all familiar with, as it contains all plants, animals, fungi, amoeba and other such critters.  All creatures in Eukarya have a particular cell structure which includes linear DNA contained within a membrane-bound nucleus.  Archaea and Bacteria are distinguished by their own cell structures.

Each domain is broken up into kingdoms.  The number of kingdoms is constantly being revised; traditionally plants, animals, fungi and protists were the four Eukarya kingdoms, but today some recognize ten kingdoms, while others lump fungi, plants and animals into a single kingdom and break the protists up into numerous kingdoms.  Expect your textbooks to be rewritten soon.

After kingdom comes phylum.  Animal phyla include sponges, vertebrates, molluscs, arthropods, etc.  Phyla are broken into classes, classes into orders, orders into families, families into genera (singular genus), and genera into species.  Each of these categories can further be refined (superfamilies, suborders, etc).

An example would be humans: phylum Chordata, subphylum Vertebrata, class Mammalia, order Primates, family Hominidae, genus Homo, species Homo sapiens.

The class Mammalia, with its 4600+ living species, can be broken into its orders as follows:

Mammal order (common names)
Number of species
Insectivores (shrews, moles, hedgehogs)
Carnivores (cats, bears, mongoose, skunks, raccoons, seals, wolves)
Artiodactyls (pigs, hippos, camels, llamas, giraffe, deer, cows, goats)
Kangaroos, possums, wombats
Rabbits, hares, pikas
Numbats, Tasmanian devils
Sloths, armadillos, anteaters
Tree shrews
Horses, tapirs, rhinos
Elephant shrews
Rat opossums
Dugongs, manatees
Platypus, spiny anteaters
Marsupial moles
Flying lemurs
Monito del monte

What do you notice about this categorization?  The first thing that jumps out at me is the diversity within this class.  We have creatures of hugely varying sizes; over 104 million of the world’s smallest mammal (the Etruscan pygmy shrew) would be required to match the weight of one of the world’s largest mammals (the blue whale).  We also have animals here that swim, fly, glide, run, burrow, climb and hop.  Represented here are animals found in the oceans, deserts, jungles, prairies.  This diversity really is amazing!

The second thing that jumps out at me is the disparity between orders.  13 out of 26 orders (50%) contain less than 20 species; two of them contain only a single living representative!   7 of 26 orders contain over 90% of all living mammal species; one order alone contains 43% of the mammal species.

The third thing that jumps out at me is the discontinuity seen in nature.  While it is true that some populations of mammals might continuously vary such that one species blends into the next, there are definite clear discontinuous groupings of mammals.  An Etruscan pygmy shrew can definitively be declared as discrete from a blue whale.

But why?  What accounts for this amazing diversity, this bizarre disparity, and this clear discontinuity?

(As an aside, in my last post I indicated why special creation was not a viable solution for diversity or discontinuity, as evidenced by its inability to form any predictions about the patterns of life on this planet. Here we see another puzzling aspect if special creation were true: God seems to have really spent His creative effort on bats and rodents, but gave up for primates or aardvarks.  This becomes even more puzzling if we consider the number of insect species or bacteria on this planet.  Also please note that 'special creation' is a very different thing than 'creation', which I will get into in a future post).

Dobzhansky, in Genetics and the Origin of Species, describes diversity and discontinuity as the central problem of biology, and says that the solution lies within a genetic understanding of evolution.

Says Dobzhansky,

‘Organic diversity is an observational fact more or less familiar to everyone…A more intimate acquaintance with the living world discloses another fact almost as striking as the diversity itself.  This is the discontinuity of the organic variation.’

Although discontinuity might seem obvious, Darwin had done a lot to discredit such a view of the natural world.  David Reznick, in The Origin Then and Now, argues that this is because Darwin was up against an establishment that believed in the fixity of species.  Darwin, by calling everyone’s attention to the fuzzy lines between species, fostered the extreme attitude that there is no such thing as a species.  Let’s say we begin with two closely related species today, and we follow their history back in time.  Those two species would blend into one; that one would merge with another species into their ancestor; and so on to the first organism.  If we ever wanted to say, ‘Now, at this moment, we officially have species X’, we would be hard-pressed to do so, as all species should insensibly blend into their earlier ancestors.

This led some to reject the notion of discontinuity entirely, and to say that the Linnaean classification of organisms was simply a human construct imposed on the natural world, rather than a real reflection of real discontinuity.

Dobzhansky, in the opening chapter of his book, rescues discontinuity. 

His argument ignores the historical, and instead focuses on what we can see today.  If we were to plot out the body shapes of all living beings, Dobzhansky argues, we would not find a single continuous distribution.  Instead, we would find ‘separate, discrete distributions’.

‘In other words, the living world is not a single array of individuals in which any two variants are connected by unbroken series of intergrades, but an array of more or less distinctly separate arrays, intermediates between which are absent or at least rare.  Each array is a cluster of individuals, usually possessing some common characteristics and gravitating to a definite modal point in their variations.  Small clusters are grouped together into larger secondary ones, these into still larger ones, and so on in an hierarchical order.’

In other words, if we were to have in front of us a two-dimensional surface on which we could plot every possible body shape of every organism, and we were to zoom way out, we would find discrete clusters with empty space between them.  We would not find each form of life melding into every other form of life.  If we were to zoom in on one of these clusters, we would find it composed of smaller clusters with space between.  We could continue to zoom in, and continue to find each apparent cluster composed of smaller clusters, such that small clusters are hierarchically nested within larger clusters.

This hierarchical clustering is reflected within the Linnaean system of classification, with the largest clusters being domains, then kingdoms, right down to species and populations.  Dobzhansky argues that this classification is indeed partially artificial, in that labels like genus, family, order etc are mostly arbitrary, but it is also natural in ‘so far as it reflects the objectively ascertainable discontinuity of variation, and the dividing lines between species, genera, and other categories are made to correspond to the gaps between the discrete clusters of living forms.’

‘Therefore, the biological classification is simultaneously a man-made system of pigeonholes devised for the pragmatic purpose of recording observations in a convenient manner and an acknowledgment of the fact of organic discontinuity…Formation of discrete groups is so nearly universal that it must be regarded as a fundamental characteristic of organic diversity.’

When studying evolution, then, we cannot simply study diversity while ignoring discontinuity; they both must be explained by evolutionary theory.  Indeed, later in this book Dobzhasnky will pioneer a definition of sexual species that uses genetics to explain this discontinuity.

How to study diversity and discontinuity

Now that we have recognized diversity and discontinuity (and, indirectly, disparity) as the primary questions driving evolutionary biology, how would one begin to answer these questions?

Diversity and discontinuity clearly both have deep histories, and can be considered ‘macroevolutionary’ processes.  Unfortunately during Dobzhansky’s day the structure of DNA had not yet been discovered, the structural study of genes was in its infancy, and no one knew how genetically similar all of life really was.  So those avenues of research were not open to Dobzhansky.  Instead, he pioneered an experimental approach of ‘microevolutionary’ processes that occurred in sister species of fruit flies.  How could such small-scale studies be used to understand diversity and discontinuity?

‘Experience seems to show, however, that there is no way toward an understanding of the mechanisms of macro-evolutionary changes, which require time on a geologic scale, other than through a full comprehension of the micro-evolutionary processes observable within the span of a human lifetime and often controlled by man’s will.  For this reason we are compelled at the present level of knowledge reluctantly to put a sign of equality between the mechanisms of macro- and micro-evolution, and, proceeding on this assumption, to push our investigations as far ahead as this working hypothesis will permit.’

In other words, he would have to assume that small-scale evolutionary changes, over long periods of time, added up to large-scale evolutionary changes.  His goal was to see how far this hypothesis could be pushed.

Dobzhansky concludes this introductory chapter by essentially revealing his research program (and thus the structure of his book).  The first step of studying small-scale evolutionary processes is to ascertain the mechanisms of evolution, those genetic processes that fuel evolution.  He identified four:

1. Mutations
2. Gene rearrangements within chromosomes, which may change gene function (position effect)
3. Reduplications and polyploidy
4. Losses of whole chromosome sets

Once the mechanisms had been studied, one then would need to study those forces that changed the fates of these genetic changes.  He identified these forces as:

1. Selection
2. Migration
3. Geographical isolation

Finally, once genetic changes have occurred and their fates have been decided, one would need to study how these genetic changes are protected (in other words, what allows discontinuity).  Dobzhansky explains:

‘Races and species as discrete arrays of individuals may exist only so long as the genetic structures of their populations are preserved distinct by some mechanisms which prevent their interbreeding.’ 

 A newly-evolved population would have new gene complexes essential for its survival, but interbreeding with closely related populations would break down such differences and prevent the formation of diversity and discontinuity.  What prevents interbreeding, protecting the gene complexes?

1. Ecological isolation
2. Sexual isolation
3. Hybrid sterility
4. others

As we follow through Dobzhansky’s book, each of these categories, and how they explain diversity and discontinuity, will become clear.

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