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AS Biology Evolution

Evolution and Variation

Define evolution Theory that describes the way in which organisms evolve or change over time as a result of natural selection. Organisms best suited to their environment are more likely to survive and pass characteristics to their offspring by genes.
Why were the finches on the Galapagos islands important in the development of the theory of evolution? Darwin noticed different islands had different finches. Were similar in many ways but claws and beaks were different shapes and sizes. Realised design linked to food available on that island, birds with beaks more suited to the food would survive longer.
Evidence for evolution: 1. Paleontology- study of fossils and fossil record. 2. Comparative anatomy- study of similarities and differences in anatomy. 3. Comparative biochemistry- similarities and differences between chemical make up of organisms.
1. What are fossils and 2. the fossil record? 1. Formed when animal and plant remains are preserved in rocks. 2. Over time sediment is deposited to form layers (strata) of rock. Most recent layer on top. In the strata fossils form sequence-oldest to youngest- shows how organisms have changed.
What evidence for evolution is provided by the fossil record? 1 and 2. 1. Simplest organisms in oldest rocks, most complex (vertebrates) in more recent rocks. Supports theory that simple life forms evolved to more complex ones. 2. Sequence matches ecological links - plant fossils before animal fossils (need them to survive)
What evidence for evolution is provided by the fossil record? 3 and 4. 3. Studying similarities in anatomy of fossil organisms can show closely related animals have evolved from the same common ancestor. e.g. horses and rhinoceros. 4. Allow relationships between extinct and living (extant) organisms to be investigated.
Disadvantages of the fossil record It is not complete, many organisms are soft-bodied and decay quickly before they can fossilise or the correct conditions for fossilisation may not have been present. Fossils are destroyed by Earths movements or volcanoes; others are not yet discovered.
Comparative Anatomy- homologous structures Appear superficially different in different organisms but have the same underlying structure e.g. PENTADACTYL LIMB of vertebrates.
Comparative Anatomy Evidence from pentadactyl limbs vertebrate limbs have a variety of functions such as running, flying, swimming. Would expect structure to be very different but basic structure in all vertebrates is similar- explanation is all vertebrates have evolved from a common ancestor.
Comparative Anatomy How are homologous structures evidence for Divergent Evolution? Divergent Evolution- Describes how different species (each with own adaptive features) have evolved from a common ancestor. May occur when closely related species diversify and adapt to new habitats e.g. due to migration.
Comparative Biochemistry Some molecules remain almost unchanged among species. Small changes can help identify evolutionary links.e.g. Cytochrome C and rRNA are studied.
Comparative Biochemistry What is the theory of NEUTRAL evolution? Most variation in a molecule does not affect function of the molecule as occurs outside of FUNCTIONAL REGIONS. Differences are called NEUTRAL as do not affect function-so not affected by natural selection and so occur at a regular rate for each molecule.
Comparative Biochemistry Evidence for evolution To see how closely related 2 species are, the molecular sequence of a molecule is compared. Differences are used to estimate when the 2 species last had a common ancestor (more closely related more similar DNA/proteins).
Variation is differences in characteristics between organisms- 2 types Interspecific variation- widest type, variation BETWEEN species. INTRASPECIFIC variation - Differences between organisms within a species e.g. height.
2 factors that cause variation Genetic variation- differences in the genetic material an organism inherits from its parents and Environmental variation- the environment an organisms lives in.
Genetic causes of variation 1. Alleles; 2. Mutations; 3. Meiosis 4. Sexual Reproduction 5. Chance
Genetic causes of variation Alleles Different alleles of a gene produce different effects on the characteristic e.g. Gene for blood groups has three alleles - A,B, O which gives the different blood groups.
Genetic causes of variation Mutations
Genetic causes of variation Meiosis
Genetic causes of variation Sexual Reproduction Offspring inherit alleles from 2 parents - so each individual is different from the parents (unlike Asexual Reproduction which produces clones which are genetically identical.
Genetic causes of variation Chance During sexual reproduction it is chance as to which 2 gametes meet and combine- RANDOM FERTILISATION. Each individual produced will be different to its siblings.
Environmental causes of variation All organisms are affected to some degree by the environment they live in. e.g. a plant of the same species with more access to sunlight than another one of the species will generally grow larger.
Most cases of variation are caused by.... a combination of genetic and environmental causes
How can genetic variation be brought about in organisms that reproduce Asexually? By Mutations
Example of variation in a characteristic that is the result of genetic and environmental variation Skin Colour- determined by how much Melanin is present in skin. Skin colour at birth is determined by genetics but when we expose skin to sunlight more melatonin is produced and skin becomes darker. Also intelligence and height.
Based on data collected by scientists studying variation (from large numbers of the population), characteristics can be sorted into those showing....... Continuous variation or Discontinuous variation
Discontinuous variation
How is Discontinuous variation usually represented graphically? Usually by a bar chart or a pie chart. e.g. Human Blood Groups.
Genetic causes of variation Meiosis Gametes are formed by Meiosis. During Meiosis 1 genetic material is mixed up by INDEPENDENT ASSORTMENT and CROSSING OVER
Genetic causes of variation Mutation Changes to the DNA sequence (a mutation) can lead to changes in the protein coded for. This may affect physical and other characteristics. Somatic (body cell) mutations just affect that individual but mutations in gametes can be passed to offspring.



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