Question 7 of 7 Map Sapling Learning macmillan learning Select the statements th

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Question 7 of 7 Map Sapling Learning macmillan learning Select the statements that accurately describe the effect of natural selection on genetic equilibrium. V t is an evolutionary force that prevents natural populations from being in genetic equilibrium. t can cause allele frequencies in a population to be in genetic equilibrium It can cause an allele to become more abundant in a population if it provides a fitness advantage over other alleles for that trait. t can cause a recessive allele for a gene to become fixed in a population. A Previous Give Up & View Solution Check Answer Next Exit 8 Hint

Explanation / Answer

Natural selection is the process by which the better adapted individuals of a heterogeneous population are favoured over less adapted individuals in a particular environment. Equilibrium is maintained if frequency of all the alleles in a population remain constant. Change in gene frequency leads to evolution. The example of Industrial Melanism shows that if environment changes,then natural selection also operates to select the best adapted individual in chnged environment. The light coloured moth Biston betularia abundant before industrial revolution was replaced by the dark one, Biston carbonaria when the environment became darker after industrial revolution due to soot, smoke etc. The white variety was visible in black environment and was eaten by predators. As natural selection favoured the dark variety, the white allele reduced in frequency and that of allele for dark trait increased. So the first statement is correct.

From the above example, it is clear that the third statement is also correct.

Second statement is wrong.

The fourth statement is also correct. Natural selection fixes a recessive allele. The allele for light bodied moth is recessive and the white colour is produced only in homozygous state while the allele for dark bodied moth is dominant. But in a particular scenario (clean environment) the recessive allele was favoured over the dominant one as the homozygous recessive individuals (light coloured moth) could avoid predation by camouflaging with the environment while their dark counterpart being visible became easy prey and their population dwindled.

Recessive deleterious mutations are normally eleminated from natural population. But in some cases, the mutant recessive allele persists in the population through favoured heterozygous individuals. Take for example, sickle cell anaemia in which the defective haemoglobin carry less oxygen than normal and the red blood cells become sickle shaped .The homozygous individuals ( HbS /HbS, Hb for haemoglobin )  for the sickle cell gene suffer from life threatening anaemia and die at an early age. But the prevalence of the heterozygous ( HbA / HbS ) persons occur in sub-Saharan Africa where deadly Falciparum malaria is common. The heterozygotes can resist the malaria attack because their deflated RBCs act as poor host for the malarial parasite. The normal individuals ( HbA / HbA ) surve as excellent host for the parasite and death occur in most of the cases. Unless the recessive allele is selected, its frequency would have remained low in the population because of its selective disadvantage for those who inherit it and might have been eleminated also with the death of the individuals who carry it . But the situation becomes different in falciparum infested regions. The deleterious recessive mutation is not eleminated, rather favoured ( fixed ) by natural selection.

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