Variation Mutation - a “spontaneous change in a gene or a chromosome” - mutations are natural phenomena most are harmful or lethal or lead to the death of the organism only a few are beneficial they are unpredictable as they are random and spontaneous they are also rare A mutation is the process of changing A mutant is the new form after the mutation has occurred. The mutation rate can be increased by: (a) Radiation X-rays, γ-rays (gamma), β-rays (beta) (e.g. leukaemia) (b) Chemicals mustard gas, nitrous oxides / acids (c) Antibiotics act as selective agents (d) For insects, mutation rate is doubled by a 10ºC rise in temperature. Some mutation enables the species to adapt to the new environments. The presence of mutant characteristics can be related to a change in the structure of the chromosome (or gene): (a) Chromosome Aberration in which a whole or part of a chromosome becomes rearranged Example: Down’s syndrome (a rare condition) there is an extra chromosome on chromosome 21 person becomes both mentally and physically retarded more likely to occur with females over 35 conceiving (b) Chromosome Multiplication in which the number of chromosomes is increased by a “multiple” of the haploid number this covers all forms of polyploidy Example: in 1870, a new species of rice grass was discovered on the mud flats of Southampton water (c) Gene Mutation a change in the chromosome at a molecular level cannot be observed under a microscope e.g. haemophilia – mutant appears unaltered, sickle-celled anaemia HbAHbA HbAHbS HbSHbS - normal blood (but can suffer from malaria) - some sickle cells, but survive OK (resistant to malaria) - die young (e.g. 6 months) because person cannot bind oxygen and the sickle-shaped red blood cells get trapped in the capillaries. N.B. It is advantageous to be HbAHbS in areas where malaria is a killing disease. The HbS allele gives resistance to malaria. Selection (a) Artificial selection “man selects individuals who’s genes will contribute to the next generation” Reasons: for increased economic yield (e.g. milk production / grain per acre) appearance (e.g. colour) food value (e.g. protein content / low fat content) resistance to disease This has been done for years – it is “applied genetics”. For example, a farmer may use a cow, which produced bad milk for meat, but a cow that produces good milk for reproduction purposes. Problems: (a) breeds die (b) organism becomes similar as they share the same genes (c) if a new disease emerges and all are susceptible, all die out (d) loss of variation reduces the chances of long-term survival Remedy: (a) seed banks (containing genes of past species) (b) rare-breed societies (b) Natural selection Thomas Malthus (1798) studied population growth. He predicted that the number of people in the world would eventually be regulated by famine, disease and war. 1858: Darwin and Wallace – read Malthus’s essay and with their own observations produced the “theory of evolution by natural selection” 1st observation Each generation of a species has more offspring than parents 2nd observation Despite the high reproduction rate, the total number of most species remain about the same once established in a particular environment. Conclusion 1 There must be something controlling the numbers of a population. (i.e. the struggle for survival) – especially for the young so that they die before the reproducing age – only strongest pass on genes. 3rd observation individual organisms in a species differed from one another in some ways – i.e. there was variation 4th observation although the offspring are different in some ways, they still inherit many parental characteristics. (Darwin found this without understanding genetics) Conclusion 2 certain variations help an organism to survive the struggle for existence – others do not. Darwin’s theory The survival of the fittest by natural selection - nature selects those organisms that are best suited for survival. - these reproduce and pass on the genes that enabled them to survive to the next generation Darwin did notice unique variations – not passed on by either parent This is now known as mutation. Conclusion variation, natural selection and inheritance limits unfavourable characteristics and leads to the accumulation of more favourable characteristics in a species. Example: Evolution of giraffe Lamark (15-20 years before Darwin) Darwin - food became short on ground giraffes turned to trees for food in the process, they stretched their necks passed on this characteristic to their offspring there was a variation of the neck length in the giraffe population the food was short on the ground (selection pressure) those with longer necks reached the leaves on the trees and survived they reproduced and passed on their genes – for longer necks – to offspring those with shorter necks starved / died – did not reproduce Natural selection depends on the pressure from the environment. These pressures are known as selection pressures and they select the better-adapted variations. Example – sickle-celled anaemia condition where some red blood cells that are abnormal are in the blood the oxygen is not supplied properly to the cells but, sickle cells provide resistance to malaria Mutant gene = ‘h’ Homozygous (HH): advantage in countries without malaria, but disadvantage where malaria is present – may die of malaria Heterozygous (Hh): some sickle cells, but survive anyway. Resistant to malaria Homozygous (hh): sickle cells only. Poor carriage of O2. Die young. Selection pressure here is the disease malaria.