Biology Reference
In-Depth Information
(i) Most polymorphic variation within a species and most amino acid substitu-
tions between species are likely to be neutral with respect to selection. This
is not to say that variant alleles have no biological function or phenotypic
effect, merely that the alleles are indistinguishable (neutral) in terms of
their function.
(ii) The amino acid substitution rate can vary between genes and this is likely
to be related to the nature and extent of structural and functional con-
straints upon the different protein products.
(iii) Those parts of a protein with the fewest functional constraints will evolve
most rapidly. Conversely, those regions of a protein which are functionally
important will be those that are most highly conserved evolutionarily.
(iv) Genes whose protein products have a stable function manifest a
molecular
clock
i.e. the amino acid substitution rate is similar for a given gene in dif-
ferent evolutionary lineages.
(v) A high level of genetic polymorphism will exist and this is predicted to
increase with population size since the probability of fixation of a specific
allele is lower in large populations.
(vi) In coding sequences, nucleotides in the third codon position evolve faster
than the first two positions. The proportion of substitutions in the third
position should be higher for proteins that are evolving more slowly.
(vii) Any change in the function of a gene sequence will serve to alter the pro-
portion of amino acid substitutions that are neutral. Genes that are no
longer capable of expression (pseudogenes) will accumulate mutations
rapidly (
Figure 7.1
) and are destined eventually to lose recognizable homol-
ogy to the original sequence.
(viii) Sequences or portions of sequences that are evolving faster than the neutral
rate for the species are likely to have been subject to positive selection.
(ix)
Genes manifesting a high rate of variation should also evolve at a higher
rate.
5
4
3
2
1
0
Gene regions
Pseudogenes
Figure 7.1.
Average rates of substitution in different parts of genes and in pseudogenes
(Li and Graur, 1991).
