What is microevolution?
Small-scale changes in allele frequencies from generation to generation (population genetics)
What is macroevolution?
Biological evolution over many generation and on the origin of higher taxonomic categories (at or above the species)
Explain the Hardy Weinberg Equilibrium:
- P + q = 1 (p being the frequency of one allele and q the frequency of the other allele)
- Thus p2 + 2pq + q2 = 1
- For three alleles, p + q + r = 1 and the same goes for its squared version.
What are the assumptions of the Hardy Weinberg equilibrium?
- No mutation
- No selection
- No migration (gene flow)
- Infinite population size
- Forces (evolutionary) that change the genetic frequency are violations of these assumptions.
What is population genetics?
The study of the allele frequency distribution and change under the influence of the four evolutionary forces: mutation, gene flow, genetic drift, natural selection.
- The ultimate source for genetic variation and thus evolution.
- Mutations might be deleterious, neutral or adaptive.
What is epistasis?
The phenomenon where the effects of one mutation are modified by one or several other mutations.
What is fitness?
An individual’s ability to propogate its genes.
What is selection coefficient?
Measures the intensity of natural selection acting on the genotypes in the population (fitness of the phenotype). Selection coefficients are environment specific.
What are the different types of natural selection?
- Positive Selection (directional selection): Favorable traits that are heritable become more common in successive generations, moving the frequency away from the average. If the trait becomes fixed, this is called a complete selective sweep.
- Negative selection (stabilizing or purifying selection): unfavorable traits that are heritable become less common. (such as disease)
Note: Genes around the selectively advantageous gene also become more common.
How to find positive selection signal in the genome?
- Reduction in diversity
- Extended linkage disequilibrium (haplotype)
Explain balancing selection
- Selection for heterozygote might be more fit to the specific environment than other two homozygotes.
- Example is the sickle cell disease trait, which provides partial immunity to malaria caused by the Plasmodium parasite in its heterozygous form. Thus, in areas where malaria is common, balancing selection operates.
Explain gene flow
- This is the exchange of genetic information between populations.
- Example: the shaping of modern human immune systems by multiregional admixture with archaic humans. Tibetan acclimation to high altitude was acquired from Denisovans.
What is genetic drift?
- Addresses the influence of random chance on the survival of alleles, due to the statistical effect.
- Example: founder effect (bottleneck)
Explain founder effect:
- The effect of establishing a new population by a small number of individuals, carrying only a small fraction of the original population’s genetic variation.
- Can cause high frequency even for deleterious alleles!
Examples of gene flow from archaic hominids
- 1. More than half of HLA genes dictating vital immune system function introgressed into Homo sapiens from neanderthal and denisovan lineages through multi regional admixture events.
- 2. The EPAS1 hypoxia pathway gene shows strong indications of positive selection in Tibetans. This gene introgressed into this population through Denisovans.
- 3. Microcephalin gene with important implications for early brain development introgressed into human populations 37,000 years ago.