If it ain't broke, don't evolve it - Module 3
Fitness's definition certainly has the connotation of being physically in shape, strong, or able to endure long periods of strenuous activity. However, biological fitness is more defined by an organism's or species' reproductive success, which is determined through how well adapted the organism is to its environment. Before fitness though, there must be variation in a population allowing the mechanism of natural selection, and thus fitness, to occur. According to Sedeer el-Showk, fitness can be measured by comparing the ratio of a specific genotype before and after selection, or simply by measuring change over a long period of time where there is some sort of major environmental change (2014).
A prime example of fitness is the order Crocodilia, a practically unchanged species since the Jurassic era. In a study by Stockdale and Benton, the body size of crocodiles was measured and compared against major environmental changes such as the Cretaceous–Palaeogene extinction period (2021). Body size is often a good variable to measure in cases like this as it is so often correlated with population size, reproductive success, resource consumption, climate change, and geographic range. The fossil record, also an accessible resource, was analyzed in this instance, and the study concluded that an increase in temperature (globally) was directly correlated with an increase in body size of crocodilians (Stockdale and Benton 2021). To quantify fitness in this example, you'd obviously have to first choose a species or closely related species to study (crocodilians in this case), ensuring that the environment in which the species lives is variable across time. Then you'd choose both an environmental variable, such as temperature, and an observable variable for the organism, such as body size, to track over time. By studying fossil records and measuring factors such as skull width or body size, change in a species phenotype, and possibly genotype, fitness can be quantitatively measured over time. Given that crocodilians have changed so little over time and that they evidently reproductively successful, it can be concluded that they are incredibly fit for the environments they occupy.
Longer red branches of the phylogeny represent high evolutionary rates while shorter bluer branches represent lower evolutionary rates (Stockdale and Benton, 2021).
Sources
Crocodile Image: © Karen Givens/Shutterstock.com
el-Showk, Sedeer. Accumulating glitches The meaning of Fitness. Nature.com. Nature Education, 10 November 2014. https://www.nature.com/scitable/blog/accumulating-glitches/the_meaning_of_fitness/
Stockdale, M.T., Benton, M.J. Environmental drivers of body size evolution in crocodile-line archosaurs. Commun Biol 4, 38 (2021). https://doi.org/10.1038/s42003-020-01561-5
Hello,
ReplyDeleteI really liked how you began your post. I think its important to differentiate the differences between all the different kinds of fitness. When we think of fitness, its often thought of as strength but biologically, its quite different. The definition you gave definitely highlighted some important factors and I like how you brought in the idea of environment. The environment shapes so many different things in evolution and its important to acknowledge how those pressures influence different factors. I hadn't looked at crocodilian examples yet but I think its a great way to represent fitness. The example I looked into involved the dark and light fur mice. When looking at fitness, studying the different factors shows how broad and narrow this study can be. Understanding the genetics and numbers behind why species evolve the way they do can help us understand natural selection and fitness better. Overall great post and I really liked the graphic, I think it helps explain the situation very well.
Caroline,
ReplyDeleteI love your title for this!! My brain initially went to the same thing you pointed out about fitness being an organisms physically strength or endurance, but in biology it is so much more than that. You definitely put a lot of effort into studying these crocodiles for the assignment, I am impressed! Using the fossil record to measure fitness is something I had not thought of but is definitely a really good point and resource for this aspect of measuring fitness from something long ago. Great work!
Caroline,
ReplyDeleteWow! I had no idea that crocodiles haven't really changed since the Jurassic era. It is crazy to think how their environment that they inhabit hasn't differed enough to make them evolve in any way. I would have thought that factors such as global warming or a drought would have occurred. Anyways, I think your steps to measure fitness are well thought out. Choosing both an environmental variable and an observable variable for the organism to track over time would be a great measure. Would you be counting how many generations this trait went? Or just how many offspring had it?
I loved this it was super cool to read, crazy how long crocs have been on this earth & how little they have changed. I really love the explanation you give for the first part as it is clear & well written.
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