Monday, December 3, 2012
As all good theories should, the theory of evolution has grown to account for new discoveries.
Its maturation has some things in common with the history of the theory of gravitation. Each accounted for phenomena that had been experienced up to the time of their formulation. Subsequently, they were amended to account for new discoveries.
It’s often said Einstein’s general theory of relativity proved Newton’s theory of gravity to be wrong. This is far from the truth, as Einstein’s work subsumes that of Newton. That is, Newton’s theory turns out to be a special case of the more inclusive picture given by Einstein.
This is also the case with Darwin’s elegant synthesis of the forces at work shaping the creatures and plants inhabiting our planet. I will discuss below two major enhancements that have shaped our understanding of evolution since Darwin’s publication of The Origin of Species a century and a half ago.
It is truly amazing Darwin was able to connect the dots from the various bits of knowledge that had been accumulated and make such a coherent and robust theory. Since it was offered to the public for the withering scrutiny it would be subjected to, nothing has shaken its central premises.
Darwin’s work was founded on keen observations in comparative anatomy of both living and extinct organisms, a burgeoning understanding of geology and rudimentary notions about domestic animal breeding. The theory reigning today fully integrates these ideas with discoveries in genetics and molecular and developmental biology.
His greatest insight, and the unifying idea behind his theory, was that natural selection enhanced certain characteristics of organisms over multiple generations. Individuals having characteristics allowing them to thrive and reproduce left more heirs than those that did not.
Rather than a breeder choosing winning traits, the environment provided the backdrop against which natural selection favored one characteristic over another. In response to new or slowly changing environments, maladapted populations of organisms migrated, died out or underwent physiological transformations that allowed them to thrive under the new conditions.
Darwin demonstrated that such transformations occurred routinely at a very slow pace in every location he examined. He could tie the adaptations to a variety of environmental factors. Many of these involved isolation of populations of organisms, often as the result of some geologic or climatological event.
So, what has happened since? Genetics was to provide an essential experimental foundation. It began with the presentation of a paper called Experiments in Plant Hybridization in 1865 by an Augustinian friar named Gregor Mendel. His meticulous study of peas demonstrated than traits inherited by offspring did not display continuous variation of the traits of their parents.
Rather, traits were passed on as discrete units, showing up in predictable ratios among offspring. In what became known as Mendel’s Laws of Inheritance, he proposed a Law of Segregation and Law of Independent Assortment. The agent of these discrete traits would later be named a gene.
Mendel’s ideas languished through most of the rest of the 19th century. They were essentially rediscovered in 1900 by Hugo de Vries. He postulated the mechanism for inheritance of traits were particles he named “(pan)genes”. William Bateson coined the term genetics in 1906.
Significant advances followed in the first three decades of the 20th century. R.A. Fisher, J.B.S. Haldane, and Sewell Wright made critical contributions in population genetics.
What would come to be known as the “Modern Synthesis” is said to have begun with Fisher’s 1918 publication of The Correlation Between Relatives on the Supposition of Mendelian Inheritance. He followed this in 1930 with The Genetical Theory of Natural Selection.
A new underpinning had been established for Darwinian evolution. Genetics could explain how natural selection worked. In 1937 Theodosius Dobzhansky published Genetics and the Origin of Species. He is famously quoted as having said, “Nothing in biology makes sense except in the light of evolution“. Ernst Mayr followed with Systematics and the Origin of Species in 1942.
The next era began with an experiment in 1944 showing DNA as the genetic material. And by 1953 James Watson and Francis Crick had demonstrated the famous double-helix structure of DNA.
In summary, the modern synthesis established a central role for genetics. It asserted natural selection occurred in small increments over long periods of time. And, though the adult organism was the object of selection, it was the genetic diversity in a population that served as raw material for divergence. And, finally, microevolution (small changes) leads to macroevolution (large changes) and new species.
Advances in understanding how DNA functioned filled the decades to come. It was well established that individual genes provided a code for cells to manufacture proteins. How could this routine task orchestrate the growth of a fertilized egg into an adult organism?
This process is the subject of developmental biology; which brings us to the second major leap in understanding evolution since Darwin.
By the early 1980s experiments had revealed a type of gene instrumental in directing the development of whole segments of an organism’s body. Hox gene’s proteins, called transcription factors, were not involved in routine cellular functions. Rather, they served as signals to turn other genes on and off.
Subsequently, these genes, and others, were found to serve similar roles in embryo development across nearly all the animal kingdom. Some orchestrated the development of eyes others the growth of limbs — legs, arms, antennae and wings.
Importantly, in the earliest stages of development, when there were perhaps no more than 100 cells, some of these genes established what would become the front/back, top/bottom and right/left axis of the growing organism.
These came to be known as “toolbox genes” because they were present in all animals and performed similar tasks in each. As a whole DNA functioned like a complex computer network. It was the cascade of genes being turned on and off in combination with other genes that determined if a group of undifferentiated embryonic cells became part of the organism’s brain, muscle or gut.
Furthermore, cell division is not always perfect. Sometimes genes in a strand of DNA get duplicated or shuffled somewhat. These copies or modifications can be of no consequence. They may languish for some time experiencing occasional mutations.
Occasionally the copied or modified gene is commandeered by the DNA “computer” for a new task. It is thought this may contribute to the origin of an organism with novel features; an organism not unlike its predecessor but different enough to constitute the beginnings of a new lineage.
Not only are many toolbox genes common to all complex organisms, studying the distribution of duplicated and modified genes in different lineages corroborates the relationships among animals and plants long established by paleontology and comparative anatomy.
With advances in molecular biology and genetics, evolutionary theory stands stronger than ever. Advances in evo-devo, as this most recent field has come to be known, sheds further light on natural selection and that it might be complemented by other mechanisms.
With all that has been discovered over the last 150 years, it is amazing to think of how well Darwin’s elegant synthesis has stood up.
Steve Luckstead is a medical physicist in the radiation oncology department at St. Mary Medical Center. He can be reached at firstname.lastname@example.org.