Year 13 - Speciation

Question 1

Describe disruptive selection

Answer 1

• Selection against intermediate traits/average​
• both extreme phenotype has increased reproductive success = improved survival/ability to reproduce and find a mate)​
• Alleles for both extreme phenotype are passed on to future generations in greater numbers. ​
• Over time, frequency of both extreme alleles, coding for extreme phenotypes, INCREASES. ​
• Allele frequency coding for mean phenotypes decrease.

Question 2

Name 3 sources of Genetic variation

Answer 2

Mutation – changes DNA nucleotide sequence forming new alleles.​

Meiosis – crossing over and independent segregation in the first meiosis division. ​

Random fertilisation of gametes during sexual reproduction.

Question 3

Define allopatric speciation

Answer 3

Formation of new species from different populations in different areas.

Question 4

Define sympatric speciation

Answer 4

Formation of new species from reproductive isolation in a population living in the same area.

Question 5

Describe sympatric speciation

Answer 5

1. Not geographically isolated (same population) but are reproductively isolated​
2. By Named isolating mechanism​
3. (If AO2-> as …..named phenotype/genotype Only mates with named phenotype/genotype And named phenotype/genotype Only mates withnamed phenotype/genotype )
4. Mutation causes (named) variation by forming different alleles​
5. Different alleles passed on (in each reproductively isolated group) ​
6. this changes allele frequencies ​
7. Example of named type of selection. ​
8. Eventually, become different species that cannot interbreed to produce fertile offspring.

Question 6

Describe allopatric speciation

Answer 6

1. Geographically isolated populations means reproductive isolation.must be first mp ​
2. If AO2-> this is allopatric speciation. ​
3. Mutation causes (AO2->named) variation by forming different alleles (in the different populations)​
4. Different (if AO2-> named) selection pressures/ description of different environmental conditions. ​
5. Means selection for different advantageous allele/s that increases survival so allele is passed on to next generation ​
6. When adapted organisms within their population breed and reproduce​
7. Changes the frequency of alleles​
8. Eventually, become different species that cannot interbreed to produce fertile offspring.

Question 7

Describe isolating mechanisms:​

Temporal ​

Ecological/behavioural ​

Mechanical​

Hybrid infertility ​

Gamete incompatibility

Answer 7

​Temporal (=time) – different breeding seasons/feeding times​

Ecological/behavioural - different courtship displays/niches/habitat/feeding areas ​

Mechanical – mismatch of reproductive parts​

Hybrid infertility – hybrid has odd number of chromosomes so bivalents and no separation of homologous chromosomes = no meiosis ​

Gamete incompatibility – sperm killed in females reproductive tracts/ cant fuse with egg.