Before we start in with out regular class, the article below should finally answer whether lemmings really jump over the cliffs...
Why cycling lemmings crash

The number of small rodents can go down as well as up. This, says John Whitfield, is because a hungry lemming gathers no moss.
1 June 2000
JOHN WHITFIELD
A Norwegian lemming on its autumn migration. Image © Lauri OksanenLemmings. Furry? Yes. Cute? Undoubtedly. Apocryphally suicidal? Goes without saying. But mighty hunters? Surely not. Perhaps this reputation is about to change, though, with a study by Peter Turchin, of the University of Connecticut, and colleagues, that asks "Are lemmings prey or predators?" and plumps for the latter.
In fact, they ask: do lemming populations boom and bust because the availability of food changes? Or does the rate at which other things eat them fluctuate? To separate the two possibilities, the researchers built a mathematical model describing how lemmings and the moss upon which they 'prey' interact, and compared several long-term records of the populations of lemmings and voles.
In northern Scandinavia, the populations of both Norwegian lemmings (Lemmus lemmus) and voles (mostly Microtus agrestis) fluctuate dramatically but approximately regularly. Vole population graphs have rounded peaks, whereas those for lemmings have a saw-toothed pattern. This shows that the vole cycle is predator-driven, Turchin and colleagues explain in Nature1. Lemming numbers, on the other hand, crash when they exhaust their plant food.
To understand the reasoning behind this, one needs to think through the population cycles. When a prey species is rare, food and space are plentiful, and its population increases rapidly. Meanwhile, the population of predators that feeds on it grows only once the prey population is big enough to fuel expansion. When this happens, numbers shoot up.
Predators could never eat enough to counteract the frenetic pace of rodent reproduction. Instead, the ceiling for prey is set by something called 'density dependence': the tendency for crowded populations to stop growing. For example, says Turchin, female voles mature more slowly in crowded conditions.
Eventually, there are so many predators that the prey population crashes. But by then, the number of prey has spent several years at or near its peak -- leading to a rounded curve.
Faced with this dearth of prey, the predators must emigrate or starve -- either way, their numbers see a sharp decline. For lemmings, says Turchin, "as soon as peak density is reached, food begins to run out, and the lemming population begins to crash" -- leading to a population trajectory with jagged peaks.
But why is the ecology of these superficially similar species so different? Diet, say Turchin's group. Voles eat grass, which recovers from damage quickly, so they don't run out of food. Their population cycles are driven by the predatory attentions of the least weasel (Mustela nivalis), which in turn shows a 'predator'-type cycle.
Although lots of lemmings fall prey to owls, arctic foxes and suchlike, this isn't what makes their numbers oscillate. Rather, this happens, say the researchers, because lemmings 'prey' upon moss. Moss regrows slowly, and so a horde of hungry lemmings can empty the larder in short order. And then hit the road in search of food.
But -- contrary to folklore -- neither hunger nor any population-control mechanism drives them over precipices. Allegedly the cameraman on the legendary 1958 nature documentary White Wilderness was able to film this phenomenon because he had a friend standing atop the cliff lobbing lemmings over the edge.
 
References
1. Turchin, P., Oksanen, L., Ekerholm, P., Oksanen, T. & Henttonen, H. Are lemmings prey or predators? Nature 405, 562 - 565 (2000).© Nature News Service / Macmillan Magazines Ltd 2001

Introduction to classical population genetics:

Each individual is endowed with a unique genetic constitution. The combined genetic variation in a population has important consequences.

1. When genetic variation causes differences in fecundity and survival among individuals, evolution results because those individuals whose characteristics enable it to achieve higher rates of reproduction at the expense of genetically less well endowed individuals will increase in number.

  • Fecundity : selection may occur in terms of more gametes, greater dispersal of gametes, or in terms of gametic competition
  • Survival: the individual must survive to reproduction and produce a viable offspring to count. Evolutionarily, f it doesn't reproduce it may as well not have survived to that stage.

Thus selection requires that ones offspring reproduce to carry those genes into the next generation.

2. Most genetic variation is harmful and is most likely to be expressed only :

  • When mating between kin ( why there is a taboo on inbreeding) or
  • Small population size, especially when there is strong subdivision ( subpopulations in which breeding occurs)

Hardy-Weinberg equilibrium:

The HW equilibrium is basically a null hypothesis- used to prove that something is occurring, though not necessarily what is... just that the population is not in equilibrium.

Assumptions of the model:

  • Large population
  • Random mating
  • No selection
  • No mutation 

If all the above assumptions hold, then the frequencies of homozygotes & heterozygotes would achieve equilibrium proportions which can be calculated from known or measured allelic frequencies

Let's say you have data for 2 alleles, a1 and a2 of a specific gene which exists in the population .If p = the frequency of the a1 allele, and q = the frequency of the a2 allele then p + q = 1, at equilibrium assuming there are only 2 alleles, If the above 4 assumptions also hold, then

a1a1 = p2

a1a2 = 2 pq

a2a2 = q2

Example: if the frequency of sharp tongues is ?

You are given the following information- in a population there are 50 sharp tongued individuals ( sharp tongue allele a1 is dominant ) and 50 flat tongued, you can they estimate the allelic frequencies given that all the assumptions listed above hold.

SS + Ss = 50 ss = 50

q2*N = 50 and q2 = 50/100 = .5 take the square root to estimate q = .7

then, 1-q = p and 1- .7 = .3

Check: if p = .3 then p2 ( .09) * 100 + 2pq (2*.3*.7)*100 = 51 which is close to the 50 sharp tongues we observed.

If the numbers don't work out ( you know the estimated 'actual' frequencies) , and the number of expected homozygotes & heterozygotes are not what you count in the field, then you expect one or more of the assumptions are actually non-binding, and you must then figure out what is occurring- is there selection?, nonrandom mating? and so on...

A hypothetical example of this: The insects you are studying survive only a year. In year one

SS (dominant phenotype

Ss

ss

300

600

300

observed # in year 1

250

600

350

# in year 2

Relative success of SS = 250/300 = .83

Relative success of Ss = 600/600 = 1

Relative success of ss = 350/300 = 1.17

By convention, the genotype with the highest relative reproductive success has a fitness equal to 1 so we redefine:

relative fitness of :

ss = 1.17/ 1.17 = 1

Ss = 1.0/1.17 = .86

SS = .83 / 1.17 = .71

and the selection coefficient for genotypes x = 1 - relative fitness of x thus SS = 1- .71 = .29 which indicates there is a high selection force against this genotype.)

 

picture from : http://www.findingchrist.org/mothsnak.htm#pepmoth

An actual example of selection comes from the work of several Englishmen, especially Kettlewell, who studied the plight of peppered moth shown above, in the 1800's. There are two forms of the moth, the dark morph and the peppered morph: the dark morph increased over a period of a century as the forests darkened from pollution of nearby industries.

To prove that there was selection against the light form, which originally had done well on the white birch, he set up the following experiment.

He set up a mark-recapture experiment:

->he released 201 peppered moths & 601 melanic ( dark) morphs, all marked

-> he recaptured 16% of the peppered form and 34% of the dark forms.

He redid the experiment in a clean forest:

->he recaptured 12.5% of the peppered moths & only 6.3% of the dark morphs

He also did blind experiments to observe predation of the 2 morphs by birds, which supported his contention that in the dark forests the dark morphs eluded predation more than the light forms, and vice versa in the clean forest. In fact his observations indicated an even bigger selection differential.

He estimated from these numbers a selection coefficient of .53 and from this he could further calculate that it would take 47 generations for the dark form to increase in alleleic frequency from .05 to .95. This estimate is in the ball park of the actual time for the morph to reach the numbers it had.

You must remember that an s = .5 is a very high selection coefficient.

In most cases in nature,

selection would be much less intense,

not as consistent,

and very often an allele being selected for or against might be associated on the chromosome with another gene with potentially the opposite selective force.

for this phenomenon to occur there can be little or no migration

Given all these requirements, most alleles unless they are incredibly destructive for the organism are not gotten 'rid of' and remain in the population in low frequencies.

 

http://genbiol.cbs.umn.edu/1201/Peppmoth/peppmoth.html

Please read the short article below. In your own words, describe how important it is to know of the genetic patterns of a species or population.

Planting pushed squirrels west
Museum exhibits' genes record British forestry policies.
21 September 2001
JOHN WHITFIELD
Museum piece: genes show where the red squirrel went.
© SPL
Commercial forest planting in the north of England drew red squirrels westwards in the 1980s say UK researchers. This migration led to big changes in squirrel numbers and genetics.
The finding has implications for the protection of the endangered red squirrel (Sciurus vulgaris). Many conservationists advocate leaving 'wildlife corridors' for animals to travel between patches of fragmenting habitat. This is the first evidence that such corridors work, says Kirsten Wolff, of the University of Newcastle upon Tyne1.
Wolff and her colleagues reconstructed squirrel history by extracting DNA from museum samples dating back to 1918, and from contemporary roadkill. They compared the genetic structure of the population with satellite and historical maps of forest cover.
Deforestation in the eighteenth and nineteenth centuries left squirrels hanging on in remaining islands of trees, and competition from the larger American grey squirrel has inflicted further damage in the past hundred years. About 161,000 red squirrels remain in the UK, three-quarters of them in Scotland.
Small isolated populations are more likely than larger ones to become extinct by chance. They may also suffer from inbreeding.
Early in the twentieth century, squirrels from the northwest and northeast of England were genetically distinct, the researchers found. In the 1950s and 1960s the planting of the Kielder forest across the north of England reunited formerly isolated squirrel populations. These animals spread rapidly west when the trees reached squirrel-friendly maturity in the 1980s, says Peter Lurz, another member of the Newcastle team.
The eastern squirrels leapt westwards from one patch of forest to the next. The genetic results suggest that, although they don't like leaving the security of the forest, squirrels can travel between habitat islands up to about 1.5 kilometres apart. This should help conservationists decide where to put nature reserves.The beneficial effects of conifers indicate a way forward for red squirrel conservation
John Gurnell, Queen Mary and Westfield College London
But joining forest fragments has drawbacks, warns Woolf: "When populations become connected, diseases can spread between them." The deadly parapox virus, for example, is one of the biggest threats to British squirrels.
Red squirrels probably benefit more from having more living space than from the chance to mix genes, says ecologist John Gurnell of Queen Mary and Westfield College in London. Red squirrels seem quite resistant to the bad effects of inbreeding, he says - their populations have frequently crashed to small numbers.
Environmentalists have frequently opposed the planting of large numbers of alien conifers in the UK. But red squirrels depend on them, says Gurnell. "Because resources are so limited, future management will have to be highly targeted. The beneficial effects of conifers indicate a way forward for red squirrel conservation."
Gurnell adds that conifers may also be a barrier to the grey squirrel, which prefers deciduous woodland.
 
References
1. Hale, M. L. et al. Impact of landscape management on the genetic structure of red squirrel populations. Science, 293, 2246 - 2248, (2001). © Nature News Service / Macmillan Magazines Ltd 2001