NATURAL SELECTION NATURAL SELECTION Natural Selection The process in which, when there is inherited variation in a population of organisms, the variants best suited for growth and reproduction in a given environment contribute disproportionately to future generations . Can be lethal · Results in change in allele frequency from generation to generation · Natural selection may be inefficient at getting rid of a deleterious allele · Based on allele's impact on survival and reproduction of individuals o If there is a recessive lethal mutation, b ... · Non-beneficial mutations will be eliminated (have no long-term impacto Natural selection does not act against heterozygotes, Bb Only acts against homozygous recessive individuals, bb · Beneficial mutations may result in adaptation to the environment · But deleterious alleles are rare and recessive Positive Selection Natural selection that increases frequency of a favourable allele Negative Selection Natural selection that decreases frequency of a deleterious allele The phrase that best describes how natural selection results in evolution ... Individuals with particular traits leave behind more offspring than individuals with other traits! · Most mutations to functional genes are deleterious · Expected frequency of homozygotes in a population under the Hardy-Weinberg equilibrium is the square of the frequency of the population! ADAPTATION & FITNESS Fitness Measure of the extent to which the individual's genotype is represented in the next generation · An organism that is better adapted to its environment is more fit · Natural selection acts over generations to increase overall fitness of a population (should a trait remain advantageous) [CASE STUDY] MALARIA Balancing Selection Natural selection that acts to maintain 2 or more alleles of a given gene in a population · Allows alleles to maintain at intermediate frequency (not totally eliminated or fixed) · e.g .: Members of a species that face different conditions depending on their habitat - some in wet and some in dry areas - so different alleles are favoured Heterozygote Advantage Form of balancing selection in which the heterozygote's fitness is higher than either of homozygotes, resulting in
selection that ensures that both alleles remain in the population at intermediate frequencies · A = Allele that codes for normal haemoglobin, resulting in fully functional, round RBCs · S = Allele that codes for abnormal haemoglobin, resulting in sickled RBCs · SS Homozygotes ® Protected against malaria BUT have sickle cell anaemia · AA Homozygotes ® No sickle cell anaemia BUT susceptible to malaria · SA Heterozygotes ® No sickle cell anaemia AND some protection from malaria