Transcript Intraspecific variation

Chapter 6
Intraspecific variation
鄭先祐
生態主張者：Ayo 工作室
Intraspecific variation
Genetic mechanisms that affect variation
 Measuring genetic variation within and
among populations
 The interaction of genetic and ecological
process.
 Environmental application
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Intraspecific variation
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Until recently, direct measurement of the amount of
genetic variation in natural populations was difficult.
In the past 20 years, however, the revolution in
molecular biology that has transformed much of
biology has also had an impact on the study of
ecology and evolution.
By applying molecular technique, we can now study
the interactions between ecological processes and
their genetic and evolutionary bases.
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Genetic mechanisms that affect
variation
Ultimately, genetic difference is derived from
mutation of the DNA.
 A second mechanism that maintains
variations is the phenomenon of heterozygote
advantage, or heterosis.
 If the heterozygote has higher fitness than
either homozygote, the heterozygote class
protects deleterious recessives from
elimination by selection. 範例：sickle-cell
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Decreasing the genetic variation
Genetic drift has the effect of decreasing the
total amount of genetic variation in a
population.
 Inbreeding also affects variation. The major
genetic effect of inbreeding is an increase in
homozygosity.
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Inbreeding 的影響
(p2 + 2pg + q2) =1.0
 (p2 + Fpq) + 2pq(1 - F) + (q2 + Fpg) =1.0
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– Where F is the inbreeding coefficient.
基因組成 p 和 q 所佔的比率不變。
 (p2 + Fpq) + (1/2) (2pq)(1-F) = p(p + q) = p
 Homozygosity increases and heterozygosity
decreases, alleles are exposed more directly to
selection, and this can lead to a decrease in
genotypic variation. (Fig. 6.2)
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Measuring genetic variation
within and among populations
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Morphological
variation
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農耕地
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地理變異的範例一
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This significant increase in growth
rate in the ab suggests that growth
rate it plastic.
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地理變異的
範例二
位於高山的，
比較低矮。
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Plants from the high-altitude population(timberline) remain
short in stature regardless of where they were grown.
On the other hand, the lower-elevation populations exhibit
some plasticity in growth, they are dwarfed at high elevation.
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Interaction of genetic and
phenotypic response
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範例：Intertidal snail, Littoria obtusata.
– 生活於浪大的地區，和浪小的地區。
– 是否會被浪捲下去，取決於shell size and shape，
以及the strength of the foot。
– 於浪大的地區生活的snail ，都有較大的foot。
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Trussell (1997) 將生活在不同浪大小地區的
snail 交換。(野外，且於實驗室內模擬)。
– 浪小地區生活的snail轉換至浪大地區，其foot
會轉變大。但是相反的轉換，卻不會縮小。
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有變異，未必然就有基因上的意義
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有型態特徵的變異，未必然就有基因上的基礎。
譬如：seed size is clearly an important component
of the reproductive strategy, we cannot necessarily
assume that seed size is under simple genetic control
such that it can be modified by natural selection.
(Wolfe, 1995)
In the biennial plant in which seed size varies by at
least a factor of 2, Wolfe found that there is no
variation on which natural selection can act to
modify seed size. The seed size is affected by the
light regime.
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Chromosomal
variation
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There is
considerable
geographic
variation in
distribution of the
chromosome types
of 果蠅 D.
psydoobscura.
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The white-footed
mouse can be
divided into two
chromosomal
races,
distinguishable on
the basis of Gbanded
karyotypes. It do
not show clines in
chromosome
types.
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Allozymes
Protein electrophoresis
has been used
extensively to measure
genetic variation.
One advantage is that a
large number of loci can
be sampled
Furthermore, we are
measuring variation at
the level of the cell’s
proteins – one step
closer to the DNA.
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硬骨魚類
Fig. 6.8 Frequencies of polymorphisms for different taxonomic groups.
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Fig. 6.8 Frequencies of polymorphisms for different taxonomic groups.
Bars represent the number of species in the group with that level of
polymorphism for electrophoretic alleles.
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Fig. 6.8 Frequencies of polymorphisms for different taxonomic groups.
Bars represent the number of species in the group with that level of
polymorphism for electrophoretic alleles.
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Fig. 6.9 percent heterozygosity
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2001 individual (H)
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Fig. 6.9 percent heterozygosity
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2001 individual (H)
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Fig. 6.9 Frequencies of heterozygosity values for different
taxonomic groups. Bar represent the number of species in
the group with that proportion of heterozygous alleles.
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• Genetic similarity
IN =  (pix piy) /  (pix2)(piy2)
• genetic distance
Dn = - ln IN
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FIt = FST + (1-FST) FIS
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FST ( variance among groups)
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DNA fingerprinting
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DNA fingerprinting has recently emerged as an
additional technique of genetic analysis that holds
great promise for ecological genetics.
This technique is based on the present in DNA of socalled minisatellites, hypervariable regions that are
composed of short segments containing multiple
copies of a short sequence.
It is thought that minisatellites are noncoding
regions.
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Fig. 6.12 The process of DNA fingerprinting
(a) DNA is isolated from cells and cleaved at specific sites
with an endonuclease.
(b) The sample containing DNA fragments from each
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individual is placed in an elctrophoresis
gel.
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© The DNA fragments are then denatured into single-stranded
segments and transferred to a nylon or nitrocellulose
membrane by a Southern transfer.
(e) A piece of X-ray film is placed over the membrane and
exposed.
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Michondrial DNA
Population biologists have also made use of
unique properties of mitochondrial DNA to
measure genetic variation in natural
populations.
 Mitochondria contain a small set of genes that
are separate from the nuclear genes and thus
do not undergo segregation at meiosis.
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The interaction of genetic and
ecological processes
Three fundamental interactions that affect
variation：
 Selection
 Dispersal
 Population size
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A classic example
land snail, Cepaea nemaralis
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The Intraspecific
variation of the land
snail is a product of
selection for color
matching to avoid
predation.
Colors of the shells
show great variety,
ranging from brown to
yellow to pink, and the
degree of banding also
varies greatly.
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A strong
association between
particular shell
patterns and certain
habitat types.
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Ecotypes
Geographic variants that result from
adaptation to local conditions are called
ecotypes.
 範例： tickle grass (Agrostis scabra) grow at
several sites near Sudbury, Ontrario, that are
contaminated by heavy metals.
 Plants from contaminated sites show higher
tolerance of heavy metals. (Fig. 6.15)
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Seed germination
and root growth in
the presence of
heavy metals was
higher in plants from
contaminated sites
than in plants from
uncontaminated sites
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範例：The wild oat, Avena barbata
Two electrophoretic genotypes associated
with extreme mesic or xeric sites.
 Each genotype is based on the allozymes
present at five loci.
 The mesic and xeric ecotypes can occur on a
much smaller geographic scale (Table 6.6).
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The wild oat, Avena barbata
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Dispersal
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範例：monarch butterfly (nonmigratory and
migratory populations) FST ( variance among groups)
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範例：Pocket gopher (Thomomys bottae)
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The mean values
of FST are
significant and are
considerable
larger than those
for migratory
butterflies.
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範例：whitefooted mouse
Ravines (深谷) and power
line rights-of-way
dissecting forest habitat
act as barriers to dispersal
in the white-footed mouse,
a forest-dwelling mammal.
 (Table 6.8)
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F
( variance among groups)
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variance among groups)
ST (2001
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範例：A perennial plant, the
blazing star (Liatris cylindracea)
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Exhibits genetic subdivision on an extremely fine
spatial scale.
The entire population occupied only 600 square
meters.
It was quite dense– as many as 175 plants per square
meter.
The study population was divided into a series of 66
quadrates, each 3 meters square.
Plants were sampled from these quadrates, and 27
allozymes were analyzed for each plant.
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blazing star
The variance among subpopulations
was significant. The values of FST,
measured on a scale of only 600
square meters,were greater than
those of the monarch butterfly
measured across all of eastern North
American.
Why?
Two factors result in extremely
small dispersal distances: pollinator
behavior and seed dispersal
distances.
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blazing star
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範例：seed dispersal mechanism
of three species of forest herbs
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Sanicular seeds
attach easily and
tighly to
mammalian fur
Cryptotaenia seeds
fall relatively near
the parent plant
Osmorhiza seeds
are intermediate.
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種子可散播最
遠的，其genetic
distance 最低。
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範例：the mangrove (Avicennia marina)
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The potential for longdistance seed dispersal.
This plant produces
seeds that germinate on
the tree and fall into the
water where tides and
currents disperse the
seeds, potentially for
great distances.
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Genetic structuring in
the Indo-West Pacific is
characterized by sharp
discontinuities in allele
frequencies.
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紅樹林的
基因循流
途徑
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範例：sugar maples (Acer saccharum)
The genetic structure of populations in
patches of forest differs from the pattern in
populations in large tracts of forest.
 結果與原本的預期，Patch populations
showed more genotypic variation than did
large tracts of forest.，相反。
 研究者的解釋：the patches of forest are
subject to increased immigrant pollen and
hence gene flow.
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於密度較低的樹林地，其
接觸外來花粉的機會較大。
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Population size
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範例：California vole, a four-year cycle.
During the peak of
the population
cycle, when
dispersal and
extensive gene
flow have
homogenized the
population, FST is
low.
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Small populations and extinction
The small populations may be subject to loss
of genetic variation by drift or inbreeding.
 Isolated populations may not have sufficient
gene glow to maintain genetic variation.
 The loss of variation has three main
detrimental effects:
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– 1. 對環境變動的適應潛能下降
– 2. Inbreeding depression.
– 3. Fixation of new deleterious mutations.
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範例：受到人為影響的生物
marsh gentian (植物)
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In the Netherlands, this species was once common, but
pollution of the soil and habitat loss have resulted in
significant population declines.
Raijman and colleagues (1994) examined allozymes from
populations ranging from 1 to more than 50,000
individuals, they discovered that most of the populations
did not contain any rare alleles, a result that suggests an
overall loss of variation.
The number of polymorphic loci and the average number
of alleles per locus were positively correlated with
population size, confirming the loss of variation in small
populations.
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Fig. 6.24 genetic variation and population size in Gentiana.
(a) proportion of polymorphic loci
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Fig. 6.24 genetic variation and population size in Gentiana.
(b) mean number of alleles per locus
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範例：The northern elephant seal
( bottleneck effect )
One of the most extreme examples of the
effects of very small population size on
genetic variation.
 This species was hunted nearly to extinction
in the 19th century – the population was
reduced to only about 20 individuals.
 Although the species had recovered to more
than 30,000 individuals by the middle of 20th
century, there was no genetic variation at any
of 24-electrophoretic loci analyzed.
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範例：the gray wolf ( Canis lupus)
and the coyote ( C. latrans)
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Restriction analysis of mtDNA indicates that eh
continent-wide coyote population is composed of a
larger number of genotypes (32) than is the wolf
population (18).
Coyote populations are not highly genetically
differentiated; their FST values were not significantly
different from zero.
FST values for the wolf, however, were large (0.76)
and highly significant, which indicates that the
North American population is highly subdivided into
significantly different subpopulations.
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範例：The wild turkey
( Meleagris gallopavo )
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The wild turkey populations were drastically reduced
by the clearing of forest land in the eastern US in the
19th and 20th centuries.
By the 1940s, the eastern populations had been
extirpated from more than 80% of their original range.
The remaining populations inhabit a highly fragmented
forest landscape.
An electrophoretic survey of turkeys revealed
extremely high FST values.
The mean FST value of 0.102 is one of the highest
reported for any avian populations.
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範例：heath hens (prairie chicken)
( Tympanuchus cupido )
Bouzat et al. (1998) extracted and analyzed
DNA from the feather roots of museum
specimens of this species collected before the
population reduction.
 These researchers used a relative new
technique of genetic analysis, microsatellite
DNA, to measure genetic variation.
 This technique is similar to DNA
fingerprinting using minisatellite DNA.
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粗體字，代表於 museum specimens有，但於Illinois
populations 沒有的 allele。 斜體字，代表只有museum
specimens 才有的特殊 allele。
The museum specimens contained nine alleles not currently
present in the Illinois populations.
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範例：the cheetah (Acinonyx jubatus)
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O’Brien and colleagues (1983) surveyed 47 allozyme
systems in samples from populations form South
Africa, they found that the 55 cheetahs in their
samples were monomorphic for all 47 allozymes.
Such extreme genetic uniformity has been found to
include the major histocompatibility complex (MHC).
This situation makes the cheetah extremely
vulnerable to disease or to a change in the
environment that would require selection to operate.
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Environmental application
Subspecies, races, and the
endangered species Act.
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In 1973 the US congress passed a piece of landmark
conservation legislation, the endangered species Act.
This legislation mandated that the Fish and Wildlife
Service determine which species in the US are at
risk of extinction.
When a species is determined to be endangered, the
government is required not only to protect the
remaining individuals but also to develop and
implement a recovery plan designed to move the
species from the brink of extinction.
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保護的範圍包含亞種，甚至是族群
In addition, the Endangered Species Act
recognizes the importance of protecting
distinct subspecies, and even populations, in
order to maintain the naturally occurring
genetic diversity so important to species
survival.
 This requirement has led to some practical
problems in the implementation of the act.
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範例：a unique subspecies of the mountain
lion (Filis concolor coryi)
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Once occupied
much of the
southeastern
US.
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Today this subspecies, is critically endangered.
Less than 40 individuals remain in the swamps of southern Florida.
Recent genetic analysis has revealed that the subspecies is
contaminated with genes from a South American subspecies, a few
individuals of which were released in the late 1950s or 1960s.
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範例：the gray wolf (Canis lupus)
Recent genetic analyses of this species have
shown that it is contaminated by genes from
coytes ( Canis latrans).
 These closely related species have very
different ecological niches, but matings
between them can produce viable, fertile
hybrids.
 因為環境的開發，gray wolf 和 coytes混交
持續增加。 → 是否要從保育類排除？

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 問題與討論！
[email protected]
Ayo 文化站 http://faculty.pccu.edu.tw/~ayo
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