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Cattle breeders have improved the quality of meat over the years by which process? A) artificial selection B) directional selection C) stabilizing selection D) A and B E) A and C

I know the answer is D. It is obvious that is artificial selection but WHY is it also directional selection? how do we know? I know the differences between directional, stabilizing and disruptive selection but there is nothing in the question that explains the quality of meat in relation to directional selection. so HELP!

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To help understand the problem, it might be good to restate the definition of directional selection that you know. What is directional selection? –  A. Kennard Feb 16 at 2:09

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There is a small issue in this question that is: everybody does not always use the exact same definition of stabilizing selection and disruptive selection. These two concepts are sometimes used to refer to phenotypic traits or to genes. For example stabilizing selection might either mean,

  • selection for the mean phenotypic trait of the population or
  • selection for the wildtype allele, getting rid of all newcoming mutations.

Under simple genetic architecture the two concepts are equal.

Although for this question I don't think we even need to give a clear definitions between these two words, we'll use the phenotypic based definition (the first one above) in order to ease the intuitive understanding of the answer.

Let's say the trait breeders select for is mean quality and for ease again, we'll imagine this trait as being one unique trait that we can map on one axis from low quality to high quality. Here is a representation of that where the x-axis represent meat quality (left = low quality) and the y-axis represents the density of cows in the population which has the associated meat quality.

enter image description here

If breeders performed a stabilizing selection, it means that they would not improve the meat but only prevent the meat to change quality. In order to shift the quality of the meat to the right (to better quality), breeders need to perform directional selection.

I think the only problem that you had for solving this problem was the conceptual issue of considering meat quality just like any phenotypic trait. And if you want to shift a phenotypic trait on one side or another, you have to apply directive selection.

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THIS WAS SO CLEAR :) I GET IT NOW!! You are awsome and amazing. Thank you very much. Have a nice day, and thanks again for your time and efforts to construct this solid explanation :) –  hil3ani Feb 17 at 5:01

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