CDS (II) 2021 General Knowledge · Question 15 of 120

Correct — B, All variations in a species have equal chances of survival. The textbook puts the question and its answer in successive sentences: do all these variations in a species have equal chances of surviving in the environment in which they find themselves? Obviously not. Depending on the nature of the variation, different individuals end up with different advantages, and the example given is bacteria able to withstand heat, which survive a heat wave better than their neighbours. Unequal survival is the whole point — if every variant did equally well there would be nothing for the environment to select and evolution would have no engine. The other three statements are all true as printed. Asexual reproduction copies DNA with only small inaccuracies, so variation there is minimal; changes in genetic constitution are precisely what variation consists of; and selection of variants by environmental factors is described as the basis of evolutionary processes.

• (a)Variation is minimum in asexual reproduction. — True. Some variation does arise even in asexual reproduction, from small inaccuracies in DNA copying, but sexual reproduction generates far more, which is why a field of sugarcane shows so little variety.
• (c)Changes in genetic constitution result in variation. — True, and close to a definition. Heritable variation arises from changes in the genetic material, whether by copying errors or by the reshuffling that sexual reproduction brings.
• (d)Variants can be selected by environmental factors. — True. Selection of variants by environmental factors is stated to be the basis for evolutionary processes; it is the same idea the false option denies.

Reproduction produces individuals that resemble their parents but differ from them in small ways, and those differences are variations. Asexual reproduction yields the least of it, since the offspring are copies apart from occasional DNA-copying errors; sexual reproduction yields far more, because each offspring receives a fresh combination of alleles from two parents. Variation is raw material only. What turns it into evolutionary change is the environment, which lets some variants leave more descendants than others.

Three of the four statements are lifted almost intact from the chapter, and only one has been quietly altered into its opposite, which is the usual shape of a not-true item. The check that works fastest is to ask what would follow if the suspect statement were true: equal survival chances for every variant would leave natural selection with nothing to act on, and the chapter that contains the statement is about how selection drives evolution. It is worth separating two things the option list runs close together. Variation exists whether or not it is useful, and a variation that helps in one environment can be neutral or harmful in another — a heat-tolerant bacterium gains nothing during a cold spell. Survival advantage is a relation between a variant and a particular set of conditions, never a property of the variant alone.

• Variations in a species do not have equal chances of surviving; the environment favours some over others.
• Bacteria that can withstand heat survive a heat wave better than their neighbours — the standard textbook illustration of unequal survival.
• Some variation arises even during asexual reproduction, from small inaccuracies in DNA copying, but sexual reproduction produces far more.
• Selection of variants by environmental factors forms the basis for evolutionary processes.
• A variation that confers an advantage in one environment may be neutral or harmful in another, so advantage is always relative to conditions.

• Reading 'variation is minimum in asexual reproduction' as 'variation is absent'; copying errors still occur.
• Assuming a beneficial variation is beneficial everywhere, rather than in a particular environment.
• Confusing variation, which is the raw material, with selection, which is the process acting on it.

As a not-true statement item on variation and selection, as a cause question on why sexual reproduction generates more variety, or as an application question on drug resistance in bacteria.

• practice — not a real PYQ

  A colony of bacteria survives a sudden rise in temperature that kills most of its members. The survivors are best described as

  • (a)individuals that acquired heat tolerance during the heat wave
  • (b)variants that already carried heat tolerance and were favoured by the environment
  • (c)individuals identical to those that died
  • (d)a new species of bacteria

  Answer(b) variants that already carried heat tolerance and were favoured by the environment — selection acts on variation that is already present.

• practice — not a real PYQ

  Which mode of reproduction generates the greater variation among offspring, and why?

  • (a)Asexual, because DNA copying is error-free
  • (b)Asexual, because only one parent is involved
  • (c)Sexual, because each offspring receives a new combination of alleles from two parents
  • (d)Both produce exactly the same amount of variation

  Answer(c) Sexual, because each offspring receives a new combination of alleles from two parents — asexual reproduction varies only through copying errors.