Question

Tammar wallabies (Macropus eugenii), a small \(6-10\) kg macropod, were introduced to New Zealand about 130 years ago. In their native Australia, they have been subject to predation by a variety of predators such as the marsupial lion (now extinct) and the marsupial tiger (also extinct), as well as large lizards and now the introduced dingo and red fox. New Zealand has none of these predators. Discuss how tammar wallabies might evolve in the absence of predation. Under what conditions would you expect antipredator behaviors to disappear from the New Zealand population of tammar wallabies? Blumstein et al. (2004) provide data on this issue.

Short answer

In the absence of predators, antipredator behaviors may disappear due to lack of selective pressure and genetic drift, especially if costly. This may happen if these behaviors provide no survival advantage and are energetically expensive.

Step-by-step solution

Understanding the Evolutionary Pressure

In their native Australia, tammar wallabies have evolved specific antipredator behaviors due to the presence of various predators like the marsupial lion and dingo. However, in New Zealand, the absence of these predators removes selective pressure for maintaining these behaviors.

Analyzing the Role of Natural Selection

Natural selection tends to favor traits that increase survival and reproduction. Without predators, there might be no advantage, and possibly even a disadvantage, to maintaining costly antipredator behaviors, such as increased energy expenditure or reduced foraging efficiency.

Recognizing Genetic Drift and Changes in Behavior

In the absence of significant selective pressure, genetic drift may lead to random changes in the frequency of genes underlying antipredator behaviors. Over time, this could lead to a reduction or even loss of these behaviors in the population.

Discussing Conditions for Behavior Loss

Antipredator behaviors would be expected to disappear when they no longer confer any survival advantage and might even pose a fitness cost. Environmental factors such as a stable food supply and absence of predators will further support this process.

Key concepts

Antipredator Behaviors

Antipredator behaviors are fascinating adaptations that many animals develop to protect themselves from predators. Tammar wallabies in their native Australia had to deal with a range of predators such as marsupial lions and dingos. These predators exerted constant evolutionary pressure on the wallabies, encouraging the development of behaviors that help them avoid being eaten.

These behaviors might include being more alert, running quickly to escape, and staying quiet to avoid detection. Such actions are crucial for survival in predator-rich environments. In Australia, antipredator behaviors would have been reinforced continually through natural selection, as wallabies with better survival strategies left more offspring behind.

But when tammar wallabies were introduced to New Zealand, they found themselves in an environment free of these predators. Here, the same behaviors that were once necessary for survival could become less essential. If there are no predators to hide from, staying constantly alert could waste valuable energy that could be better used for foraging or reproduction.

Natural Selection

Natural selection is a fundamental concept in evolutionary ecology. It explains how certain traits become more or less common in a population over time. Traits that improve an organism's chances of survival and reproduction tend to be preserved and amplified across generations.

For tammar wallabies living in New Zealand, the environmental change of predator absence significantly affects natural selection. The absence of predators means the traits that protect against them, like antipredator behaviors, may lose their evolutionary advantage. If these behaviors reduce foraging efficiency or increase energy expenditure without providing any real benefit, wallabies might start to forgo them over time.

• Survival of the fittest: Wallabies conserving energy might thrive better.
• Increased reproductive success: More energy could lead to more offspring.
• Lack of predator-driven selection pressure: Removes the need to maintain antipredator traits.

This shift underscores how adaptable species can be. Adapting to a predator-free environment involves a subtle balance between retaining useful traits and letting go of previously advantageous ones.

Genetic Drift

Genetic drift is another driving factor in evolutionary change. Unlike natural selection, which is non-random, genetic drift refers to random changes in the frequency of alleles (versions of a gene) in a population.

In small populations, like the one in New Zealand, genetic drift can have a significant impact, especially when there's little to no selective pressure. If antipredator behaviors no longer improve survival in New Zealand, they might start to diminish due to genetic drift.

Here's how it works:

• Random mating means some wallabies may not pass on genes for antipredator behaviors.
• Over generations, being randomly omitted, these genes may become less common.

While genetic drift can sometimes lead to the loss of advantageous adaptations, in this case, it could signify the gradual disappearance of unnecessary traits. So, in environments like New Zealand, we could eventually see wallabies evolving behaviors that are better suited to their predator-free lifestyle.