Parallel evolution

Parallel evolution is a biological process where distinct species develop similar traits. These species are not closely related to one another. However, they often share a similar original trait from a common ancestor. They also face similar evolutionary pressures in their environments. This leads them to evolve in a similar direction. Scientists use this term to describe how different lineages reach similar solutions to life's challenges.

Understanding the mechanism requires looking at how traits change over time. Imagine a hypothetical common ancestor that has a specific amino acid at a certain position in a protein. One lineage might change that amino acid to serine. In parallel evolution, a second, distinct lineage also changes that same position to serine. This is different from convergent evolution, where the species might reach the same result from a different starting point. Scientists often distinguish them by checking if the ancestors shared the similarity. If the ancestors shared the trait, it is parallel evolution. If they did not, it is convergent evolution. This distinction can be difficult because all organisms share common ancestors to some degree.

We can see these patterns clearly in the history of mammals. Roughly 100 million years ago, the large land-mass known as Gondwanaland began to break up. This split mammals into two main branches: placentals and marsupials. For a long time, most mammals remained small and occupied limited roles in their ecosystems. This changed 65 million years ago during a mass extinction of the dinosaurs. Following this event, mammals on different continents began to evolve a wide variety of forms. They started to fill many different ecological roles across the globe.

Many remarkable examples of this process exist across different continents. In South America, marsupials and placentals once shared the same ecosystems. This led to the emergence of surprisingly similar animals in different places. For instance, the placental sabre-toothed cats, known as Machairodontinae, developed similar features to the South American marsupial sabre-tooth, Thylacosmilus. We also see this in the Tasmanian wolf and the European wolf. Other examples include moles, flying squirrels, and even mice, which show similar patterns in different lineages.

Parallel evolution also occurs in the specialized world of nectar-feeding birds. Hummingbirds in the New World and sunbirds in the Old World are separate lineages. Despite this, they have both evolved a suite of specialized anatomical and behavioral traits. Both types of birds possess long, needle-like bills. These bills allow them to reach deep into flowers to access nectar. This shape is a specialized adaptation for their ecological niche. Some birds even practice nectar robbing by piercing the base of a flower's corolla tube. This allows them to get nectar without performing the service of pollination.

This relationship between birds and flowers is a form of mutualism. In this system, the birds eat nectar and collect pollen on their bills. They then transfer this pollen to the next flower they visit. This coevolution has happened in parallel between birds and flowers in both the Old and New Worlds. The birds' digestive systems have also changed to match the nectar of their specific flowers. This creates highly specialized ecological guilds where plants and pollinators are perfectly adapted to one another.

One specific biological detail involves the enzyme sucrase-isomaltase. This enzyme is responsible for hydrolyzing, or breaking down, sucrose. Nectarivorous birds have a much higher concentration of this enzyme than other types of birds. This allows them to digest sucrose more rapidly to meet their energy needs. The sucrase activity per unit of intestinal surface area is notably higher in these birds. Interestingly, the Adaptive Modulation Hypothesis does not seem to apply here. This means the high enzyme concentration is not simply a required response to eating sugar. Instead, the parallel acquisition of this digestive capability is a distinct evolutionary result.