Real-life "Alien" jaws - Darien Satterfield
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However, through time the ocean became increasingly filled by organic material which could be used as sources of nutrition and energy. As marine life was evolving, natural selection led to the evolution of oral jaws which opened up a wide variety of new food sources for fish to consume. Though oral jaws can be used for capturing prey with biting, and processing prey with chewing using one structure to both capture and process prey can be limiting in that the oral jaws cannot become too specialized for any one task. The functions of prey capture and processing became decoupled when natural selection lead to another gill arch moving forward into the pharynx to become a separate secondary set of jaws, called pharyngeal jaws. With this development fish could specialize the oral jaws for capture and the pharyngeal jaws for prey processing.
In the theory of evolution, new traits such as pharyngeal jaws which open up new niche space are called novel or key innovations. Novel innovations are a key ingredient to adaptive radiations (the term for when a clade of organisms rapidly diversifies, and many new species arise in a relatively short period of time). Fish are a very large an extreme example of a clade that is has had many adaptive radiations and bursts of diversity. Many scientists believe much of this diversity can be attributed to the development of pharyngeal jaws. In order for a new species to come about there must be space and resources in the habitat which aren’t already exhausted by an existing species. With the development of pharyngeal jaws fish were able to invade unused niche space, consuming food sources that were previously too challenging to consume with just the oral jaws. For a close-up view of the use of pharyngeal jaws in feeding, take a look at this video.
Through time, the pharyngeal jaws became more and more diverse, ever evolving into new morphs and functional rolls. For example, some fish developed an adaptation in the pharyngeal jaws called pharyngognathy, the fusion of the pharyngeal jaws to the skull. These fused pharyngeal jaws are strong, well suited for crushing and grinding, and were a major advancement in the versatility of pharyngeal jaws. Incredibly, this innovation didn’t just evolve once in fish, it actually was a product of natural selection in at least six different and independent lineages (Check out this paper by Peter Wainwright et. Al. 2012). This innovation was so successful because it allowed for even more previously inaccessible prey to become available as a food supply. This paper by Darrin Hulsey gives great detail about the effects of Pharyngognathy as a key innovation in the cichlid adaptive radiation.
It is unclear, how much of fish diversity is solely attributable to pharyngeal jaw evolution. For example in this article by Michael Alfaro et. al showed that the evolution of novel color patterns drove the speciation of parrotfish more so than pharyngeal jaw diversification. We also know that other adaptations in fish were novel innovations to living in aquatic environments, such as the evolution of gills. Thus, it is difficult to say whether or not pharyngeal jaws can be considered the reason why fish are the most diverse taxa of animals. Fish biologist have more tools now than ever before to study evolution and fish. There is still much that can be learned by continuing to study pharyngeal jaws, and the ways in which this solution to eating without limbs has allowed fish to inhabit every ocean habitat.
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