Download Natural Selection Survival of the Fittest? The process in nature that

Natural Selection
Survival of the Fittest?
The process in nature that causes evolution through differential reproductive
success among members of a population; that success depends on genetically
based and heritable variation in characteristics that confer relative advantage or
disadvantage to the bearer.
Differential Survival and Reproduction.
Early 1900’s
Evidence for Natural Selection, but Darwinians thought NS was too slow to
observe.
Population Biologists predicted that very rapid changes could occur.
How to test natural selection?
Peppered Moth, Biston betularia
Shift from light to dark color noted during short period of time. This change was
correlated with industrialization, and was called Industrial Melanism.
Dark form of B. betularia, which was first noted in 1848, reached 90% or higher
in urban areas by 1900.
Explanation?
1920’s, J.W. Heslop Harrison
Fed one group of caterpillars leaves coated with toxic compounds commonly
found in soot and another group of caterpillars normal leaves.
1) toxic leaves --> 53 light winged, 3 dark winged.
2) normal leaves --> all light winged
Harrisons’s Conclusion: Toxins induced mutations that resulted in dark wings.
Claim of documented case of inheritance of acquired characteristics.
•Very high rate of mutation required
•Could not be replicated
1930’s, E.B. Ford
Industrial melanism was a result of natural selection acting on rare mutations.
- a few black individuals were always produced by mutation.
- black were usually eliminated by NS.
- adaptive in polluted areas, so increase by NS.
Contrast two Explanations
Harrison - mutations as direct result of pollution, occurred simultaneously in many
members of the pop.
Ford mutation not caused by pollution, always occurred in very small number.
Harrison - evolution occurred because many members of the population
simultaneously mutated.
Ford evolution resulted from high survival and reproduction of rare mutants
compared with normal moth.
What was advantage of melanism?
•Camouflage?
•Physiological advantage linked to color?
•Could predators distinguish light and dark?
•Could natural selection account for such a rapid increase?
1950’s, H.B.D. Kettlewell (worked with Ford)
Applied Fisher’s statistical methods to problem.
Kettlewell’s Hypothesis:
light wings + lichens --> dark eaten
dark wings + soot --> light eaten
Were birds really fooled?
Did wing color provide significant advantage?
Behavior of Peppered Moths
•Females rarely fly, often spend their life on a single tree branch.
•Males fly at night and sit on tree trucks during day.
•Both sexes rest on tree trunks with wings spread.
Do moths choose a background?
Enclosure experiment.
•Large cider barrel with overlapping strips of black and white cloth.
•Release moths and count where they rest.
Results:
Black cloth
White cloth
Dark-winged
38
21
Light-winged
20
39
Highly significant difference by P2
Subsequent field studies did not support this observation, moths landed at
random.
Could birds select prey?
Large enclosure with light and dark tree truncks. Add equal number of light- and
dark-winged moths and insectivorous birds.
For two hours, no moths eaten, then birds actively searched for conspicuous prey.
Conclusion:
Birds can act as selective agent, but predation is a learned behavior.
Ornithologists and Entomologists denied that birds would actively search on the
basis of wing coloration.
Ethologist Niko Tinbergen documented birds’ behavior on film.
Field Experiments in polluted forest near Birmingham and pristine forest in Dorset.
Mark-Release-Recapture
Marked males
•Released large number at sunset
•Recaptured by attraction to light and pheromones every night for a week.
Results
Light-winged
Dark-winged
Recapture Rate
Polluted
Pristine
18/137 (13%)
62/496 (12.5%)
136/493 (27.5%)
34/488 (7%)
Provided convincing evidence that shift in frequency was a result of natural
selection.
As pollution has decreased, the melanic form has decreased in frequency.
Recent studies have shown that the melanic form of the moth decreased in
many areas before the gray lichen flora returned. In fact, the early developing
lichen flora is typically green.
The switch in frequencies may also be due to other factors, perhaps sulfur
dioxide.
Some simple conclusions from the industrial melanism example:
• You can measure selection.
• What you measure matters.
• What you measure is going to be important in evolution.
• Change in gene frequency, as seen here, is fundamental to evolution.
• Once it is demonstrated that changes in gene frequency can occur rapidly then
all the other evolutionary processes follow.