The Making of the Fittest: Evolving Switches, Evolving Bodies | HHMI BioInteractive Video

Summary

This video explores the evolutionary adaptation of the three-spined stickleback fish, which transitioned from ocean to freshwater habitats after glaciers receded. Over thousands of years, these fish lost their pelvic spines, a trait beneficial in the ocean but a liability in predator-free freshwater lakes. Scientists discovered that a mutation in a regulatory genetic switch, controlling the Pitx1 gene, caused the loss of spines. This phenomenon was found to repeat in different populations across the world, showing how evolution can follow similar paths. The study illustrates the role of genetic mutations in shaping the diversity of animal forms through time and environment.

Takeaways

• 😀 The retreat of glaciers 10,000 years ago opened up new ecosystems in Alaska, providing opportunities for species like the three-spined stickleback to explore new spawning grounds.
• 🐟 The three-spined stickleback, a fish common in the northern ocean, adapted to freshwater environments after being cut off from the sea by rising land and disappearing glaciers.
• 🏞️ Sticklebacks in freshwater lakes underwent significant physical changes over thousands of years, including smaller body size, altered coloring, and changes to their skeletons.
• 🧬 The study of stickleback bones provides insights into how animal bodies evolve, helping biologists understand broader evolutionary processes.
• 🔬 Mike Bell, a biologist, studies the evolution of sticklebacks in Alaska's Bear Paw Lake, where the fish adapted to the absence of large predators, causing them to lose their pelvic spines.
• 👾 In freshwater lakes, the pelvic spines of sticklebacks became a liability, as dragonfly larvae used the spines to capture them, reducing their survival and reproductive success.
• 🦴 The evolution of the pelvic spine in sticklebacks is a striking example of how species adapt to their environment, even losing important physical traits for survival.
• 🔍 Geneticists, including David Kingsley, used crossbreeding experiments to pinpoint genetic factors responsible for the loss of pelvic spines in freshwater sticklebacks.
• 💡 The discovery of a mutation in the Pitx1 gene's regulatory switch was key to understanding why some sticklebacks lost their pelvic spines, revealing how gene expression changes can lead to evolutionary changes.
• 🧪 After identifying the genetic switch that causes pelvic spine loss, researchers were able to reverse the process by reintroducing the functional switch into sticklebacks, causing them to regrow their spines.
• ⏳ The same genetic switch responsible for pelvic spine loss has been repeatedly activated in sticklebacks in different parts of the world (Alaska, Iceland, Scotland), showcasing convergent evolution in action.
• 📚 The study of sticklebacks, combining genetics, developmental biology, and the fossil record, is revolutionizing our understanding of how evolution works at the genetic and environmental level.

Q & A

What environmental changes led to the evolution of the three-spined stickleback fish?

The retreat of glaciers created new freshwater lakes in Alaska, isolating populations of three-spined sticklebacks from the ocean and presenting them with new environmental pressures that led to evolutionary changes in their body structure.

What is the significance of the pelvic spine loss in the stickleback fish?

The loss of pelvic spines in freshwater sticklebacks is a key adaptation to their environment, where the spines were a liability, making the fish more vulnerable to dragonfly larvae predators. This loss is a clear example of evolutionary adaptation driven by natural selection.

How did the stickleback fish adapt to life in freshwater lakes?

Freshwater sticklebacks evolved smaller bodies, different coloration, and most notably, a loss of pelvic spines. This change made them better suited to the freshwater environment, where they faced different predators and survival challenges compared to their oceanic ancestors.

How do geneticists determine which genes contribute to the evolutionary changes in sticklebacks?

Geneticists use crosses between marine and freshwater sticklebacks to map the genetic differences between the two populations. By identifying which genes are linked to the observed traits, such as the presence or absence of pelvic spines, they can pinpoint the specific genes responsible for the changes.

What role does the Pitx1 gene play in the evolution of stickleback fish?

The Pitx1 gene is crucial for the development of pelvic structures in sticklebacks. However, in freshwater populations that have lost their pelvic spines, a mutation in the regulatory switch for this gene prevents it from being expressed in the pelvic region, leading to the absence of spines.

Why was the discovery that the Pitx1 gene’s coding sequence remained unchanged surprising?

It was surprising because scientists had expected that evolutionary changes in body form would result from mutations in the protein-coding regions of genes. However, the mutation that led to the loss of pelvic spines occurred in the regulatory switch controlling the expression of Pitx1, not in the coding region itself.

How did researchers identify the regulatory switch responsible for the loss of pelvic spines?

Researchers identified the regulatory switch by injecting fragments of the Pitx1 gene into stickleback embryos and observing where the gene was expressed. After testing many fragments, they discovered a specific region of DNA that controlled Pitx1 expression in the pelvic area, which was deleted in the spine-less fish.

What was the result when researchers reintroduced the Pitx1 regulatory switch into freshwater sticklebacks?

When researchers reintroduced the functional Pitx1 switch into freshwater stickleback embryos, the fish that would normally lack pelvic spines grew them, confirming that the loss of spines was due to the deletion of this specific regulatory switch.

How does the repeated loss of pelvic spines in different populations of sticklebacks demonstrate evolutionary convergence?

The repeated loss of pelvic spines in different populations of sticklebacks from various regions—such as Alaska, Scotland, and Iceland—shows that similar environmental pressures can lead to the same genetic changes across different populations. This is an example of evolutionary convergence, where different populations independently evolve similar traits.

What can the study of stickleback fish teach us about evolution in general?

The study of stickleback fish illustrates how environmental changes can drive evolutionary adaptations through genetic mutations, particularly in regulatory switches that control gene expression. It demonstrates the role of gene regulation in shaping body forms and how evolution can work through changes in the timing and location of gene activity, rather than just through mutations in the gene sequences themselves.