Answer :
The presence of a greater number of low-armored sticklebacks in freshwater is due to a process known as adaptation by natural selection.
When sticklebacks move from the ocean to freshwater, they encounter new challenges, such as the lack of predators or different food sources. Over time, some of the sticklebacks will develop genetic mutations that allow them to better survive and reproduce in their new environment.
In this case, the low-armored variety of sticklebacks has a genetic advantage because it grows more quickly and becomes too big and fast to be preyed upon by dragonfly larvae, which are common predators in freshwater environments. As a result, low-armored sticklebacks are more likely to survive and pass on their genes to the next generation. Over time, the proportion of low-armored sticklebacks in the population will increase, leading to a greater number of low-armored individuals in freshwater.
This process of adaptation by natural selection is a key aspect of evolution and helps populations of organisms to better adapt to changing environments over time.
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Final answer:
Freshwater sticklebacks become low-armored due to evolutionary adaptations for survival in environments with different predators, such as dragonfly larvae. Genetic mutations, like those affecting the Pitx1 gene, lead to these changes, resulting in a greater number of low-armored sticklebacks in freshwater habitats.
Explanation:
Fully armored sticklebacks are common in oceans; however, when these fish move to freshwater, they begin to show genetic variation over the years and become low-armored. This evolutionary change is largely due to the different environmental pressures in freshwater ecosystems compared to marine environments. For example, in freshwaters, the presence of predators like dragonfly larvae, which prey on smaller or less agile fish, has led to the evolution of a low-armored variety of sticklebacks. These low-armored sticklebacks grow quickly, becoming too big and fast for these predators, thus having a better chance of survival and reproduction. The greater number of low-armored sticklebacks in freshwater habitats can be attributed to this evolutionary adaptation to local conditions. Genetic variations, such as a mutation in an enhancer upstream of the Pitx1 gene, play a crucial role in the development of these traits. This mutation results in the absence or significant reduction of pelvic spines, contributing to a low-armored morphology that offers advantages in freshwater environments. This phenomenon is an example of adaptive evolution, where species diverge and adapt based on the specific demands of their environment. In the case of sticklebacks, the adaptation to freshwater conditions exemplifies how genetic variations can enable species to optimize their survival in new or changing ecosystems.
