Selection in Extreme Environments

I. Natural selection is the only evolutionary agent that leads to adaptation—what is adaptation? Traits that increase survival/reproduction and appear to fit or function well in environment that organisms lives.

Examples of adaptations: Biochemical adaptation: Bacteria that live in volcanic pools and have enzymes that are stable near 100 C where as in typical organisms enzymes are inactivated and breakdown at 60 C.
Physiological adaptations—Desert rodents have physiological adaptations to conserve water. Kidneys concentrate urea to higher levels than other rodents that live in more benign climates.
Morphological adaptations: Hummingbirds have adaptations to extract nectar from tubular flowers

Darwin used adaptations as evidence for his theory of evolution by natural selection, but he also recognized that there were some traits which seems contrary to his idea of natural selection. These traits often occurred only in males and females had little or no expression—some examples included such traits as bright plumage coloration of male birds or long extravagant tails, such as a peacocks. How did Darwin resolve this paradox?
Sexual selection emphasizes success or failure in competing for mates. Two ways to compete:
1. Physical contests, male elephant seals
2. Competition to be most attractive, chosen—females prefer these bizarre traits

Although organisms are often adapted to their environments, extreme environments and infrequent environmental changes interspersed with normal conditions and the evolutionary history of most organisms has included such fluctuations.

II. Evolutionary consequences of extreme environments

A. Environmental extremes influence phenotypic expression

1. Heat shock in Drosophila cause abnormal wing veination and homeotic gene expression, bithorax.
2. Pollen tube length after exposure to ozone and UV-B.

B. Extreme environments may influence genetic variation

1. Increases in mutation rates; ionizing radiation such as x-rays or UV radiation; other environmental stresses can as well such as extreme temperature, humidity, salts. Increased mutation rates in some organisms may be associated with transposons, mobile genetic elements that insert into an organisms genome. Following heat shock in Drosophila, transposition rates were as low as 10-2 compared to less than 10-4 for controls
2. Changes in recombination rates; rates increase at temperature below and above normal.

3. Increased mutation rates and recombination rates may be adaptive—increase chance of producing progeny with favorable mutation or new combination of genes that will more effectively deal with adverse condition, may be just a fortuitous results of living under stress.

C. Extremes can reduce population size, leading to drift and/or extinction; human habitat alterations have created endangered and threatened species.
D. Environmental extremes cause intense natural selection. Observable over a few generation and provide powerful evidence for evolutionary change by natural selection.

1. During a long-term study by Peter and Rosemary Grant, Geospiza fortis, a ground finch in the Galapagos, underwent two evolutionary changes in bill size—measured by bill depth.
a.Bill depth is heritable—parents with large bills produce progeny with large bills.
b. Normal pattern of seasons: Jan – May hot, wet and rest of year cool, dry.

c. First evolutionary change occurred during a drought that occurred from mid 1976 through early 1978. As a consequence, finch population crashed.

In particular small birds died at a higher rate. Usually individuals of all sizes take small seeds, but as drought continued, these small seeds were in short supply and average seed size available was larger than usual.

This favored individuals with larger bills that could eat the large, hard seeds more efficiently. As a consequence these individuals survived and reproduced better. In 1978 G. fortis born were 4% larger than before the drought.

d. The second evolutionary event occurred 4 years later during 1982-3 which was an El Nino year. In the Pacific, El Ninos are accompanied by exceptionally heavy rainfall, which resulted in an excess of small seeds—which should favor individuals with smaller bills. As predicted, finches born in 1985 had beaks about 2.5% smaller than those prior to El Nino indicating that individuals with smaller beaks were surviving and reproducing more during the El Nino surplus of small seeds.

2. Pollution

a. Industrial melanism - pepper moth (Biston betularia)
i. Light morph common prior to industrial revolution. First melanic form recorded in 1948.
ii. Melanic forms went from 0.01 to >0.90 from 1848-1908 in industrial regions of Great Britain, in unpolluted areas, light morph remained common
iii. Kettlewell identified one main locus involved: CC is melanic, cc is typical. Cc is usually melanic
iv. Conducted mark-recapture studies—release a group of individuals and then return in a week or two and recapture as many as possible. With total number recaptured, calculate what expected based on original proportion released. Compare to what observed. Dark forms survived better where soot covered lichens

light dark (Cc, CC)
Polluted: obs 18 140
exp 36 122
surv 0.5 1.15

Unpollut obs 67 32
exp 53 46
surv 1.26 0.69

v. When pollution levels began to decrease in the 20th century, melanic forms have declined in recent years

vi. Melanic forms have remained common in some areas (East Angelia), apparently due to some inherent advantage. In the lab viability of melanic morph 30% higher than light morph.

3. Pesticide and herbicide resistance

i. 32 chemicals in use in 1939. Now 860 active ingredient and 20,000 different products. US annual production ins 2.23 billion pounds

ii. By 1950, fewer than 20 species showed signs of pesticide resistance. By 1960, 137 spp showed resistance. By 1990, 504 spp showed resistance.

iii. Crop loss due to insect damage was 7% in 1940s, 13% in 1980s

iv. In 1945 no insecticides were used on corn and losses were 3.5%, 1000x increase in use of insecticides, yet losses are now 12%
v. DDT

First synthesized in 1874, used in WWII to halt typhus epidemic in Naples. Chlorinated hydrocarbon which interferes with electrical conductance of neurons in insects. Typhus is transmitted by fleas, lice and mites. Then sprayed over Pacific islands to halt Malaria. Sprayed Cape Cod for several years in early 50's to eradicate gypsy moths. In 1952 was added to dry cleaning process to mothproof woolen garments. By 1950 DDT residues were in all produce and allowed in babyfood. In 1958 mosquito control program resulted in massive death of songbirds. Silent Spring was published in 1962. Rachel Carson died of cancer in 1964. Has 1/2 life of 7 years, e.g. takes 7 years to clear 1/2 out of body. Was banned in 1972 in the US. But, is still being used in other countries!

Caused malaria to decline to 75 million cases/yr by early 1960s. DDT was effective in India for 10-11 years. First resistant mosquitoes found in India in 1959. Now resistance appears in months. Malaria reached 200 million/yr by 1972 and 300 million/yr now. Malaria kills 3.5 million/yr. Resistance appears rapidly now because resistant mosquitoes are present around the world.

Don't know genetics underlying resistance in mosquitoes. For other insects can involve genetic changes that increase detoxification, such as gene amplification of esterases, decrease sensitivity of target (change number of binding sites); decreased cuticular penetration or behavior—avoid insecticide
vi. Mechanisms associated with pesticide resistance:
Behavioral - avoid places where insecticide is placed, shown for DDT
Detoxification - by gene amplification of esterases, shown for DDT
Decreased sensitivity of target, e.g. receptor change - DDT
Decreased cuticular penetration