Biology 1001 Spring 2008 (B. Fall), Class notes, topic #11—Genetic variation in populations;
mutation as the ultimate source of variation.
complete the reading assignments in your text (Freeman, Biological Science, 2
ed.): pp. 264-
265, 318-323, 355-357, 433-436. View the three assigned segments of the NOVA video, Cracking the Code of
Life (part V.E. below).
Appreciate some ways that scientists use to detect genetic variation in populations.
Distinguish the effects on phenotype of different kinds of point mutations, and understand why
these differences occur.
Appreciate how and where mutations occur, and that only germ-line mutations have evolutionary
Appreciate that mutations are random, and what random means in this context.
of evolution. Without such variation there would be no
natural selection and no evolution. How much variation is there is populations, and where does it
refers to the existence of two or more discrete, phenotypically different variants
) in the same population at the same time (e.g., light and dark peppered moths; human
A-B-O blood groups). A polymorphic gene locus is one with two or more alleles in a population.
Two different models for the nature of genetic variation in populations are:
The former “classical” and now
view was that populations are invariant (i.e.,
monomorphic) for most characteristics, and variants arise only as rare mutations.
The current, modern view is that populations have abundant genetic variation and many genes
are polymorphic (i.e., have two or more forms or alleles). Advances in molecular biology
have resulted in this change in understanding of variation.
is a standard technique used to separate macromolecules (including
nucleic acid fragments and proteins). Since the 1960s, electrophoresis has been used to detect
differences in gene products (enzyme proteins) without actually determining their specific
amino acid sequences (primary structure). From electrophoresis studies, the general
conclusion is that polymorphic genes are common in most populations.
More recently it has become feasible (and now routine) to determine
the actual genetic
makeup of genes—the specific sequence of bases (A, T, C, G) in the DNA. This is the most
direct way of determining genetic variation in populations. Within the last decade,
sequencing techniques have improved dramatically in speed and decreased in cost.
Human Genome Project (HGP)
, a collaborative effort started in 1990 and involving
thousands of scientists, reached a milestone in 2001 when the draft sequence of the entire
human genome was published [
= all the genetic material in a haploid set of
chromosomes of an individual].