Question

for each of the following examples, discuss whether the observed result is due to neutral mutations or mutations that have been acted on by natural selection, or both: A. When comparing sequences of homologous genes, differences in the coding sequence are most common at the wobble base (i.e., the third base in each codon). B. For a protein-encoding gene, the regions that encode portions of the polypeptide that are vital for structure and function are less likely to display mutations than other regions of the gene. C. When comparing the sequences of homologous genes, introns usually have more sequence differences than exons.

Short answer

A. Neutral mutations because the base wobble doesn't often affect the amino acid sequence. B. This is natural selection, as vital structures are conserved via stabilizing selection. C. These are likely neutral mutations since introns aren't subject to the same selective pressures as exons.

Step-by-step solution

Analyzing the Examples

For each example, look at them in the context of genetic mutations. Use your understanding of coding sequences, mutations that encode for vital structure and function, and exons and introns to ascertain whether the mutations observed are neutral or acted upon by natural selection or both.

Example A

In this case, the wobble base (3rd base of a codon) often doesn't affect the amino acid sequence, because it can be variable due to the degenerate nature of genetic code. Hence, mutations at this position could be mainly neutral.

Example B

The regions in a gene that encode for vital structure and function will likely be conserved, as mutations in these regions would be deleterious and less likely. Mutations are not common in these regions due to stabilizing selection, indicating action of natural selection.

Example C

Introns don't usually code for proteins, so they are not subject to the same selective pressures as exons. Thus, differences between sequences are likely due to neutral mutations.

Key concepts

Natural Selection

Natural selection is a fundamental mechanism of evolution. It is the process by which heritable traits increase an organism's chances of survival and reproduction. These advantageous traits tend to accumulate over generations as individuals with these characteristics are more likely to thrive and pass them on to their offspring.

For instance, in the context of genetic mutations, natural selection will favor those mutations that provide a survival or reproductive advantage to the organism. Mutations that lead to resistance against a disease, an improved ability to obtain nutrients, or enhanced camouflage are examples of beneficial mutations that may be promoted through natural selection.

Coding Sequences

Coding sequences are regions of DNA that contain the genetic information necessary for building proteins. These sequences are transcribed to mRNA and then translated into the corresponding amino acid sequence, which folds into a functional protein.

In the context of mutations, changes in the coding sequences of a gene can lead to alterations in the protein produced. Some mutations do not affect the protein's function, considered neutral mutations, while others can severely affect the organism's phenotype. Such mutations can be disadvantageous, or in some cases advantageous, resulting in a change in an organism’s fitness and possibly subjecting the mutation to natural selection.

Wobble Base

The 'wobble base' refers to the third nucleotide in a codon of mRNA, which through a concept known as the 'wobble hypothesis', is understood to have a certain level of redundancy or 'wobble' when pairing with the corresponding anticodon of tRNA during protein synthesis.

This flexibility allows for mutations at the wobble base to often be 'silent' or synonymous—meaning they do not change the resulting amino acid and thus typically do not impact the protein's function. These neutral mutations are less likely to be affected by natural selection since they do not generally influence the organism's survival or reproduction.

Gene Conservation

Gene conservation refers to the phenomenon where certain gene sequences remain relatively unchanged throughout evolution. This typically happens with genes that are crucial for the basic cellular functions and structures.

As these genes are often essential for survival, any mutations that alter their function can be highly detrimental. Therefore, they are subject to strong purifying or stabilizing selection, which removes disadvantageous variations, resulting in high conservation of these sequences across different species.