Evolution

Darwin’s Postulates & Firemouth Cichlids

Why cichlids?

Cichlid fish are one of the most biologically diverse groups of vertebrates on the planet. This diversity (as well as population size, reproduction rate, etc) allows scientists to study the cichlid evolution, and the role of natural selection, more closely than other populations. Many aspects of cichlid characteristics have undergone (and are currently undergoing) selective pressures. Cichlid evolution can be found at multiple levels, including behavioral changes, molecular adaptations, size and coloration variation.

Screen Shot 2014-07-24 at 10.42.50 AMThorichthys meeki is a species of cichlid commonly known as the “firemouth”, because of its bright red-orange coloration on the jaw. This specific coloration is unique to the firemouth, and is used in mating, competition and defense, and is therefore strongly affected by selective  pressures. In general, male firemouth cichlids have prominent  jawline coloration (10). The female Firemouth cichlids have specific molecular adaptations that that allow them to see the male coloration very clearly (6).

In the case of the firemouth cichlids, jawline coloration in males (and the subsequent female response) plays an important role in the evolution of the species due to the pressures of sexual selection. Sexual selection has been shown to be a mechanism of significant biological change for firemouth cichlids, even if the environment stays very stable (9), and research confirms that male coloration corresponds to sexual selection by the females (8). The females tend to mate with the more dominant males, so any characteristic that improves one’s chance of winning in a male-male competition scenario will be selected for. In any cichlid population, males who display the same color will compete more intensely. In firemouth cichlid populations, all of the males have a distinguishing red color, so male-male competition is very strong (4).

firemouthIt has been shown that smaller, less dominant males have physiological and behavioral changes during any social interaction with a more dominant cichlid. (3).  Many times, cichlids will engage in “pre-fight behaviors” (7) and  whichever male is smaller and duller will back down (5).    The coloration allows the more dominant males to emerge victorious from a fight with other males, without even having to engage in actual physical contact. The firemouth cichlids open their mouths and expose more of the red jawline. This behavior enlarges the head considerably when seen from the front and the side. In the event that there is no definitive pre-fight winner, they will engage in the more dangerous physical form of fighting. (1) In many cases, coloration is directly correlated to body size, which also plays a role in determining the winner of the conflict. (2) (more…)

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Homo sapiens, Neanderthals and Denisovans… did they interbreed?

Recent discoveries in DNA have shed light on the relationship of Homo sapiens and extinct hominids, Homo neanderthalensis and Denisova hominins. There is evidence to suggest that all three lived around the same time, and possibly in the same area (Krause et al., 2010)(Meyer et al., 2012). If these three groups lived close together, it should be possible to detect whether or not theyinterbred. If H. sapiens interbred with H. neanderthalensis and D. hominins, we should be able to find genetic evidence of this interbreeding in modern H. sapiens genomes. Using modern genetic examination (Gibbons, 2010), scientists have determined that H. sapiens did interbreed with both H. neanderthalensis (Green et al, 2010)(Hawks, 2013)(Meyer et al, 2012) and with D. hominins (Hawks, 2013)(Meyer et al, 2012).

article-1058538-02B984B100000578-348_468x342Homo neanderthalensis fossils were first discovered in 1856 in Germany, and ever since, scientists have been exploring the relationship of Neanderthals to modern humans. (Gibbons, 2012) The first “draft sequence of the Neanderthal genome” was published in 2010 (Green, et al), and that has given us insight into the relationship of Neanderthal DNA to the DNA of modern humans. Using data obtained by the Human Genome Project (Intl Human Genome Sequencing Consortium, 2001), scientists can compare human and Neanderthal DNA in order to discover the genetic relationship between the two.  The evidence suggests that Neanderthals lived in both Europe and Asia before going extinct around 30,000 years ago (Green et al, 2010). However, many of the Neanderthals interbred with H. sapiens before going extinct. According to Green and colleagues (2010) andHawks (2013), modern humans living outside of Africa represent between 1% and 4% of ancestry from Neanderthal populations. It is possible, with modern genomic technology, for the average person to send a sample of her own DNA to a lab (ex: ‘23andMe’) and get results showing her ancestry. If she has European ancestry, it is possible that she will also have some small percentage of Neanderthal ancestry (Gibbons, 2012). These sorts of results would only be possible had the Neanderthals interbred with direct ancestors of modern humans. Not all of the evidence suggests that Neanderthals interbred with the ancestors of modern humans, however. A study showed that there were no contributions from Neanderthal mtDNA to modern human mtDNA from a specimen recovered from Mezmaiskaya Cave in the northern Caucasus. (Ovchinnikov, 2000) This is not necessarily contradictory data from the other studies; it shows that not all Neanderthal populations interbred with modern human populations.
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Rethinking Aristotle: The Unwarranted Rejection of ‘Final Causation’ in Modern Evolutionary Biology

Why is the polar bear’s fur white? Why does the snake have the ability to unhinge its jaw? At first consideration, the answers to these questions are fairly straightforward. A polar bear has white fur for camouflage and the snake can unhinge its jaws to eat large prey. However, behind these questions lies a larger question, a question that is not directly answerable by describing the function of a certain feature. This question is of final causation, purpose or teleology. Does the polar bear have white fur because camouflage was the purpose of white fur? Is the snake’s unique unhinging jaw a result of a purposed process, with eating as a goal? Or are these features just the accidental by-products of the purposeless mechanism of evolution? Aristotle was under the impression that you do not fully understand an object unless you understand all of its explanations, including teleology, which Aristotle referred to as ‘the final cause’. Is that standard of explanation accepted today? And if we don’t embrace a teleological explanation today, is that rejection justified?

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“Aristotle was one of the greatest philosophers and scientists the world has ever seen”. (Dunn, 2005) He was one of the first people to propose a formal logical system, a functioning ethical system, a methodology concerning causality and a systematic way of studying the natural world. During his study of the natural world, he spent a large portion of his time studying life; a field that we now know of as biology. “Aristotle’s studies encompassed the entire world of living things. Many of his descriptions and classifications remain sound today” (Dunn, 2005)

In addition to his study and classification of organisms in the natural world, Aristotle had a very specific way of looking at natural and man-made objects. In Metaphysics, Aristotle explains the 4 different types of aition, often translated as ‘explanations’ or causes’. He believed that in order to fully understand something, you have to understand it in light of the four causes. If you didn’t understand all four of the causes, you didn’t actually understand the object in question.
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