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AQA Biology Unit 4 with Anon_98.

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Global warming.

The concentration of carbon dioxide in the atmosphere is affected by:

- The removal of carbon dioxide from the air by plants for use in photosynthesis.

- The addition of carbon dioxide to the air by all organisms as a result of respiration.

- The addition of carbon dioxide to the air as a result of volcanic eruptions + natural fires.

The levels of CO2 in the atmosphere fluctuate as overall rates of photosynthesis + respiration change. In winter, the amount of photosynthesis is reduced due to:

- Cooler temperatures.

- Shorter day length.

- Loss of leaves by many plants.

In summer, the balance is reversed.

What is the greenhouse effect?

- A natural process.

- Keeps averaging surface temperatures around 17C

- Heat radiation from the sun is trapped by gases around. (CO2 is a greenhouse gas)

Carbon dioxide may be a factor in global warming by contributing to the greenhouse effect BUT remember that the evidence is correlational so no definite cause and effect can be proved.

ALSO, the increased temperature will cause carbon dioxide dissolved in water to escape therefore global warming might be causing the increase in carbon dioxide concentration.

Reply 41

Anon_98

Succession - Topic 7.

Alright, so basically organisms that survive in an environment are adapted to the environment. If the environment changes then those organisms will be less well adapted to the changed environment.

Other organisms may be better adapted to the changed environment + out-compete the original organisms, which may become locally extinct.

Like, the ecosystems that exist today did not always exist but they developed from other previous ecosystems by succession.

Reply 42

Anon_98

So, you need to know that succession often involves an initial colonisation of a hostile environment and the colonisers cause changes in the abiotic environment. These changes allow other species to enter the area because it is now less hostile. In turn, these new species FURTHER MODIFY the environment, making it less suitable for the colonisers and more suitable for yet, newer species. As a result, further changes in the community occur. Therefore the complexity of the food webs, which make up the community of the developing ecosystem, increases.

The succession leads to a final state called the climax community.

Reply 43

Anon_98

Where a succession starts from bare, previously uncolonised, ground or from a newly formed pond with no life, the succession is a primary succession.

However sometimes communities are destroyed (e.g. by fire) so when a new succession begins in such an area then it is a secondary succession.

Reply 44

Anon_98

Succession + conversation of ecosystems.

So, as we've gathered, succession leads to a change in an ecosystem, whereas conservation might require the ecosystem to be maintained in more/less the same condition.

People conserving ecosystems must interfere with, and manage the process of succession in such a way that the ecosystem does not change significantly.

An example of managing succession is the management of heather moorland.

Alright so:

Heather moorland is usually maintained as a habitat for grouse, but it also provides a habitat for other animals that would not exist elsewhere.

Heather plants are small, woody shrubs + where there is a dense cover of heather, it provides an ideal habitat for grouse.

BUT

When the heather grows taller, the cover becomes less dense which means it is now LESS SUITABLE for grouse.

There are several stages in the life cycle of a heather plant, one of which is a degenerate phase where trees begin to grow in the gaps of heather plants and this phase is basically the start of a succession.

HOWEVER A SUCCESSION IS PREVENTED... by burning the heather at regular intervals.

The fire is carefully managed so that it doesn't become too intense + damage the underground parts of the plants. These then re-grow and the cycle starts again.

Succession is prevented and the heather moor is now conserved!

Reply 45

Anon_98

Inheritance - Topic 8.

Genotype- The genetic constitution of an organism.

Phenotype- The expression of this genetic constitution and it's interaction with the environment.

Alleles - One or more alternative versions of the same gene.

(Alleles at a specific locus may be either homozygous or hetreozygous and alleles may be recessive, dominant or co-dominant. There may also be multiple alleles of a single gene.)

You get practice with monohybrid crosses + sex linked characteristics, by doing past papers + examples so I can't really make notes on that.

Reply 46

Anon_98

The Hardy-Weinberg principle- Topic 8.

Gene pool.- The alleles of all the genes that are present in a population.

Populations with a large gene pool have a high genetic diversity (they can withstand changes in the environment + have the necessary adaptions to survive) + ofc, populations with a small gene pool therefore have a small genetic diversity. (more likely to become extinct as a result of environmental changes)

So, within a gene pool every allele or gene has a particular frequency.

Frequency of an allele - the number of occurrences of that allele in that population as a proportion of the number of occurrences of all the alleles of that gene. (usually expressed as a decimal.)
The sum of the frequencies of all alleles of a gene must therefore equal to 1.

The Hardy-Weinberg principle states that the frequency of the alleles of each gene will NOT change from generation to generation.

Assumptions of the Hardy-Weinberg principle/conditions which must be met are:

- Population is v large.

- Mating is random.

- No selective advantages for genotypes.

- No mutation, migration or genetic drift.

The Hardy-Weinberg equation is as follows:

p² + 2pq + q² = 1

Where p represents the frequency of the dominant homozygote,

q represents the recessive homozygote.

And 2pq represents the hetreozygote.

Reply 47

Anon_98

Selection- Topic 8.

Alright so one of the main assumptions of the Hardy Weinberg principle was that an individual's genotype does not affect it's chances of breeding successfully. This is usually not the case -- one allele can have a distinct advantage over other alleles. As a result, the frequency of the favorable allele increases: This is natural selection.

Overall, natural selection can be summarised as follows:

- A gene mutation produces an allele that has an advantage.

- Organisms with this allele reproduce more successfully than those without it

- As a result, they produce more offspring which, in turn, inherit the favourable allele.

- In this way, the frequency of the favourable allele increases.

Directional selection.

If the environment changes, individuals at one extreme may have an advantage while those at the other extreme may have a disadvantage.

Over time, selection operates against the disadvantaged extreme + in favour of the other extreme.The mean + range of values shift towards the favoured extreme. (e.g. the cacti spines example)

Stabilising selection.

In a stable environment, selection operates both extremes of a range. It operates to maintain the status quo in the population and to make the population more uniform. (e.g. the baby weight example)

Reply 48

Anon_98

Speciation- Topic 8

Alright, so you need to understand that as long as two populations are able to interbreed, they are unlikely to evolve into distinct species. In order for speciation to occur, they must undergo a period of isolation where they are prevented from interbreeding.

There are several ways in which populations can be reproductively isolated.

Geographical isolation occurs when two populations are physically separated. Interbreeding is impossible + speciation may result. Speciation as a result of geographical isolation is called allopatric speciation.

The stages for this are as follows:

1) A physical barrier divides the species into populations.

2) In different environments, different features have a selective advantage.

3) The two populations show increasing genetic differences.

4) The two populations are now distinct species ; even after the barrier has gone, they cannot interbreed.

Other types of isolation need not involve a physical separation. The two diverging populations may inhabit the same area, but be prevented from interbreeding in one of the following ways:

- Seasonal isolation -- members of the two populations reproduce at different times of the year.

- Temporal isolation -- members of the two populations reproduce at different times of the day.

- Behavioural isolation -- members of the two populations have different courtship patterns.

Speciation following any of these methods of isolation is called sympatric speciation.

Reply 49

Anon_98

So, I've covered the spec which means I've finallyyyy finished making all my notes. - Thread complete. :woo:

Reply 50

Supermanxxxxxx

Original post by Anon_98

So, I've covered the spec which means I've finallyyyy finished making all my notes. - Thread complete. :woo:

Wooo I hate this exam too I just feel like even if you know the content the mark scheme is so precise you could still end up with a b or even worse a c

Posted from TSR Mobile

Reply 51

Anon_98

Original post by Supermanxxxxxx

Wooo I hate this exam too I just feel like even if you know the content the mark scheme is so precise you could still end up with a b or even worse a c