Evolution of Sex Flashcards
Modes of Reproduction
Asexual and Sexual
Asexual Reproduction
Amixis - No mixing
Mitotically based reproduction with no syngamy(fusion of gametes with reduced ploidy)
- Colonial
- Clonal
Amixis
No mixing
Syngamy
Fusion of gametes with reduced ploidy
Asexual Reproduction:
Colonial
Cell division adds to existing colony
- coral reefs
Asexual Reproduction:
Clonal
Reproduction with an individual not connected to you.
- Budding
- Polyembryony
- Apomixis
Asexual Reproduction:
Clonal - Budding
Vegetative reproduction
- seeds fall to ground and grow
- hydra grows a second organism on its side that splits and goes on its own way.
Asexual Reproduction:
Clonal - Polyembryony
One fertilized egg develops into multiple individuals
- Different morphologies but start as 1 embryo
- armadillos
- human twins
Asexual Reproduction:
Clonal - Apomixis
Single unfertilized egg develops into adult.
- an egg never needs sperm to come in and fertilize.
- typically there used to be reproduction but the female population decided to get rid of males.
- males are unnecessary from a biological perspective
Sexual(mixis) Reproduction
Meiotic reduction of ploidy followed by syngamy.
- Automixis
- Amphimixis
Mixis
Mixing of genes
Sexual(mixis) Reproduction:
Automixis
Syngamy within the same parent
- syngamy within mother fuse and creates a new offspring that is diploid
- shuffled the genome still which means it is not a clone
- being Asexual without partner BUT still sexual as you are shuffling your own genome.
Sexual(mixis) Reproduction:
Amphimixis
Syngamy between parents.
- two sexes each produce a miotically reduced gamete which then come together in syngamy to produce diploid offspring.
- Human reproduction
When did sex evolve?
Proterozoic Eon.
1.2 BYA.
Bacteria and Archaea did not engage in sexual reproduction.
How did sex evolve?
Unknown.
3 working models:
- DNA repair strategy(of single chromosome organisms)
- Selfish Genetic Elements
- Transformation(bacterial)
How did sex evolve?
DNA repair strategy(of single chromosome organisms)
Like crossing over except for DNA repair.
Holiday junction resolution.
Haploid genome
- Grab another strand of DNA and borrow it to fix yours but some crossing over happens
How did sex evolve?
Selfish genetic elements
Bacterial conjunction is one way in which sex began.
- must have an F factor to create a “sex pilius”(grabs hold of another bacteria and wont let go)
- In the process F factor leaves the plasmid copy so genetic material is swapping and the new plasmid containing bacteria can go on and repeat the process to others.
How did sex evolve?
Transformation(bacterial)
Upon stressful conditions, bacteria incorporate foreign DNA into genome.
- Stress induce bacteria to go through transformations.
- grab “naked” DNA and insert randomly into genome to attempt to survive the stress via recombinant chromosome.
Asexual VS Sexual reproduction
Asexual always chosen UNLESS stress forces Sexual.
The Cost of Sex
Most common reproduction mode among animals but comes with significant costs compared to Asexuality.
- Cost of Males
- Cost of Meiosis
The Cost of Sex:
Costs of Males
Asexual reproduction produces twice as many offspring as sex.
- 2 partners to create 1 offspring
- Asexual population grows exponentially
- males create a disadvantage in the ratio of offspring produced
The Cost of Sex:
Costs of Meiosis
- recombination breaks apart co-adapted gene complexes
- co-adapted genes work well together in certain environments and meiosis destroys this
Why Did Sex Evolve?(and how its currently maintained)
Advantages
- DNA Repair
- Purge Deleterious Mutations
- Promote Genetic Variation
Why Did Sex Evolve?(and how its currently maintained):
DNA Repair
- Allows the repair of damaged DNA by using one of the two chromosomes as a template
- Damage occurs at a rate of 1,000 to 1,000,000 molecular lesions per cell per day.
- Double stranded breaks are bad
Why Did Sex Evolve?(and how its currently maintained):
Purge Deleterious Mutations
- Muller’s Ratchet - as asexual individuals multiply, deleterious(harmful) mutations accumulate.
- over time this creates an unfit individual.
- Sex allows deleterious mutations to be removed from the genome.
- Must get rid of increasingly deleterious mutations
- fit individuals outcompete those with deleterious mutations and thus removes them from the population.
Why Did Sex Evolve?(and how its currently maintained):
Promote Genetic Variation
Similar to Muller’s Ratchet.
- Increases the rate of adaptive evolution.
- Increases offspring diversity to exploit/defend against temporally heterogeneous environments.
- Increases offspring diversity to exploit spatially heterogeneous environments.
- Allows progeny to either escape of escalate an antagonistic co-evolutionary dynamic(Red Queen Hypothesis)
Why Did Sex Evolve?(and how its currently maintained):
Purge Deleterious Mutations
Muller’s Ratchet
as asexual individuals multiply, deleterious(harmful) mutations accumulate.
- over time this creates an unfit individual.
Why Did Sex Evolve?(and how its currently maintained):
Promote Genetic Variation:
Increase rate of adaptive evolution
Brings together beneficial alleles in fewer geneations.
- genes A, B, and C come together much quicker than Asexual reproduction.
Why Did Sex Evolve?(and how its currently maintained):
Promote Genetic Variation:
Increases offspring diversity to exploit/defend against temporally heterogeneous environments.
Tomorrow is a mystery.
- produce more offspring than the environment can handle to increase diversity. - those fit for tomorrows environment will flourish while the unfit die off.
- Asexual are either really fit or really unfit for an environment AND can not adapt quickly enough to change.
Why Did Sex Evolve?(and how its currently maintained):
Promote Genetic Variation:
Increases offspring diversity to exploit spatially heterogeneous environments.
Entangled Bank - so much competition of species requires development of specific niche to survive.
- Sex allows this diversification to happen where Asexual does not.
- Sex has a greater “Niche Breadth”(ways to exploit a niche) than Asexual.
Why Did Sex Evolve?(and how its currently maintained):
Promote Genetic Variation:
(Red Queen Hypothesis) Allows progeny to either escape of escalate an antagonistic co-evolutionary dynamic
- it takes all the evolution we can do to keep everything else at bay
- to stay ontop you keep evolving or die
- Stable limit cycle
- fluctuation of parasite VS host as the constant recombination of alleles swings the fight in favor of one and then the other as they adapt to each other
- Co-evolutionary dynamic
Example:
- Snails reproduce Asexual without parasites as it is more efficient(more offspring).
- When parasites present, snails produce Sexually to recombine the genome and produce offspring who can survive the parasites.
- Escape the weaknesses the parasites are exploiting.
Entangled Bank
so much competition of species requires development of specific niche to survive.
Niche Breadth
Sex has a greater “Niche Breadth”(ways to exploit a niche) than Asexual.
= higher likelihood of survival
Eukaryotes tend to be _______.
The vast majority are Sexual.
Persistence of Sex
- Asexuality is great in the short term but terrible in the long term due to environment instability.
- Stressful Situations force the necessity for sex.
Persistence of Sex:
Why do Asexual lineages continue to erupt from Sexual lineages?
- Asexuality is great in the short term but terrible in the long term due to environment instability.
- Stable environments promote Asexuality
- Produce offspring quickly and exploit the niche
- BUT if the environment changes the entire species goes extinct.
Persistence of Sex:
Stressful Situations
Sexual reproduction happens to hope one of the offspring will survive the stress.
- you don’t have the genome to survive so you adapt
How do you force something to go from Asexual to Sexual reproduction?
Take away its food.
Imminent death = great necessity to change ASAP.
“Best Man Hypothesis”
Asexual progeny have a higher mean fitness in one environment. however, environments are temporally unpredictable. Thus, progeny diversity is highly adaptive.
Asexual higher in one environment.
Sexual greater survival in variety of environments.
