Chapter 23 Evolution

» A population has been described as a group of individuals of the one species that breed together in an environment (fertile offspring) » Geneticists are interested in the “gene pool” and its changes over time, when looking at a species » Gene pool= sum of all the alleles in a population » An allele is the different forms of a gene i.e. blue vs brown eyes

» Changes in a gene pool can be brought about by several factors: ˃Mutations: +Changes to an organisms DNA +Causes new allelic forms of a gene +Gene mutations at one locus are rare and unlikely to be beneficial +They are the only source of new genes +(Usually mutations are fatal)

» Natural selection –a powerful force discussed in e8.3 » Gene flow: » The exchange of alleles between populations » Can occur by movement of organisms with different genotypes in and out of populations ˃Changes the frequency of genes in the gene pool ˃DO GENE POOL ACTIVITY SHEET IN TEACHERS FOLDER

» the exchange of alleles between populations » IF FLOW IS REDUCED BETWEEN POPULATIONS- greater diversity between the populations arises » If populations are small-disasters have great effects on the survival of the gene pool of that population » Often non selective and random-drastically altering the gene pool

» Variation between members of a species is essential for evolution to occur » This allows organisms to change to a changing environment » VARIATION is the raw material on which natural selection can act » Variation in a species is dependent on their reproductive method- asexual vs sexual » Asexual reproduction only has variation via mutation

» The process of meiosis and gamete formation gives rise to greater combinations of alleles and therefore greater variation in a population » Mutations are still the only source of new alleles and is rare, as usually fatal » Environment can cause some variations, but these are not inherited- must be genetic variation to be passed on from one generation to the next ˃i.e an athlete can lower heart rate by exercise and training but this can not be passed on

» We easily see phenotypic differences- makes us all distinguishable form the next person » There are other differences not so obvious » Many behavioural characteristics have inherited components » As does intelligence » Biochemical characteristics-coded for by the DNA and do represent a source of variation » Genetic diseases like phenylketonuria has a mutant gene that leaves someone with the inability to break down amino acids….and can be passed on » DNA IS UNIQUE TO EVERYONE EXCEPT IN THE CASE OF IDENTICAL TWINS

» Read pages about colour blindness to refresh your memory on inherited characteristics

» The theory of evolution suggests: ˃All life forms have evolved from a single, simple common ancestor ˃That all species change through time ˃Evolution in a species can be recognised by the changing frequencies of genes in populations

» Charles Darwin- most influential in this theory » Hypothesised that for all populations there were more individuals born than ever survive to reproduce » And that environmental factors kept things in check » First suggested his theory of natural selection in Origin of the Species » Nature selects those best suited to survive the environmental conditions- long necks/legs/colouring etc

» He challenged the creationist view by stating that this variation in a species allowed some to be selected to survive and that they then further adapted ….eventually becoming new organisms » Darwin at that time had no knowledge of DNA/GENES

» A Characteristic in an organism we know is an expression of their genes/alleles » If “nature” selects a characteristic- it selects a particular gene » Changes the frequency of that allele in a population » ?? Evidence that evolution is occurring » Natural selection is said then to act on characteristics expressed in the phenotype » Selection is made by biotic or abiotic factors

» Biotic- living factors- predators/food/disease causing organisms » Abiotic- non living-rainfall, temperature, nutrient levels of soil, light availability and wind » Evolution is slow- but if selective pressures are strong can be faster- the moth( Biston betularia) – industrialisation in England read pg. 211/212 –the rabbits

 Natural selection using abiotic factors can be seen in the evolution of antibiotic resistant strains of bacteria  Some bacteria in a population are resistant to certain antibiotics- due to their prolific reproduction a new generation of resistant bacteria is quickly created  Can be because of transfer of plasmids between one type of bacteria and another as plasmids are circular DNA= genes AND can be passes between bacteria conferring immunity or resistance to the same type of antibiotic

» Found in yeast and bacteria » DNA can replicate independently of the main DNA » The transfer of these plasmids between bacteria is called CONJUGATION » see diagram pg 213

» Work in a variety of ways » Interfere with the cell wall production » Blocking protein synthesis » Affects the functions of the cell membrane » Penicillin was hailed to be the wonder drug when introduced- but wide usage has meant that several strains of bacteria have developed immunity to this antibiotic » Antibiotic resistance can mean patients have had limbs amputated/ and death (READ PAGE 215- worrisome strain of staph…)

» In populations there are phenotypic variations between individuals. Variation is a result of environmental effects or genetic differences » In natural populations there is a struggle for survival- over resources, mates or resistance to antibiotics. When competition is severe the effects of natural selection are more likely seen » Those organisms best adapted to the environment survive and reproduce and pass on those favourable genes » Over many generations the population may evolve, showing changes in the gene pool, which reflect the effects of natural selection