Question Key
-
As we have seen, inbreeding can reduce offspring fitness by exposing
deleterious recessive alleles. However, some animal breeders practice
generations of careful inbreeding within a family, or “line-breeding,”
and surprisingly many of the line-bred animals, from champion dogs to
prize cows, have normal health and fertility. How can it be possible to
continue inbreeding for many generations without experiencing inbreeding
depression due to recessive alleles? (Hint: Consider some of the
differences between animal breeders and natural selection in the wild.)
Generally, if a small population continues to inbreed for many
generations, what will happen to the frequency of the deleterious
recessive alleles over time?
Inbreeding does not always
produce homozygosity for deleterious recessives – it can also produce
homozygosity for non-harmful alleles and thereby remove harmful alleles
from the population. In fact, if you breed two parents who are
heterozygous for the same harmful recessive allele (Tt × Tt), then 75%
of the offspring will have at least one T allele and only 25% will be
homozygous for the harmful recessive. And breeders won’t use the
individuals with low fitness to breed the next generation so they will
tend to select parents that are less likely to carry the harmful
recessive allele. Eventually, that can mean that the harmful recessives
are bred out of the population.
For the next five questions, name the evolutionary process(es)
being described in each (hypothetical) example, and describe how allele
frequencies are likely to change in succeeding generations.
-
A beetle species is introduced to an island covered with dark basaltic
rock. On this dark background, dark beetles, TT or Tt, are much more
resistant to predation than are light-colored beetles, tt. The dark
beetles have a large selective advantage. Both alleles are relatively
common in the group of beetles released on the new island.
There is natural selection
occurring – we have variation in beetle color, connected to genotype at
the T locus, and color is related to survival (resistance to predation).
The dark beetles have an advantage, this is directional selection
favoring dark color. Over time, we expect the frequency of the t allele
to decrease (and T to increase) and thus to see more dark beetles in the
population. In the long term, we expect fixation of the T allele, p=1
and extinction of the t allele.
-
Another beetle population, this time consisting of mostly light beetles
and just a few dark beetles, is introduced onto a different island with
a mixed substrate of light sand, vegetation, and black basalt. On this
island, dark beetles have a selective advantage only on the black
basalt.
This is a founder event,
with only a few dark beetles and possibly not that many founding
individuals. The founders have different allele frequencies than the
source population did. Here, there is not much advantage for the dark
beetles due to the environmental heterogeneity (only an advantage on
basalt but there is also lots of non-basalt) so the allele frequencies
may change just at random by genetic drift. In fact, it’s possible that
the T allele could be lost due to chance. Alternatively, all alleles
might be maintained by having advantages in different
environments.
-
A coral reef fish has two genetically determined types of male, one of
which is much smaller (aa) and “sneaks” into larger males’ (AA or Aa)
nests to fertilize their females’ eggs. When small males are rare, they
have a selective advantage over large males. However, if there are too
many small males, large males switch to a more aggressive strategy of
nest defense, and small males lose their advantage.
This is also natural
selection but it’s frequency-dependent selection, where you only have an
advantage when you are rare. The phenotype is male size (or behavior),
there is variation associated with genotype, and the less common
phenotype has an advantage in reproduction. Over time, we expect to see
the most common phenotype fluctuate back and forth between small and
large males.
-
In a tropical plant, CC and Cc plants have red flowers and cc plants
have yellow flowers. However, Cc plants have defective flower
development and produce very few flowers.
This is an example of
underdominance or heterozygote disadvantage, a type of selection where
the heterozygote has the lowest fitness. Over time, we expect to mainly
see red flower and yellow flowers, with few heterozygotes. In fact,
selection could end up favoring the red and yellow flowers to avoid
mating (such as by having different flowering times or different
pollinators).
-
In a species of bird, individuals with genotype MM are susceptible to
avian malaria, Mm and mm birds are resistant to avian malaria, but the
mm birds are also vulnerable to avian pox.
When both malaria and pox
are present, this is heterozygote advantage or overdominance, a type of
selection where the heterozygote has higher fitness than either
homozygote. Over time, we’d expect to see all three genotypes persist
(if conditions stayed the same)… how common each would be depends on how
well the homozygotes (MM and mm) survive and reproduce compared to each
other.
What if avian malaria is
present but avian pox is not? In that case, we would have directional
selection favoring mm birds, where m is dominant to M for resistance to
malaria. Over time, the frequency of m would increase until fixation, if
the conditions stayed the same.
What if pox is present but not
malaria? Then we have directional selection favoring MM birds, where M
is dominant to m for resistance to pox. Over time, the frequency of M
would increase until fixation, if the conditions stayed the
same.
The next four questions are about the evolution of resistance to
tetrodotoxin in garter snakes. Use the following info to help you answer
the questions.
The toxic newt Taricha granulosa and its predator, the garter
snake Thamnophis sirtalis, live in forests in western North
America. The newt produces the neurotoxin tetrodotoxin (TTX) in its
skin; TTX blocks transmission of signals through the nervous system and
a newt can produce enough to kill many potential predators (including
humans!). The predatory garter snake is the only known potential
predator NOT killed by consuming the newt… though it may be affected by
the toxin, losing muscle control and mobility for up to 7 hours. Studies
in this system have found the following:
Snake populations living in the same geographic areas as the
toxic newt are resistant to TTX, while garter snake populations in areas
without the toxic newt eat other prey and are not resistant to
TTX.
In the snake populations with resistance (Benton and
Tenmile), the heritability of TTX resistance in garter snakes was
estimated to be h2 = 0.65 in Benton and h2 = 0.8
in Tenmile.
Experimental attempts to induce resistance by exposing
sensitive garter snakes to small amounts of TTX have had no
effect.
Resistant snakes have amino acid changes in the SCN4A gene
encoding a skeletal muscle voltage-gated sodium channel protein
(Nav1.4). These amino acid changes result in greatly reduced binding by
TTX in snakes with resistant genotypes.
-
Is TTX resistance caused by genetic differences or by the environment?
How do we know? Be as specific as possible, using the given information.
We know that resistant
snakes have changes in the amino acid sequence of the SCN4A gene and
also that exposure to TTX does not induce resistance. These things both
suggest that resistance is caused by genetic differences between
sensitive and resistant snakes. In addition, the estimates of
heritability tell us that resistance is caused by genetic variation and
that individuals within populations vary in resistance. Finally, the
fact that the heritability values are not 1.0 tells us that the
environment must also play some role in resistance.
-
What might explain why the Tenmile (h2 = 0.8) and Benton (h2 = 0.65)
populations have different values of heritability for TTX resistance? Do
you think all of the snakes in a single population have the same level
of resistance?
Remembering that \(h^2 = V_G/V_P\) and \(V_P = V_G +V_E\), we can say Benton and
Tenmile have different heritability values due to genetic differences
and environmental differences. For example, they could have different
allele frequencies for the SCN4A gene. Or they could have different
alleles for the gene! Alternatively, there could be environmental
factors present in Tenmile that are absent in Benton that influence a
snake’s response to TTX. For example, the availability of extracellular
potassium ions is known to influence the degree of paralysis experienced
such that snakes without the resistance mutations might fare better in
an environment where their diet allowed higher levels of extracellular
potassium.
-
Do you think there is evidence that natural selection is acting (or has
acted) on the garter snake populations in terms of resistance to TTX?
Explain. (Perhaps discuss the criteria for selection…)
When we want to decide if
there is evidence that natural selection could be acting, we need to
look for 3 things: (1) there is variation in the phenotype of interest;
(2) the phenotypic variation looks to be inherited, there’s a genetic
basis for the phenotypic variation; and (3) variation in the phenotype
is correlated with fitness (survival and reproduction).
In this case, (1) our
phenotype is TTX resistance and there is variation – we know not all
pops have resistance and there is variation within pops since we have
estimates of heritability (must have \(V_P\) to get non-zero values). (2) The
phenotypic variation appears to be inherited. We know of a related gene
(SCN4A) for which resistant snakes have different alleles compared to
non-resistant snakes. Even better, we know that the phenotypic variation
is inherited because we have pretty high heritability values for the
Benton and Tenmile populations. (3) This one is more speculative. We do
not seem to have direct evidence that resistant snakes have higher
fitness (probably survival) than non-resistant snakes. It seems likely
that this would be true but we can’t be certain from this data. If
snakes can eat something other than newts, it might not matter whether
or not they have resistance so there could not be selection favoring
resistance.
I would conclude that we
have pretty good evidence that TTX resistance could be experiencing
natural selection with the caveat that we haven’t decisively shown that
survival and/or reproduction are related to TTX resistance
phenotype.
-
What do you think would happen to the frequency of resistance in the
Tenmile and Benton populations if the toxic newt went extinct? Explain.
If TTX resistance IS favored
by selection, then that’s only true when the toxic newt is around to be
the source of the selection. If the newt goes extinct, then there is no
longer a benefit to having TTX resistance and it’s likely that TTX
resistance is costly to the animals, so I’d expect that those without
resistance would have higher survival and reproduction in the absence of
the newt. Thus, I would predict that extinction of the newt would cause
the frequency of resistance in Benton and Tenmile to gradually decrease
over time until there are no more resistant snakes in those
populations.