snail+of+the+world

The rate of speciation in snails can be dated using molecular evidence and information from geologists on the age of islands. They provide evidence about possible modes of selection for reproductive isolation.
 * __Partula on Moorea__**

__**When did Crampton make his observations?**__

• Tahiti- 1916 • Moorea- 1932

__**What does Prof Bryan Clarke wear on collecting trips?**__

He wore pyjama top due to the hot conditions, the pyjama top pocket allows him to collect the snails.

//P. suturalis (right and left coiling)// //P. taeniata// //P. mirabalis// //P. tohiveana//

//The allozyme information can be used to reconstruct evolutionary history by within and between species comparisons.//

//The whole anatomy including the position of the genital aperture is affected by right and left coiling. Within species, right and left coiled morphs have reduced success in mating.//

__**Why should there be such a pattern?**__

Difference in right and left coiling arose due to a mutation that occured from a mother with left coiling on her shell, who had an offspring with right hand coiling (not clear, but is wrong or muddled). The phenotype is determined by the genotype of the mother, so in the case that a mutation occurs in the germ line of a female, her offspring will have the cryptic change in genotype and the grand-offspring will actually show in the change in coiling) . It is thought thar positive frequency dependant selection acted upon the right hand coil, but what was the frequency dependent selection?? a selection that result in increasing the number of "rare" individuals. In this case the offspring with the right hand coiling was favoured why?? so it increased in population.

The species P. suturalis has both right handed and left handed individuals that can mate with fertile offspring. When studying the behaviour of the snails when choosing mates, there is a higher chance that a snail will prefer to mix and mate with other snails that have phenotypes similar to its mothers, no, similar to itself. regardless of incompatibility that arise due to morphological differences between the left and right handed coiling.

However species that are distinctly left handed or right handed such as P. taeniata or P. mirabalis can not physically mate yes they can. Mating frequency is reduced though, due to the differences in position of the genital aperture. This barrier of reproduction is advantageous as it prevents the species from wasting their gametes, as the offspring will not be viable. If a hybrid is produced, then offspring would either be infertile or have low fitness (not really infertility is a form of low fitness - broadly low fitness can be due to reduced fertility, or reduced survival to breeding age (there are other), hence they will have low survival not if the effect is lower fertility rate therefore their alleles are removed from the gene pool.

__**How do Crampton's observations corroborate this story?**__

Crampton and Clarke both studied the species of Partula in snails found on the islands of Moorea and Tahiti. Crampon visited the islands to collect the snails and studied their phenotypes in relation to their locations on the island. On these islands he classified the snails according to shell coiling, right handed coiling was dextral, the left hand coiling was named sinistral and found that the right handed coiling was found in the south of the island whereas the left handed ones were found in the north. No this is not the relevant points. His key observation was that, where species overlap, they tend to differ in the direction of their coiling

On the other hand Brian Clarke analysed the molecular composition of allozyme by using gel electrophoresis. He found that the family of genes responsible for the allozyme composition between snails on different islands were from the same family of genes as they were significantly similar to each other, suggesting they were related. No its nothing to do with the gene families. The differences in allozymes were difference in the allele detected at the same locus.

These two observations are linked as the genetic data sho//w//s that they have a common ancestor which accounts for the similarities in their genome, yet they have different phenotypes due to adaptations to different environments.

OK

The chains of islands that are located on the pacific ocean develop at different times and at different rates. When a new island forms, the conditions are heterogeneous to the previous islands evidence? , hence when a snail of the same species are introduced to a new island, they become genetically different to the original population because the new island has different selection pressures acting upon them, such as differences in food availability, predation and ecological environment. what about drift?
 * __How does the molecular evidence show that speciation occurred in situ on the islands?__**

As the islands continue to develop at different paces, the environment on the virgin islands will be less stable when they are younger, this results in a higher variation of alleles no. Selection will tend to cause the fixation of new (or at least different alleles) in the population as the organisms need to continuously evolve to keep up with the new changes. In comparison, the older islands are more stable so the gene pool remains almost stable. Not likely. The rate of change may be slower. If evolution stopped in a stable environment, species would not continue to diverge after they had split. The rate of substitution does not directly link to speciation, but produces genetic diversity within a population. No. Substitution is the accumulation of differences between species. Often fixed within each species. Genetic diversity within each species is due to mutation since the common ancestor. Therefore allopatric speciation occurs as a result of individuals from the same species being physically isolated by being located on different islands, hence they can not interbreed, so gene flow is prevented. Perhaps. But the whole point of the observation that the species on Moorea share a common ancestor is the speciation has occurred ON THE ISLAND

The molecular evidence can be obtained from analysing allozyme composition from snails on different islands. Variation in the DNA is correlated to the relationship between the species, not clear as species on different islands show they have the same family of enzymes no, see above as would be expected from species with a common ancestor. However due to the different mutation rates on the different islands, the allozyme composition is not identical. we don't know that the mutation rate is different. We assume not, and that the differences are due to differences in the rate of fixation.

__**What are the ages of Tahiti and Moorea?**__

• To work out the ages, they looked at the rocks which are used as molecular clocks that suggest Tahiti- 1mya and Moorea- 1.5 mya.

__**What are the relative rates of evolution on these islands and the Society islands?**__ Possible theories to explain the difference in the rate of evolution on these islands and society islands include:


 * Snails on new islands evolve faster because new islands require new adaptations as their conditions are unstable, they are subjected to high selection pressures acting upon the new snail species resulting different genotypes being selected for.


 * Another theory explaining the difference in rate are founder effects which lead to genetic drift eventually resulting in the fixation of mildly deleterious alleles

Finally, as new virgin Islands are developing or changing, the snails require new adaptations to keep up with the new environment, the changes occur more readily in smaller populations leading to a higher rate of genetic drift. However, in the older islands the environment is more stable, therefore the snails do not need to adapt- slower rate of change. you need to separate what is an observation (the differences in allozymes) and what is interpretation which may be wrong (that the patterns are due to differences in selection intensity, or rate of drift). So you could say 'Estimates of the substitution rate between species on the older islands is lower than that on younger islands (over the shorter time period since they were formed' [that is a uncontroversial fact]. 'This could be due to the rapid rate of adaptation when an island is first colonized, or the more rapid changes in the environment on young islands leading to changes in the selection regime, which in turn would lead to more rapid changes in the genotype' [note the use of 'could be' and 'would'] 'or it could be due to more rapid genetic drift acting on the founders genetic variation, due to the small initial population size' [note the use of 'could' again, and the fact that we are presenting various alternatives.


 * __Why is it difficult to explain these rates of evolution if selection is responsible for substitutions?__**

As mentioned before, the islands all have different rates of substitution, which is a result of the islands developing at different times, which in turn leads to a high genetic variation in the population no... variation between species within the island - not variation within a population. it is difficult to explain that selection is solely responsible for substitution because, if genetic load occurs, i.e. the individual dies before passing on its deleterious genome, then the population will only consist of advantageous alleles which will eventually become fixed. As a result, the variation will reduce as these genotypes are being removed from the gene pool. However when observing the islands, a high genetic variation is present therefore selection can not be the only process acting upon evolution of the snails on these islands.

group members: Tara Arif, Alaa Aziz, Victoria Estacio, Asem Azid, Farha Sonvadi, Ashia Mahboob

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––– ––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––– Partula on Moorea

The rate of speciation in snails can be dated using molecular evidence and information from geologists on the age of islands. They provide evidence about possible modes of selection for reproductive isolation.

When did Crampton make his observations?

Crampton made twelve separate expeditions over the course of his career to Moorea near Tahiti to study the land snail genus Partula, while years more were spent measuring and cataloguing his specimens. In all, he dedicated nearly half-a-century to the study. These were the dates of his findings published onstudies of partula.

• Crampton H. E. 1916. Studies on the variation, distribution and evolution of the genus Partula. The species inhabiting Tahiti. Carnegie Institution of Washington, 228: 1-311. • Crampton H. E. 1925. Studies on the variation, distribution and evolution of the genus Partula. The species of the Mariana Islands, Guam and Saipan. Carnegie Institution of Washington, 228a: 1-116. • Cooke C. M. & Crampton H. E. (1930) "New species of Partula". B. P. Bishop. Mus. Occ. Papers 9: 3-5.

Specifically the Tahiti findings were in 1916 and the Moorea in 1932.

What does Professor Bryan Clarke wear on collecting trips?

Professor Bryan Clarke wears pyjama tops due to the humid conditions. P. suturalis (right and left coiling) P. taeniata P. mirabalis P. tohiveana

The allozyme information can be used to reconstruct evolutionary history by within and between species comparisons.

The whole anatomy including the position of the genital aperture is affected by right and left coiling. Within species, right and left coiled morphs have reduced success in mating.

How do Crampton's observations corroborate this story? nd click paper…or find his paper… Something to do with molecular clock etc.

No answer? What are the ages of Tahiti and Moorea. Tahiti:

Fifty-eight new K-Ar age determinations from Tahiti and Moorea indicate that these islands were volcanically active during the past 2 m.y. The ages from Tahiti are clearly younger than the more westward island, Moorea. What does Prof Bryan Clarke wear on collecting trips?

P. suturalis (right and left coiling) P. taeniata P. mirabalis P. tohiveana

The allozyme information can be used to reconstruct evolutionary history by within and between species comparisons.

The whole anatomy including the position of the genital aperture is affected by right and left coiling. Within species, right and left coiled morphs have reduced success in mating.

Why should there be such a pattern?

There should be such a pattern because if they have different apertures it will reduce interbreeding which can lead to sterility and reduced fitness of offspring. As they are from different habitats,

Not really. The point is that sometimes the range of the two species extends they come into contact in the SAME habitat. Furthermore it is assumed that the two species initially were of the same coiling direction... there was then selection in favour of the mutations that changed coiling direction in one of them

overlap between species lead to an overlap between right and left handedness.

Usually rare alleles are selected against BUT in areas of overlap the selection against rare alleles may be outweighed as Right handed species will not waste their gametes on inappropriate mating.

How do Crampton's observations corroborate this story?

Crampton went to Moorea and Tahiti and studies the species Partula on the two islands. He studied their phenotypes and locations on the island in comparison to different landscapes.  This does not answer the questions. Crampton found that the species tended to have different coiling directions when they overlapped. Those observations conforms to the prediction generated by Clarke's theory. Brian Clarke wanted to find out the molecular composition. He found that the family of genes responsible for the allozyme composition between snails on different islands were from the same family of genes and that the species were actually related. These two observations are linked as the genetic data shows they have a common ancestor which accounts for the similarities in their genome, but their different phenotypes are due to their adaptations to different environments.

How does the molecular evidence show that speciation occurred in situ on the islands.

It occurred in situ as the molecular clock runs at slightly different rates from island to island. If speciation did not occur in situ due to the close proximity of these islands the rate of the molecular clock should be the same. No. The above is true, but not relevant to the question. The key observation to show in-situ speciation is that the tree reconstructed from allozymes shows that all species on the islands share one common ancestor on the island.

What are the ages of Tahiti and Moorea?

Fifty-eight new K-Ar age determinations from Tahiti and Moorea indicate that these islands were volcanically active during the past 2 m.y. Rocks and Molecular clocks were analysed and from this the ages of were established. Tahiti was formed about 1m.y.a. and Moorea about 1.5m.y.a.

What are the relative rates of evolution on these islands and the Society Islands?

Rapid changes occurred in the new islands and so mildly deleterious alleles were established in a relatively new population. There was increased dramatic genetic drift due to there being a small population this could lead to these alleles being selected for.

Why is it difficult to explain these rates of evolution if selection is responsible for substitutions?

This may be because if it was due to selection you would have one or the other being selected for at each site and not a mixture of two when there is an overlap.

This answer is not clear. You would expect some convergent evolution if substitutions were all due to selection, which would make tree building very difficult, as genetic similarity would not be a simple function of time since a common ancestor

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