Genetics and the Origin of Species

The mule is a famous example of a sterile hybrid.  Photo circa 1937

We are officially almost finished our summer-long read through Dobzhansky’s evolutionary classic, Genetics and the Origin of Species.  We have spent an awful lot of time talking about genetics – about mutations, and how their value is context-dependent; about chromosomes, and how breakdowns during cell replication and gamete production can lead to large-scale changes; about natural selection, and how it affects gene frequencies; but now it is time to actually roll up our sleeves and delve into the ‘origin of species’ part of Dobzhansky’s book title.  Last week we explored polyploidy, a form of instant speciation that is solely due to reproductive incompatibilities brought about by chromosomal mismatch.

In other words, last week we saw that a simple genetic cause (chromosomal duplication) led to instant speciation, simply because the polyploid and its parental form could not successfully reproduce (or, more precisely, their offspring could not reproduce); that is, the polyploidy event caused a reproductive barrier to form between the parent and its polyploid offspring.  They were reproductively isolated.

Even You Can Create a New Species - The Curious Case of Polyploidy

There are currently 1.7 million named species of plants, animals and algae on this planet.  This does not include the vast array of bacteria and other such single-celled creatures.  And those are only the ones we know about – based on rates of discovery, the National Science Foundation estimates that we have only described 10% of the world’s true diversity.  Most of these organisms will likely be small or marine, but not all; twenty-five species of primates have been discovered since 2000 alone.  And then this diversity really only scrapes the surface of the total diversity that has ever existed, with a mind-boggling number of plant and animal species known only from the fossil record.

Theodosius Dobzhansky, in Genetics and the Origin of Species (1937), presents, in chapters 7-9, an introductory treatment on the topic of speciation.  He brings genetics and natural selection and various observations together to provide a compelling account of how new species are formed.  This would be the groundwork used by Enrst Mayr, who would develop the modern Biological Species Concept.

Natural Selection

We continue with our (seemingly unending) walkthrough of Dobzhansky’s evolutionary classic, Genetics and the Origin of Species.  Today we will finish his chapter on Selection; the rest of the book will be a cakewalk in comparison.

Natural selection was not unique to Darwin, having been recorded by numerous others, including the watchmaker-hypothesis’ William Paley.  But for these pre-Darwinian writers (with two minor exceptions, noted in the opening of the sixth edition of the Origin of Species), natural selection was simply a way of preserving the created species.  Forces would try to alter the species into less-adapted products; natural selection would winnow out those mutant forms, allowing the species to remain relatively pure.  Selection, then, was like a hangsman, killing off what nature abhorred.  Darwin’s major insight was to raise the status of natural selection from preserver and destroyer to the primary creative force in nature.  (I am indebted to Stephen Jay Gould’s book The Structure of Evolutionary Theory for much of this discussion).

Variation

Humans come in different shapes, sizes, colours, temperaments, propensities to disease and the like.  Although the human population continues to increase, we seem to never run out of variations on the human ‘type’.  All of the dog breeds of the world ultimately descend from the same wolf ancestor.  The bulldog, the terrier, the great Dane, the German shepherd, were all produced when humans took existing varieties and made some choice selections.  The same goes for our agricultural products: kale, cabbage, broccoli and cauliflower all descend from the same wild cabbage species.  The fruit fly Drosophila pseudoobscura, according to Dobzhansky, has seven different types of Y chromosomes; there are strains of wheat with 7, 14 or 21 chromosomes.

Without variation, there can be no evolution.  Dobzhansky spends the first four chapters of his book driving this point home, because it is that essential.  If variation was not continually being produced, there would be no differences between individuals; without differences between individuals, there would be no selection; and without selection, there would be no adaptation.

The Joy of Chromosomes (Part 3)

Last week we talked a bit about chromosomes, the process of sperm/egg/pollen formation, and how missteps during meiosis can lead to chromosomal mutations in terms of numbers of chromosomes.  We defined haploidy and polyploidy as the loss or gain of an entire set of chromosomes, and monosomy and polysomy as the loss or gain of a single chromosome.   We also saw how species and races contain variability, not just in the content of their DNA, but also in the number of chromosomes that they have.  Humans, for instance, have one less chromosome than the other great apes; a single species of Iranian climbing weed has populations with vastly different numbers of chromosomes.

Today I’d like to continue following the layout of Dobzhansky’s book, Genetics and the Origin of Species, by continuing to categorize chromosomal mutations according to changes in their structure.  Structural mutations do not affect the number of chromosomes within an organism, but rather alter the layout of genes within a chromosome.  Structural mutations can be classified as follows:

The Joy of Chromosomes (Part 2)

Chromosomes are the carriers of genes.  Each chromosome acts as an individual, faithfully passing its structure on to its offspring each time a cell divides.  But ‘faithfulness’ has its limits – occasionally a ‘mutant’ chromosome is produced that will then be replicated.  The number of chromosomes within a cell can also be faithfully replicated down the generations, but occasionally they too can change.

When we talk about chromosomal mutations, then, we need to differentiate between changes in chromosome number and structure.  Today, we will talk about number.

The Joy of Chromosomes (Part 1)

So, you have found yourself transported in time to the 1930s, and you have a strong desire to study mutations?  But those uncivilized brutes have yet to develop methods for examining changes in DNA?  Never fear!  You can do the second-best thing, and rediscover everyone’s favourite pastime: karyotyping!

All Species Are the Products of Mutations

Summary so far

To recap our story: evolutionary biology’s ultimate quest is to explain the diversity and discontinuity we see in the natural world.  A whale and a mouse are two very different things; they do not insensibly blend into one another.  Yet despite their discontinuity, they also have very similar body structures.  What accounts for this?  And why in less extreme cases is this discontinuity at times fuzzy, such that we have difficulty telling if two different populations are different species, or merely varieties of the same species?

Mutations: The Good, the Bad, and the Neutral (Part 5)

Lesson 5: A single mutation cannot create a new species

There was some debate during Dobzhansky’s time about whether new species could be formed through the birth of rare ‘monsters’.  Such monsters would be the product of a mutation, and would be effectively shut off from the rest of the population, forming their own species.  This theory was called saltationism.  Saltationists argued that a single mutation can have a very large effect.  A mutation somehow produces fundamental changes to the species, such that a new species is formed.

Mutations: The Good, the Bad, and the Neutral (Part 4)

We have so far learned from Dobzhansky that:

1.       Mutations are common in nature and are the source of all diversity

2.       Mutations can be really bad, to the point of being lethal, but there is a gradient from bad to good; bad mutations can hide as recessives within a population

3.       Environmental change (or a change in the genetic background) can turn a ‘bad’ mutation good, and a ‘good’ mutation bad.  Mutational value is contextual.

Lesson 4 – A single mutation can have a multitude of effects

Mutations: The Good, the Bad, and the Neutral (Part 3)

Lesson 3 – The value of a mutation (good, bad, or neutral) changes with the environment.  A mutation that appears to be bad in one context, might be good in another.

So far we have seen that mutations are common enough in nature, that bad mutations can hide as recessives within a population (contributing to the population’s overall genetic diversity), and that mutations can have anywhere from hugely negative effects (ie lethal mutations) to almost negligible effects.

What Dobzhansky has not yet shown is that mutations can be beneficial to the organism.

Mutations: The Good, the Bad, and the Neutral (Part 2)

Lesson 2: ‘Bad’ is a relative term.  Some mutations are worse than others, and even really bad mutations can hide in a population.

As we saw yesterday, mutations occur frequently in nature.  Inheritance of parental traits is conservative, in that it is usually quite faithful; but if it is completely faithful, there can never be variation in a trait.  Mutations counter this ‘copy fidelity’, acting as an opposing ‘force’ that produces variation.  Dobzhansky inferred from this that all of the heritable variation that we see in nature is due to mutations that have occurred either recently or in the remote past.

Mutations: The Good, the Bad, and the Neutral (Part 1)

Chapter two of Dobzhansky’s Genetics and the Origin of Species is a summary of lessons learned surrounding the enigmatic ‘mutation’.  What is so fascinating about this is that, first of all, he addresses a number of misconceptions that the public still have about mutations today, and, secondly, he did this without knowing the mechanisms behind mutations.  For example, today we know that point mutations can occur in DNA.  This is when one of the bases in a DNA molecule gets swapped for another one.  Thus a C (the molecule cytosine) get could replaced with a T (thymine), which could potentially affect the appearance or behaviour of the organism.  In 1937, everyone knew that biological information was stored in the chromosomes.  It was known that chromosomes were somehow composed of genes (stuffed into the chromosomes like a sausage, according to Dobzhansky), and that sometimes parts of chromosomes could break off, switch around, go missing, get doubled.  But no one knew that genes were made of DNA.  No one knew that there were four 'letters' to DNA.  No one knew that mutations could occur in the DNA.  Yet Dobzhansky could still with full confidence talk about point mutations!  He knew they had to exist, even if he could not explain how.

Today I would like to give a snapshot of the lessons Dobzhansky learned when studying mutations.

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.

Why Special Creation Failed - An Introduction to Genetics and the Origin of Species

Special creation, defined as the direct creation of fixed (unchanging) species by God, was the predominant view of the origin of species among scientists during Darwin’s time.  Writes David Reznick in The Origin Then and Now, ‘Today we think of the advocates of special creation as representing non-scientific, religious opponents to evolution.  In Darwin’s day, they were the scientific establishment.  Virtually everyone, ranging from his professors at Cambridge to all those who had the greatest influence on Darwin’s intellectual development, advocated some form of special creation.’

God in the Origin of Species

It is an interesting and much-observed phenomenon that the language of technology can be appropriated for describing the world in new ways.   The best example of this today comes from philosophy of mind and consciousness, where analogies from the computer sciences have helped us envision how the mind operates.  It is interesting to ponder how our concept of mind would have developed without the advent of computer processors.

In the 1600s one of the most remarkable pieces of technology was the clock.  The clock had indeed been around for hundreds of years, but by the late 1500s it had gained an unprecedented level of complexity.  The second Strasbourg clock, for example, which was completed in 1574, contained moving statues and automata, played music, and could track both the time and the movement of celestial bodies.  One of the great ironies of history was that a device whose history began as a simple shadow caused by the movement of the sun, would become one of the most powerful images for the movement of the universe.

What is Bathybius? A Look at the Origins of Life in 1870

Bathybius

I have found a treasure trove of old science papers – every paper every published in the American Naturalist, from 1867 to today.  These papers are fascinating, giving me some much-desired insight into the state of biology shortly after the publication of the Origin of Species.

Today I wanted to share a story that is partially revealed in the 1870 paper, ‘What is Bathybius?’ by W.C. Williams.

The Other Side of Fundamentalism - James Orr's 'Science and the Christian Faith'

When people think of Fundamentalist Christians, they (I'm sorry to say) tend to think of closeminded, loudmouthed, arrogant people who wouldn't recognize valid evidence or a rational argument if it hit them upside the head.  Although in my experience there are definitely some of these, this, like all stereotypes, is a dangerous mischaracterization of the richness of viewpoints that exist among Fundamentalists.  Fundamentalist Christians are just like everyone else - they are eager to know the world, and they are prone to making mistakes.  

Last week I remarked on an article published in 1911 in The Fundamentals, a Protestant journal that would come to define Fundamentalist Christianity.  The author, who remained anonymous, joyfully bashed both evolution and those who would preach it, but he (or she) at times confused scientific controversy over the role of natural selection, with a scientific disavowal of evolution itself.  The article is certainly insightful, witty and quite well-written - there is nothing slack-jawed or unintelligent about it - but by posting it on this site I may have led you to believe that all Fundamentalists have been historically opposed to evolution.

The Birth of Fundamentalism

Between 1910 and 1915, the Bible Institute of Los Angeles published a series of essays collectively referred to as The Fundamentals.  They were written as a response to various perceived threats against an orthodox Protestant faith, and reaffirmed those doctrines that were considered fundamental to a vibrant Christianity.  The denounced philosophies included liberal theology, the 'higher criticism' of scripture that was reaching America from Germany, Catholicism, Mormonism, atheism and many others.  It is from the title of these essays that we get the term 'fundamentalism' and 'fundamentalist Christianity.'

Although at least one of the early Fundamentals articles was sympathetic towards evolution, this changed with the article 'Evolutionism in the Pulpit', published in 1911 by an anonymous author.  The entire text is below.  It is a witty and scathing critique of Darwin's theory of evolution, and should give you a good sense of some of the reasons why the Fundamentalist movement was so successful.  Before you read it, though, I wanted to highlight a few items of interest:

Debunking Design - A Conclusion to Hume

We have finally reached the end.  I know it has been a long journey, but at last we today finish David Hume’s An Enquiry Concerning Human Understanding.  I thank you all for your patience as we go through this remarkably important book.  You may wonder why I have bothered with this book on a science-religion blog; the answer is that, by refusing to be content with knowledge derived solely from philosophy or revealed religion, Hume paved the way for the empirical experimental method.  More specifically, though, Darwin cited Hume as a ‘central influence’.  You can readily see why: Darwin was up against a standard view of the world, which saw both the design of God in the complexity of nature and the goodness of God in the harmony of nature, as revealed through the opening lines of Genesis.  To question the standard interpretation of revealed religion, to wonder if our experience of the world could possibly address the question of life’s origins, was a very Hume-ian thing to do.