Hostility to evolutionary theory is nothing new; Darwin’s modest proposal has been consistently attacked over the last 150 years. What makes this theory so remarkable is how bulletproof the idea of evolution is; like a block of granite, each new discovery nicks away at surface of the theory, but at its core, the concepts are solid. But there was a time when evolution wasn’t so secure in the minds of scientists. For one thing, the mechanism by which natural selection would operate was completely unknown. Evolution seemed to be odds with the newly-rediscovered field of genetics. And so evolution by natural selection fell out of favor for a generation of biologists after the death of Charles Darwin in 1882. It wasn’t until Gregor Mendel’s research was exhumed from its obscurity that scientists once again began to look upon evolution with favor. A new generation of biologists managed to reconcile evolution with the engine of heredity, and in doing so, led the way for one of the most profound paradigm shifts in the field of biology.
Gregor Mendel was a monk who had spent years working out the laws of heredity on pea plants that he grew in the garden of a monastery. By meticulously breeding parent pea plants together, Mendel was able to predict the characteristics of the offspring. After years of plant breeding, Mendel discovered the same basic 3:1 ratio popped up every time in the second generation. We now understand that pea plants have a pair of alleles for each trait like flower color and stalk height, which come from each of their parents. If an organism inherits the same allele from both parents, we call that organism homoyzygous. And if an organism inherits a different allele from each parent, the organisms is heterozygous. Mendel was looking at homozygous parents giving rise to heterozygous offspring, but without a concept like genes, it was hard for anyone in the nineteenth century to see the mechanism at work here. Suffice it say, Mendel did not understand the significance of his laws of inheritance, and neither did the scientific community at large. For over thirty years after his death, Mendel’s work on pea plants languished until they were rediscovered by scientists like Hugo De Vries.
The pioneering ideas of Hugo De vris led the way for the modern evolutionary synthesis of evolution and Mendelian genetics. De Vries is credited for independently discovering Mendel’s laws of inheritance and for his ideas that linked Darwin’s ideas of natural selection to the mutation of genes. Darwin himself believed that parents possessed tiny particles called gemmules that were passed on to their offspring in a theory he coined pangenesis. But he could not explain why some traits were passed onto offspring while others were not. Nor could he explain the role that the supposed gemmules played in the development of the offspring. So when De Vries published his own work on pangenesis in 1889, he proposed that particles called pangenes carried only one specific trait and that when parents passed their pangenes onto their offspring, the traits of the offspring were determined by which pangenes they picked up (later scientists truncated this term to gene for short). De Vris also coined the term mutation to describe new traits that arose between the parent generation and the offspring generation.
The division of chromosome pairs in meiosis supported the two Mendelian Laws of Heredity and helped to explain how traits were passed from parents to offspring. Mendel’s first law of Inheritance states that each gamete cell receives only one of the two copies of an allele in each diploid cell. Walter Sutton was researching the process of meiosis in grasshopper cells; in meiosis, a cell’s chromosomes are divided equally into haploid gamete cells. Thanks to Sutton’s research he correctly identified that chromosomes were the carriers of genetic information in the form of alleles. But Sutton wasn’t alone. Across the planet, another researcher named Theodor Boveri too found Mendel’s research on inheritance and applied Mendel’s laws of inheritance to his research on the embryonic development of sea urchins. In the end, both Sutton and Boveri discerned that chromosomes come in pairs, one inherited each from the maternal and paternal parent; they share credit for its discovery. It was Sutton who independently discovered Mendelian laws of inheritance could be applied to homologous chromosomes and to the genes that they carried. In 1903, Sutton published his discovery that chromosomes contained genes, but his research was at first viewed with skepticism.
T.H. Morgan thought that Mendel’s laws of inheritance were dubious and instead focused his research on De Vris’s mutation theory. By inducing mutations in fruit flies, Morgan wanted to prove that mutations in alleles were heritable and that these mutations were responsible for the formation of new species. Morgan’s focused on a male fruit fly with mutated white eyes; curiously he found that when he bred that male, the offspring didn’t follow the Mendelian laws of inheritance. The first generation yielded 1,237 non-mutated offspring and three white-eyed flies, all males. The second generation produced 2,459 non-mutated females, 1,011 non-mutated males, and 782 white-eyed males. Morgan correctly surmised that this mutation was carried on the y-chromosome and that only males could inherit the white-eyed mutation. This discovery perhaps single-handedly refuted the idea that offspring were just a random mix of both parents’ DNA; there were clearly sex-linked inheritance of some traits that resided on only a specific chromosome (in this sex, the chromosome that determines the sex of the offspring). This meant that the Morgan’s work ultimately vindicated Darwin’s theory of evolution by showing that random mutations of a gene on an organisms’ chromosome can be inherited in the next generation when the mutated gene is carried on a gamete cell. This had profound implications for biology because it proved that evolution involves the transmission of traits from parents to offspring through a known mechanism of genetics.
The great synthesis of modern biology highlights something about science that I want to harp on; discovery is a contentious issue. Scientists often have rivals within their own field who (sometimes even unknowingly) stumble upon the same concepts like when Boveri and Sutton independently discovered the relationship between chromosomes and genes. And so it becomes a race to publish results. The notion that scientists are colluding with each other in a great conspiracy to hide the truth is asinine; the pressure to publish and the level of scrutiny applied to a scientist once the work is published ensures that research meets an certain level of integrity. The Great Modern Synthesis also highlights how scientific research is vindicated. Like pieces of a puzzle, new discoveries are more likely to be true if they corroborate accepted evidence in more than one field. By uniting evolution with genetics, two fields that were highly specialized a century ago, scientists like De Vris, Boveri, Sutton, and Morgan helped to reconcile Mendelian genetics with evolution by means of natural selection.