DIVERSITY
AND THE PUREBRED DOG
(The
Poodle and the Chocolate Cake)
by John Armstrong
The Nature of Diversity
Think of genes as recipes. They
carry the instructions for the various components that go into making up an
organism. Each recipe specifies a particular component, and different
individuals may carry different versions of the same recipe. (In the jargon of
genetics, we say that they carry different alleles of a particular gene.)
Individuals within a population often carry similar or identical recipes, for
example, chocolate cake for a Poodle, lemon cake for a Beagle, and white cake
for a Samoyed. A different canine species might be represented by a fruit cake.
When you consider animals that are quite different, such as frogs and chickens,
you will generally find "homologous" recipes, say for pies or
puddings. Thus, there is more diversity among mammals than among carnivores,
more among the carnivores than among the Canidae, and
more among the Canidae than among the wolf group.
An organism carries a collection of recipes, and the collection
defines the organism. The great diversity in the possible collections of
recipes is the reason for the great diversity in the animal and plant kingdoms.
The more closely related two individuals are, the greater the similarity in
their collections. The number of combinations is huge, and during evolution,
the recipe collection was undoubtedly reshuffled many times. The combinations
that worked well survived and multiplied. Those that did not work quickly died
out. In theory, one may make a meal of
One definition of a species is that members of two different
species bred to each other cannot produce a fertile hybrid. However, a more
modern definition is that two species are geographically, physiologically, or
behaviourally isolated such that they do not normally produce hybrids.
Additionally, they should have features that differ sufficiently to allow them
to be distinguished from each other. The domestic dog, wolf, coyote, and jackal
can all mate with each others (barring size constraints) to produce viable and
fertile hybrids. Yet, they have been considered different species (within the
genus Canis) because they normally live in different
places, behave differently, and can usually be told apart. (Though there has
been a recent move to change Canis
familiaris to a subspecies of Canis lupus.) However, a jackal will
not mate with a dog unless they have been raised together from pups (presumably
due to a learned behavioural difference). Furthermore, no Canis
species can produce a hybrid with a fox. This is not because the kinds of
genetic recipes are greatly different, but because foxes do not share the same
number of chromosomes. (In other words, their recipes are filed under a
different, incompatible system — somewhat akin to filing one under DOS and the
other on a Mac.)
Genetic recipes may get modified when they are passed on. Many of
the modifications will make no noticeable difference, or only a very subtle
one. Some may improve the recipe and others will not. If we are making a
chocolate cake and a critical ingredient is forgotten, or the cake is baked too
long or at the wrong temperature, we end up with a disaster. (If we don't
understand what has gone wrong, we will likely throw out the recipe and look
for a new one.) We may even make deliberate modifications in an attempt to get
a more memorable cake. Among the "chocolate cake" population, there
will be a variety — or diversity — of recipes and, therefore, of cakes.
This, I would say, is a "good" thing. Do we always want
the same
chocolate cake? Surely we will tire of it, and even if we don't, we lose the
pleasure of anticipation. If, for some unforseen
reason, everyone suddenly loses their taste for THE chocolate cake, it will surely go extinct.
To
have the potential for evolution and adaptation, we must risk the possibility
of the bad. That is the "cost."
In a large, naturally breeding population, we will end up with a
number of versions (alleles), some so slightly different that we will never
notice, some perceptibly different (but still functional), and some that just
don't work at all. However, if we remove the diversity we lose the potential
for evolution and for surviving unexpected change. To have the potential for
evolution and adaptation, we must risk the possibility of the bad. Geneticists
call that cost genetic load. This "bad" group
persists because every individual carries two copies of every recipe, and often
having just one "good" copy is enough for normal function. In most
populations, every individual carries a portion of the load — three to five bad
recipes out of several thousand. The load is so well distributed that if two
individuals compare their recipe collections they will generally not have two
copies of the same bad recipe.
Loss of Diversity
Suppose we start a new
population with only six or eight founders. (A number of breeds have started
with that few.) We will get rid of hundreds of bad recipes, but the remaining
dozen or two will be encountered much more frequently. Furthermore, if there
are several good or excellent recipes, the chance of dropping one of these from
the collection grows greater as the number of founders diminishes, and the risk
of losing one remains high as long as the effective population size remains
low. Working with small numbers will inevitably decrease the diversity, simply
because individuals do not pass on their recipes equally to the next generation
and some recipes are accidentally lost. This has the superficially desirable
result of giving a more reproducible phenotype, but at the expense of an
overall reduction in quality, health, and longevity.
If breeders had the ability to recognize each individual recipe and
choose only those that were excellent, breeds could be produced with a small
number of individuals that lacked genetic problems. However, what we see (the
phenotype) is the product of all the recipes and, for the most part, we cannot
distinguish the individual recipes. Moreover, we do not have the option of
selecting recipes individually. When we select an animal for breeding, we are
forced to accept a complete set. Even in those few cases where we now have a
DNA test for a bad recipe (allele), we do not possess the ability to correct or
selectively discarded it. We are therefore forced to work around it, or to
discard the whole collection, with the attendant risk of discarding something
excellent along with it.
The common practice of almost everyone rushing to breed to the
currently-popular male show champion is probably the most significant factor
reducing whatever diversity remains. Consider your own breed (the situation for
most breeds is similar). Can you find one or more males that appear in most
pedigrees? Almost everyone decides they like the recipes of (insert name) — or at least the
ones they can see readily — and abandons other recipes with little thought to
the eventual consequences. In a few generations, almost everyone has a
substantial number of his recipes, though not necessarily his exceptional ones, and many excellent alternatives are very hard to find.
How precious is the individual that comes along with some of the
missing recipes and relatively few of the "popular" collection? Do we
hesitate because there are also a few bad recipes in this alternate collection?
Are we now so accustomed to dealing with the more-popular collection that we
have lost the vision of the "memorable" chocolate cake?
Population Genetics and
the Breeder
What is often called Mendelian genetics deals with the outcome of specific crosses. Population genetics deals with the distribution of alleles in a population and the
effects of mutation, selection, inbreeding, etc., on this distribution. As a
breeder, you are a practicing geneticist. A knowledge
of both Mendelian genetics and population genetics is
critical, not only to your own success, but also to the survival of your breed.
Because many early geneticists believed that there were only two
possible alternatives for a gene — "good" alleles that functioned
normally and "bad" alleles that didn't — they expected to find little
genetic variability in a population. The majority of individuals were expected
to be homozygous for the good allele for most genes.
But with the advent of modern biochemical and molecular tools,
geneticists studying populations found far more variability (diversity) than
they had expected. There are a number of possible reasons for this, and even
the experts are not in total agreement on the most likely reason(s). However,
geneticists have also discovered that populations lacking genetic diversity
often have significant problems and are at greater risk from disease and other
changes in their environment. The conclusion is that genetic diversity is
desirable for the health and long-term survival of a population.
Are purebreds dogs genetically diverse? Some may regard that as a
contradiction in terms. The very concept of creating a breed with
characteristics that are distinctly different from other breeds implies a
certain limitation on diversity. Nevertheless, within the standards for a
breed, diversity should still be possible for genes that do not affect the
essential characteristics that distinguish one breed from another. If, in order
to maintain breed identity, one has to compromise on genes that relate to
general structural soundness, good health, intelligence, and temperament,
perhaps this breed should not exist. However, as long as these essentials are not
compromised, I see no reason why one cannot have different breeds with
different appearances and different talents.
For those genes that establish breed identity, there will be
markedly less variability within a breed than within Canis familiaris
as a whole. The tricky bit is restricting variability for those genes that make
a breed distinctive without sacrificing the variability/diversity that is
necessary for good health and long-term survival of the breed. In many cases,
this has not been achieved, and we are now paying the price in terms of high
incidence of specific genetic diseases and increased susceptibility to other
diseases, reduced litter sizes, reduced lifespan, inability to conceive
naturally, etc.
Why has this happened?
- Many breeds have been
established with too few founders or ones that are already too closely
related.
- The registries (stud
books) are closed for most breeds; therefore you cannot introduce
diversity from outside the existing population.
- Most selective breeding
practices have the effect of reducing the diversity further. In addition,
the wrong things are often selected for.
- Even if the founders
were sufficiently diverse genetically, almost no one knows how their
genetic contributions are distributed among the present day population.
Consequently, breeding is done without regard to conserving these
contributions, which may be of value to the general health and survival of
the breed.
Do we have to accept it as an inevitable consequence of creating a
breed? I don't think we do.
A role for the breed
clubs
Each breed needs a database
with all the breedable animals recorded with all
their ancestors back to the founders. This would most appropriately be the
task of the breed club. Are any actually doing this (outside some of the rare
breeds)? Such a database would enable breeders to identify which
individuals are most likely to carry the genes from each founder. At the
level of the individual breeder, it would enable him/her to make intelligent,
informed choices when selecting mates. Measures might also be considered to
re-balance the breed, in order to ensure that the remaining diversity is more
evenly distributed and that, therefore, there is
less risk of loss. |
Reference
Hartl, D.L. A Primer of Population Genetics,
Sinauer,
Notes
A population is regarded as genetically
diverse if a substantial proportion of the genes are polymorphic. A polymorphic
gene is one for which the most common allele has a frequency of less than 0.95
(95%). Mammals are about 15% polymorphic.
A gene that is not
"polymorphic" is called "monomorphic",
but this does not imply only one allele. Most monomorphic
genes have rare alleles, generally occurring at frequencies below 0.005.
Copyright 1998, 2001 John
B. Armstrong. Reprinted with permission.
It may be reprinted providing it is not altered and appropriate credit
is given.
Return Home