Chap 22D - Evolution II

Describe advantages of molecular methods

1. Amino acid and DNA sequences can be easily obtained/assessed from electronic databanks for easy comparative study and classification

2. Studies involving the DNA sequence data of all the organisms can be conducted as all known life is based on nucleic acids

3. Some molecular differences may not be visible/ expressed in the phenotypes

Define convergent evolution

• (Def.): refers to the independent evolution of similar traits or structures in different evolutionary lineages (similarities as analogous structures) 

Explain how convergent evolution affects tree

• Similarities observed at phenotypic level -> underlying DNA sequences differ 

• Can mislead phylogenetic analyses because it can mimic patterns expected from shared ancestry -> phylogenetic tree may appear falsely reticulate or bushy

Define Hardy-Weinburg Model

• (Def.): Describes how allele and genotype frequencies behave in an ideal population where no evolution is occurring

Hardy-Weinburg Model shows…

• It shows that:

  • Dominant alleles do not automatically become more common

  • Allele frequencies stay the same unless evolutionary forces act

• If a population is in Hardy–Weinberg equilibrium, it is not evolving at that gene locus

Define Hardy-Weinburg Model

p + q = 1

p2 + 2pq + q2 = 1

2pq = number of He individuals 

p = dominant allele frequency 

q = recessive allele frequency 

Individual organisms/phenotypes = question talking about p2, 2pq, q2 

16% population is unable to taste PTC, they are recessive for tasting gene

Find q2, p + state conclusions

Example: 16% population is unable to taste PTC, they are recessive for tasting gene 

• q2 = 0.16 -> q = 0.4 

• p = 1 - 0.4 = 0.6

• Expected frequencies: use p2 + 2pq + q2 = 1 formula 

• Expected number (He): 2pq x total population 

Conclusion: 

• The values are very close, with only minor differences

• The population approximately obeys Hardy-Weinberg equilibrium, suggesting no strong evolutionary forces are acting on this gene

Describe significance of Hardy–Weinberg model

• Real populations rarely meet all Hardy–Weinberg conditions

• Each condition corresponds to a mechanism of microevolution: genetic drift, non-random mating, mutation, gene flow and natural selection

• Any violation of Hardy–Weinberg assumptions leads to evolutionary change

State conditions of Hardy-Weinburg Model

1. Large population size

2. Mate without regard to genotype

3. No mutation

4. No migration

5. No Natural selection

Decribe Hardy conditions (Large population size, Mate without regard to genotype)

1. Large population size 

• This minimises effects of random choice 

• Small population: cause genetic drift (changes in allele frequencies from 1 generation to the next) -> loss of 1 allele

2. Mate without regard to genotype

• Non-random mating disrupts Hardy–Weinberg equilibrium by altering genotype proportions

• Effects 

  • Assortative mating: Individuals preferentially mate with others of similar traits (Eg. Height)

  • Consanguinity: Mating between closely related individuals who share a recent common ancestor

• Consequences 

• Increase in Ho

• Decrease in He

• Genotype frequencies change, but allele frequencies remain the same

Decribe Hardy conditions (no mutation)

3. No mutation (No new alleles arise through mutation)

• Mutation -> genetic variation -> alter allele frequencies

• Mutation rates are usually low and may be balanced by selection -> populations appear close to equilibrium

Decribe Hardy conditions (no migration, no natural selection)

4. No migration (no gene flow)

• Population must be genetically isolated 

• Gene flow introduces new allele -> alters existing allele frequency -> evolution 

• Disproportionate migration between populations can disrupt equilibrium as migrants introduce alleles at frequencies different from those in the resident population

5. No natural selection (all genotypes equally likely to survive and reproduce)

• All genotypes must have equal fitness

• If some genotypes are favoured or selected against, allele frequencies will change

• Selection pressures prevent equilibrium from being maintained 

Hardy assumptions SHORTENED

1. The population is not genetically isolated, as migration (gene flow) can introduce or remove alleles from the population.

2. Mating may not be random, as individuals may exhibit mate preferences.

3. Natural selection can still take place, such that individuals with different genotypes have different survival and reproductive success