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 Champagne with tacos and Yorkshire pudding, but they don't
really belong together. As time passed, exchange of recipes became
difficult between animals that differed substantially in their physical
and behavioural characteristics. Different groups, therefore, became
constrained to work with only a subset of the total possible collection of
recipes.
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.
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