lecture 6- origin novelty

Two-fold cost of sex

A female passes only 50% of her genes to each offspring (cost of meiosis), and producing males reduces reproductive rate (cost of males)

Recombination disadvantage

can break up adaptive combinations of genes

Fitness comparison

Asexual females can produce more offspring than sexual females

Asexual mutant advantage

can spread quickly in a sexually reproducing population due to higher fitness

Question: Why sex evolved if asexuality increases fitness?

Despite fitness advantages, asexuality has long-term disadvantages

Sexual reproduction advantages

1. Eliminates deleterious mutations via recombination + selection 2. Produces genetic variety (defense against pathogens)

Sexual reproduction DNA repair

Allows repair of damaged DNA by using the homologous chromosome as a template

Genetic load

in asexual species, deleterious mutations accumulate and can’t be removed except through lineage extinction

Muller’s ratchet

process where harmful mutations build up in asexual populations with each generation

Muller’s ratchet (consequence)

Only death of the lineage can eliminate the mutational load in asexual species

Sex and mutation avoidance

Sex allows offspring to carry fewer deleterious mutations than their parents

Effect of sex on allele frequencies

Sex reshuffles alleles but does not change allele frequencies directly

How do novel traits arise?

Through recombination, lateral gene transfer, and gene duplication

Lateral gene transfer

Genes, organelles, or genome fragments move between lineages

Methods of lateral gene transfer

1. DNA fragments from environment 2. Viral genome transfer 3. Hybridization

Benefit of lateral gene transfer

allows incorporation of novel, advantageous genes

Antibiotic resistance example

Genes for resistance are often transferred among bacteria (e.g., MRSA)

Lateral transfer across domains

e.g., Wolbachia bacteria and insects—may increase resistance, cause sex-ratio distortion, or create incompatible males

Gene duplication

major source of new genetic material in evolution

Gene duplication outcomes

1. Both copies retain function 2. Subfunctionalization 3. Pseudogene formation 4. Neofunctionalization

Whole genome duplication

sometimes entire genomes are duplicated, increasing genetic content (e.g., 2n to 8n)

Globin family example

different globin genes arose through duplication and divergence in vertebrate evolution

Applications of molecular evolution

understanding gene functions, relationships, resistance mechanisms, and developmental processes