Adaptive Radiation: Videos & Practice Problems

Adaptive radiation is an evolutionary process where a single lineage diversifies into many species, often filling ecological niches after mass extinction events. A prime example is the Cambrian explosion, which introduced nearly all animal phyla around 530 million years ago, leading to complex body plans and diverse life forms. Key factors include ecological opportunities and evolutionary innovations, such as the development of homeotic genes like Hox genes, which regulate body structure and promote rapid diversification. This highlights the dynamic nature of evolution and the intricate relationships between organisms and their environments.

Adaptive Radiation

Adaptive radiation is a significant evolutionary process where a single lineage rapidly diversifies to produce many distinct species, effectively expanding the tree of life. This phenomenon often occurs in response to ecological opportunities, particularly after mass extinction events, which create open niches for species to fill. For instance, following a mass extinction, a lineage that survives can undergo multiple speciation events, leading to a bushier representation of life on the phylogenetic tree.

One classic example of adaptive radiation is observed in the Hawaiian Drosophila, a group of fruit flies that has diversified into approximately 1,000 species on the Hawaiian Islands. This represents about a quarter of all Drosophila species globally. These flies have adapted to various ecological niches, with some laying eggs on rotting fruit, while others have specialized to lay eggs on rotting bark or even in spider egg sacs. Such diversification illustrates how species can evolve to exploit available resources in their environment.

Another driver of adaptive radiation is evolutionary innovation, which refers to the emergence of new traits that enhance an organism's fitness, allowing it to outcompete others in its niche. A prime example of this is the angiosperms, or flowering plants, which evolved around 100 million years ago during the Cretaceous period. Angiosperms possess unique adaptations, such as seeds enclosed in protective fruits, which facilitate wider dispersal and longer viability. Their reproductive strategies, including the development of flowers, further contributed to their success over other plant groups like ferns and conifers.

Understanding adaptive radiation highlights the dynamic nature of evolution, showcasing how life can rapidly diversify in response to environmental changes and innovations. This process not only enriches biodiversity but also illustrates the intricate relationships among different species within the ecosystem.

Adaptive Radiation Example 1

The phylogenetic tree illustrates the relationships among modern mammal orders, particularly focusing on placental mammals, which are highlighted in a blue box. Within this classification system, mammals are organized hierarchically into orders, families, genera, and species. Placental mammals represent a significant group, often encompassing numerous species depending on the order. Additionally, the tree includes marsupials, known for their pouches, and monotremes, which are egg-laying mammals like the duck-billed platypus.

The Cretaceous-Paleogene (K-Pg) extinction event, occurring approximately 66 million years ago, is a pivotal moment in this evolutionary narrative. This event is marked on the timeline, which spans from 50 to 100 million years ago. The branches representing placental mammal orders begin before this extinction event, indicating that adaptive radiation at the order level was already underway prior to the K-Pg event.

In contrast, diversification within these orders, represented by triangles on the tree, occurs after the K-Pg extinction event. This suggests that while some adaptive radiation was present before the extinction, the event itself catalyzed a significant increase in diversification among existing orders, leading to the emergence of new species within those orders.

To identify specific orders where the K-Pg extinction event is closely linked to adaptive radiation, one can observe where the triangles intersect with the extinction line. Notable orders include primates, rodents, and artiodactyls (even-toed ungulates), all of which show diversification that aligns closely with the timing of the extinction event. This pattern indicates that the extinction of dinosaurs opened up ecological niches, allowing mammals to rapidly diversify and adapt to new environments.

In summary, the K-Pg extinction event played a crucial role in shaping the evolutionary trajectory of mammals, facilitating a surge in diversification within existing orders and leading to the establishment of new species. This adaptive radiation underscores the dynamic nature of evolution in response to significant environmental changes.

Which of the following statements are true?


I) Adaptive radiations often follow mass extinction events as organisms adapt to fill previously occupied niches.
II) Adaptive radiations are common on new volcanic islands where organisms can occupy many new environments.
III) The evolution of a new trait can trigger adaptive radiation as organisms are able to outcompete resident organisms in many different environments.

• A. I & II only.
• B. I & III.
• C. II & III.
• D. I, II, & III.

Answer: (D)

Which of the following would be considered an example of an adaptive radiation?

• A. Finches on the Galapagos are thought to have evolved into many different species from an original founder population.
• B. The peppered moth evolved to have dark coloration in response to an environmental change in industrial England.
• C. Native human populations tend to have less genetic diversity the further away they are from Africa.
• D. Mass extinction events are usually caused by worldwide habitat destruction from things such as meteorites or extreme volcanic activity.

Answer: (A)

Which adaptive radiation is correctly matched to the extinction event that preceded it?

• A. Adaptive radiation of dinosaurs: late-Devonian extinction event
• B. Adaptive radiation of flowering plants (angiosperms): Ordovician extinction event
• C. Adaptive radiation of mammals: end-Cretaceous extinction event
• D. Adaptive radiation of Cambrian animals: end-Permian extinction event

Answer: (C)

Cambrian Explosion

The Cambrian explosion marks a significant adaptive radiation event that occurred around 530 million years ago, introducing nearly every major animal phylum. A phylum is a classification level just below kingdom, encompassing major groups of animals. Notable phyla that emerged during this period include chordates, which are characterized by having a nerve cord along their back, mollusks such as snails, clams, squids, and octopuses, and arthropods, which include crustaceans and insects. This explosion of diversity is believed to have unfolded over approximately 10 million years, initiating the Paleozoic Era, which lasted until the end of the Permian extinction event.

Prior to the Cambrian, animal life was predominantly soft-bodied, with few known predators. Most organisms were likely sessile, remaining fixed to the ocean floor, and included simple sponge-like creatures. The Cambrian period, however, transformed life dramatically, introducing hard-bodied animals with complex structures, including limbs, jaws, and the first nervous systems. This era also saw the emergence of the first eyes, allowing for enhanced interaction with the environment. Trilobites, a prominent group from this time, exemplify these changes with their hard exoskeletons and distinct features.

The causes of the Cambrian explosion are thought to stem from a combination of ecological opportunities and the evolution of new traits that increased fitness. In the millions of years leading up to the Cambrian, there was a significant rise in algae, which are photosynthetic organisms that produce oxygen. This increase in oxygen levels allowed for larger and more mobile animals, while also providing a new food source that expanded the base of the food chain. The rise of predators further drove diversification, as prey species developed defenses against predation, leading to the evolution of hard and spiky body forms.

Additionally, the diversification of life created new ecological niches. As organisms became more mobile, they could occupy different layers of the ocean, leading to new interactions and predator-prey dynamics. The evolution of a new set of developmental genes during or just before the Cambrian also played a crucial role, enabling the rapid evolution of complex body plans across many of the newly emerging animal phyla.

In summary, the Cambrian explosion represents a pivotal moment in the history of life on Earth, transitioning from a relatively simple and static biosphere to a dynamic and diverse ecosystem characterized by mobility, predation, and complex interactions among organisms.

Adaptive Radiation Example 2

The Cambrian explosion marks a significant period in evolutionary history characterized by a rapid increase in the diversity of life forms. Several factors contributed to this phenomenon, facilitating adaptive radiation among various organisms.

One major factor was the rise of predators. The emergence of predators created a new dynamic in ecosystems, prompting prey species to evolve various defensive adaptations. This evolutionary pressure led to the development of hard shells, spiky exteriors, and enhanced mobility among prey organisms. As these adaptations arose, they allowed species to exploit different ecological niches, resulting in a diversification of life forms—a hallmark of adaptive radiation.

Another contributing factor was the increase in atmospheric oxygen levels. Higher oxygen concentrations enabled organisms to grow larger and become more active. This increase in size and activity allowed animals to explore and exploit new ecological niches that were previously inaccessible. The ability to occupy diverse habitats further spurred the diversification of species, as organisms adapted to their specific environments.

Lastly, the introduction of new animal developmental genes played a crucial role in facilitating adaptive radiation. These genes allowed for the evolution of more complex body structures and forms, which in turn enabled organisms to adapt to a wider range of ecological roles. The emergence of varied body plans and functions contributed significantly to the overall increase in biodiversity during this period.

In summary, the rise of predators, increased oxygen levels, and the introduction of new developmental genes collectively fostered an environment ripe for adaptive radiation, leading to the remarkable diversity of life observed during the Cambrian explosion.

Which of the following statements about the Cambrian Explosion are true?


I) Before the Cambrian Explosion, most organisms had soft bodies.
II) The Permian extinction created new ecological niches that were exploited during the Cambrian Explosion.
III) After the Cambrian Explosion, new morphological features like limbs and jaws are observed.

• A. I & II.
• B. I & III.
• C. II & III.
• D. I, II, & III.

Answer: (B)

Developmental Genes & Regulation

Understanding developmental genes and gene regulation is crucial for grasping how rapid evolutionary changes occur, particularly during adaptive radiation. A significant aspect of evolution involves alterations in gene regulation rather than just mutations in coding regions of proteins. Many evolutionary changes stem from mutations in regulatory regions or regulatory genes, which influence when and how much proteins are expressed during development. This means that small changes early in development can lead to substantial differences in the organism's overall development.

To illustrate this concept, consider the homology observed in vertebrate limbs, such as bat wings, human arms, and whale fins. Despite having the same underlying bone structure, the differences in their forms arise from variations in growth regulation. For instance, bat wings have long, thin bones, while whale fins feature thicker, shorter bones. This phenomenon, known as heterochrony (where "hetero" means different and "chrony" refers to time), indicates that changes in the timing or rate of development can lead to significant evolutionary changes without the need for entirely new structures.

Central to gene regulation are homeotic genes, which dictate the specific organization of body parts in organisms. These genes function as transcription factors, controlling the expression of other genes and influencing various organismal patterns. For example, in plants, homeotic genes determine the development of flower parts, such as petals. Changes in these genes can lead to dramatic alterations in flower morphology.

In animals, a specific group of homeotic genes known as Hox genes plays a pivotal role in determining body plans. While not all animals possess Hox genes, many of those that emerged during the Cambrian explosion do. These genes are crucial for establishing the head-to-tail orientation of an organism's body plan. Hox genes are organized in a specific order on chromosomes across different phyla, such as mollusks, arthropods, and chordates, indicating a shared evolutionary heritage. Although there are variations, including duplications and losses, the fundamental organization remains consistent.

During development, Hox genes send signals to cells, informing them of their positional identity within the body. This signaling allows cells to develop into specific body parts, contributing to the complexity of vertebrate body plans. The presence of multiple Hox genes in vertebrates provides detailed positional information, enabling the evolution of intricate body structures. Consequently, changes in these regulatory genes can lead to significant alterations in animal body plans, a phenomenon that was particularly evident during the Cambrian explosion.

In summary, the interplay between developmental genes, gene regulation, and evolutionary processes highlights the importance of understanding how organisms can rapidly adapt and diversify through changes in gene expression rather than solely through new protein coding. This insight into the mechanisms of evolution underscores the significance of regulatory genes, particularly Hox genes, in shaping the complexity of life.

The evolution of the Hox gene cluster likely allowed for what key evolutionary innovation?

• A. Genes that control development.
• B. Complex body plans in animals.
• C. Hard-bodied animals in the Cambrian.
• D. Transcription factors that control gene expression.

Answer: (B)

In arthropods, the Hox genes Ubx and abd-A are largely expressed in the same tissues. The illustrations below show in red which tissues both Ubx and abd-A are expressed in three species of arthropods. Based on this illustration, which statement is consistent with the data?





_{Images adapted from: Jennifer K. Grenier, Theodore L. Garber, Robert Warren, Paul M. Whitington, Sean Carroll
Evolution of the entire arthropod Hox gene set predated the origin and radiation of the onychophoran/arthropod clade,
Current Biology, Volume 7, Issue 8, 1997, Pages 547-553,
ISSN 0960-9822,
https://doi.org/10.1016/S0960-9822(06)00253-3.}

• A. The gene Ubx induces the growth of legs while abd-A inhibits the growth of legs.
• B. Legs can only grow in tissues with Ubx and abd-A expression.
• C. Ubx and abd-A repress leg formation in insects but not in crustaceans or centipedes.
• D. Duplications of Ubx and abd-A allowed organisms such as crustaceans and centipedes to grow more legs.

Answer: (C)

Here’s what students ask on this topic:

What is adaptive radiation and how does it differ from extinction?

Adaptive radiation is an evolutionary process where a single lineage diversifies into many species, often filling various ecological niches. This typically occurs after mass extinction events, where many niches become available. In contrast, extinction is the process where species die out, reducing biodiversity. While extinction prunes the tree of life, adaptive radiation makes it bushier by rapidly increasing the number of species. Adaptive radiation can also occur due to evolutionary innovations that increase fitness, allowing species to outcompete others in their niches.

What are some examples of adaptive radiation?

Examples of adaptive radiation include Darwin's finches on the Galapagos Islands and the Hawaiian Drosophila. Darwin's finches diversified into multiple species, each adapted to different ecological niches on the islands. Similarly, the Hawaiian Drosophila, with about 1,000 species, have evolved to fill various niches, such as laying eggs on rotting fruit, bark, specific leaves, or even spider egg sacks. Another significant example is the Cambrian explosion, which introduced nearly all animal phyla around 530 million years ago, leading to diverse and complex body plans.

What caused the Cambrian explosion?

The Cambrian explosion was likely caused by a combination of ecological opportunities and evolutionary innovations. Before the Cambrian, a rise in algae increased oxygen levels, allowing animals to grow larger and become more mobile. The emergence of predators drove diversification as animals evolved defenses. Additionally, the evolution of new developmental genes, such as Hox genes, enabled complex body plans to evolve rapidly. These factors created a feedback loop, promoting rapid diversification and the emergence of nearly all major animal phyla.

How do homeotic genes like Hox genes contribute to adaptive radiation?

Homeotic genes, including Hox genes, play a crucial role in adaptive radiation by determining the specific organization of body parts in organisms. Hox genes control the body plan from head to tail, providing detailed information about cell positions during development. Changes in these regulatory genes can lead to significant morphological changes, enabling rapid diversification. For example, the evolution of Hox genes before the Cambrian explosion allowed for the development of complex body plans, contributing to the rapid emergence of diverse animal phyla during this period.

What is the significance of ecological niches in adaptive radiation?

Ecological niches are crucial in adaptive radiation as they provide opportunities for species to diversify. After mass extinction events, many niches become vacant, allowing surviving species to evolve and fill these roles. This leads to rapid speciation and increased biodiversity. For instance, on volcanic islands like the Galapagos and Hawaiian Islands, new niches are created, and species that arrive first diversify to exploit these opportunities. The availability of diverse niches drives the adaptive radiation process, promoting the evolution of new species adapted to different environmental conditions.