I have to say, at the moment I’m feeling a bit like this chap:
Mainly because we are currently ‘between’ registrars & so I don’t seem to have a life! But I shall press on – with a question from the 2006 paper on human evolution.
The question provided a hominin phylogenetic tree (something like this, perhaps) & followed this with the following information:
Although there are differences in interpretation of the evidence, it is generally accepted that human biological evolution has not been a linear progression from one species to the next. In fact, there are now thought to have been between 5 and 7 hominin genera, and at several points there have been 2 or more species and/or genera in existence at the same time.
Significant factors involved in the biological evolution of humans include:
genetic drift
natural selection
cultural evolution.
Using selected named hominins, discuss how each of these three factors has contributed to the biological evolutionof humans.
I rather like this question. First, because it’s one of those lovely ‘big picture’ questions (like all Schol Bio questions, in fact), applying evolutionary concepts to our own family line in a more in-depth way than the L3 examiner can. And second, because it makes it clear that natural selection isn’t the only player in evolutionary change: genetic drift can (& often does) play a significant role. (Darwin, of course, with his lack of knowledge of how heredity works, could have no concept of genetic drift. Yet another example of how science moves on as new information comes to hand.)
Anyway, what was the examiner after?
Well, you need to show that you know what these terms mean. And because there are 3 terms needing definitions, you could really split the essay into three main sections & deal with each in turn. First up, genetic drift: what is it, how is it relevant in a named hominin, and how might it have contributed to human evolution? It would be a really good idea to set these points – & any related ideas – out in an essay plan before you get started on your actual answer.
Genetic drift is where you get changes in allele frequencies in a population due to chance events (e.g. all the mice carrying a dominant allele giving them long fur shelter under a tree during a storm – to keep dry, of course. Anyway, lightning strikes the tree, it falls on the mice, & behold – the long-hair allele is removed from the population…) It’s often the result of a population bottleneck event: if a small group of individuals leaves a parent population (or is all that’s left after some catastrophe – like the mice in my example), those migrants are unlikely to be representative of the parent population’s gene pool, simply by chance. Homo floresiensis was isolated on a small island & probably had a fairly small population, so genetic drift could well have played a role in this species’ evolution. The same could be said for populations of H.sapiens migrating out of Africa, & could possibly have have influenced the distribution of A/B/O blood groups around the world.
Natural selection – you can probably give a good definition of this. The examples you could use to demonstrate how this affected named hominins could include: changing environmental factors & their possible impact on the evolution of bipedalism, or the effect of climate on body form. This last one’s quite well known, & in fact has a couple of ‘rules’ describing how it works. Allen’s rule (or law) states that warm-blooded animals in a cold environment will tend to have shorter protruding body parts (including limbs & digits) than members of the same species in a warmer region. The corollary, Bergmann’s rule, says that a species population in a cold area will have a larger body (greater mass) than a population from a warm place. Both are adaptations that reduce heat loss through a reduced surface area: volume ratio. For human evolution – the shorter, squat Neandertal physique may well have been the result of selection pressure from the harsh, cold climate of Europe 300,000 years ago.
And cultural evolution? It involves the transition of learned behaviours from one generation to the next, and cumulative change in these behaviours. You could choose from a range of cultural innovations to explain the relevance of cultural evolution in our own lineage. Tools, for example: the development of tools by Homo habilis offered the possibiity of processing food more efficiently, & perhaps of accessing new food sources. After all, with their relatively small teeth, habilis individuals wouldn’t have had much luck getting the meat off any carcase that they managed to scavenge. But even very simple stone tools would allow them to cut or scrape meat from the bones, thus providing a whole new source of protein. And the greater nutrition (in terms of both energy & nutrients) would have been quite an advantage & could well have underpinned changes in brain size, for example. (Which in turn could lead to more complex tool technologies, greater processing ability… you get the picture.)
Jim Thomerson says:
In discussing genetic drift in a small population which becomes separated from a larger population, we understand that sampling error will make the small population not an exact analog of the larger population. However, the small population cannot be an exact analog just because it is small. I have never seen this discussed, however.
Suppose the small population is 50 diploid individuals. Possible gene frequency ranges from a maximum of 100/100 to a minimum of 1/100. In the larger population, there probably are rare genes with frequencies like 1/1000, 1/10,000, etc. These frequencis are impossible in the smaller population. Suppose that one of the 1/10,000 frequency genes just happens to be present in the smaller population. Its frequency has suddenly increased to 1/100. That strikes me as significant.
david w says:
Hi Alison,
We’ve just finished up our 3rd year labs on simple Popgen simulations and experiments down here.
One of the things that the students find interesting and might be relevant here is that in small populations drift can counteract selection becaue even beneficial mutations are more likely to be lost than fixed, you can actually do the simulations with the Panda’s Thumb’s cool popgen simulator
If you set up a simulation with:
population size: 1000
initial allele frequency of A: 0
fitness of A: 1.5 and a
forward mutation rate: 1×10-8
You have a model of a population ‘waiting’ for a mutation that will be selected for when it arrives. With a population of 1000 A is almost always fixed within a few generations of the mutation arising in the population but if you take the population size down to 50 or so new A alleles are actually more likely to be lost than to be fixed (you see them arrive in the population, wriggle around a little the fall back out).
This is a prediction borne out in genome data. If you compare the proportion of substitutions that have a signature of selection between Drosophila species and the same between primate species you find much, much more evidence for selection in the fixation of Drosophila mutations than primate ones.
So, as well as changing allele frequencies genetic drift in populations of early hominids might well have prevent slightly beneficial mutations from fixing (there is a nice summary of neutral evolution here ).
Jim,
What you talking about is the basis of some of Ernst Mayr’s ideas about speciation following bottle necks and “genetic revolutions” – it’s probably not talked about that much because it’s not clear that it really matters in speciation (though getting at the genomics of that process is very much in field in its infancy)
Alison Campbell says:
Thanks, David – sounds like a useful tool; I’ll suggest to our first-year tutor that we look at incorporating it in one of our popgen labs 🙂
Jim Thomerson says:
Take a look at this and shake your head over something published in 1971, perhaps before you were born.
http://www.kendallhunt.com/samples/663.pdf
The concept of natural selection is based on a correlation between high fitness and genetic make up. I think this is much more likely to be true in a large population than in a small. In a small population the noise of “luck” could be more important than genetics in determining fitness.