Author: Ellie Wang
Editors: Emily Chen and Ivan Feng
Artist: Jenny Luo
When you first hear the word “evolution,” most would think of a single species gradually changing over time, similar to the popular image of an ape slowly morphing into a human. However, this isn’t the full picture. There are many ways that evolution can happen, involving variations in the types and lengths of time for it to occur. Evolution generally takes place when speciation—the formation of new and distinct species—occurs. This can happen in a number of different ways that prevent members of two species from mating. This means that humans and chimpanzees- our closest cousins in terms of shared DNA- branched off from a common ancestor. The core idea is that evolution is branched, not linear, meaning that every organism in the human evolution lineup should be a different branch from a previous one.
There are three types of evolution: divergent, convergent, and parallel. The more well-known divergent evolution is where one species splits into two separate species. A famous example of this is Darwin’s finches: the result of an ancestral finch species migrating from South America to one of the Galapagos Islands and eventually evolving into the 13 species seen on the islands today. This is known as adaptive radiation, or the speciation of one species into multiple to fill ecological niches. Different conditions on each island would have provided the selective pressure necessary for natural selection to take the course, altering the finch populations that survived and got to pass on their DNA and traits to future generations. Research done by Rosemary and Peter Grant has revealed just how quickly this process can happen. In 1977, when a terrible drought plagued one of the islands, the medium-ground finches had to compete for seeds to eat. As the supply of smaller seeds dwindled, the medium-ground finches with larger beaks had better chances of surviving because they were able to break open and feed upon the harder and larger seeds. With higher-than-average beak sizes compared to the other finches in 1976, these survivors went on to reproduce and pass on their traits. Consequently, the offspring in 1978 had a 4% increase in average beak size, demonstrating how evolution can happen in real-time. This sudden evolution is called punctuated equilibrium, where long periods of genetic stability in the species are intermittently disrupted by events that cause the gene pool to change, thereby fast-forwarding the process of evolution during these events. Therefore, evolution doesn’t necessarily always follow gradual equilibrium.
Another type of evolution is convergent evolution. This can be seen as the opposite of divergent evolution since it describes multiple species evolving into similar forms. This is often portrayed through the difference between homologous and analogous structures. Homologous structures are body structures that developed from a shared ancestral structure, even though they might currently be used for different purposes, such as a human arm and a bat's wing. On the other hand, analogous structures have similar functions but have not evolved from shared structures, such as the wings of a bird or butterfly. The former case describes divergent evolution, while the latter fits convergent evolution. One example of convergent evolution is carcinization—otherwise known as “one of the many attempts of Nature to evolve a crab.” An almost absurd number of crab species have evolved from non-crab-like forms into crab-like forms, making them more related to their ancestors than to each other. For example, porcelain crabs are more closely related to squat lobsters and hermit crabs than any other crabs. These crabs that underwent carcinization may have evolved separately but eventually converged into similar forms. This also holds true in certain features that evolve independently across species, which include the similar eyes of the octopus and the human.
Finally, parallel evolution is the evolution of more than one species that shares at least one trait. They develop into different forms that also share at least one trait due to similar selective pressures. To simplify, similar species evolve into similar species. Placental and marsupial mammals are excellent examples as they have evolved in parallel into similar mammalian body plans and traits despite their geological separation after Australia drifted away.
Evolution is the driving force behind this planet’s ever-changing biosphere. With these different types of evolution, who knows what species the future will bring?
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