Molecular Evolution and Origin of New Features

Chapter 15

Nucleotide substitution

Change in one nucleotide in agene sequence (point mutation)

Synonymous substitutions

Most don’t affect phenotype because most amino acids are specified by more than one codon.

Nonsynonymous substitutions

The specified amino acid does change; deleterious (harmful), selectively neutral, or advantageous

Substitution rate are __ at positions that __ __ the amino acid being expressed.

highest; don’t change

Synonymous and Nonsynonymous substitutions are similar

The corresponding amino acid is likely to be under neutral selection = NO selective forces

Synonymous < Nonsynonymous substitutions

The corresponding amino acids are under positive selection (an allele is favored by natural selection)

Synonymous > Nonsynonymous substitutions

The corresponding amino acids are under purifying selection (a type of natural selection that acts to eliminate or reduce the frequency of deleterious (harmful) mutations within a population)

Neutral theory (introduced 1968)

The majority of variants in most populations are selectively neutral. They become fixed through genetic drift.

Rate of fixation (m) of neutral mutations by genetic drift is independent of population size.

m=p

If mutation rates are similar

long term neutral substitution rates will be too

# of nucleotide changes be used

as a “molecular clock” to calculate evolutionary divergence times between species

Some noncoding DNA consists of pseudogenes

they have lost their original function

Some noncoding sequences may help maintain chromosome structure and some consist of transposons

they are jumping genes that move from one location to another.

Sexual reproduction

involves combining gametes from two individuals

Asexual reproduction

involves producing genetically identical copies of oneself

Disadvantages of Sexual Reproduction

Females only pass 50% of her genes to each offspring, dividing offspring into genders reduces a female’s overall reproductive rate, and recombination can break up adaptive combinations of genes.

Asexual mutant will have __ fitness in a sexually-reproducing population.

a higher

Advantages of Sexual Reproduction

Elimination of deleterious mutations through recombination, increases genetic variation and facilitates repair of damaged DNA.

Muller’s ratchet

Mutations accumulate (genetic load) or “ratchet up” at each replication

Lateral (horizontal) gene transfer

Individual genes move horizontally from one lineage to another. Species may pick up DNA fragments directly from the environment.

Hybridization

results in the transfer of many genes

Gene duplication

New genetic material is generated.

Different fates for gene duplication

1) Both copies look like the original function.

2) Each copy may look like the original function, or may express different tissues, or may be expressed at different times.

3) One copy may become nonfunctional.

4) The two copies may diverge and acquire new functions.