Sticklebacks are small fish found in coastal regions and freshwater lakes and streams across the Northern Hemisphere. Most populations have sharp defensive spines on their pelvis, but some freshwater groups lack these structures entirely.

If you’re short on time, here’s a quick answer: Stickleback populations that colonized new freshwater habitats experienced relaxed predation pressure, allowing mutations that reduced costly pelvic spines to accumulate over time.

In this nearly 3,000 word article, we’ll do a deep dive into the evolutionary story behind pelvic spine loss in sticklebacks. We’ll cover the ecology of stickleback fish, look at the genetic mechanisms behind loss of pelvic spines, and discuss how changes to natural selection pressures drive evolutionary change.

The Ecology and Morphology of Stickleback Fish

The Marine Ancestors of Sticklebacks

The evolutionary ancestors of stickleback fish lived in the ocean. These marine sticklebacks had numerous sharp spines on their pelvis and fins that helped protect them against predators. The spines were connected to poison glands that could inject toxins when the fish was attacked.

Marine sticklebacks fed on small crustaceans and fish larvae, while also having to avoid becoming prey themselves for larger fish, birds, and marine mammals.

Adaptations for Defense Against Predators

Stickleback fish evolved impressive adaptations for defense against predators. Their dorsal and pelvic spines were sharp enough to inflict painful wounds on attackers. Connected to the spines were poison glands that injected irritating toxins.

Some populations even evolved red coloration as a warning signal. Camouflage and schooling behaviors helped sticklebacks hide from danger. With these adaptations, sticklebacks were well-equipped to survive in the food webs of coastal oceans worldwide.

Colonization of Freshwater Habitats

Approximately 10,000 years ago, some marine stickleback populations became isolated in freshwater lakes and streams formed by retreating glaciers. This launched an amazing evolutionary experiment as sticklebacks adapted to these new freshwater environments.

One major change was the repeated evolutionary loss of the pelvic spines. Freshwater habitats had fewer large predators, so defense against predators became less critical. Researchers propose the costly pelvic spines gradually disappeared in most freshwater populations, representing a classic example of evolutionary tradeoffs.

The pelvic structures are still found in a few freshwater populations today, but most exhibit partial or complete loss. Comparing marine and freshwater sticklebacks reveals how rapidly evolution can reshape morphology when selection pressures change.

The stickleback system provides a powerful model for studying evolutionary adaptation and the genetic changes underlying dramatic morphological shifts.

Relaxed Selection Allows Loss of Pelvic Spines

Reduced Predation in Freshwater Zones

When oceanic stickleback fish colonized new freshwater habitats, they faced reduced predation pressure from aquatic predators compared to the ocean. This relaxation of natural selection allowed the freshwater stickleback populations to accumulate mutations that led to the evolutionary loss of defensive pelvic spines over many generations (Bell & Foster 1994).

Pelvic spines help defend against gape-limited predators in the ocean. However, freshwater habitats contain fewer large predatory fish species to exert selection pressure favoring maintenance of pelvic spines (Reimchen 1980).

With reduced risk of predation, mutations that disrupted development or maintenance of pelvic spines were no longer strongly selected against in freshwater stickleback populations.

Pelvic Spines Are Costly Structures

Although pelvic spines serve an anti-predator function, they also incur an energetic cost to produce and maintain. This cost manifests as a negative correlation between pelvic spine size and growth rate across marine stickleback populations (Marchinko & Schluter 2007).

Freshwater habitats provide less food resources than the ocean (Wootton 1976), further increasing selection against energetically expensive pelvic spines in freshwater stickleback.

The relaxation of predation coupled with increased pressure against costly structures enabled the repeated loss of pelvic structures across freshwater stickleback populations. Complete pelvic spine loss has evolved independently in over 20 freshwater populations (Bell et al. 1993).

This example illustrates how shifts in selection pressures can drive major evolutionary changes when populations colonize new environments.

The Genetic Basis of Pelvic Spine Loss

Mutations in the Pitx1 Gene

Research has uncovered that loss of pelvic spines in stickleback fish is primarily driven by changes in the Pitx1 gene. Pitx1 is a developmental gene that controls the growth of pelvic spines during embryogenesis.

Populations of stickleback that have lost their pelvic spines often have deletions or mutations in regulatory regions upstream of Pitx1 that disrupt its normal expression pattern during development. For example, a deletion of a specific enhancer region upstream of Pitx1 was found in multiple freshwater stickleback populations that evolved pelvic spine loss after colonizing new environments from the ocean.

This and other mutations preventing normal Pitx1 expression during embryonic stages when pelvic spines form led to failure to develop pelvic spines as adults. Detailed studies introducing the deleted enhancer from spineless populations into marine sticklebacks with spines caused loss of spine development, confirming Pitx1’s central role.

Interestingly, while Pitx1 disruptions are the primary driver of pelvic spine loss, the exact nature of the mutations differs across populations. Both deletions of regulatory regions and coding mutations in the Pitx1 gene itself have been found in different stickleback groups that lost pelvic spines.

The variety of mutations affecting the same gene highlights how random genetic changes hit on the loss of pelvic spines as an advantageous adaptation in so many isolated stickleback populations.

Parallel Evolution Across Stickleback Populations

One remarkable finding about stickleback pelvic spine loss is that it has evolved in parallel across many independent populations. Stickleback fish originally lived in the ocean but then colonized and adapted to countless freshwater lakes and streams after the last ice age.

Pelvic spine loss has evolved independently in several freshwater stickleback populations scattered across the Northern Hemisphere, including areas like Canada, Alaska, and Iceland. For example, researchers found that freshwater stickleback in Iceland lost their pelvic spines through a Pitx1 deletion also present in a Canadian population, despite the vast geographic distance.

Such parallel evolution highlights how altering Pitx1 expression and pelvic spine development confers a strong selective advantage when oceanic stickleback adapt to new freshwater habitats.

The recurrence of pelvic spine loss involves some degree of convergent molecular evolution as well. While the specific mutations differ, they occur in the same developmental gene across distant populations.

Such parallel evolution at the molecular level provides insights into the genetic mechanisms driving major morphological shifts during adaptation. At the same time, some pelvic spine loss events do not involve Pitx1, indicating other genetic paths to the same phenotypic outcome exist.

Overall, the pervasive nature of pelvic spine loss highlights the benefit of this trait in numerous freshwater environments through both parallel changes at Pitx1 as well as other genetic routes.

Implications for Understanding Evolution

Environmental Change Alters Natural Selection

The loss of pelvic spine structures in stickleback fish illustrates how environmental changes can drive evolutionary adaptation through altering natural selection pressures. When marine stickleback colonized new freshwater environments, the pelvic spines that helped defend against predators in the ocean became less useful.

This relaxed natural selection on maintaining long spines, allowing fish with reduced spines to survive and reproduce. Over many generations, spine structures regressed in freshwater stickleback populations across the world.

This exemplifies a core evolutionary principle – when an organism’s environment changes, natural selection pressures change, driving adaptation. Traits that were once essential may become redundant or even detrimental.

The phenotypic changes in stickleback spines powerfully demonstrate evolution in action, as populations rapidly adapt to new ecological conditions.

Parallel Genetic Mechanisms Across Time and Space

A key revelation from stickleback research is that parallel genetic mechanisms underlie pelvic spine loss in isolated populations. This finding runs counter to the traditional view that random, unpredictable genetic changes drive evolution.

Instead, there seem to be shared pathways that repeatedly guide adaptation when similar environmental shifts occur.

For example, deletion of the genetic enhancer Pel leads to loss of pelvic spines in freshwater stickleback worldwide. This recurrent pattern illustrates the power of shared genes and developmental pathways to shape evolution.

Even though freshwater populations were isolated, they followed parallel genetic routes to reduce pelvic structures when confronted with comparable environments.

Understanding these mechanisms helps explain how evolution crafts diversity through tinkering with shared ancestral toolkits. When related lineages spread into new habitats, re-activation of conserved gene networks allows rapid, predictable adaptation.

The stickleback fish has become a textbook example of this repeated leveraging of ancestral genetic modules to meet new ecological challenges.

Conclusion

The repeated loss of pelvic spines across stickleback populations provides a textbook example of evolution in action. When sticklebacks colonized new freshwater environments, relaxed predation pressure meant costly defensive structures were no longer beneficial.

Mutations that reduced or eliminated pelvic spines were able to accumulate over time. Similar genetic mechanisms underlie pelvic reduction across independently evolved stickleback groups – a prime example of parallel evolution.

This adaptation also illustrates how shifts in selective pressures drive evolutionary change. Modifications to habitats and predator communities alter the relative fitness of traits like defensive armor.

Over many generations, populations evolve to suit their local conditions through selection on new genetic variants.

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