Evolution, Natural Selection and Speciation

It also covers reproductive and geographical isolation. The empty boxes are left for you to draw diagrams (out of the edexcel A2 biology guide). Hope it helps!

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Evolution, natural selection and speciation
Evolution is a change in allele frequency
1. The complete range of alleles present in a population is called a gene pool
2. New alleles are usually generated by mutations in genes ­ these are changes in the base sequence of DNA
that occur during DNA replication
3. How often an allele occurs in a population is called the allele frequency. It's usually given as a percentage of the
total population, e.g. 35% or a number, e.g. 0.35
4. The frequency of an allele in a population changes over time ­ this is evolution
Evolution occurs by natural selection
1. Individuals within a population vary because they have different alleles
2. This means some individuals are better adapted to their environment than
3. Individuals that have an allele that increases their chances of survival (a
beneficial allele) are more likely to survive, reproduce and pass on their genes ( including the beneficial one),
than individuals with different alleles
4. This means that a greater proportion the next generation inherit the beneficial allele
5. Hey, in turn, are more likely to survive, reproduce and pass on their genes
6. So the frequency of the beneficial allele increases from generation to generation
7. This process is called natural selection
Speciation is the development of a new species
1. A species is defined as a group of similar organisms that can reproduce to give fertile offspring
2. Speciation is the development of a new species
3. It occurs when populations of the same species become reproductively isolated ­ changes in the allele
frequencies cause changes in the phenotype (physical characteristic) that mean they can no longer breed
together to produce fertile offspring
Reproductive isolation occurs in many different ways
Here are some of the ways changes in the phenotype prevent two populations from successfully breeding:
A population could become reproductively isolated due to geographical isolation or random
mutations. Random mutations could occur within a population, resulting in the changes mentioned
above, preventing members of a population breeding with other members of the species.

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Geographical isolation and natural selection lead to reproductive isolation
1. Geographical isolation happens when a physical barrier divides a population of species ­ floods, volcanic
eruptions and earthquakes can all cause barriers that isolate some individuals from the main population
2. Conditions on either side might be slightly different. For example there might be a different climate on each
3. Because the environment is different on each side, different characteristics (phenotypes) will become more
common due to natural selection:
4.…read more


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