Evolution of Dolphins

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Dolphins evolved from terrestrial animals into aquatic animals throughout generations of evolution. The dolphins share numerous ancestors with the various cetacea. https://sites.google.com/site/obenscience7e/unit-1-why-do-organisms-look-like-that/a-case-study-how-modern-whales-came-to-be.

The evolution of dolphins, or Delphinus, is believed to have started with the Pakiectus a four legged, land walking mammal. The Pakiectus dates back to approximately 50 million years ago. Throughout the centuries, these animals have gone through drastic changes to become the modern day dolphin. Along with the Pakiectus, the dolphin is thought to have evolved alongside or from the Ambulocetus, Remingtonocetidae, Protocetid, Basilosaurudae, Dorudontid, Squalodontidae, Aetiocetidae, and Kentriodontidae to eventually become the Delphinus which is found in the Cetacea infraorder.[1][2] The Pakiectus lived near the shallow waters and began to feed on organisms that lived in these waters, which began the transition from terrestrial to aquatic animals. The bone structure of the flipper in the modern dolphin is very similar to the structure found in the Pakiectus legs and hooves, confirming the link between the two organisms.[3]

Over the 50 million years of evolution, the ancestors of dolphins adapted from being terrestrial to aquatic. A characteristic that is mainly found in terrestrial animals is that of a vertical spine. Dolphins have vertical spines leaving them swimming with vertical movements, while fish movements are horizontal.[4] In comparison to the Pakiectus, dolphins hav much larger skulls with a shape that has adapted to accommodate for a padded lower jaw and extended middle ear. Having an extended middle ear allows the dolphins to use echolocation for communication. As the ancestors of dolphins moved toward living aquatically, the nostrils evolved into a blowhole to expend less energy while surfacing to breathe. The ancestors of dolphins that lived terrestrially had legs to walk. Dolphins have two small pelvic bones that are rod shaped that are now vestigial legs from their land walking ancestors.[5]

Adaptation Genetics

To adapt to living in an aquatic environment, the dolphins what to change their body and way of living. Throughout millions of years of evolution, the dolphin body form changed from the walking mammal with four limbs, Pakicetus, to the swimming mammal with flippers and a tail, Delphinus. https://dolphin-academy.com/learn/evolution

The common dolphins underwent centuries of evolution to move from terrestrial to aquatic mammals. These mammals share much of their evolution with the other members of the Cetacea. Various genes went through positive selection, or Darwinian selection, to be in the form they take today. Numerous studies show the about 2.26%-4.8%, around 376 genes, of genes were selected for with positive selection. This gene selection is believed to help the overall system development, pattern of specification process, and mesoderm development.[6] Morphological evolution is evident as the Delphinus form is significantly different than the body of the Pakicetus. In dolphins, there is a overrepresentation in genes of MET, FOXP2, TRIM63, FOXO3, CD2, and PTCH1 which are possibly responsible for the morphological evolution. Evidence shows that there could be changes in transcription factors, expression patterns, and in protein function that is also responsible for the morphological changes.[7]

Adapting to the aquatics lifestyle required changes to the internal body and function of the the Delphinus. Being that dolphins live in the water, there is a much higher need of kidney function. The amount of water that flows through and around the dolphin is significantly more than the terrestrial mammals. The genes SMAD1, NPNT, LEF1, SERPINF1, and AQP2 are critical for the modifications of the kidney.[8] These genes allow for the dolphins to have a reniculate kidney. Reniculate kidneys are found in aquatic mammals. They allow for a large surface area for removing toxins from the body more efficiently. Aquatic mammals such as Delphinus, don’t have specialized glands for excreting salts from the body as terrestrial mammals do - sweating. With the reniculate kidney dolphins are able to gain water from seawater without any harm.[9][10] The heart of a dolphin is approximately 33 times larger than that of a cox, a terrestrial mammal. Specific genes were adapted to allow for the diving motion required for the aquatic lifestyle. Genes ADAM9, NKX2, CAD15, CRFR2, GDF9, CADH3, TAB2, and PLN are the specific genes that have allowed for the dolphin to adjust the new environment. All eight of these genes allowed for anastomoses, or cross-connection, between the dorsal and ventral inter ventricular arteries. The anastomoses allows for the Delphinus to either conserve or dissipate body heat.[11] Another adaptation that the genes contribute to is hypertrophy, an increased growth of muscle cells, of the right ventricle.[12][13] Living below the surface of water requires an adaptation to the lungs due to the lack of oxygen in the environment. As the Delphinus, along with other Cetacea, breath through their lungs, not gills as many other aquatic mammals do. Among all the organisms in the world, the Cetacean has the most modified lung. Increased cartilage support, reinforcement of peripheral airways, loss of respiratory bronchioles, and presence of bronchial sphincters are all adaptions from the genes RSPO2, LEF1, and FOXP2. Many of these adaptions are in response to the new diving motion from living in the aquatics.[14] In order to breath in oxygen, dolphins use their “nostrils” that are located at the top of their body, also known as a blowhole. The bronchiole sphincters make it possible for the dolphin to seal off the water from entering the lungs and hold in oxygen for extensive amounts of time.[15]

The external part features of the Delphinus adapted to living in the marine environment. Homeotic development involves genes that regulate the anatomic structure of organisms. The 5’HoxD genes, homeotic genes, are three times higher in Cetaceans than in terrestrial animals when comparing the ratio of non-synonymous to synonymous substitutions.[16]

Section 2 Microbiome

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  1. Bapor Kibra, Willemstad, Curaçao, “Learn About Dolphin Evolution” 2019. Dolphin Academy
  2. Mark Caney, “Evolution of Dolphins” 2019. Dolphin Way
  3. Dolphins-World, “Dolphin Evolution”, April 25, 2017. Dolphins-World
  4. Dolphins-World, “Dolphin Evolution”, April 25, 2017. Dolphins-World
  5. Dolphins-World, “Dolphin Evolution”, April 25, 2017. Dolphins-World
  6. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3686761/ Mariana F. Nery, Dimar J. Gonzalez, and Juan C. Opazo, “How to Make a Dolphin: Molecular Signature of Positive Selection of Cetacean Genome” 2013. National Center for Biotechnology Information
  7. Carroll SB “Chance and Necessity: The evolution of morphological complexity and diversity” 2001. Nature 409: 1102-1109
  8. Hill DA, Reynolds JE “Gross and microscopic anatomy of the kidney of the West Indian manatee, Trichechus manatus (Mammalian: Sirenia), 1989. Acta Anat 135: 53-56
  9. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/dolphin Bruno Cozzi and Helmet Oelschlager “Urinary System, Genital systems, and Reproduction” 2017. Anatomy of Dolphins
  10. Hill DA, Reynolds JE “Gross and microscopic anatomy of the kidney of the West Indian manatee, Trichechus manatus (Mammalian: Sirenia), 1989. Acta Anat 135: 53-56
  11. https://jeb.biologists.org/content/205/22/3475 Erin M. Meagher, William A. McLellan, Andrew J. Westgate, Randall S. Wells, Dargan Frierson, Jr, D. Ann Pabst “The relationship between heat flow and vasculature in the dorsal fin of wild bottlenose dolphins Tursiops truncatus 2002. Journal of Experimental Biology
  12. https://www.healthline.com/health/muscular-hypertrophy Daniel Budnis “Muscular Hypertrophy and Your Workout” 2019. Healthline
  13. Kooyman GL “Diving physiology” 2008. Ln: Perrin MR, Wursig B, Thewissen JGM, editors. Encyclopedia of Marine Mammals San Diego: Academic Press, pp 327-332
  14. Kooyman GL “Diving physiology” 2008. Ln: Perrin MR, Wursig B, Thewissen JGM, editors. Encyclopedia of Marine Mammals San Diego: Academic Press, pp 327-332
  15. https://us.whales.org/whales-dolphins/how-do-whales-and-dolphins-breathe/ Whale and Dolphin Conservation “How Do Dolphin and Whales Breath?” 2019. Whale and Dolphin Conservation
  16. Wang Z, yuan L, Rossiter SJ, Zuo X, Ru B, et al. “Adaptive evolution of 5’HoxD Genes in the origin and diversification of the Cetacean flipper” 2009. mol Biol Evol 26: 613-622

Edited by [Carolyn Herbosa], student of Joan Slonczewski for BIOL 116 Information in Living Systems, 2019, Kenyon College.