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发帖数: 21627 | 1 http://en.wikipedia.org/wiki/Outbreeding_depression
A concept in selective breeding and zoology, outbreeding depression refers
to cases when offspring from crosses between individuals from different
populations have lower fitness than progeny from crosses between individuals
from the same population. (In selective breeding, the concept is opposed to
inbreeding depression). This phenomenon can occur in two ways.
First, selection in one population might produce a large body size, whereas
in another population small body size might be more advantageous. Gene flow
between these populations may lead to individuals with intermediate body
sizes, which may not be adaptive in either population. It might be that, in
a certain environment, having either a large or a small body is advantageous
, whereas an intermediate-sized body is comparatively disadvantageous.
A second way outbreeding depression can occur is by the breakdown of
biochemical or physiological compatibilities between genes in the different
breeding populations. Within local, isolated breeding populations, alleles
are selected for their positive, overall effects on the local genetic
background. Due to nonadditive gene action, the same genes may have rather
different average effects in different genetic backgrounds--hence, the
potential evolution of locally coadapted gene complexes. However, on the
other hand, according to the overdominance hypothesis in genetics, it is
believed that certain combinations of alleles (which can only be obtained by
outbreeding) are especially advantageous when paired in a heterozygous
individual, which is one explanation for the existence of hybrid vigor.
A third, but neutral, effect of outbreeding is the loss of allopatric
speciation of a particular group, that lends its distinctness and
contributes to the diversity of said types in either group by the exclusive
retention of select traits.
Considering the second manner of outbreeding depression cited above,
individuals from Population A will tend to have alleles selected for the
quality of combining well with allele combinations common in Population A.
However, alleles found in Population A will not have been selected for the
quality of crossing well with alleles common in Population B. Therefore
outbreeding can undermine vitality by reducing positive epistasis and/or
increasing negative epistasis. The sterility and other fitness-reducing
effects often seen in interspecific hybrids (such as mules) can be
considered an extreme case of this type of outbreeding depression, involving
not only different alleles of the same gene (as in distinct populations of
a single species) but even different orthologous genes.
However, it is critical to understand that reduced inbreeding depression in
first generation hybrids can, in some circumstances, be strong enough to
more than make up for outbreeding depression. Because of this and because of
the uniformity and predictable outcome of a first generation hybrid (F1
hybrid) farmers keep purebred strains for the purpose of outcrossing.
Crossing the hybrids will give unpredictable outcomes and outbreeding
depression will remain or worsen so that is not common practice.
As a general rule of thumb, hybrid vigor (another way of saying a reduction
of inbreeding depression) is strongest in first generation hybrids and gets
weaker over time. In contrast, outbreeding depression can be relatively weak
in the first generation. But outside the context of ruthless selective
pressure, outbreeding depression will increase in power through the further
generations as co-adapted gene complexes are broken apart without the
forging of new co-adapted gene complexes to take their place.
It is important to keep in mind that these two mechanisms of outbreeding
depression can be operating at the same time. However, determining which
mechanism is more important in a particular population is very difficult.
生物学上，既有inbreeding depression，又有outbreeding depression
混血只有第一代有优势，后面会逐渐减弱
从长远看，哪一种因素占dominant作用，还是没有办法证明
一个明显的例子，如果混血个个聪明漂亮，那么广大老莫应该是又聪明又漂亮的
因为老莫是杂交最最充分的。但事实上并非如此 |
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