Evolution, Individuals, and Populations 41For the sake of argument, let’s say 35
percent. In other words, your DNA is
over one-third the same as a banana’s.
Yet, of course, there are few ways other
than genetically in which a human could
be shown to be one-third identical to a
banana.
That context may help us to assess
the 98 percent DNA similarity of hu-
mans and chimpanzees. The fact that
our DNA is 98 percent identical to that
of a chimp is not a transcendent state-
ment about our natures, but merely a
decontextualized and culturally inter-
preted datum.
Moreover, the genetic comparison is
misleading because it ignores qualitativedifferences among genomes. Genetic
evolution involves much more than
simply replacing one base with another.
Thus, even among such close relatives
as human and chimpanzee, we find
that the chimp’s genome is estimated
to be about 10 percent larger than the
human’s; that one human chromosome
contains a fusion of two small chimpan-
zee chromosomes; and that the tips of
each chimpanzee chromosome contain
a DNA sequence that is not present in
humans.
In other words, the pattern we en-
counter genetically is actually quite
close to the pattern we encounter ana-
tomically. In spite of the shock the figureof 98 percent may give us, humans are
obviously identifiably different from, as
well as very similar to, chimpanzees.
The apparent paradox is simply a result
of how mundane the apes have become,
and how exotic DNA still is.CHRONICLE OF HIGHER EDUCA-
TION by Marks. Copyright 2000 by
CHRONICLE OF HIGHER EDUCATION,
INC. Reproduced with permission of
CHRONICLE OF HIGHER EDUCATION,
INC. in the format Textbook via
Copyright Clearance Center.Evolution, Individuals,
and Populations
At the level of the individual, the study of genetics shows
how traits are transmitted from parent to offspring, en-
abling a prediction about the chances that any given indi-
vidual will display some phenotypic characteristic. At the
level of the group, the study of genetics takes on additional
significance, revealing how evolutionary processes ac-
count for the diversity of life on earth.
A key concept in genetics is that of the population, or
a group of individuals within which breeding takes place.
Gene pool refers to all the genetic variants possessed by
members of a population. It is within populations that
natural selection takes place, as some members contribute
a disproportionate share of the next generation. Over gen-
erations, the relative proportions of alleles in a population
change (biological evolution) according to the varying
reproductive success of individuals within that popula-
tion. In other words, at the level of population genetics,
evolution can be defined as changes in allele frequencies
in populations. This is also known as microevolution. Four
evolutionary forces—mutation, gene flow, genetic drift,
and natural selection—are responsible for the genetic
changes that underlie the biological variation present in
species today. As we shall see, variation is at the heart of
evolution. These evolutionary forces create and pattern
diversity.
In theory, the characteristics of any given population
should remain stable. For example, generation after gen-
eration, the bullfrogs in a farm pond look much alike,
have the same calls, and exhibit the same behavior when
breeding. The gene pool of the population—the genetic
variation available to that population—appears to remain
stable over time.
Although some alleles may be dominant over others,
recessive alleles are not just lost or destroyed. Statistically,
an individual who is heterozygous for a particular gene
with one dominant (A) and one recessive (a) allele has a
50 percent chance of passing on the dominant allele and
a 50 percent chance of passing on the recessive allele. Even
if another dominant allele masks the presence of the reces-
sive allele in the next generation, the recessive allele none-
theless will continue to be a part of the gene pool.
Because alleles are not “lost” in the process of repro-
duction, the frequency of the different alleles within a
population should remain exactly the same from one gen-
eration to the next in the absence of evolution. In 1908,
the English mathematician G. H. Hardy (1877–1947)
and the German obstetrician W. Weinberg (1862–1937)
worked this idea into a mathematical formula called the
Hardy-Weinberg principle. The principle algebraically
demonstrates that the percentages of individuals homozy-
gous for the dominant allele, homozygous for the reces-
sive allele, and heterozygous will remain the same from
one generation to the next provided that certain speci-
fied conditions are met. Among the conditions are these:
Mating is entirely random; the population is sufficientlypopulation In biology, a group of similar individuals that can
and do interbreed.
gene pool All the genetic variants possessed by members of a
population.
evolution Changes in allele frequencies in populations; also
known as microevolution.
Hardy-Weinberg principle Demonstrates algebraically
that the percentages of individuals that are homozygous for
the dominant allele, homozygous for the recessive allele, and
heterozygous should remain constant from one generation to
the next, provided that certain specified conditions are met.