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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The Unit of Selection

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