Monday Article #74: Your Theory of Evolution is Evolving?!
For those that don’t know, the title is a Pokémon reference, implying “theory of evolution” is a Pokémon. Only me? I’m only 18, am I that old? Anyways, out with the jokes in with the content. Evolution, the basis of life adapting and flourishing on Mother Earth, the cornerstone of modern biology, has not always been explained the way it is now. Over the years, the theory has been refined over and over, filling in gaps in our understanding and providing us with a deeper comprehension of the beauty known as life!
The first fully formed theory of evolution, the theory of transmutation of species, was proposed by Jean-Baptiste Lamarck back in the early 19th century. He believed that there were two forces driving evolution, the adaptive force, and the complexifying force. In his theory, the complexifying force is an innate life force (this was back in the 1800s when science wasn’t as fancy as today) that increased species complexity, while the adaptive force causes changes to the organs of animals based on the use or disuse of it. He argued that these changes would be inherited by the next generation and produce adaptation to the environment. A simple analogy of his theory would be the giraffe. According to Lamarckian theory, if a giraffe stretches its neck throughout its life to reach higher trees, its offspring will inherit a longer neck.
Figure 1: Lamarck’s two-factor theory. Image taken from Wikipedia.
When Charles Darwin went to places such as the Galapagos Islands, the biogeographical patterns observed there caused him to doubt the fixity of species. This led Darwin to view transmutation as a process of divergence rather than a ladder-like progression proposed by Lamarckians. Eventually, the groundbreaking publication of Charles Darwin’s On the Origin of Species transformed the discussion around evolution. Darwin’s idea of natural selection (co-discovered by Wallace) suggests that the driving force of evolution is selection pressure. To go over it briefly, natural selection states that in a population of organisms, those that happen to be born with traits more capable of surviving in their environment will have higher chances of reproducing and hence will be more represented. For example, in a population of short-neck giraffes, a long-neck giraffe is born by pure chance. This long-neck giraffe will have a higher chance of reproducing and eventually will represent the majority of the population of giraffes. Back then, because Darwin didn’t know about DNA, genomes, or genetics, he knew nothing about how said variation occurred in the first place and he knew nothing about horizontal gene transfer. Instead, he focused his attention on what he could observe, which was natural selection.
Figure 2: Natural selection in action. Image taken from SolPass.
In the early 20th century, biologists began to understand the source of variation, thanks to work by Thomas Hunt Morgan and Gregor Mendel. Gregor Mendel and his work on pea plants showed that there is the smallest unit of inheritance he called genes. Through his work, he proposed the law of segregation and the law of independent assortment. The law of segregation states that only one alternative of a specific gene (alleles) is present in gametes (reproductive cells) while the law of independent assortment states that each gene is segregated independently of each other. These 2 laws contribute to variation as it means every offspring would inherit different combinations of genes. Thomas Morgan on the other hand worked with fruit flies and showed that by introducing radioactive elements to the food of the fruit flies, he could produce mutated traits that weren’t originally present in the parental generation. Essentially, he brought genetic mutations into the picture of variation. As the field continued to develop, many more cross-disciplinary consensuss led to the development of the modern synthesis by Julian Huxley. It combined the ideas of natural selection, Mendelian genetics, and population genetics. Through statistical consideration of genetics and mutations, the modern synthesis theory proposed that natural selection is the only factor that could contribute to the direction of evolution, whereas factors like mutation could only contribute to variation, not direction. To this day, modern synthesis remains the dominant theory taught in most schools, though some may not think it is the most accurate.
Figure 3: Modern synthesis theory. Image taken from Wikipedia.
Since then, the field of molecular biology has been rising in popularity and has led to many discoveries that could potentially add even more explaining power to the modern synthesis theory. In 1953, Watson and Crick made the revolutionary discovery of the double helix structure of the functional unit of inheritance, our DNA. This further led to the discovery of jumping genes, ie: transposons by Barbara McClintock. Lynn Margulis also discovered the role of symbiosis in cell evolution.
Most recently, a new theory of evolution has emerged, the extended synthesis theory. This theory attempts to unify fields and shine light on unconventional factors in evolution such as epigenetic inheritance, plasticity, evolutionary development, cultural evolution, and many more. Going into details of this theory would require an article in of itself, but the jist of it is that scientists are suggesting that the modern synthesis theory requires a grand reevaluation. They think that evolution and inheritance are much more than just genes and may involve the inheritance of environmental modifications to genes. This is why all of you should tune in next week when we will publish an article specifically discussing what the extended synthesis theory is about. With that, I hope to see all of you back here in a week’s time!
Figure 4: Extended evolutionary synthesis theory on the bottom compared to traditional modern synthesis theory on top. Image taken from Royal Society Publishing.
Article prepared by: Jared Ong Kang Jie, R&D Director of MBIOS 2023/2024