Seminar 2 2011 Discussion Page Challege 2

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LENScience Senior Biology Seminar Series 2011
Rethinking Polynesian Origins Challenge 1
Challenge 3

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South City High School - Student MB    16th April 2pm

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Rethinking Polynesian Origins - Discussion Page

Challenge Question 2

Some scientists now think that Neanderthals, modern humans and Denisovans should all be considered sub-species of Homo sapiens. The problem is biologists have different ways of defining species. Three of the most common are the biological, morphological and phylogenetic species concepts.
Compare and contrast the three methods. Discuss the strengths and weaknesses of each method and discuss the limitations of using only DNA evidence to decide on whether two populations are a different species.

Onehunga High School - Cameron Whiteman and Sarah Darroch 15-4-11

The morphological species concept supports the view that members of a species are individuals that look similar to one another. The biological species concept defines a species as members of populations that actually or potentially interbreed in nature, not according to similarity of appearance. The Phylogentic concept of a species as an irreducible group whose members are descended from a common ancestor and who all possess a combination of certain defining, or derived traits. The biological concept is supported by most biologists because two individuals belong to the same species if their gametes can unite with each other under natural conditions to produce fertile offspring, morphological species concept is not preferred by biologists because individuals of two populations very hard to tell apart would not mate with one another, suggesting that they were in fact different species. The phylogenetic concept because slight differences can be found among virtually any group of organisms, the concept tends to encourage extreme division of species into ever-smaller groups.

Viewing a population by their mtDNA and nuclear DNA is very different looking at them through their biological and morphological aspects. Two populations may look very different in appearance but be biologically defined as the same species because they can breed together. Their genetics may be close enough to produce fertile offspring. Comparing mtDNA and nuclear DNA can show us the genetic similarities between populations and allow us to see if they are the same species.

mtDNA is looking at the DNA produced in the mitochondria of the person. One mitochondrial DNA is passed down through the mother. They mutate faster and do not recombine. Benefits of looking at mtDNA are that you can see the linage and pass down of the genetics through the mutations present in the cell. You also have a more pure version of the DNA because mtDNA doesn’t recombine or crossover. This means that new material isn’t being mixed with the inherited mtDNA. Nuclear DNA is also useful for finding out the genetic relationship between populations but it does have weaknesses. Nuclear DNA is a mixture of both parents. This means that it is less pure in definition than mtDNA.

Lisa Matisoo-Smith, Allan Wilson Centre, 5 May

Very nice answer – but you missed one important point – the limitations of using mtDNA alone is that it only tells you about what is happening on the maternal side. As in the case with the Neanderthals, the mtDNA showed no introgression of Neanderthal DNA in modern humans but when they did the analysis of nuclear DNA they did find a small contribution. This may suggest that the admixture was from Neanderthal males mixing with Homo sapiens sapiens females.
A major problem with the Biological Species concept is how do you determine the species of fossils? It has only been very recently that we could get DNA out of samples as old as Neanderthal and Denisova, so we can make some judgements about interbreeding. It is unlikely that we will ever be able to get DNA out of Australopithecines for example – so we are stuck with morphological analyses alone!

Michal Denny, LENScience, 6 May

Good explanations of the three species concepts.Thinking about what the question asked, you need to develop the section on the strengths and weaknesses of each method. doing this is how you would should evidence of compare and contrast. 

Conor N - Horowhenua College - 6/05/2011

The biological species concept defines a species based on breeding. The concept states that a species is defined based on whether or not they interbreed and produce fertile offspring. This contrasts to the morphological definition of a species which defines a species based on their appearance. If two organisms appear similar in appearance, then the morphological concept defines them as the same species. As a result, these two definitions can contradict each other. For example, the species of ant, Pheidole barbata, shows two very distinct ants; that while look different, do in fact come from the same species.

The problem with the biological concept is that it relies on sexual reproduction. Therefore it becomes useless when trying to define the species of an asexual organism. Also, it cannot be used when trying to define the species of an extinct organism. If only fossils remain, then morphological definitions become more useful than biological. However, the morphological concept is flawed because many organisms have been shown to look different, but successfully interbreed. The morphological concept is based on bringing together a species, where the biological concept is based on defining the lines between the species.

The phylogenetic species concept defines a species based on whether or not they have a common ancestor. As an extension of this, this species will also share common traits, passed down through the ancestry. The theory is strong because it restricts less than the biological concept; allowing for species interbreeding. Despite this, it still presents problems because tiny variations are always seen in a group of organisms. Therefore, the phylogenetic concept would cause these variations to become another new species, and then another, and so on, until there is just a huge amount of miniscule species.

In using DNA evidence, the only species concept that could be applied, is the phylogenetic. By DNA only, the morphological and biological definitions become redundant as it is impossible to determine appearance and/or interbreeding habits. Also, if mutations had occurred through the ancestry, this could further complicate the process of sorting a species. Since the phylogenetic concept does not take reproductive boundaries into account, it may differentiate between species which are actually connected by gene flow. Since gene flow has such a significant role in evolution, species can begin to take on new traits without actually becoming a new species. This is something that the phylogenetic species concept does not account for.

Michal Denny, LENScience, 6 May

Conor, thanks for your contribution. Your answer is comprehensive and clearly written. I like the way you have structured your answer with the first 3 paragraphs dealing with the three species concepts and then the strengths and weaknesses of each. This makes the logic of your argument easy to follow and understand especially when you address the last part of the question. But is there something missing from the end? the last sentence seems to be unfinished :-) Haha, thanks for pointing that out Michael. Completely missed that.

Lisa Matisoo-Smith, Allan Wilson Centre, 12 May

Nice answer, but – what about looking at the mtDNA and nuclear data for Neanderthals (which are extinct) and humans? The mtDNA initially suggested that they are different species, but we see from the whole genome study that there is a component of Neanderthal nuclear DNA in modern humans indicating that there was interbreeding. And similarly with the Denisovans!