Genetics and Evolution


The History of Genetics

- in the mid 19th century, Gregor Mendel carried out breeding experiments on plants, one of his observations was that the inheritance of each characteristic is determined by 'units' that are passed on unchanged to descendents

- in the late 19th century, behaviour of chromosomes during cell division was observed

- in the early 20th century, it was observed that chromosomes and Mendel's 'units' behaved in similar ways. this led to the idea that the units, now called genes, were located on the chromosomes

- in the mid 20th century, the structure of DNA was determined and the mechanism of gene function was worked out

- this scientific work by many scientists led to the gene theory being developed

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Theories of Evolution

- earlier theories of evolution, including that of Jean-Baptiste Lamarck, are based mainly on the idea that changes that occur in an organism during its lifetime can be inherited. we now know that in the vast majority of cases this type of inheritance cannot occur

- charles darwin proposed his theory of evolution by natural selection, largely as a result of observations made on a round the world expedition

- the individual organisms in a particular species tend to show a wide range of variation for each characteristic

- reproduction always gives more offspring than the environment can support. the organisms that have inherited the characteristics most suited to their environment - the 'fittest'- are more likely to survive and breed successfully

- when they breed, they pass on the characteristics that have enabled them to survive to the next generation

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Accepting Darwin's Ideas

- darwins theory of evolution by natural selection was only gradually accepted for a number of reasons

- these include:

    ...conflict with the widely held belief that God made all the animals and plants on Earth

    ...insufficient evidence mechanism for explaining variation and inheritance (genetics were not understood for anohther 50 years)

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Evolution and Speciation

- Alfred Russel Wallace independently proposed the theory of evolution by natural selection. he published joint writings with Darwin to publish 'The Origin of Species' the following year (1859)

- Wallace worked worldwide gathering evidence for evolutionary theory. he is best known for his work on warning colouration in animals and his theory of speciation

- a build up of evidence over time has led to our current understanding of the theory of speciation. new species arise as a result of:

    ...isolation: where two populations of a species become separated 

    ...genetic variation between the populations

    ...natural selection that operates differently on the two populations

    ...speciation: where the populations become so different that successful interbreeding is no longer possible

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Evidence for Evolution

- fossils are the remains of organisms from millions of years ago that can be found in rocks, ice and other places

- fossils may be formed in different ways including the absence of decay, parts replaced by minerals as they decay, and as preserved traces of organisms

- fossils give us information about organisms that lived millions of years ago

- it is very difficult for scientists to know exactly how life on Earth began because there is little valid evidence

- early forms of life were soft bodied so left few traces behind and mant traces behind and many traces of early life have been destroyed by geological activity

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Fossils and Extinction

- you can learn from fossils how much or how little organisms have changed as life has developed on Earth

- extinction may be caused by a number of factors including new preadators, new diseases or new, more successful competitors

 - extinction can be caused by a variety of factors including changes to the environment over geological time and single catastrophic events, such as massive volcanic eruptions or collisions with asteroids

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Antibiotic Resistant Bacteria

- bacteria can evolve rapidly because they reproduce at a fast rate

- mutations of bacterial pathogens produce new strains

- some strains might be resistant to anitibiotics and so are not killed. they survive and reproduce, so the population of the resistant strain increases by natural selection. the resistant strain will then spread because people are not immune to it and there is no effective treatment

- MRSA is a resistant to antibiotics

- to reduce rate of development of antibiotic resistant strains, it is important that doctors do not prescribe antibiotics inappropriately, patients complete each course of antibiotics

- the development of new antibiotics is costly and slow and is unlikely to keep up with the emergence of new resistant strains

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- traditionally, living things have been classified into groups depending on their structures and characteristics in a system described by Carl Linnaeus

- Linnaeus classified organisms into kingdom, phylum, class, order, family, genus and species

- organisms are named by the binomial system of genus and species

- as evidence of the internal structures of organisms became more developed due to improvements in microscopes, and the understanding of biochemical processes progressed, new models of classification were proposed

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New Systems of Classification

- studying the similarities and differences between organisms allows us to classify them into archaea, bacteria and eukaryota

- classification also helps us to understand evolutionary and ecological relationships

- models such as evolutionary trees allow us to suggest relationships between organisms

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