Download 5: Insect Microevolution

5: Insect Microevolution
Let’s start with a huge problem…
The complex adaptive nature of ecosystems means that evolutionary forces are strongest at lower levels of
organization; we have learned that the hard way in our continual battles with the evolution of resistance to
pesticides and antibiotics, and the unwillingness of [insects and] microorganisms to take a reasonable
approach to making things easy for us. (Levin 2001, p. 17)
Why do insects evolve so rapidly? A major reason is that they can reproduce rapidly; in just one year, insects
may have multiple generations. Some of the best-documented examples of contemporary evolution are those
from insects; humans have rapidly changed the environment on this planet and insects have evolved rapidly in
response (McKenzie 1996). But other groups of animals can also evolve rather rapidly, too…
It has the menacing sound of an Alfred Hitchcock movie: Millions of
rats aren't even getting sick from pesticide doses that once killed
them. In one county in England, these "super rats" have built up
such resistance to certain toxins that they can consume five times
as much poison as rats in other counties before dying. From insect
larvae that keep munching on pesticide-laden cotton in the U.S. to
head lice that won't wash out of children's hair, pests are evolving
genetic shields that enable them to survive whatever poisons
humans give them.
Rachel Carson predicted such resistance in her groundbreaking book Silent Spring, published soon after the
chemical insecticide glory days of the 1950s. And the problem is getting worse. Farmers in the U.S. lost about
seven percent of their crops to pests in the 1940s. Since the 1980s, some 13 percent of crops are being lost -and more pesticides are being used. Now 60% of crops are being lost due to pests in many areas.
It's a huge problem, but the pests are only following the rules of evolution: the best-adapted survive. Every time
chemicals are sprayed on a lawn to kill weeds or ants for example, a few naturally resistant members of the
targeted population survive and create a new generation of pests that are poison-resistant. That generation
breeds another more-resistant generation; eventually, the pesticide may be rendered ineffective or high doses
may even kill other wildlife or the very crop it was designed to
protect.
In countless ways, human actions are hastening pests'
evolution of resistance. Farmers spray higher doses of
pesticide if the traditional dose doesn't kill, so genetic
mechanisms that enable the pests to survive the stronger
doses rapidly become widespread as the offspring of resistant
individuals come to dominate the population. Farmers often
also use only one pesticide, when using a combination would
slow the evolution of resistance.
Dr. Miller ENY3005/5006 Principles of Entomology, University of Florida Includes material from the intro. entomology course taught by Dr. Douglas Emlen and from The Insects, Authors: Gullan & Cranston. Pest management worldwide is currently dominated by broad-spectrum toxic
chemicals. The limited adoption of other forms of pest management (we’ll talk
about some of these later in the semester) suggests that concerns about toxic
effects on humans and the environment
are not as great as a desire to save
money and control pests rapidly.
However, recent initiatives in Europe
have reduced agricultural pesticide
usage by 50% because of concerns for
human health. For the most part,
though, chemical pesticides are applied
liberally to crops, and one result has
been the evolution of pesticide
resistance. This phenomenon is similar
to antibiotic resistance in bacteria.
In 2001, the Food and Agriculture Organization reported more than 700 pest
species were resistant to pesticides. A parallel may be drawn between the way
that the presence of an antibiotic and the presence of a pesticide both select
for resistance in their target species. Bacteria and insects often acquire
resistance in the same way, through a single mutation. In the common fruit fly
Drosophila, change in just one gene allows the fly to overproduce an enzyme
that breaks down dichloro-diphenyl-trichloroethane (DDT) and other poisons.
As seems to be the case in at least some instances of antibiotic resistance,
removal of the selective agent (the global decline in the use of DDT in this
case) does not necessarily cause the insects to revert back to a susceptible
state (Major et al. 2003).
What have we learned about pesticide resistance? Ways that we can minimize
pesticide resistance include the following: 1) Rotating pesticides so that a
single population of insects is not constantly bombarded with the same
chemicals, 2) Using mixtures of chemicals, because the mutant insects that
can survive one pesticide are unlikely to also survive the second or third pesticide, 3) “Spot spraying”, or
spraying very carefully to only the affected areas, limits exposure for other organisms, including beneficial
insects and pest insects not presently targeted. 4) Avoiding pesticides except when absolutely necessary.
Pesticide resistance will not evolve if the selective forces (i.e. pesticides) are not sprayed! Evaluating the
results of treatment can also help farmers to choose the most
effective treatment. All of this parallels the plans to slow the
evolution of antibiotic resistance… understanding evolution in
one context helps us to understand it in another context.
The overuse of pesticides has caused some problems that
were not expected. For example, broadcast spraying of
synthetic organic insecticides has induced resistance not only
in their intended targets—crop pests—but also in vectors of
disease. Agricultural spraying, especially on cotton and rice,
has contributed greatly to the evolution of pesticide resistance
in the Anopheles mosquitos that transmit malaria, and thus to
Dr. Miller ENY3005/5006 Principles of Entomology, University of Florida Includes material from the intro. entomology course taught by Dr. Douglas Emlen and from The Insects, Authors: Gullan & Cranston. the worldwide resurgence of that disease.
The continuing battle against resistance is analogous to the evolutionary concept of arms races, in
this case pitting human ingenuity in discovering new toxins and using them appropriately against the
adaptive capacity of pest species.
DDT, the powerful synthetic pesticide once thought to be harmless unless ingested, was used
liberally on boll weevils and humans alike. Typhus outbreaks in the countries entered by the U.S.
army during World War II were stopped cold by DDT, sprayed directly on people, clothing and
bedding -- even sprayed by air over entire cities.
REFERENCES
This reading draws directly from the following sources:
Denholm I, Rowland MW. 1992. Tactics for managing pesticide resistance in arthropods: theory and
practice. Annu. Rev. Entomol. 37:91–112
Levin SA. 2001. Immune systems and ecosystems. Conserv. Ecol. 5:17
http://www.consecol.org/vol5/iss1/art17
Major J, Bogwitz M, Perry T. 2003. Lone gene could force rethink on pest insect control. Presented at
Int. Congr. Genet. Poster, 19th, July 6–11. Referenced in http://news.bio-medicine.org/biology-news2/ Lone-gene-could-force-re-think-on-pest-insect-control-4295-1/
Orzech KM, Nichter M. 2008. From resilience to resistance: political ecological lessons from antibiotic
and pesticide resistance. Annual Review
of Anthropology 37: 267–282.
http://www.pbs.org/wgbh/evolution/library/10/1/l_101_02.html
Dr. Miller ENY3005/5006 Principles of Entomology, University of Florida Includes material from the intro. entomology course taught by Dr. Douglas Emlen and from The Insects, Authors: Gullan & Cranston. How does microevolution differ from macroevolution?
Earlier in the semester we went over macroevolution: changes at or above the species level.
Macroevolution is “big picture” evolution, including the formation of new species. Now we will discuss
microevolution, or genetic change in populations. Microevolution happens all the time – nearly all
populations experience genetic change on an almost daily basis as members of that population are
born and die. Some of these genetic changes result in obvious differences (e.g. size, color, and
pesticide resistance), while other genetic changes may not change the phenotype of organisms.
Macroevolution also involves genetic changes (though they are usually more massive).
Important Definitions
Theory
The common use of the term theory implies speculation or assumption that has not been verified or
has limited proofs. However, in science, a theory is a well-substantiated explanation or a set of
statements that have been confirmed over the course of many independent experiments. In
comparison, theories are more certain than hypotheses but less certain than laws. And in science, an
unproven idea or a mere theoretical speculation is regarded as a hypothesis rather than a scientific
theory.
Hypothesis
A supposition or tentative explanation for (a group of) phenomena, (a set of) facts, or a scientific
inquiry that may be tested, verified or answered by further investigation or methodological
experiment. A scientific hypothesis that has been verified through multiple scientific experiments and
other research may be later considered a scientific theory.
Contemporary evolution
Evolution often occurs on contemporary timescales, often within decades, or even more quickly for
organisms with short generation times, such as insects and (especially) bacteria. Contemporary
evolution is evolutionary change occurring now or within a human lifetime.
Human activity is generating selection across the planet, so much that rapid evolution is becoming
the new normal. Until recently it seemed that science progressed quickly while nature stood still, but
now it seems the reverse is true.
In today’s world, it is crucial for people to understand how evolution functions. Huge problems loom
including antibiotic resistant bacteria, pesticide resistance in insects, and species extinction. These
Dr. Miller ENY3005/5006 Principles of Entomology, University of Florida Includes material from the intro. entomology course taught by Dr. Douglas Emlen and from The Insects, Authors: Gullan & Cranston. problems can only be solved (or at least mitigated) by using the insights and tools of evolutionary
biology.
Forces that can result in evolution (genetic changes in populations) are the following:
1.
2.
3.
4.
5.
Natural selection
Sexual selection
Genetic drift
Gene flow
Mutation
NATURAL SELECTION
Evolution by natural and sexual selection was Darwin’s big idea. Change in organisms over time was
already a known fact (animal and plant breeding for desirable characteristics had occurred for
centuries), but natural mechanisms responsible for these changes were not known. Natural selection
is selection resulting from differences in the survival of organisms in a population. Examples of
natural selection pressures include predation, extreme temperatures, and food limitation.
Three elements need to be present for natural selection to result in evolution:
1) There is phenotypic variation, and these differences affect survival.
• E.g. Big animals die while small animals live when food is in short supply.
2) The differences are heritable
• E.g. Small adults tend to produce small offspring
3) Reproduction must occur
Artificial selection is a type of natural selection, but humans are responsible
for creating the selection. For example, humans have bred wild dogs to result
in everything from Chihuahuas, Boxers, and Terriers, to Huskies and Great
Danes. Similarly, Native Americans bred a type of grass that was good to eat,
and by selecting only those with big grains to reproduce, created corn.
Mustard plants were bred to produce broccoli, cauliflower, cabbage, and kale
(they are all the same species!).
SEXUAL SELECTION
Sexual selection is selection due to differences in mating success.
Sexual selection consists of two components: male-male competition,
which results in the evolution of weapons, and mate choice, which
results in the evolution of ornaments. Sexual selection is generally
stronger in males than in females because of great variation in mating
success – some males mate many times, while most males mate only
a few times or not at all. This difference in selective pressures occurs
because most animals are not monogamous. Because sexual
selection can be so strong, elaborate ornaments and weapons can
Dr. Miller ENY3005/5006 Principles of Entomology, University of Florida Includes material from the intro. entomology course taught by Dr. Douglas Emlen and from The Insects, Authors: Gullan & Cranston. evolve, even if they are bad for survival (i.e. natural selection). The long eye-stalks on stalk-eyed flies
and the long tails of birds of paradise are just a couple of examples of such ornaments and weapons.
GENETIC DRIFT
Genetic drift is simply random genetic changes in populations and is
more likely to occur in small populations. One example of genetic
drift occurs when populations go through a “bottle neck” where
population size becomes tiny, and the number of genotypes
represented is small. For example, if a couple of insects get trapped
inside a cargo ship and are transported to another continent, they
may breed and colonize the new habitat. However, the genetic
composition of the new population will be different from the old
population. E.g. insects in the native habitat may be red, grey, and
white, while in the new habitat, they are only white (only white ones
caught a ride on the boat)
GENE FLOW
Gene flow is simply mating among individuals from different populations. Gene flow occurs when
seeds are dispersed, animals immigrate into a new population, or when a lovebug catches a ride in
your car to a new location where it then finds a mate.
MUTATION
Mutation is the alteration of a DNA sequence (thus, genetic change). Mutations are fairly rare, though
exposure to certain types of radiation and toxic chemicals (carcinogens) can increase the rate of
mutations.
It is fairly common to come across statements like “insects reproduce at such a high reproductive rate
to ensure the survival of the species,” or “male beetles rarely fight to the death because such
aggressive interactions might lead to the extinction of the species.” However, these statements show
an important misunderstanding of what evolution is and how it operates.
When insects are stressed (due to pesticides, food limitation, competition, etc.) some will succeed
and produce lots of offspring while others will die before producing offspring (this is individual-level
selection). If the difference between those that succeed (i.e. those with high fitness) and those that
fail (i.e. those with low fitness) is genetically heritable, then offspring will resemble parents, and the
genetic make-up of the population will change (this is evolution).
An altruistic trait – a feature that reduces the fitness of an individual that bears it for the benefit of the
population or species – cannot evolve by individual selection. An altruistic genotype amid other
genotypes that were not altruistic would necessarily decline in frequency, simply because it would
leave fewer offspring per capita than the others. Likewise, if a population were to consist of altruistic
Dr. Miller ENY3005/5006 Principles of Entomology, University of Florida Includes material from the intro. entomology course taught by Dr. Douglas Emlen and from The Insects, Authors: Gullan & Cranston. genotypes, a selfish mutant – a “cheater” – would increase to fixation, even if a population of such
selfish organisms had a higher risk of extinction.
Image to the right: Lemmings once had the reputation
of being altruistic. When population densities got too high,
as the story goes, then some lemmings would drown
themselves to save others in the population. But, as the
cartoon shows, cheaters (here, the one with the inflatable
tube) will survive, produce offspring, and if the cheating
phenotype is heritable, the cheater adults will produce
cheater babies. Soon, nearly everyone in the population
will be watching out for themselves, and altruism will
disappear.
So what about behaviors that appear altruistic?
Some insects and other animals do have behaviors that benefit their immediate group – such as
alarm pheromones that alert others of danger. Why would they do this? One major cause for such
behaviors is kin selection – that individuals should have behaviors that benefit their relatives (since
their relatives share many of the same alleles). Another possibility is that even though an individual’s
behaviors benefit the group, the individual itself also achieves many selfish benefits. Some species
with highly evolved social networks (such as primates) commonly help one another. In these groups
“reciprocal altruism” is common – one individual helps another, and this behavior is later reciprocated.
If individuals do not reciprocate, they are commonly ignored or shunned (and thus achieve lower
fitness). Reference:
Futuyma, D.J. 2005. Evolution. Sinauer Associates: Sunderland, MA.
In the Sensory Systems lecture we discussed how pheromones
can result in nymphal grasshoppers developing into locusts.
Just because of the presence of pheromones and other cues
during development, the color, morphology, and behavior of
these insects can change. This is an example of phenotypic
plasticity, not evolution. Evolution requires genetic change, but
here, these Orthoperans may be genetically identical.
Phenotypic plasticity is the capacity of a single genotype to
produce more than one phenotype. Phenotypic plasticity is very
common. For instance, if you had an identical twin who was
raised in a far-off country, you two could speak different
languages, choose different clothes to wear, prefer different foods… all because of your environment.
You would be different not because of genetic differences, but because you were raised differently.
Granted, some things would be the same! Some traits are phenotypically plastic while others are
“canalized”, where differences among individuals are due to genetic differences.
Dr. Miller ENY3005/5006 Principles of Entomology, University of Florida Includes material from the intro. entomology course taught by Dr. Douglas Emlen and from The Insects, Authors: Gullan & Cranston. 5: Insect MicroEv
M
volution
Stud
dy Questio
ons and Ob
bjectives
Draw
w a diagram
m to explain
n how insec
cts evolve pesticide
p
re
esistance. N
Now label th
he diagram and
provvide a shortt essay in support of th
hat diagram
m. In your la
abels and e
explanation,, answer the following
quesstions: wha
at is the sele
ective press
sure involve
ed? How m
many pest sspecies are resistant to
o
pestticides? If an
a insect ge
ets sprayed with pestic
cide, what tthree eleme
ents are needed for se
election to
resu
ult in evolutiion? Finally
y, provide fo
our ways th
hat growerss can minim
mize pesticid
de resistancce.
Base
ed on the CBS
C
news reading,
r
ho
ow long did bednets re
esult in a dro
op in malarria attacks?
? What is
the p
pesticide us
sed in the bednets?
b
Base
ed on the NPR
N
reading and radio
o story (visit http://www
w.npr.org/2011/01/19/133057071
1/bed-buggeno
ome-revealls-pesticide
e-resistance
efor the story), describ
be the evide
ence that be
edbugs havve evolved
pestticide resisttance? Wha
at was the mechanism
m
m thought to
o be respon
nsible for the evolution
n of
pestticide resisttance (i.e. how
h
did the
e resistant bedbugs
b
tolerate the p
pesticide)? S
Some bedb
bugs can
tolerrate high levels of pyre
ethroids. Th
hey can withstand a do
ose of up to
o how many times the
e dose that
wou
uld usually be
b lethal?
Explain the diffference betw
ween selec
ction and ev
volution. Exxplain a hyp
pothetical ccase where selection
doess not resultt in evolution and why..
Explain how ge
enetic drift is responsib
ble for the invasive “su
uper-colonie
es” of the A
Argentine A
Ant. From
the rradio broad
dcast, what is one way
y that huma
ans might co
ontrol these
e super-colonies witho
out typical
pestticides?
Explain group selection,
s
based
b
on the course
note
es, and why
y it is largely
y a faulty co
oncept.
Whyy do most in
ndividuals rarely
r
do an
nything
“for the good off the specie
es” unless it is likely
to diirectly bene
efit them as
s well?
Stud
dy Terms
Theo
ory, hypoth
hesis, Biolog
gical Evolution
Pestticide resisttance, Natu
ural Selectio
on,
Artifficial Selecttion, Sexual Selection,, Genetic
Driftt
Gen
ne, Allele, Locus
Gen
notype, Phe
enotype, Fittness, Herittability
Con
ntemporary evolution, Adaptation
A
Grou
up selection
n, Phenotypic plasticitty
Dr. M
Miller ENY300
05/5006 Princciples of Entom
mology, University of Floriida Includees material from the intro. entomolo
ogy course taughtt by Dr. Douglas Em
mlen and from Thee Insects, Authors : Gullan & Cransto
on. Malaria fears up as mosquitoes show pesticide resistance - - CBS News
Page 1 of 1
August 18, 2011 12:27 PM
Malaria fears up as mosquitoes show
pesticide resistance
By David W Freeman
(CBS) Malaria experts are worried in the wake of an
ominous new study from Senegal showing that the
mosquitoes that spread the deadly disease can develop
resistance to insecticide-treated nets.
Researchers studied malaria infections in a village in the
West African nation and found that the disease may be
rebounding because the insects are becoming resistant
to the increasingly resistant to the insecticide
deltamethrin.
Malaria-carrying mosquito,
Anopheles gambiae
(Credit: CDC Public Health Image
Library)
Despite decades of efforts to beat malaria with
insecticides, indoor spraying, bednets, and drugs, the
disease kills nearly 800,000 people a year, Reuters
reported. Most of the victims are babies and young
children in sub-Saharan Africa.
In recent years, the nets have been widely distributed in Africa, and the World Health
Organization says that when properly deployed they can cut malaria rates by half, the BBC
reported. But will the nets lose their effectiveness if mosquitoes are resistant to the pesticide
used to treat them?
To find out, researchers looked at malaria cases in the Senegalese village of Dielmo before
and after the bednets were distributed there. During the two years from August 2008 to
August 2010 after bednets were distributed, there was a sharp drop in malaria attacks,
according to Reuters. But between September and December 2010 - 27 to 30 months after
the nets had been given out - malaria attacks in adults and older children rose to even
higher levels than before.
The rise in cases seemed to mirror the increasing proportions of malaria-carrying
mosquitoes resistant to deltamethrin.
What does it all mean?
"Strategies to address the problem of insecticide resistance and to mitigate its effects must
be urgently defined and implemented," concluded the authors of the study, which was
published in The Lancet Infectious Diseases.
The CDC has more on malaria.
http://www.cbsnews.com/2102-504763_162-20094086.html?tag=contentMain;contentBody
9/30/2011
Bedbug Genome Reveals Pesticide Resistance : NPR
Page 1 of 2
Bedbug Genome Reveals Pesticide Resistance
by JON HAMILTON
January 19, 2011
text size A
A A
Scientists have found some new genetic hints that
could help explain why bedbugs are so hard to kill.
Some bedbugs appear to have evolved a
mechanism that helps them break down toxins,
including the ones in many pesticides, a team
from The Ohio State University reports in the
journal PLoS ONE.
Enlarge
Orkin LLC/AP
Modern bedbugs are increasingly resistant to pesticides.
Some populations, in fact, can survive 1,000 times the
amount of pesticide that would be needed to kill a traditional
bug.
The finding comes after earlier research found
genetic changes in bedbugs that help protect
nerve cells from specific pesticides.
Together, the findings suggest that current efforts
to curb bedbug infestations will be more difficult
than in the past, says Susan Jones, an urban
entomologist at Ohio State University in Columbus.
"We're dealing with a different bug than what we were decades ago," Jones says, one that's harder to
exterminate.
The latest genetic explanation for bedbugs' ability to resist pesticides comes from a study comparing
genes from modern bedbugs with those from a colony started decades ago by a military bug expert
named Harold Harlan.
Harlan's colony has been kept completely isolated, Jones
says. "So it has had absolutely no exposure to
insecticides," she says." When you expose it to
insecticides, the bugs just keel over."
Breaking Down Pesticides
Bugs from Harlan's colony were compared with bugs from
an apartment complex in Columbus that had been treated
repeatedly with insecticide, Jones says.
We're dealing with a
different bug than what
we were decades ago.
- Susan Jones, urban
entomologist, Ohio State
University
Researchers focused on genes known to be involved in
breaking down toxins and removing them from the body,
says Omprakash Mittapalli, an entomologist from The Ohio State University in Wooster.
Mittapalli suspected modern bedbugs had genes that encouraged their bodies to produce more of the
enzymes that break down pesticides. And, he says, that's just what the team found.
http://www.npr.org/2011/01/19/133057071/bed-bug-genome-reveals-pesticide-resistance
9/30/2011
Bedbug Genome Reveals Pesticide Resistance : NPR
Page 2 of 2
"These enzymes are indeed higher in the pesticide-exposed populations compared to the pesticidesusceptible population," Mittapalli says.
Bedbugs' ability to eliminate toxins and protect nerve cells has become quite common, says Ken
Haynes, an entomologist at the University of Kentucky who has studied many populations of modern
bedbugs.
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Aug. 21, 2010
He says nearly all of the populations he's studied
have some resistance to common pesticides
known as pyrethroids. And many "have a level of
resistance that's quite extraordinary," he says,
meaning they can withstand up to 1,000 times the
dose that would usually be lethal.
A New Approach To Killing Bedbugs
All the new information about resistance suggests
it may be time to try a different approach to killing
bedbugs, Haynes says.
"Instead of relying on the same insecticide generation after generation of the bedbugs," Haynes says,
"you'd rotate to a different class of pyrethroid or a different class of insecticide altogether with a different
mode of action."
Bedbug control also should include nonchemical approaches like heat to kill the bugs and vacuuming to
remove them from a room, says Louis Sorkin, an entomologist at the American Museum of Natural
History in New York.
"It's not just one chemical approach and that's the end of it," Sorkin says.
http://www.npr.org/2011/01/19/133057071/bed-bug-genome-reveals-pesticide-resistance
9/30/2011