Adaptive radiation refers to an evolutionary event when an opportunity allows for a lineage of species that share a common ancestor to rapidly diversify. Broken down, adaptive refers to adjusting to a new environment or opportunity and the word radiation refers to the numerous species that evolve and spread out in the opening niches. Adaptive radiation is a specific phenomenon of natural selection when opportunities and pressures act simultaneously to cause a rapid divergence of species. Occurrences of adaptive radiation have significantly contributed to the wealth of diversity on planet Earth.
But what does adaptive radiation have to do with beetle diversity?
Turns out, nearly everything! There are about 1.9 million animals known to science, and, yes, more than 400,000 of those are beetles! But with all the other taxa in the animal kingdom, how did beetles (Coleoptera) become so prominent? The key to their success lies in good timing and an opportunity well-seized. During the Cretaceous period, angiosperms rapidly diversified. Hungry, herbivorous beetles evolved alongside flowering plants, each in their own concurrent instance of adaptive radiation. With each divergence of angiosperms, the beetles found a new food source. Beetle species put pressure on angiosperms to evolve in ways to avoid herbivory.
To make the processes of adaptive radiation easier to understand, we look to phylogenetic trees. Phylogenetic systematics is the field of evolutionary biology that reconstructs the steps a species has taken during the process of speciation. Scientists have long been using techniques to map taxonomy, but the use of genetic data to map genomes has added some much-needed clarity to phylogeny in recent years.
Phylogenetic trees help visualize adaptive radiation. Like the sketch pictured below, each tree maps out both how species are related along with the approximate time they evolved and went extinct (as shown on the x-axis by the branches and clade dots, respectively).
To identify occurrences of adaptive radiation versus evolution in general, certain criteria must be met. The species in question must have a common ancestry, available niches to fill, phenotype-environment correlation, trait utility, and rapid speciation. For each of the criteria, we’ll take a close look at the case study of Darwin’s Finches for a tangible example of adaptive radiation.
Adaptive radiation starts with a single, recent, ancestral species. Essentially, this means that all the component species that need to have the same grandma. Not the same great-great-great-great-great-great grandma (that particular primordial goop is the same for all of us), but a more recent common ancestor. Remember, the phenomenon of adaptive radiation has a time element to it. The component species speciate from this common ancestor. Speciation is the development of new, distinct species through the evolutionary process.
In the case study of the finches, there are currently 14 species of finch that live on the Galapagos Islands and Cocos Island. But 14 species of finch didn’t all happen to fly from the mainland to the equatorial island chain. A single species, thought to be the Blue-Black Grassquit finch (Volatina jacarina) or an extinct relative, made the journey from the Pacific coast of South America. This single ancestor species then paved the way for the incredible biodiversity of finches the Galapagos has today!
The next step of adaptive radiation is for the common ancestor to be exposed to the right conditions. Ecological opportunity and speciation both rely on niches opening up. A niche includes all the fundamental conditions for a species to survive and reproduce. For example, elf owls hunt for invertebrates during the crepuscular hours and live in woodpecker cavities. Their niche necessitates that they live in a habitat that overlaps with woodpeckers and has a year-round bug supply. What’s more, only the elf owl occupies this specific niche! When a species occupies a niche, there aren’t resources available for another species with the same needs.
But from time to time, certain conditions arise that cause new niches to open up. Some of these conditions could develop from a loss of a predator or competitor, dispersing to a new environment, or the evolution of an important resource or key innovations. The Cretaceous-Paleogene mass extinction of the non-avian dinosaurs is a great example of this. Three-quarters of life on Earth went extinct, leaving abundant room for life to rapidly evolve and diversity. Everything from mammals to sharks took the opportunity for diversification.
The different conditions to create ecological opportunity can result in two types of speciation: allopatric or sympatric. Allopatric speciation occurs when a geographic boundary (like two different islands) causes species to separate. On the other hand, sympatric speciation occurs without any pressures from biogeography or physical separation. In this case, different resources or competitive pressures might cause a species to branch off.
Once the Blue Black Grassquit Finch made it to the Galapagos, it settled down in the new wide-open environment. A population of finches developed on each of the islands – a separation that’s key to the story. The Galapagos Islands are close enough to each other that the ancestral finch dispersed across the archipelago, but far enough that interbreeding between the islands’ populations is rare. As a result, each island’s population of finches was in an evolutionary test tube. Each island presents a unique set of ecological opportunities, and, one by one, the finches adapted to fill the open niches.
After a niche opens up and organisms begin to speciate, we start to see this next trend of adaptive radiation. The physical or behavioral traits of the new species are closely tied to the aspects of the novel environment. This is called the phenotype-environment correlation.
The phenotype-environment correlation can also be viewed from a more macro level through niche partitioning. Not only are the organisms responding to the environmental factors, but they also respond to competition within their own community.
Take the classic story of Darwin’s finches on the Galapagos Islands as an example. Each of the 15 different finch species has a very specific bill (e.g. short & stubby, pointy & narrow, or long & curved) that is adapted for a specific type of food (e.g. seeds, insects, or nectar from flowers). The finches’ unique phenotypic morphology correlates with the food available and allows each species to uniquely exploit its environment.
In terms of niche partitioning and the finches, we see evolution push the finches one step further. Not only did they diversify between habitats (shrubland, forest, etc…), but they also diversified within single habitats. For example, there are three species of ground finches that share a habitat. Competition among the finches for food caused three species to evolve, each with a bill specialized for a certain type of seed or nut.
Next up on the adaptive radiation criteria docket also has to do with traits. The phenotype-environment correlation shows us that the evolving species’ morphology and behavior correspond with the environment. One step further is trait utility. Trait utility means that the specialization improves the species’ fitness in their corresponding environments.
For the Galapagos finches, we see trait utility as the different bills help each species to survive. The three species of ground finches all eat a mixture of seeds and nuts during the wet season when food is abundant. But during the dry season, each of the three species focuses its diet to specialize on the seeds they are most adept at eating. The Large Ground Finch eats the largest seeds that it can crunch with its giant bill while the Small Ground Finch picks out the tiny seeds and the Medium Ground Finch finds the moderately sized snacks.
The final distinguisher between adaptive radiation and more general evolutionary radiation is the time frame. While all life constantly (albeit slowly) responds to the pressures of natural selection, adaptive radiation calls for an explosive emergence of new species. The rise of the mammals after the Cretaceous-Paleogene mass extinction is called the Cambrian Explosion after all! This burst of evolutionary diversification is known as rapid speciation. The theory around ecological opportunity indicates that after this initial rapid diversification, speciation will significantly slow down as niches are filled. Because of the nature of rapid speciation, examples of adaptive radiation more clearly reveal the causes and processes of evolution.
In the case of the finches, arriving at the islands opened a whole new world of habitats and unoccupied niches. Once they settled on the archipelago, the finches encountered a unique set of evolutionary pressures, depending on which island they landed on. Based on these factors, the finches quickly filled distinct ecological niches. Today, there are 14 different species of finch found in the Galapagos!
Most of the research on adaptive radiation focuses closely on the aforementioned criteria. However, modern critiques of a phylogenetic approach to studying adaptive radiation point out that the period after rapid speciation is often overlooked. If there’s one consistency across evolution, it’s that conditions never remain consistent. Evolution responds to environmental and genetic pressures that continue to change. As a result, species will either continue to diverge and specialize, hybridize, or go extinct (with exceptions that prove the rule, of course, like the immutable horseshoe crabs).
Extinction can either happen with a significant event like a volcano eruption, or, in most cases, it happens more gradually as a species struggles to adapt to a changing environment. Extinction causes a loss of phylogenetic data points. Essentially, when a species goes extinct, it’s like a piece is missing from the adaptive radiation puzzle. Extinction erodes patterns of adaptive radiation.
Darwin’s finches continue to be some of the most well-studied examples of adaptive radiation. Since the 1980’s Dr. Rosemary Grant and her husband Peter have dedicated their life’s work to understanding the avian community on the Galapagos. Their research provides a critical piece. We are learning how the finches are reacting to the post-burst life. And the Grants have uncovered some fascinating findings. At its core, they observe a community in flux. With climate change increasing the food stress on the finch populations, the Grant’s have seen bill size change in a species in as little as two generations. Additionally, a recent feathered immigrant to the archipelago has bred with the local finches resulting in an entirely new species.
The takeaway here is a simple reminder that communities are constantly evolving. Though not as rapid as a case of adaptive radiation, species constantly respond to the pressures of natural selection. For the finches, we are lucky enough to have a wealth of observation. For other less-studied species, however, these changes can obscure the story of adaptive radiation.
For a few other examples of adaptive radiation, we turn to another island chain. When the Hawaiian archipelago was still forming, volcanic activity caused major changes to the landscape. With literal new land forming, open niches were a dime a dozen. Simply put conditions were ripe for adaptive radiation.
The Hawaiian honeycreepers (family Drepanididae) feed on flower nectar, have brightly colored plumage, and sing beautiful songs. But they’re more than just charismatic characters of the canopy. Honeycreepers are endemic to Hawaii, meaning they are found nowhere else in the world. Scientists believe that the honeycreepers radiated from a common ancestor into over 50 divergent species. Ironically, scientists believe this common ancestor may have been the European Rose Finch. After the finch’s dispersal among the islands, it began to evolve. Unfortunately, the arrival of humans in Hawaii didn’t do the birds a service. Less than half of that number of species are extant today. We only know about the plethora of extinct species through bones and fossil evidence.
Similar to the Galapagos finches, the honeycreepers have morphological differences in their beaks. Different clades of honeycreepers demonstrate different dietary preferences and foraging behavior. Each honeycreeper has both a common name in English and Hawaiian to go along with its scientific name. A classic nectar drinker, ʻiʻiwi’ or Scarlet Honeycreeper (Drepanis coccinea) has a long curved bill that helps it reach into the long tubular lobelia flowers. Another stunner, ʻakiapōlāʻau’ (Hemignathus wilsoni) has an asymmetrical beak with a longer top that allows it to probe for insects in tree bark. On the other hand, there’s an entire clade of seed-eating honeycreepers, like the Laysan Finch (Telespiza cantans) with tough short bills perfect for cracking open nuts.
It’s not all about the vertebrates! We can’t talk about the honeycreepers without also highlighting some of their favorite foods: Lobelia flowers. Lobelioids are members of the Campanulaceae family and a group of flowering plants that are mostly endemic to Hawaii. With over 2,400 species, studies on lobelioid ecology are not short on sample size! Considered one of the most spectacular examples of adaptive radiation, lobelioids have provided unique insight into the evolution of flowering plants. Members of the family take on an incredible diversity of forms and occupy a wide range of ecosystems. In many cases, species of lobelia co-evolved with species of honeycreeper, the curve of the bird’s beak mirroring the curve of the flower’s corolla.
In another example of fascinating evolutionary history, we find a second Hawaiian plant group. The silverswords are named for a member of their family that sports long silvery leaves (Argyroxiphium sandwicense). The silversword plant builds up nutrients for 20 years before flowering only once, then subsequently dying.
Along with its related species, the silversword alliance is actually a member of the sunflower family (Asteraceae). But not all the related species have this same unusual behavior as the alliance’s namesake. The alliance includes trees, shrubs, vines, herbaceous plants, and more.
Thanks to instances of adaptive radiation, we live on an incredibly diverse planet. For millennia, life has been susceptible to the pressures of natural selection, evolving and growing as the Earth changes. Today, however, irresponsible use and abuse of the Earth’s resources has put much of the beautiful biodiversity in jeopardy. Human interference plays a disproportionate role in the loss of species. The first step to preserving Earth’s delicate ecosystems is understanding more about the biological sciences that explain how ecology works.