6.1.6 - Mutation, gene flow, and genetic drift on changing gene pools Flashcards
Mutations as evidence for evolution
- Spontaneous mutations occur at a rate of around 1 in every 100,000 genes per generation. This is measured in germline mutations
- DNA sequencing allows geneticists to determine how many generations ago two individulas or species last shared a common ancestor
DNA sequencing: how many generations since two individuals/species last shared a common ancestor.
E.g. LUCA
Genetic Drift
- A mechanism of evolution where the allele frequencies of a population change over generations due to chance events. These events are random and are not selection pressures, as they do not select in favour of or against individuals with or without certain traits
- Genetic drift can cause the loss of some alleles and the fixation (rise to 100% frequency) of others
- Effects are strongest in small populations
- The ratio of alleles in the remaining population do not represent the original population. This is due to the bottleneck effect and the founder effect
E.g. a bushfire destroys a koala habitat, but some happened to survive because they were not in the vicinity of the fire. These individuals are not better adapted, but lucky
Compare genetic drift and natural selection as mechanisms of evolution
Genetic drift: more lucky than fit, evolve through bottleneck/founder effect rather than adaptation due to selection pressures
Fixation
100% frequency of alleles due to genetic drift in which by chance other alleles are lost/not passed down
Bottleneck effect
- When a random event causes a drastic reduction in population (genetic drift), the allele frequency of that population is also reduced. This reduced population is the bottleneck population
- The allele frequency of the bottleneck population is often significantly different to the parent population. When the population recovers to original numbers, the gene new allele frequency will reflect the ratio of the bottleneck population
Founder effect
- When a small group of individuals break off from a larger population to establish a new, isolated colony, the allele frequency of the new population will reflect the frequency in the founder population
- Alleles in the founding population may be present at different frequencies than the original populations and some may have been lost altogether
E.g. Dandelion seeds that landed on the Galapagos may not represent the allele frequency of the original population on the mainland
Ensuring accuracy when analysing genetic drift
- Sampling errors can mislead researchers to suspect genetic drift in a population if the allele frequencies within it are different from the parent population’s
- To ensure genetic drift has truly occurred, it is necessary to use multiple large samples of the population being studied
Gene flow
- Introduction/removal of new alleles in a population or gene pool by the movement of fertile individuals or gametes from one population to another.
- Alleles across populations may be different because if the populations are geographically isolated, each would have undergone allopatric evolution by different selective pressures, which favour different alleles for optimal survival and reproduction
- If the geographical barrier, like a mountain range, is penetrable enough, individuals will be able to move between the neighbouring populations. Interbreeding between neighbouring populations allows for gene flow, as offspring will inherit a combination of traits from both populations
Mutations introducing new alleles into a population
- Mutations can bring new alleles into a population, but not as quickly as genetic drift (changing allele frequencies), gene flow, or natural selection (changing allele frequencies)
