Yacoubb's Bio 3+4 Questions

[Yacoubb]

Is a selecting agent the same thing as a selection pressure?

And is it accurate to state that members of a population with a high genetic fitness have a selective advantage to members of the same population with a low genetic fitness?

Thanks!

[vox nihili]

Is a selecting agent the same thing as a selection pressure?

And is it accurate to state that members of a population with a high genetic fitness have a selective advantage to members of the same population with a low genetic fitness?

Thanks!

Essentially. They're slightly different, but the difference I don't think is particularly important. Intuitively, selective agent is the thing doing the selecting, selection pressure is the thing causing selection. In most cases, you'll find that these are identical.

I'd be weary about using the term genetic fitness. Like I said in my previous post on this, HW equilibrium is not part of the course and language like genetic fitness is very typical of population genetics, something that isn't explicitly studied in the course. You'd be better to go with selective advantage. And you will not be asked things about heterozygous advantage and so on. The questions will not require a level of complexity such that you need to talk about things like genetic fitness.

[Yacoubb]

Essentially. They're slightly different, but the difference I don't think is particularly important. Intuitively, selective agent is the thing doing the selecting, selection pressure is the thing causing selection. In most cases, you'll find that these are identical.

I'd be weary about using the term genetic fitness. Like I said in my previous post on this, HW equilibrium is not part of the course and language like genetic fitness is very typical of population genetics, something that isn't explicitly studied in the course. You'd be better to go with selective advantage. And you will not be asked things about heterozygous advantage and so on. The questions will not require a level of complexity such that you need to talk about things like genetic fitness.

Fair enough Thanks for the heads up. Could someone check whether my definitions are correct

Gene pool:
Refers to the total genetic material of a population, in a given environment.

Fitness:/b]
A measure of genetic contribution to the next generation of a particular phenotype compared to other phenotypes controlled by the same gene.

Selective advantage:
A higher genetic fitness of one phenotype relative to other phenotypes in a given environment, of a trait controlled by the same gene.

Selecting agent:
An environmental factor that acts differentially on various phenotypes in a given population.

Allele frequency:
The frequency of a particular alleles in a given population, living in a given environment.

Phenotype Frequency:
The frequency of particular phenotypes in a population, living in a given environment.

[psyxwar]

Essentially. They're slightly different, but the difference I don't think is particularly important. Intuitively, selective agent is the thing doing the selecting, selection pressure is the thing causing selection. In most cases, you'll find that these are identical.

so in the example of the industrial revolution and moths and all that (hopefully you're all familiar with this example lol...):

The selecting agent would be the change in the color of the tree trunks (due to all the soot from the industrial revolution)

The selecting pressure would be predators of the moths (eg. birds), which would be killing of those who expressed phenotypes that did not camouflage them well in these new conditions.

Is that right?

[vox nihili]

so in the example of the industrial revolution and moths and all that (hopefully you're all familiar with this example lol...):

The selecting agent would be the change in the color of the tree trunks (due to all the soot from the industrial revolution)

The selecting pressure would be predators of the moths (eg. birds), which would be killing of those who expressed phenotypes that did not camouflage them well in these new conditions.

Is that right?

I don't think so. It's kind of a tricky one, so I've done a little bit of googling to try and get it straight in my head. In most cases they're essentially synonyms.

In the case of the moths:

The selection agent is predation, the selective pressure is to avoid predation. So the fact that the birds want to eat the moths (agent) will increase the frequency of the wildtype allele (because of the selective pressure of the predation).

[Yacoubb]

I don't think so. It's kind of a tricky one, so I've done a little bit of googling to try and get it straight in my head. In most cases they're essentially synonyms.

In the case of the moths:

The selection agent is predation, the selective pressure is to avoid predation. So the fact that the birds want to eat the moths (agent) will increase the frequency of the wildtype allele (because of the selective pressure of the predation).

For VCE Bio, are we expected to know the difference between selective pressure and selective agent?

[alondouek]

Yes, because examiners have historically been very picky with terminology. You will lose marks if you call a pressure an agent, and vice versa.

[Yacoubb]

Yes, because examiners have historically been very picky with terminology. You will lose marks if you call a pressure an agent, and vice versa.

Selecting pressure causes the selection, selection agent is what does the selecting.

E.g. Tall plants vs short plants

Tall plant phenotype is favourable over short plant phenotype due to taller plants being able to photosynthesise more effectively.

Selecting agent: Light intensity
Selection pressure: Increased photosynthetic rate of the tall plants

^ Could someone please fix that up. I'm pretty sure its wrong, but I'd just like someone to explain what the right answers are and why.

Thanks

[vox nihili]

Yes, because examiners have historically been very picky with terminology. You will lose marks if you call a pressure an agent, and vice versa.

Was my definition right?

Selecting pressure causes the selection, selection agent is what does the selecting.

E.g. Tall plants vs short plants

Tall plant phenotype is favourable over short plant phenotype due to taller plants being able to photosynthesise more effectively.

Selecting agent: Light intensity
Selection pressure: Increased photosynthetic rate of the tall plants

^ Could someone please fix that up. I'm pretty sure its wrong, but I'd just like someone to explain what the right answers are and why.

Thanks

Assuming what I said was correct.

The agent is correct, the pressure is to harvest more energy from it though. The pressure isn't the result, it's the thing that's "pressuring" selection to occur.

[alondouek]

The selecting agent exerts the pressure on the population, if that helps clarify things. The agent selects for or against a phenotype; the pressure is what results from that.

Edit: t-rav, I'm not confident that your definition is correct. I'd say that the selection agent is the bird, and the selection pressure is predation.

[vox nihili]

The selecting agent exerts the pressure on the population, if that helps clarify things. The agent selects for or against a phenotype; the pressure is what results from that.

Edit: t-rav, I'm not confident that your definition is correct. I'd say that the selection agent is the bird, and the selection pressure is predation.

I think what we've clearly got here is that the difference is bloody tricky.

I just had a quick look at a textbook and it said "the selective agent is the environmental or biotic factor that causes selection to occur", and gave climate change, predation and bacteria/viruses as examples.
Another source though, which was a journal article, gave a really specific selective agent, it was an antibiotic.

So now I've got no idea :| haha!

[alondouek]

I'd go with the journal hahaha, probably a bit more reliable and reviewed than a textbook in most cases

I tend to think of selecting agents as entities, and selecting pressures as occurrences/events. It fits to most credible definitions that I see, an it helps draw a distinction between the two

[vox nihili]

I'd go with the journal hahaha, probably a bit more reliable and reviewed than a textbook in most cases

I tend to think of selecting agents as entities, and selecting pressures as occurrences/events. It fits to most credible definitions that I see, an it helps draw a distinction between the two

Selection pressure: constraints upon phenotype from the environment that produce differential gene transmission.

That's from the penguin dictionary of Biology. To be honest though, it's a relatively unclear definition and there's nothing provided for agent.

In my opinion, selective agent is factor that results in selection and the selective pressure is the effect of that factor upon the organism, in terms of gene transmission. So with that, back to the moths, predation is both the pressure and the agent, to some extent. I think in most cases they're just accepted as being the same thing to be honest. The distinct lack of resources that Google seems to shoot back suggests that there probably isn't a significant difference, if any.

[alondouek]

haha we biologists have more important things to worry about

[vox nihili]

haha we biologists have more important things to worry about

Hahah exactly. I kind of got half way through the last reply and thought "actually, who gives a shit?".