Evolution May Lead To Speciation

Question 1

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

Answer 1

○ Mutations
○ Crossing over between homologous chromosomes during meiosis
○ Independent segregation of homologous chromosomes during meiosis
○ Random fertilisation of gametes during sexual reproduction
● Environmental factors

Question 2

What is evolution?

Answer 2

● Change in allele frequency over time over many generations in a population
● Occurring through 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, ie. natural selection

Question 4

Explain the principles of natural selection in the evolution of populations

Answer 4

Random gene mutations can result in new alleles of a gene
Due to selection pressure, the new allele might benefit its possessor [explain why] → organism has a selective advantage
Possessors are more likely to survive and have increased reproductive success
Advantageous allele is inherited by members of the next generation producing fertile offspring
Over many generations, allele increases in frequency in the gene pool

Question 5

Explain the effects of stabilising selection

Answer 5

● Organisms with alleles coding for average variations of a trait have a selective advantage (eg. babies with average weight]
● So frequency of alleles coding for average variations of a trait increase and those coding for extreme variations of a trait decrease
● So range / standard deviation is reduced

Question 6

Explain the effects of directional selection

Answer 6

● Organisms with alleles coding for one extreme variation of a trait have a selective advantage (eg. bacteria with high resistance to an antibiotic)
● So frequency of alleles coding for this extreme variation of the trait increase and those coding for the other extreme variation of the trait decreases

Question 7

Explain the effects of disruptive selection

Answer 7

● 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 of the trait decrease
● This can lead to speciation

Question 8

Describe speciation

Answer 8

1. Reproductive separation of two populations (of the same species)
2. This can result in accumulation of differences in their gene pools
3. New species arise when these genetic differences lead to members of the population not being able to interbreed and produce fertile offspring

Question 9

Describe allopatric speciation

Answer 9

1. Population is split due to geographical isolation
2. This leads to reproductive isolation, separating gene pools by preventing interbreeding / gene flow between populations
3. Random mutations cause genetic variation within each population
4. Different selection pressures and environments act on each population
5. So different advantageous alleles are selected for and passed on in each population
6. So allele frequencies within each gene pool change over many generations
7. Eventually different populations cannot interbreed to produce fertile offspring

Question 10

Describe sympatric speciatiation

Answer 10

1. Population is not geographically isolated
2. Mutations lead to reproductive isolation, separating gene pools by preventing
   interbreeding / gene flow within one population, eg.
   ● Different courtship behaviour preventing mating
   ● Body shape / size changes mating
3. Different selection pressures act on each population
4. So different advantageous alleles are selected and passed on 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 11

What is genetic drift

Answer 11

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

Question 12

Why is genetic drift important in small populations

Answer 12

● As some alleles are passed onto offspring more / less often by chance
○ Regardless of selection pressures and whether alleles give a selective advantage
● So there is a strong effect in small populations as gene pool is small and chance has a greater influence
○ Eg. when a population is sharply reduced in size
● This can reduce genetic diversity - some alleles can become fixed or lost entirely