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8/10/2019 The Unit of Selection
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THE UNIT OFSELECTION
Part 3 Adaptation and Natural Selection
Fathan Hadyan R
3415106786
Rahmat Fadrikal
3415096616
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a en es ene rom eadaptations produced by
selection? It is a familiar idea that life can be divided intoa series of levels of organization : nucleotide
to gene, through cell, organ, and organism, to
social group, species, and higher levels In some cases, what benefits an organism
may not also benefit its species,
in these cases the evolutionary biologistneeds to know which level natural selection
most directly benefits
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If we are seeking to understand why an
adaptation evolved, we need to know what
entities adaptations in general evolve for the
benefit of.
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Lions often hunt alone, but they can improve theirchance of success by hunting in a group
When prey have been detected, a wildebeest herdperhaps, the lions start to stalk towards them. Asthey get close, they take different routes, somegoing on straight ahead, and some to the sides, sothe prey herd is approached by lions stalking them
from different directions. . . . Eventually one liongets close enough to make a rush at a wildebeest,or else a lion is detected by the prey Bertram(1978).
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The cooperative behavior of the hunting party ishere an adaptation for catching food, but it is notthe lions only feeding adaptation. The lionsmuscular jaws and limbs, their teeth and five
senses, all contribute to the success of the hunt
Each time a hunt is successful, there will be asmall incremental increase in the species chanceof survival, or avoiding extinction.
The survival probability will also be increased, ifby a smaller amount, for the genus Felis and thecat family Felidae.
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The beneficial effect of the hunt spreads
downwards as well as up from the individual
lion
As the survival of the lion is increased, so isthe survival of its constituents ( the organs,
cells, proteins, and genes)
The levels of organization, from gene throughindividual lion to Felidae, are to a large extent
bound together in their evolutionary fate, and
what benefits one level will usually also benefit
the others
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However, this is not always so. Male lions can
only join a pride by forcibly evicting the
incumbent males. In the fight, lions may get
killed or wounded, and in any case lions havea low rate of survival after they have been
evicted from a pride
The survival of the species may be little
affected by the death of male lions, because
the mating system is polygynous and has
plenty of males to spare; but the effect is
clearly not positive
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the levels of living organization are bound
together, if natural selection produces an
adaptation to benefit one level, many other
levels will benefit as a consequence if it benefits one level, which is it?
What is the unit of selection?
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Natural selection has produced
adaptations that benefit variouslevels of organization
Segregation distortionbenefits one gene at
the expense of its allele
Selection may sometimes favor some cell
lines relative to other cell lines in the same
body Natural selection has produced many
adaptations to benefit organisms
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Natural selection has produced
adaptations that benefit various
levels of organization
Natural selection working on groups of closegenetic relatives is called kin selection
Whether group selection ever producesadaptations for thebenefit of groups has beencontroversial, though most biologists now think itis only a weak force in evolution
Which level in the hierarchy of organization levelswill evolve adaptations is controlled by which levelshows heritability
t t t t
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egregat on stort on ene tsone gene at the expense of its
allele With normal Mendelian segregation at a
genetic locus, on average half of an
organisms offspring inherit one of the alleles
and the other half the other allele.
Mendelian segregation is so to speak fair in
its treatment of genes: genes emerge from
Mendelian segregation in the sameproportions as they went in
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some curious cases in which Mendels laws
are broken in which one of the alleles, instead
of being inherited by 50% of a heterozygotes
offspring, is consistently overrepresented The segregation distorter gene of Drosophila
melanogaster is an example of this
phenomenon, which is also called meiotic
drive.
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A heterozygote for the segregation distorter is
then sd/+. The majority (90% or more) of offspring
from male heterozygotes have the sd gene
because the sperm containing the + gene fail todevelop. Female heterozygotes have normal
Mendelian segregation.
A segregation distorter gene can have a great
selective advantage. The allele that gets into morethan half of a heterozygotes offspring will
automatically increase in frequency and should
spread through the population.
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Segregation distortion sets up an interestingselection pressure in the rest of the genome.On average, all other genes at other loci suffera disadvantage because of segregationdistortion.
When selection acts in conflicting ways ondifferent genes in the same individual body, it
is called intragenomic conflict. The sd/+ fruitflyhas intragenomic conflict, because selectionon the sd gene favors segregation distortionand selection on other genes favors restoring
normal segregation.
S l ti ti f
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Selection may sometimes favor
some cell lines relative to other
cell lines in the same body In organisms like ourselves, a new individual
develops from an initial single-cell stage and
that single cell derives from a special cell line,the germ line, in its parents
In a Weismannist organism, most cell lines
(the soma) inevitably die when the organism
dies; reproduction is concentrated in aseparate germ line of cells.
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The separation of the germ line limits the
possibilities for selection at the suborganismic
level, between cell lines. One cell may mutate
and become able to out-reproduce other celllines and (like a cancer) proliferate through the
body.
Any somatic cell line comes to an end with the
organisms death. For this reason, cell
selection is not important in species like
ourselves.
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However, Buss (1987) pointed out thatWeismannist development is relativelyexceptional among multicellular organisms
We tend to think of it as usual becausevertebrates, as well as the more familiarinvertebrates like arthropods, develop in aWeismannist manner. However, more than half
the taxa listed in Table 11.1 have the capacityfor somatic embryogenesis : a new generationmay be formed from cells other than those inspecialized reproductive organs.
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Tabel 11.1 s: somatic ; p:preformationistic ;
e:epgenetic ; u:unknown (developemental
mode)
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In a Weismannist species, that cell line will diewhen the organism dies and any selectionbetween cell lines will be unimportant. However, ifany cell line in the body has some chance of
giving rise to the next organismal generation, themutant cell line would increase its chance of beingin an offspring and be favored by selection.
Whitham & Slobodchikoff (1981) argued that in
plants selection between cell lines enables theindividual to adapt to local conditions more rapidlythan would be possible with strictly Weismannistinheritance
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The process is at present more of a theoretical
possibility than a confirmed empirical fact, but
it may well be important in non-Weismannist
species
a ura se ec on as pro uce
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a ura se ec on as pro ucemany adaptations to benefit
organisms the beaks of the woodpecker
the Galpagos finches
the peppered moth Biston betularia
mimetic butterflies
are all adaptations that benefit the individual
organism. It can hardly be doubted, therefore,that organismal adaptations exist, and naturalselectioncan favor them.
N t l l ti ki
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Natural selection working on
groups of close genetic relatives is
called kin selection In species in which individuals sometimes
meet one another, such as in social groups,
individuals may be able to influence each
others reproduction
Altruistic behavior often takes place between
genetic relatives, and when it does the most
likely explanation is the theory of kin selection. Under what condition will natural selection
favor altruism?
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The altruist still pays a cost of c for performing
the act, and the recipient receives a benefit b
However, the chance that the altruistic gene is
in the recipient is r. When rb exceeds c therewill be a net increase in the average fitness of
the altruists
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The condition for natural selection to favor
altruism among relatives is that it should be
performed if :
rb > c This is the theory of kin selection. It states that
an individual is selected to behave altruistically
provided that rb > c. The condition itself iscalled Hamiltons rule
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Whether group selection ever
produces adaptations for the
benefit of groups
A group adaptation is a property of a group of
organisms that benefits the survival andreproduction of the group as a whole
group adaptations that did not evolve by kin
selection
Many characters are beneficial at the group
level, but also benefit all the individuals in the
group
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The controversial group adaptations are those
that benefit the group but not the individual
A hypothetical example is Wynne-Edwards
theory, put forward inAnimal Dispersion inRelation to Social Behaviour (1962), that
animals restrain their reproduction in order not
to overeat the local food supply.
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Many models of group and individual selection
exist, but they can mainly be reduced to a
common form (Figure 11.1).
The groups are supposed to occupy patches innature. As before, some patches are occupied by
altruisticand others by selfishgroups. There are
also emptypatches. A selfish group in the model
drives itself extinct by overeating its patchsresources. The result of the model depends on
whether a selfish group can infect an empty or
altristic patch before going extinct.
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Figure 11.1
Maynard Smithsformulation of groupselection models. A
patch may changestate in the directionof the arrows.Redrawn, bypermission of the
publisher, fromMaynard Smith(1976). 1976University of ChicagoPress
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Maynard Smith
(1976) defines the number m as the number of
successful migrants produced by one selfish
group on average between its origin andextinction. (Successful means that the migrant
establishes itself in another group and breeds.) If
m= 1the system will be stable;
if m < 1 the selfish groups decrease in number,and if m > 1 they increase. In other words, a
selfish group only needs to produce more than
one successful emigrant during its existence for
the selfish trait to take over
Which level in the hierarch y o f organization
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Which level in the hierarch y o f organization
levels w i l l evo lve adaptat ion s is con trol led by
which level show s her itab i l ity
Most adaptations appear to benefit the
individual organism
Heritability is the key to which levels of
organization show adaptations
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