Tag Archives: evolution

Modern Evolutionary Synthesis: Natural Selection and Genetics Together Forever

So Darwin had A Big Idea ™ which has caused a ridiculous amount of debate/screaming/studying over the past 150 years. Everyone knows that, right? It’s caused so much uproar that recently the Texas school board has had near never-ending debates about the merits of teaching such Big Ideas to poor innocent children.

I live here! Awesome.

But I (sarcastically) digress. My main point for this essay is to discuss an interesting development that occurred nearly 50 years after Darwin wrote his abstract “On the Origin of Species by Natural Selection.”

(Did you notice that I called it an “abstract?” You see, Darwin had been working on his book for over 20 years when it finally came out–and it only came out when it did because another young scientist had happened upon the same idea and Darwin didn’t want 20 years of thought and work to go down the drain. So instead of publishing the 1200 page opus he originally planned, Darwin cut it down to a svelt few hundred pages and called it an abstract. Just an historical tidbit).

Oh Kanye...will this never get old?

Anyhow, Darwin’s ideas have not been taken at face value since the moment his pen hit paper. As with most scientific ideas, they have been analyzed, discussed, tested, reworked, and improved upon until they are almost unrecognizable in their original form. This is why I love science so much–the ideas are constantly changing and improving, and each member of the community has the opportunity, nay, duty, to suport or falsify the concepts that came before.

But that’s a bit too “philosophy of science” for this post. No, what I’m attempting to explain here is the concept of modern evolutionary synthesis, commonly called “the new synthesis” or “neo-Darwinism.”

So, in 1859 Darwin published On the Origin of Species, and it sparked a huge reaction. His main ideas can be summed up in four major points:

1. There is variation within species

2. These variations can be passed on to offpsring

3. Each generation sees too many offspring born than can possibly survive

4. The survival and subsequent reproduction of offspring isn’t random; those that do survive long enough to reproduce, or those that reproduce the most have the most favorable characteristics. Those characteristics are passed on to offspring. Those characteristics are naturally selected.

What’s interesting is Darwin did not put for the idea of evolution–in fact, the word “evolved” is only used once in the entire book, and is the last word of the last chapter. Evolution as a concept had been around since the mid 1700’s, and was generally accepted even by the devoutly religious as a plausible explanation of animal life. What people didn’t know was HOW evolution worked, and there were many scientists putting forth various hypothesis to explain the mechanism.

Darwin came up with his ideas after years on board a ship, and collecting and observing all manner of life forms across the world. He then let the idea fester in his mind for over 20 years and slowly developed the over arching idea that evolution occurs through small variation from generation to generation, and that the most beneficial variations are conserved through natural selection.

The biggest problem with the idea of natural selection back in Darwin’s day, however, was no one could quite figure out how it actually worked. The evidence made sense (and Darwin does a beautiful job of beating the reader over the head with chapter after chapter of evidence for natural selection), but the actual mechanisms of inheritance still eluded the best scientists of the time. The debate raged on.

Meanwhile, off in Germany, a little known monk by the name of Gregor Mendel was doing some experiments with pea plants.

I eat my peas with honey, I've done it all my life....

Mendel figured out that traits were passed from generation to generation in a mathematically predictable manner. The problem was few people knew about his work. He published it in 1865, about 6 years after the first edition of On the Origin of Species, but all evidence suggests that Darwin never read the paper. Both Darwin and Mendel died before someone figured out to link the two ideas together.

But get linked together they did (Yay for literature searches!!). The 20th century saw some clever person pick up both pieces of work and think “Huh. This seems to work well together!” Mix in the discovery of genes and the idea of population genetics, and you get an explosion of evolutionary ideas that united a formerly divided group of scientists.

After a few decades of experiments and nobel prizes, the New Synthesis was born, and incorporated the ideas of both natural selection and genetics:

1. Populations contain randomly derived genetic variation accomplished through mutation and recombination

2. Gene frequency within populations changes through genetic drift (allele frequency change due to random sampling), gene flow (the transfer of alleles from one population to another), and natural selection

3. Most adaptive genetic variants have slight phenotypic effects, so change in phenotype is gradual

4. Speciation involves gradual evolution of reproductive isolation among populations, and is what causes diversification

5. The processes of speciation over a long enough time cause changes so great that higher taxonomic levels are necessary (genera, families, orders, etc.).

Basically, the modern evolutionary synthesis is the application of Darwin’s idea of natural selection to Mendel’s idea of inherited characteristics, with a little bit of Watson and Crick mixed in.

Hey! We discovered the structure of DNA!

Of course, listing the 5 major ideas of any paradigm simplifies it a little too much. These ideas were created by numerous scientists over the course of a decade, and represent some of the greatest ideas evolutionary biologists have had.

The major names associated with modern evolutionary synthesis are Julian Huxley, R.A. Fisher, J.B.S. Haldane, Sergei Chetverikov, Theodosius Dobzhansky, E.B. Ford, Ernst Mayr, Sewall Wright, George Gaylord Simpson, G. Ledyard Stebbins, and Bernhard Rensch.

Julian Huxley is credited with coining the term “modern evolutionary synthesis” in a book he wrote in 1942 Evolution: The Modern Synthesis. Julian Huxley was the grandson of Thomas Henry Huxley, the man known as “Darwin’s bulldog” due to his dogged defense of Darwin’s ideas. So I suppose it’s no surprise that Julian Huxley would carry on that tradition two generations later by bringing Darwin’s ideas into the 20th century. Julian’s book merges the revolutionary ideas of population genetics and genetic inheritance with natural selection by approaching genes from a natural selection standpoint. The book has two major editions, and has just recently been rereleased.

Of course, Huxley did quite a bit more than simply write a book–the years up to the writing of this work were filled with evolutionary research and the years after were filled with the same. He will forever be known as the man who coined the term, however.

R.A. Fisher is the guy responsible for such statistical break throughs as Fisher’s exact test and the ANOVA. I blame him for making stats such a pain in the butt! And so very useful. Thanks Fisher! Anyhow, Fisher was an active mathematician and geneticist during the first half of the 20th century. His interest in eugenics, mathematics, and evolution gave him the tools to become the founder of quantitative genetics. He spent much of his time attempting to calculate the distribution of gene frequencies among populations. Geez! I also had problems with those calculations in  my molecular ecology courses! Apparently this guy is responsible for all the difficult homework I’ve had over the last two years. Thanks, Fisher.

JBS Haldane is the second of three important mathematicians that helped to quantify rates of changes in gene frequencies (the first being RH Fisher). In 1932 Haldane penned a book entitled The Causes of Evolution, which summarized his work on the mathematical theory of natural selection.  Haldane went on to write an essay called On Being the Right Size, which postulates that physical size is often what determines the equipment necessary for life in a given species. This idea is referred to as Haldane’s principle by modern biologists. On a humorous note, Haldane famously answered the question “What can be inferred about the mind of the Creator form the works of His Creation” with “An inordinate fondness of beetles.”

Funny and sarcastic scientists are the best!

Sergei Chetverikov was one of several Russian scientists that ventured into the world of evolutionary biology in the early 20th century. Chetverikov worked with fruit flies, and in 1926 wrote a paper that bridged early theories of genetic evolution with real world populations. He found that the process of mutation was the same in natural, laboratory, and domesticated populations; most mutations are deleterious but there  ere some that do not reduce viability; a randomly mating population is stable; new mutations are absorbed by heterozygous individuals; mutations gradually spread through the population by chance (a very early concept of genetic drift); and isolation and genotypic variability lead to differentiation. These concepts are just a few put forth in his major work, which, unfortunately, was only published in Russian. Luckily, Haldane had a translated copy and was able to make use of these concepts during the new synthesis.

Theodosius Dobzhansky is another influential Russian scientist, although he had the distinction of moving to America during his seminal research years. Dobzhansky, a geneticist by trade, is credited with writing a major work on modern evolutionary synthesis: Genetics and the Origin of Species. This work defines evolution as “a change in the frequency of an allele within a gene pool” and is responsible for spreading the idea that natural selection takes place through mutations on genes. He is also credited with the famous phrase “Nothing in Biology Makes Sense Except in the Light of Evolution,” which is the title to a famous article in which he articulates the conflict of evolution and creation.

E.B. Ford was a student of Julian Huxley at Oxford, and spent his career working on insects. He is responsible for the field of ecological genetics, and was on good working terms with Dobzhansky and Fisher, with whom he exchanged numerous letters and ideas. Ford formalized the definition of genetic polymorphism (when two or more clearly different genotypes exist within a single species), and used his knowledge of Lepidoptera to test and eventually prove many of Fisher’s predictions. Ford’s most famous student was Bernard Kettlewell, who conducted experiments on the evolution of melanism in the peppered moth, an experiment which delights and annoys students to this day.  On a side note, Ford campaigned for the legalization of male homosexuality in his native Britain. Just an FYI.

Ernst Mayr (pronounced “Mire”) was a taxonomist and naturalist who was the youngest of the modern synthesis boys (he died only recently in 2005). His major book was Systematics and the Origin of Species in 1942, in which he helped to define the biological species concept: a species is not just a group of morphologically similar individuals, but a group that can breed exclusively among themselves. He suggest the concept of peripatric speciation, in which populations of adjacent yet isolated individuals will, though genetic drift and natural selection, evolve into distinct species over a period of time. Mayr was a voice of dissension among the modern synthesis crew, insisting that natural selection acted upon the whole organism and not individual genes. He also criticized molecular evolutionary studies. Huh. Maybe he was kind of a jerk. A smart jerk, but a jerk none the less.

Sewall Wright is the third mathematician that I blame for all those damn calculations I had to do in molecular ecology. He worked with Fisher and Haldane on the concept of population genetics, and discovered the inbreeding coefficient.

George Gaylord Simpson (hee! Gaylord) was an influential paleontologist during the early 20th century, and lent a much needed historical perspective to the new synthesis. He wrote several works including The Meaning of Evolution and The Major Features of Evolution, and was an expert on extinct mammals and their migratory patterns. He is known for coining the term hypodigm (a sample from which the characteristics of a population may be inferred), and he predicted the concept of punctuated equilibrium years before it was officially put forth by Dawkins.

G. Ledyard Stebbins is the lone botanist in this crew, and studied the evolutionary biology of plants at Berkeley. He wrote Variation and Evolution in Plants, which, by many, is considered a core of the evolutionary synthesis. His work on polyploidy and speciation in plants has influenced nearly all botanists since his time. When speaking of his own work, he never considered his contributions unique; he simply thought he was applying evolutionary biology and genetics to plants, and describing how these concepts affected botany as a whole.

Bernhard Rensch is the lone German of this group, and is responsible for popularizing the new synthesis in his native country. His primary work involved explaining how the concepts that drove speciation could be used to describe higher taxa. He also worked in areas of animal behavior.

It’s almost magical how just the right people with the right backgrounds in the right areas all were studying at the same time, isn’t it? It’s a tribute to the beauty of the scientific method, I think, to see how each man’s ideas were disseminated and used by the others to eventually create a paradigm shift in the way biologists think about evolution.