3.7.3 Evolution may lead to speciation (Genetics , population , evolution and ecosystems 3.7)

Question 1

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

Answer 1

genetic factors :
- mutations
- crossing over
- independent segregation
- random fertilisation
- environmental factors

Question 2

what is evolution

Answer 2

change in allele frequency over time / 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

Question 4

explain the principles of natural selection in the evolution of populations

Answer 4

mutations - random gene mutations can result in [ named] new alleles of a gene
advantage - due to [named] selection pressure , the new allele might benefit its possessor [explain why] => organism has selective advantage
reproduction - possessors are more likely to survive and have increased reproductive success
inheritance - advantageous allele is inherited by members of the next generation ( offspring)
allele frequency - over many generations , [named] allele increases in frequency in the gene pool

Question 5

explain effects of stabilising selection

Answer 5

• organisms with alleles coding for average variations of a trait have a selective advantage (e.g babies with an 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 selective advantage e.g bacteria with high resistance to antibiotic
• so frequency of alleles coding for this extreme variation of the trate increase and those coding for the other extreme variation of the trait decrease

Question 7

explain the effects of disruptive selection

Answer 7

• organisms with alleles coding for either extreme variation of a trait have selective advantage
• so frequency of allele 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 ( how new species arise from existing species )

Answer 8

1) reproductive separation of the two populations
2) this can result in accumulation of differences in their gene pools
3) new species arise when these genetic differences lead to an inability of members at the populations 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 isolating separating gene pools by preventing interbreeding / gene flow between populations
3) random mutations cause genetic variation within each population
4) different selection pressures
5) so different advantageous alleles are selected for / passed on in each population
6) so allele frequencies within each gene pools change over many generations
7) eventually different populations cannot interbreed to form fertile offspring

Question 10

describe sympatric speciation

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 e.g gamete incompatibility, different breeding seasons , different courtship behaviours , body shape
3) different selection pressures act on each population
4) so different advantageous alleles are selected for / 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

explain genetic drift and its importance in small populations

Answer 11

Genetic drift :
a mechanism of evolution in which allele frequencies in a population change over generation due to chance
- Some alleles are passed onto offspring more / less often by chance regardless of selection pressures and whether alleles give a selective advantage
- so strongest effects in small populations with no interbreeding with other populations ( no gene flow ) as gene pool is small and change has a greater influence
- this can reduce genetic diversity - some alleles have much have frequencies