It's a bit strange to me that speciation is covered in unit 4 and classification in unit 3 since these topics are so connected. (everything in bio is connected but these are very tightly linked).
So, I thought I'd put together this guide on speciation and classification. Each topic is listed under what unit it's for so you
can just jump to the unit 3 ones but if you become confused I recommend going back and reading the unit 4 sections as well.
For unit 3, you should be familiar with the biological species concept and the phylogenetic species concept, as well as what a clade is. Speciation isn’t covered until unit 4 but since it links in closely with classification so let’s take a look at those together now.
How do populations change over time? (unit 4 content)We can split changes that occur in a population into two different types: heritable and non-heritable. For example, a population of birds might change over time if a new type of plant is introduced to the area. For example, there could be behavioural changes like using the plant for nesting material and/or their beaks might change size and/or shape. Something that non-biologists tend to overlook sometimes is that even without a new change to adapt to, populations are always changing.
Just by luck there might be a few generations where by random chance birds with larger beaks tend to have more children, and if beak size is inherited this will mean that the average beak size in the population increases during these generations. This is an example of genetic drift (in this case, from demographic stochasticity – don’t worry if you’re unsure what that means).
Of course, it might also be that beak size does actually matter for how many children a bird will have (e.g. if a bird with a small beak is more likely to starve to death it’s less likely to have lots of kids). In this case, we would expect the birds with the “best” beak size to have more children, so if beak size is inherited we would expect the average beak size to become more similar to “best” one over generations. This is an example of natural selection.
To talk about this properly, we say that organisms with a trait that increases how many offspring they are likely to have experience a selective advantage. Organisms with a trait that reduces the number of children they are likely to have experience a selective disadvantage. The selection pressure picks which organisms are likely to have more children, and therefore have their heritable traits be present in more of the next generation.
If we quickly accept that alleles (versions of a gene) are the units of heritability then we have:
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Genetic drift: random change in allele frequencies across generations
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Selection: change in allele frequencies across generations due to differences in expected number of offspring
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Microevolution: changes in allele frequencies across generations
Speciation (unit 4 content)When we talked about the changes in a population earlier we kind of assumed that the whole population changes together but what if it didn’t? For example, maybe there’s a bird population split across two islands. If the islands are really close then all the birds might move between them and interbreed a lot. You would then expect the frequencies/percentages of different gene types (alleles) to be kept very similar on both islands, since if it changes on one island the change will be carried over to the other one too.
If it’s harder to get from one island to the other, we would expect there to be less movement and interbreeding, meaning less gene flow, and therefore changes in allele frequency on one island are less likely to be carried over to the other one.
We know from looking at genetic drift and selection that there are random and non-random changes in populations occurring all the time. These changes stack up over generations, changing populations, and without high gene flow differences will accumulate.
This means that our birds on our two islands without much gene flow will eventually accumulate so many differences that even if the islands were right next to each other, those differences would make it hard for birds from different islands to produce offspring together. For example, there might have been changes that influence sperm-egg compatibility, physical changes that reduce compatibility, or changes in what traits are viewed as attractive.
Once the groups are so different that they cannot produce viable offspring together speciation has occurred. I.e. there are now two populations, each belonging to a different species
Is that the one true species definition then? (unit 3 content)
It’s a bit more complicated than that.
Biological species concept (unit 3 content)The biological species concept says that if two populations cannot produce viable offspring together they are not the same species (this makes sense as a rule for sexually reproducing species) and it also says that if two populations can produce viable offspring together then they are the same species (oh no).
The biological species concept seems straight forward so you might be question the “oh no” but the problem is that there are situations where it just doesn’t work.
One limitation I’ve hinted at is that it doesn’t work for classifying organisms which don’t experience sexual reproduction. If you have a group of organisms that only experiences asexual reproduction, then you would need to say that every single organism is a different species – even if the are genetically identical. Obviously this isn’t very practical for grouping organisms together as part of a species.
Another limitation is that there are cases where the biological species concept doesn’t work even for organisms which sexually reproduce. Let’s consider the example of some fish where there are a bunch of small populations which mix together, but only fish from populations close to each other can produce viable offspring together (since the ones further away accumulate more genetic differences to each other). Then you could have a scenario where under the biological species concept the fish far away from each other are both considered to be the same species by also not considered to be the same species – clearly that doesn’t work. This is easier to show with a diagram:
Finally, we often know whether organisms
do interbreed but for organisms that don’t interbreed we don’t know if they actually
can.
Phylogenetic species concept (unit 3 content)To address some of these issues, you might instead use the phenotypic species concept, which looks at genetic relatedness. This looks at the smallest monophyletic group / clade. This might be new language to you so let’s take a moment to examine what that means:
• before we showed an “upside down y” showing how one population can split into 2 species
• this happens again and again and again throughout macroevolutionary time, resulting in a phylogenetic tree with many branches.
• a monophyletic group / clade is a group containing all of the descendants of one common ancestor
I’m a big fan of basing classification on phylogeny but there are some issues with this approach.
One is that sometimes morphology changes faster than the genetic markers researchers look at, so there might be clear physical differences between two populations but their genetics might be too similar to separate them using the phylogenetic species approach. The same thing can happen with location, where populations are more genetically similar to a population across the other side of the world than their next door neighbour they mix with all the time.
When it comes to asexually reproducing organisms, there’s an accepted range of genetic similarity used to determine if they are the same species. Speciation in bacteria is often caused by horizontal gene transfer (rather than just passing on your genes to your children, giving some to another organism), which complicates phylogenetic relationships.
That's the guide done! If you have any questions please feel free to ask - to do that you'll need to
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Topic: Speciation and classification (Read 3690 times)