7.3 Evolution May Lead To Speciation

Question 1

Explain why individuals within a population of a species may show a wide range of variation within a phenotype

Answer 1

Genetic factors:
- mutations
- crossing over between homologous chromosomes during meiosis
- independent segregation of homologous chromosomes during meiosis
- random fertilisation of gametes during sexual reproduction

Question 2

What is evolution?

Answer 2

• change in allele frequency over time in a population
• occurring though the process of natural selection

Question 3

Describe factors that may drive natural selection

Answer 3

• Predation, disease and competition for the means of survival
• these result in differential survival and reproduction

Question 4

Explain the effects stabilising selection

Answer 4

• Organisms with alleles coding for the modal variations of a trait have a selective advantage
• So frequency of alleles coding for average variations of the trait increase and those coding for extreme variations of the trait decreases
• so range/ standard deviation decreases

Question 5

Explain the effects of directional selection

Answer 5

• Organisms with alleles coding for one extreme variation of a trait have a selective advantage
• so frequency of alleles coding for this extreme variation of the trait increases and those coding for the other extreme variation of the trait decreases

Question 6

Explain the effects of disruptive selection

Answer 6

• Organisms with alleles coding for either extreme variation of a trait have a selective advantage
• so frequency of alleles coding for both extreme variations of the trait increase and those coding for the average variation decrease
• this can lead to speciation

Question 7

Describe speciation

Answer 7

• reproductive separation of 2 populations of the same species
• this can result in accumulation of differences in their gene pools by preventing interbreeding/ gene flow between populations
• New species arise when these geneticc differences lead to an inability of members of the population to interbreed and produce fertile offspring

Question 8

Describe allopatric speciation

Answer 8

1. Population is split due to geographical isolation
2. This leads to reposductive isolation, separating gene pools by preventing interbreeding/gene flow between populations
3. Random mutations cause genetic variation within each population
4. Different selection pressures act on each population
5. So allele frequencies within each gene oool change over many generations
6. Eventually different populations cannot interbreed to produce fertile offspring

Question 9

Describe sympatric speciation

Answer 9

1. The population is not geographically isolated
2. Mutations lead to reproductive isolation, separating gene pools by preventing interbreeding/gene flow within one population
3. Different selection pressures act act on each population
4. So different advantageous alleles are selected for and passed in in each population
5. So allele frequencies within each gene pool change over many generations
6. Eventually different populations cannot interbreed to produce fertile offspring

Question 10

What is genetic drift?

Answer 10

a mechanism of evolution in which allele frequencies in a population change over generations due to chance

Question 11

Explain genetic drift and its importance in small populations

Answer 11

• some alleles are passed onto offspring more/less often by chance
• So strongest effects in small populations with no interbreeding with other populations hence no gene flow as the gene pool is small and chance has a greater influence
  -This can reduce genetic diversity- some alleles have much higher frequencies, others are lost

Question 12

Effects of genetic drift

Answer 12

Bottleneck effect-> when a population is sharply reduced in size
Founder effect-> When a small, new colony forms from a main population