EVOLUTION Fall 2004: Review Key 1
This
answer key was produced using the answers from the highest scoring tests. Some answers are brilliant, others simply
adequate. But since there is no
cumulative exam, I felt that it was not necessary to belabor minutia.
a. Autosome: a
non-sex-determining chromosome.
b. Background
selection: the idea that a mutation’s chance of being spread is dependent on
the overall quality of the genome it arises in—a good mutation can kept from spreading due to being stuck in a low-fitness
genome. This is more important in
asexual lineages because in sexual lineages recombination can to some extent
overcome this pressure.
c. Background
trapping: the idea that if a beneficial mutation arises in the living dead, it
will not spread to fixation in a population.
d. Clonal
interference: different beneficial mutations in different asexual lineages
cannot combine, and so the beneficial mutations compete against each other to
spread themselves, instead of combining as they would likely do in sexually
reproducing lineages.
e. Coadapted
gene complex: genes that work together.
They may or may not be closely linked.
Recombination often shuffles these genes,
positive epistasis wants to keep these genes together.
f. Cost of
males: in sexually reproducing species, males cannot give birth, but make up
half the population, thereby cutting sheer number of those reproduced by
one-half
g. Cost of
meiosis: in meiosis only half of an organism’s genome ends up in the
gamete. Therefore, the offspring will
only have .5 related to a parent, rather than the 1.0 relationship that an
asexual lineage provides.
h. Eusocial: a
type of social animal (such as honeybees, ants, and arguably naked mole rats)
in which organisms are split into castes, one caste of which (the workers) do
not reproduce. This is frequently
explained by haplodiploidy or other mechanisms that make it more advantageous
for a worker to care for the breeder’s offspring than
to produce her own.
i.
Frequency-dependent selection: selection that varies based on frequency of a
given trait. For example, in sex ratios,
when frequency of males becomes high, selection favors females that produce
females, when the females ratio becomes high, it
favors females that produce males. This
can help to keep genetic variation in a species.
j. Genic
capture: in the production of a sexually selected trait, a large proportion of
the genome is involved; thus the phenotype displayed is an accurate
representation of the entire genome of the individual.
k. Genomic
imprinting: the non-Mendelian process in which it matters which parent gave you
a gene. The copy of the gene the other parent passed on to you is turned off.
l. Good genes
hypothesis: Sexual selection for “good genes” by a female. Female selects male based on overall
phenotype, with the effect that her progeny will possess those traits which
have made the parner that she selected successful.
m.
Haplodiploidy: in some organisms one sex (generally the male) is haploid,
arising from an unfertilized egg, while the other sex is diploid. In eusocial animals this can play into power
conflicts and sex ratio conflicts.
o. Lek paradox:
persistent female choice for particular male trait values should erode genetic
variance in male traits and thereby remove the benefits of choice; and yet
choice persists.
p. Linkage
disequilibrium: the statistical tendency for alleles at different loci to occur
together, thus departing from the frequency of combinations that would be
expected from the overall frequencies of the alleles themselves in the
popualtion
q. Living dead:
the portion of an asexual population whose accumulated deleterious mutations
doom it to eventual dying out as selective sweep brings the advantageous
genomes to fixation. Even if an
advantageous mutation arises in this population, it will die out because of
background selection.
r. Meiotic
drive: the tendency of a certain gene to show up more frequently than would be
expected, indicative of some selfish genetic element (like a segregation
distorter) attempting to pass itself on.
s. Muller’s
rachet: the lineage with the fewest deleterious mutations in an asexual lineage
is necessarily small. It is therefore
vulnerable to dying out through genetic drift and other random acts of chance.
t.
Mutation-selection balance: the idea that there is a balance between the
frequency of mutations arising and the removal of variance in a population via
selection. If selection occurred faster
than mutations, genes would often spread to fixation in a population.
u. Mutational
load: the fitness reduction in a population due to accumulated deleterious
mutations.
v. Progenitor
tail: the small portion of an asexual population whose few deleterious
mutations keep it from dying out with the living dead. It then accumulates deleterious mutations of
its own, and part of the progenitor tail eventually becomes part of the living
dead itself as a new progenitor tail arises.
w. Selective
sweep: the selective process that leads to the fixation of a positive trait in
a population. It is responsible for the
living dead’s disappearance in asexual populations.
x. Sexy son
hypothesis: closely related to the Fisher process, this hypothesis states that
if a female with a preference for a trait mates with a male with a trait, their
offspring will have the trait and a preference for that trait. The trait may not provide any selective
advantage, but females will want to mate with males that have the trait,
particularly if the preference for that trait is widespread, because the trait
will be passed on to their sons, who will then have higher reproductive success
and spread more genes.
y. Twiggy
distribution: the majority of asexual lineages show up as relatively new
additions, to sexual lineages, on phylogenetic trees. This twiggy distribution indicates that
asexual lineages do not have high long-term fitness and do not tend to speciate
into other lineages.
z.
Worker policing: workers in some eusocial organisms will kill the eggs of other
workers (which they can identify by chemical differences from the queen’s
eggs). This occurs because they are more
related to their brothers than they will be to their nephews, particularly if a
queen was multiply mated. Hence,
although it is advantageous for each worker to sneak in eggs of her own (since
she’ll be more related to her son than her brother), it is advantageous for the
other workers to kill eggs that are not the queen’s (or their own.) Oddly enough, this happens even in slave
ants, when the slaves should have no genetic reason to get rid of non-queen eggs
(since they’re not related to any of the ants in the colony.)
2. Referring to the provisioning of jellyfish babies with stinging
cells from mom: “if these vertically transmitted cnidocytes are capable of
de-differentiating into multipotent cells, then parental cell lineages could
compete for access to their offspring’s germ line.” Explain. (3 pts)
This quote refers to the prevalence of a unicellular propagule. It both shows how parental provisioning can
help overcome some of the disadvantages associated with the creation of a
unicellular propagule (the stinging cells should prevent predators from eating
all the propagules) and how the commonly held ideas about why there are
unicellular bottlenecks (specifically that the creation of a unicellular
propagule reduces competition among the cells of the organism) are confounded
sometimes. In this instance, the
maternal cells are not only provisioning the propagule, but also competing to
reproduce again by becoming part of the propagule germ line.
3. Some intracellular parasitic bacteria feminize males. Some ladybugs have a
parasitic endosymbiont bacteria that causes male embryos to die. When females hatch, they gorge on the eggs of
their dead brothers. Why is cannibalism
important in this system? (3 pts)
The ‘goal’ of the parasitic elements is to increase
the likelihood that they will be passed onto the next generation. In order to
be passed on their hosts must survive. The female ladybugs stand a better
chance of survival if they are well-nourished enough to live until they
reproduce. Eating the male embryos thus gives the females, and the parasites a
better chance at surviving to reproductive age.
Although feminizing males might be a better way to increase the fitness
of this selfish bacteria, providing infected females with a hearty first meal
is a pretty savvy strategy.
4. It was long
thought that the loss of eyes in cave fish was due to either lack of selection
for eyes OR selection for lack of eyes.
a.
Briefly
explain the difference. (3 pts)
If
there was a lack of selection for eyes, the loss of eyes in cave fish would
occur because there was no selection against mutations in genes that coded for
eyes. Deleterious mutations simply
weren’t selected against, added up, and the eyes were lost. If there was a selection for lack of eyes, it
would indicate that the lack of eyes would somehow be advantageous for the
organism, and therefore any mutation that created lack of eyes would be
selected for and spread through the population.
b.
Then
explain the selection pressures now known to have resulted in eye loss in
Mexican tetras (3 pts)
It
appears that the gene coding for eyes in Mexican tetras is connected to the
genes for lateral lines. A mutation that
was advantageous to the lateral lines was deleterious to the eyes. However, this loss of eye function wasn’t
deleterious in this instance because the eyes could do little in the
environment anyway, so the gene was not selected against on that account. The improved lateral lines, on the other
hand, offered a distinct advantage, and so the gene was selected for. In some ways, the eye loss in cave fish can
be seen as being both a lack of selection for eyes, because the mutation that
caused eye loss was not selected against because of lack of need for eyes, and
selection against eyes, because the advantageous affects on the lateral lines
necessitated lack of eyes.
5.
Interpopulation crosses can provide extremely interesting data.
a. When an
eyeless female Mexican tetra from cave A was crossed with an eyeless male from
cave B, the offspring had semi-developed eyes.
Explain. (3 pts)
The eyelessness of the tetra parents must
have arisen during separate evolutionary events and arise from mutations on
different genes. The offspring tetra
will have inherited a good copy of each gene from one of the parents, while
getting the mutated copy of each gene from the other parent, allowing it to
have more-developed eyes than either of its parents.
b. Females fruit
flies mated with males from a different population died younger than did
females mated with males of their own population. Explain.
(3 pts)
The male fruit fly has probably been
undergoing sexually antagonistic evolution with the females from his
species. He has evolved some way to mix
fluids in his seminal fluid that kill other sperm. However, this is toxic to the female. In his species, the female has evolved ways
to negate the toxicity of this fluid.
However, the female from the other species, having been under no such
evolutionary pressure, has not developed resistances and is vulnerable to the
full toxicity of the fluid.
c.
Interpopulation crosses in some insects with even sex ratios produce
female-biased broods. Explain. (3 pts)
It is likely here that there is some
selfish genetic element at work (perhaps a bacteria) that kills the male
embryos, or biases sex ratios in favor of the female. However, members of the sex with the selfish
element at work have developed compensatory mutations that allow them to keep
their sex ratios even. Crossing with
members of another species, which doesn’t have this compensatory mutation,
however, allows the selfish genetic element to rear its ugly head and express
itself in a female-biased ratio.
d. Mixed strains
of slime molds possess higher spore:stalk ratios, even though the parental
strains show no bias. Explain. (3 pts)
The likelihood here is that one of
the strains of slime mold is a cheater, which manages to infiltrate into the
spores more often than it should (advantageous since this allows that strain to
reproduce itself to the detriment of the other strain). However, the cheater trait is fixed in the
population, and so is not apparent when only that strain is present, because
all the organisms in that strain are cheaters, and cheaters can’t cheat unless
there is a naïve strain. When mixed with
a naïve strain, however, the cheater trait reemerges as it takes advantage of
the vulnerable strain, leading to a higher spore:stalk ratio. Also it is possible that the strains are both
cheaters, who cheat in a different ways, which would lead to hugely high
spore:stalk ratios as each strain tried to put their organisms into the
stalk.
6. In flowering
plants, pollen lands on the stigma, “germinates”, and a pollen tube grows down
through the style and into the ovary for fertilization of an individual
ovule. Because many sperm may land on
the stigma during its receptive phase, there is intense selection on the growth
rate of the pollen tube. In fact, the strength
of this selection is reflected in the fact that pollen tubes are the fastest
growing tissue in the world (deer antlers are the fastest non-cancerous
mammalian tissue).
Such
a mutation would spread throughout the population because it acts to cause
advantage earlier in the life cycle than seed production. If the growth rate of pollen tubes increases,
the chances of the sperm out-competing others to fertilize the egg increases,
and selection favors mutations that provide an advantage early on, even to the
detriment of later life. However, if the
affect on seed production was too high (i.e. such a mutation caused almost no
seeds to develop) the mutation would not spread because it affects reproductive
fitness too highly.
Each
sperm nucleus wants to reproduce, and get its genes into the germ line for the
next generation. Unless the sperm nuclei
are identical genetically, there will be competition of some kind between the
two sperm nuclei over fertilizing the egg.
Both want to make it into the germ line, neither wants to be stuck in
the endosperm where it will be unable to reproduce itself. Any mutation that allowed for a sperm to have
better success at fertilizing the egg would be selected for.
7.
Why would a clade with a high propensity to generate high fitness asexual
lineages go extinct more quickly than clades that do not? (3 pts)
High fitness asexual lineages can
out-reproduce and outcompete sexual lineages in the short term (because
beneficial genes are passed on intact, not shuffled up in recombination and
because there are no males, allowing more reproduction.) The sexual lineages will end up dying out,
and eventually the asexual lineages will accumulate too many mutations and die
out too. Therefore, the clade that tends
to produce high fitness asexual lineages will go extinct via outcompetition of
sexual species and increased mutational load.
On the other hand, clades that tend to produce lower quality asexuals do
not see their sexual lineages die out because of competition with asexuals, and
so have no problems when the asexual lineages eventually die out.
8. What is the
general purpose genotype hypothesis? (3
pts)
Successful asexual lineages tend to
have genotypes that are not too specific.
They tend to work for a niche in a generalist community, and to be able
to survive temporal changes, and also tend to incorporate some form of
phenotypic plasticity and a low mutation rate.
These lineages may be boring, but they are able to stick around for long
periods of time by not specializing so much that they will be greatly affected
by minor environmental change.
9. What did
Morgan, Fisher, and Muller all realize about recombination and the
co-occurrence of beneficial mutations (in other words, why are multiple
beneficial mutations more likely to be present in the same individual in sexual
populations than in asexual populations)? (3 pts)
In sexual populations recombination
can lead to beneficial mutations being in the same gamete. This gamete can combine with another gamete
with its own beneficial mutations.
Asexuals can’t share genes in this way, and so beneficial mutations must
arise independently, a rather rare occurrence.
Moreover, the asexual lineages undergo clonal interference as different
lineages with beneficial mutations try to compete with each other to be the
lineage to “win,” reproduce, and become part of the progenitor tail. Essentially, they are competing rather than
working together to create an even more successful organism.
10. We said that
lunglessness in plethodontid salamanders and asexuality in aphids are unlikely
ever to be reversed but winglessness in stick insects has been reversed
multiple times. Explain. (4 pts)
In stick insects the muscles and
developmental pathways that produce wings are still being used for similar
activities, therefore mutations will not pile up due to lack of selection. Also, the same (or very similar) pathways are
there, easing the way for any reversion mutation. In the salamander example, the pathways that
used to lead to lung development are now being used for a projectile tongue, a
different path from the original. Any reversion
back to the ancestral lunged state would mean losing the useful projectile
tongue, and would probably never work because the salamanders have gone too far
down another developmental path. In
aphids, the genes that are responsible for sexual pathways are unused, and it
is probable that mutations have built up due to lack of selection against
deleterious mutations.
11. Explain the
informational basis of power asymmetries in gamergate ants. (3 pts)
In gamergate ants the gamergate
knows that her rival is laying haploid eggs, and that those haploid eggs are
chemically different from her own.
Therefore she can kill off the beta’s eggs, which are less related to
her than her own sons, with little cost to herself. The beta, on the other hand, knows the
gamergate’s eggs are chemically different than her own, but can’t tell which
are male and unwanted and which are female, which she does want because she’s
very related to them. When the
gamergate’s eggs hatch the beta can tell the difference between male and
female, but the chemical signal is gone and she might kill her own kids. So the gamergate has more information than
the beta, allowing her to control reproduction of the beta at very little cost
to herself and bolstering her own position, thus proving that knowledge is
indeed power.
12. New Zealand
researchers, in an attempt to bolster the reproductive success of the kakapo (a
flightless lekking nocturnal parrot), provided extra food for the females. But to their surprise, the females didn’t
make more babies; they made more of one sex.
Which sex? Explain. (3 pts)
According to the Trivers-Willard
model, the sex who has to compete for reproductive success will benefit more
from having more resources, because in this instance the higher resources mean
a higher quality kid, more likely able to compete successfully and have many
kids. Most likely in this situation
it’ll be the males, because lekking tends to be about strong male-male
competition.
13. Briefly
describe the costs and benefits of a unicellular bottleneck. (4 pts)
The costs of a unicellular
bottleneck are that it creates a smaller, vulnerable propagule which needs
either parental care or provisioning.
There is also a loss of specialized tissue, which leads to a longer
development time for the propagule as it rebuilds the specialized tissues lost
in the unicellular bottleneck. However,
a unicellular bottleneck also allows for the expedited purging of deleterious
mutations, since a cell with serious mutations cannot hide among less-mutated
cells like it could in a multicellular propagule. This purging keeps highly mutated cells from
getting into another germ line and reproducing again. Also, the unicellular bottleneck causes all
cells in one organism to have the same genome.
This is beneficial because it prevents competition between differing
lineages to get into the germ line, probably preventing cheater cells from
taking over an organism’s germ line.
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