Molecular Evolution Flashcards
What is genetic drift
Genetic drift is the random change in allele frequencies in a finite population due to chance.
Why is genetic drift stronger in small populations
Because random events have a larger impact when fewer individuals are present
What are the two possible outcomes of an allele in a finite population
Fixation or extinction
What is the probability of fixation of a new neutral mutation
1 / 2N, where N is the population size
How long does it take for neutral mutations to fix
On average, 4N generations
What happens to most new mutations in a drift only scenario
They are lost due to chance
Why does genetic drift contribute to molecular evolution
Because even neutral mutations can fix over time by chance
What is positive selection
Selection for beneficial alleles, increasing their frequency
What is purifying (negative) selection
Selection against deleterious alleles, decreasing their frequency
How does selection impact genetic diversity
It reduces diversity by favoring or removing alleles
What causes mutations
Errors in DNA replication
Name three types of mutations
Base substitutions, insertions/deletions, large chromosomal changes
What determines the likelihood a mutation fixes in a population
Its initial frequency and whether it is neutral, beneficial, or deleterious
How does a one way mutation (e.g. yellow to green) behave over time
The green allele accumulates as yellow mutates and green never reverses
How did electrophoresis change our understanding of variation
It revealed much higher genetic diversity (allozyme variation) than expected
What is clock-like divergence
Steady accumulation of protein differences between species over time
Why does clock-like divergence challenge selection theory
Too much consistency and diversity for selection alone to explain
What is the Neutral Theory of Molecular Evolution
Most molecular changes are due to neutral mutations fixed by drift
What drives molecular evolution under neutral theory
Random genetic drift of neutral mutations
How does neutral theory explain molecular clock observations
Fixation rate = mutation rate (u), leading to constant divergence
What does the neutral theory say about selection
Selection mainly removes deleterious alleles; beneficial ones are rare
What do selectionists believe
That fixed differences often reflect adaptive, positively selected changes
What do neutralists believe
That most polymorphisms and fixed differences are neutral and drift-driven
What is a synonymous mutation
A DNA change that does not alter the amino acid - usually neutral
What is a non - synonymous mutation
A mutation that changes the amino acid - may be neutral, harmful, or beneficial
What is the dN/dS ratio used for
To detect selection by comparing rates of synonymous vs. non-synonymous changes
Interpret dN/dS >1, <1, =1
> 1 = positive selection;
< 1 = purifying selection;
= 1 = neutral evolution.
What limits dN/dS analysis
It only detects repeated or strong positive selection across lineages
What is a selective sweep
A rapid rise and fixation of a beneficial mutation, reducing nearby diversity
Why is parallel evolution evidence for positive selection
Because the same mutation appearing independently is unlikely by drift alone
What did the E.coli long term evolution experiment show
Repeated, parallel adaptations across separate populations - evidence of selection
What is neofunctionalism
After gene duplication, one copy mutates into a new function while the other retains the original
How do bacteria acquire new genes
Through Horizontal Gene Transfer (HGT) using plasmids - enables transfer of entire operons and resistance genes
