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Chapter 3 Natural selection and evolution EXTRA WORKSHEET 1: SCIENCE UNDERSTANDING Evolution of icefish LOGICAL/MATHEMATICAL Name: _____________________ Class: _________ Date: __________ Fish of the Family Channichthyidae, found in the waters around Antarctica, are very unusual. They have evolved in extremely cold water, and are commonly called ‘icefish’ (Figure 1). There are 25 species of icefish. Scientists have been interested in these fish since they were first discovered, because of their amazing ability to avoid freezing to death in extremely cold waters. Research has shown that icefish’s body contains special ‘antifreeze’ chemicals called glycoproteins, which seemed to work by preventing ice crystals growing. The antifreeze chemical coats ice crystals so water cannot come into contact with the surface and add to the crystal. These chemicals are so effective that they are being researched for possible applications where freezing needs to be prevented. Possible applications include increasing the ability of crop plants to withstand frost, lengthening the shelf life of frozen foods and potential applications in surgery, storing organs for transplant and treating hypothermia. Figure 1 Ocellated icefish, Chionodraco rastrospinosus. Icefish blood Another amazing fact about icefish in this family is that they have almost no red blood cells. Red blood cells contain a chemical called haemoglobin, which Copyright © Pearson Australia 2012 (a division of Pearson Australia Group Pty Ltd) ISBN 978 1 4425 2366 1 Page 1 bonds to oxygen and transports it in the blood of all other vertebrates. But these fish contain no haemoglobin and their blood is almost transparent. So scientists wondered how oxygen was carried in the blood of these fish, and how their blood can carry enough oxygen to the cells to maintain respiration. Research discovered that: • the antifreeze production and the haemoglobin loss were both shown to be specific genetic mutations • icefish have large hearts that pump out a high volume of blood each beat • icefish have a very high volume of blood in their body • very cold water holds a lot more oxygen than warm water. Evolution and icefish These icefish seemed to present an evolutionary puzzle. Scientists wondered why fish without red blood cells were not removed by natural selection. The conclusion of scientists studying these fish is that they evolved in the following way: • A mutation in an ancestral species allowed formation of the antifreeze chemicals. • A much later second mutation occurred which caused a loss of haemoglobin. This would normally be fatal in any vertebrate organism. • The ancestors of the icefish were present in Antarctica as the climate began cooling. • The loss of haemoglobin was not fatal as colder water could carry more oxygen and the antifreeze helped them cope with the cold water. • There would not be much competition from other fish since those fish lacked the antifreeze chemicals and most seem to have been unable to survive the cold. Copyright © Pearson Australia 2012 (a division of Pearson Australia Group Pty Ltd) ISBN 978 1 4425 2366 1 Page 2 Questions 1 Identify the adaptation of icefish that enables them to maintain liquid water in their bodies despite the extreme cold. ___________________________________________________________ ___________________________________________________________ 2 Describe three adaptations of the icefish that enabled them to survive without haemoglobin. ___________________________________________________________ ___________________________________________________________ 3 Explain how natural selection could have acted on all the Antarctic fish and selected in favour of the icefish. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ 4 Justify the view that the antifreeze mutation came before the haemoglobin mutation. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ 5 Propose whether the adaptations in the circulatory system of the icefish came before or after the loss of haemoglobin. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ Copyright © Pearson Australia 2012 (a division of Pearson Australia Group Pty Ltd) ISBN 978 1 4425 2366 1 Page 3