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Chapter 17: Variation and Evolution Leaving Certificate Biology Higher Level Evolution • Evolution is the way in which living organisms change genetically to produce new better adapted forms of life over a long period of time Evolution (cont.) • Evolution occurs over a long period of time as a result of variation within species caused by sexual reproduction and mutations – Sexual reproduction is the main source of variation within a species – Mutation is a change in the sequence of bases in DNA – caused by loss, gain or swapping of bases within DNA Evolution (cont.) • Sexual reproduction causes variation by: – Independent assortment of chromosomes into gametes at meiosis – Crossing over (swapping of alleles between homologous chromosomes) during meiosis – Fertilisation – combining of a homologous chromosome from the gamete of one individual with the homologous chromosome from a gamete of another individual Evolution (cont.) • Mutations cause variation by: – Spontaneous mutations that occur during replication of DNA and where the cell does not repair it – they are totally random, vary rare and not caused by any one factor – Mutagens are chemicals that speed up the process of spontaneous mutations – all mutagens are potential carcinogens (cancercausing chemical) • Examples of mutagens include: asbestos, benzene, radiation (UV light, X-rays, gamma rays, cosmic rays, α-particles, neutrons), arsenic, acrylamide, human papilloma virus, Helicobacter pylori, hepatitis B and C viruses Evolution (cont.) • Types of mutations: – Gene mutations: change in the sequences of bases within a single gene causing the resulting protein to be non-functional • Common genetic disease caused by gene mutations include: sickle-cell anaemia, cystic fibrosis, haemophilia, and albinism – Chromosome mutations: changes in the structure or number of chromosomes of cells • A common example of a chromosome mutation is Down’s Syndrome (name after the physician who described this condition) where the individual has 3 number 21 chromosomes Darwin and Wallace Theory of Natural Selection • Darwin worked in the Galapogos Islands • He made three observations: 1. Organisms overbreed 2. Population remains constant 3. There is limited resources for organisms • As a result of these observations he made two conclusions: 1. There is variation amongst members of the same species 2. Those organisms with the most beneficial variations (adaptations) will survive and pass on those characteristics to their offspring Speciation • Speciation is where accumulated genetic changes over many millions of years that result in an organism being better adapted to its habitat results in the formation of organisms that can no longer interbreed with each other and a new species is formed Evidence for Evolution • Comparative anatomy: – Involves studying the anatomical (structural) links between organisms of separate species – Pentadactyl limbs of different vertebrate animals have a similar pattern of arrangement (i.e. five digits). But there is no need for this to be the case if evolution hadn’t occurred (i.e. wings of insects are completely different structurally from that of a bat’s). Thus the conclusion is that all variations of the pentadactyl limb arose through natural selection (evolution) from a common ancestor Pentadactyl limb Evidence for Evolution • Fossil Records: – A fossil is the preserved remains of an organism or part of an organism or an imprint related to an organism – Fossil records support evolution because: • They are very old (dated by carbon dating and rock depth measurement) and show how organisms have been slowly changing and adapting over the millennia • More recent fossils show increased complexity when compared to older fossils e.g. the height of the horse has increased over a 60 million year period • Many fossils are of organisms now extinct but have modern-day ‘relatives’ Dinosaur fossil Evidence for Evolution • Comparative Embryology: – Study of structure of early embryos of organisms of different species • Embryos of species as diverse as fish, tortoise, chick, rabbit, and human all have a similar structure at the early developmental stage • This shows evidence for evolution as this developmental mechanism must have been come from a common ancestor Early embryos of the following animals show similar structure