some advantage in being second because those males contribute to the other 25%,
but subsequent males receive no benefit. What advantage is this mating with several
males to the female? Since males are intolerant of juveniles that are not their own
offspring, is this a tactic of females aimed at ensuring the cooperation of all surrounding
males?
Waterbuck (Kobus defassa) in Africa defend territories through which female herds
pass while grazing. The male mates with any estrous females when the female herd
is in his territory. He also has to defend his territory against other territorial males,
and bachelor males that have no territory. In some areas a territorial male allows one
other male into his territory (Wirtz 1982). What advantage is there to the territory
holder in allowing the second male in? One suggestion is that the second male helps
to defend the territory and so allows more opportunities for the primary male to mate.
In return the second male may be able to “steal” some matings when the primary
male is occupied elsewhere. A similar situation is described for long-tailed manakins
(Chiroxiphia linearis), a small neotropical bird. Two males defend a territory on a
lek, one being dominant and obtaining almost all matings. The unrelated subordin-
ate may benefit by inheriting the territory and obtaining a few matings in the mean-
time (McDonald and Potts 1994).
So far we do not have the answers to most of these questions. To obtain the answers
we must identify the individual parents of offspring. Recent genetic techniques have
allowed us to do this.Allozyme gel electrophoresis
Until recently the standard technique for detecting genetic variability within and between
individuals was to measure differences in amino acid composition of allozymes or
proteins encoded by different alleles at a locus. Blood or tissue homogenate from
individuals is placed on a gel matrix, such as cellulose acetate, and an electric charge
applied. The proteins migrate along the gel at rates dependent on their total electric
charge. Changes in amino acid composition, the result of mutation, are often
reflected as changes in electric charge. The electric current is switched off and the
gel is stained for a particular protein after a set time. Differences between indi-
viduals are evident as different configurations of the protein bands on the gel.
The method has been used to measure differences between races and between species
by assessing the variability in many proteins from several individuals in each popu-
lation. It is useful because inheritance patterns are generally known. Phylogenetic
trees have been constructed by this method.
The technique has several limitations. First, some proteins with different muta-
tions can move at the same rate, thereby appearing to be the same. This problem
becomes greater the more distant the relationship between individuals or species. Second,
much of the genetic variability is not evident at the protein level because of the redun-
dant nature of the genetic code. Other techniques assess the genetic diversity pre-
sent in the individual’s DNA itself. We examine these next.Polymerase chain reaction
Taxonomy, population genetics, and molecular ecology have advanced rapidly as
a result of a technique called the polymerase chain reaction(PCR). This allows
millions of copies of a particular target sequence of DNA to be produced so that DNA
amplification can now be used easily to identify individuals or groups of organisms.28 Chapter 3
3.7.1Methods for
studying genetic
variability