J. Biomedical Science and Engineering, 2010, 3, 109-113 JBiSE
doi:10.4236/jbise.2010.32016 Published Online February 2010 (http://www.SciRP.org/journal/jbise/).
Published Online February 2010 in SciRes. http://www.scirp.org/journal/jbise
Novel method for discerning the action of selection during
evolution
Ming Yang1, Ada Solidar2, Gerald J. Wyckoff1
1Division Molecular Biology and Biochemistry, University of Missouri-Kansas City, Kansas City, Missouri, USA
2VaSSA Informatics, LLC Kansas City, Missouri, USA
Received 10 October 2009; revised 10 December 2009; accepted 15 December 2009.
ABSTRACT
A common problem in molecular comparative geno-
mics is the identification of genes that are under posi-
tive, adaptive selection [1]. Such genes are likely to be
crucial for speciation, species differentiation, and func-
tional specialization. However, discerning the differ-
ence between positive selection and relaxation of func-
tional constraint can be difficult using current meth-
ods. Both processes generally increase the rate of ami-
no acid change relative to synonymous changes within
coding regions, and unless the amino acid rate is over-
whelmingly high across an entire gene, the signature
of positive selection can be obscured [2]. Some meth-
odologies do not explicitly determine the difference be-
tween a relaxation of functional constraint and posi-
tive selection, leaving researchers to determine via other
means whether the trajectory of a gene has been spe-
cialization or creation of a new function, or removal
from the genome via a process of degeneration.
Keywords: Utilizing Information Theory; Action of
Selection during Evolution
1. INTRODUCTION
Most current methods evaluate the possibility of positive
selection based on the exchangeabilities of amino acids.
The rationale is that if an observed amino acid substitu-
tion has a low probability in terms of their amino acids’
physio-chemical properties, then it is more probable that
the substitution may be driven by selection events. There
are several kinds of matrices that can be used to evaluate
the probability of substitutions. Function, charge, and
amino acid structural properties (via Karlin and
Ghandour [3]) and genetic and structural similarity
(from Feng et al., [4]) are common methods. However,
Dayhoff's PAM-250 matrix is easily the most common.
Based on evolutionary distance measures from a 1,572
amino acid change data set in 71 closely related proteins,
PAM stands for “percent-accepted matrix” [5]. It set the
path for most matrices to come.
Henikoff and Henikoff proposed the BLOSUM (BLO-
cks of Amino Acid Substitution Matrix) matrices based
on a large number of proteins to get a better measure of
differences between two proteins specifically for more
distantly related proteins [6]. To create the matrices, the
BLOCKS database was searched for ungapped, highly
conserved protein domains within protein families and
amino acid frequency substitutions were determined,
scaled by relative amino acid frequency. [7] They then
calculated a log- odds score for each of the 210 possible
substitutions of the 20 standard amino acids. BLOSUM
was designed for search algorithms when relatively close
protein relationships are being examined, such as
FASTA and BLAST. However, this work set the stage
for other research looking at more fine-grained matrices
for evolutionary comparison and ultimately led to the
work described in this paper.
Contrasting both PAM and BLOSUM matrices that
are based on amino acids, Tang and co-workers proposed
a universal evolutionary index (EI) for amino acid chan-
ges based on the genetic code [8]. The EIs are defined as
the observed/expected amino acid changes based on the
transition and transversion rate between related codons.
The high correlations between EIs derived from genes
with various functions in divergent species suggest that
the amino acid properties are strong determinants of
their substitution patterns. The EIs can be used to clas-
sify proteins based on their exchangeability and detect
the positive selection in each of the groups.
There is another category of methodologies that are
based on the sequence information content at specific
sites. In an alignment of DNA or amino acid sequences,
the information content for each position is calculated
based on the distribution of the variations at that site,
and they are measured in bits [9]. The information con-
tents are smaller for divergent sites and larger for con-
served sites. It can therefore be thought of as giving a
measure of the tolerance for substitutions at the position:
higher information content indicates that the site can

M. Yang et al. / J. Biomedical Science and Engineering 3 (2010) 109-113
110
tolerate less replacements and so is more conserved, and a
lower information content in a site means it can tolerate
more substitutions and has been subjected to more muta-
tions. Sequence LOGOS are graphical representations of
sequence alignments [10]. Each LOGO consists of
“stacks” of nucleotide or amino acid symbols, with the
overall height of the stack representative of the “total”
information content at that position. The height of each
symbol corresponds to the relative contribution to in-
formation content of each symbol at that position within
the alignment.
Although the above methods are useful, only one evo-
lutionary variable is examined. Further sequence logos,
though useful, are essentially graphic methods of illus-
trating the sequence conservation for the sites in an align-
ment, but not for the each individual sequence. Given the
above problems, we are aiming at utilizing two indepen-
dent parameters to access the nature of the amino acid
substitutions more reliably. As Tang’s EI is included, ano-
ther parameter should be evolution-independent; protein
structure for example, however the structure data are ex-
pensive to collect and there is no proven methods to jus-
tify the differences. Linear sequence complexity is a pro-
mising parameter as the technique is inexpensive and can
be quantified for comparison across a wide range of data
types.
We argue that information theory allows us to deter-
mine the gain or loss of entropy within a sequence mar-
ried to evolutionary methodologies that look at the like-
lihood of amino acid change and rate changes allow us to
determine whether a gene is evolving in an essentially
neutral fashion, whether it is specializing it’s function,
likely gaining a new function, or heading towards non-
functionalization. While information theory has been ap-
plied to non-coding regions to examine transcription fac-
tor binding sites and regulatory elements and to coding
regions to examine intron/exon boundaries and alterna-
tive protein splicing [11-13], its application to comparative
genomics in combination with other proven methodologies
yields an interesting analysis tool for further study.
2. METHODS
2.1. Universal Evolutionary Index
For each pair-wise protein alignment, we adopted the
universal evolutionary index to quantify the likelihood
of the amino acid substitution [8]. This index is a uni-
versal ranking of the likelihood of amino acid change
and was proposed based upon the high correlation of EIs
from different sets of genes of different taxa. Comparing
with other indexes, the universal evolutionary index is
scaled such that its weighted average is 1, and it is easy
for comparison and can be adjusted to specific species
by multiplying the average Ka/Ks ratios of the given
dataset.
2.2. Information Content Analysis
We adopted the program VaSSA program from VaSSA
Informatics, LLC to examine linear information content.
The change of information content in aligned sequences
is checked and their functional meaning is accessed. The
sequence subsections with fixed size are scanned across,
and their linear information content is measured. The con-
tribution of each single position in the sequence to the
total information content of the sequence is evaluated.
Information content, in this specific example, is es-
sentially a measure of the entropy rate of a particular
sequence (vis a vis Shannon [14]); that is the measure of
the ability to compress the sequence via some encoding.
In our usage, this measure is then normalized by the
channel carrier capacity of the sequence; that is, given
the lexicon and it’s representation, how complex could
the sequence be if the symbols were arranged such that
they were minimally subject to compression. Formally,
the channel carrier capacity is the limiting rate for infor-
mation transmittal in the medium. While over an entire
genome, this rate can be calculated and would be rela-
tively fixed, it varies within the genome based on codon
usage, representation of the lexicon, and other factors
(such as rate of duplication). In this case, then, we’re
examining what the local channel (gene or locus) could
have carried across evolution vs. what the observed en-
tropy rate within that channel is at a given point in time.
This is rather different than standard definitions of bit-
wise information content used in LOGOS and BLOCKS
(and other usage), as in those cases information is said to
be transmitted across species and the measure of the data
transmittal rate is measured as a function of the frequency
of inter-species change for a particular point within a
sequence.
By combining the information content and universal
evolutionary index, we can examine each amino acid
change between sequences and plots them on a two-axis
chart (Figure 1); the chart is broken into quadrants, and
where the majority of amino acid changes sit within the
chart determines the likely evolutionary pressures acting
upon a gene. This quadrants division is based on an em-
pirical study that shows the sequences without functions
(e.g. introns, intergenic sequences) are less complicated
than the functional ones. Thus an unlikely amino acid
change (low EI) that increases the complexity of the se-
quence (positive information content change) is more
probable to be driven under positive selection; similarly
an unlikely amino acid change (low EI) that decreases
the complexity of the sequence (negative information
content change) is more probable to be driven under
non-functionalization of the protein. A likely (high EI)
amino acid change is within the constraint. Positive in-
formation change may indicate it is within functional con-

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111
Figure 1. Functional sorting of the amino acid substitution using
the information content and evolutionary index.
Figure 2. The information content of the 28 zinc finger pro-
teins.
straint, while a negative one may hint the protein is los-
ing some of the unrelated functions.
3. RESULTS
Zinc finger proteins are a group of protein families clas-
sified based upon their conserved sequence motif, and
they are capable of binding DNA, RNA, protein and/or
lipid substrates following their coordination with one or
more zinc atoms [15-17]. The primary amino acid se-
quences, the folding, the number of fingers and their
spatial arrangement jointly determine the protein’s bind-
ing properties. Among the many zinc finger families
with various binding modes and unique functions, the
Cys2/His2(C2H2) zinc fingers were the first group to be
characterized [18,19]. This subset of zinc finger proteins
plays pivotal roles in DNA transcription and develop-
ment in organisms. About 400 C2H2 zinc finger proteins
known exist in humans, which makes them one of the
largest protein families in animals. The C2H2 zinc fin-
gers are identified by their conserved sequence motif
(CX2–4FX8HX3–5H). A zinc atom can be coordinated
with the two cysteines and two histidines within the mo-
tif to form a compact structure that can bind sequence
specifically to DNA in its major groove. More recently,
Laity and co-workers [20] found a sub-set of C2H2 zinc
fingers that contains two interacting fingers, and they are
evolutionarily distinct. We performed an evolutionary
analysis of these data using our information theory based
methods.
The data set contains 28 interacting two-finger C2H2
zinc fingers, and there is a conserved tryptophan be-
tween the first two Cysteine residues of the proteins. The
information content of each sequence is calculated using
VaSSA (Figure 2). To illustrate the site-specific change
of information content, we align the DNA sequence 26
and 27 based on their peptide alignment (Figure 3), and
calculate their information content (Figure 4).
The site-specific comparison of the proteins is plotted

M. Yang et al. / J. Biomedical Science and Engineering 3 (2010) 109-113
112
Figure 3. The DNA alignment of the zinc finger sequence 26 and 27 from Figure 2.
Figure 4. Change in information content along and between two sequences, the gaps show
as “no information”.
in Figure 4. The comparison is directional with se-
quence 26 as the basis for comparison. The sequence 26
has been posited to have a regulatory function through
the interaction between fingers mediated by zinc con-
centration. For sequence 26, the pattern of changes is
versatile: there are some regions of likely changes tied to
a gain in information content, several regions of unlikely
changes with an information content gain, and a few
areas of unlikely changes with loss of information. The-
se various combinations of the substitution likelihood
and the change of information content may indicate the
different regions of the proteins are under different kinds
of evolutionary effects.
Using information content change between sequences
as an evolution-independent variable will allow us to
determine what factors drove sequence diversification.
The method is highly reliable but data intensive, which
is not currently an obstacle as the program can run in
distributed mode across a cluster computer. While this
paper only explores nucleotide data, we have adapted
this method to protein data as well through the addition
of protein semantics. In addition, we have developed this
methodology further for organic molecules in general
using an alternative lexicon. Our overall goal is to allow
evolutionary methods to work in conjunction with in-
formation content measures within proteomics without
the need of making evolutionary conclusions based on
nucleotide sequences.
4. DISCUSSION
We here propose a novel method for fine mapping dif-
ferent evolutionary effects within proteins by simulta-
neously checking two independent parameters. This is
promising to solve the classical problem in evolutionary
studies: the difficulty of distinguishing the relaxation of
functional constraints and positive selection. This met-
hod is currently in testing and development with over
50,000 protein domains for stability. The broad applica-
bility of this method for coding region and non-coding
region genomic analysis is being tested, and proteomic
analysis and integration with polymorphism scoring
pipelines is being developed.

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113
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