Disadvantages of Asexual Reproduction continued. Disadvantages of asexual reproduction continued 3. Müller’s Ratchet (an hypothesis) Asexual reproduction.

Disadvantages of Asexual Reproduction continued

• Disadvantages of asexual reproduction continued

• 3. Müller’s Ratchet (an hypothesis)• • Asexual reproduction is like a ratchet mechanism.

• Most mutations are deleterious• These should accumulate and result in a one-way decline

in fitness.

• Counter argument: diploidy (and especially triploidy) in asexuals would reduce the effects of deleterious mutations.

• 4. Destabilizing hybridization.

• 4. The establishment of new parthenogenetic species will be difficult.

• Parthenogenetic females can mate with male sexual ancestors.

• This destabilizes a functional genome.• Sterile hybrids are the usual product.• Distributions of parthenogens should be allopatric to those

of gonochoristic congeners.• But this is not the case.• Destabilizing hybridization is presently taking place

between A. tesselata (P) and A. marmorata (G) at Arroyo del Macho, Chaves County, NM.

• What are the effects?

Testing the hypothesis from a serendipitous discovery

Genus CnemidophorusBecame genusAspidoscelis in 2002

5 entity guild

Aspidoscelis exsanguis

A. inornata

Aspidoscelis tesselata

Hybrid

A. marmorata

Would it be easy to overlook a hybrid?

A numericaladvantage basedon micro-habitatutilization

10 hybridsexaminedelectrophoretically

Product of nineseparate hybridization

A reproductive advantage

The gonochoristic species is morphologically more variable

• GAB, COS, LSG (scale count characters)• Variation of all three included in a composite

character: the 1st principal component from a PCA (68% of variation).

• C. tesselatus: s2 = 0.519 (N =10*)• C. tigris: s2 = 1.211 (N = 9+)• * those with genetic data (34 loci)• + females

• The Frozen niche hypothesis• Like its genotype, the niche of a parthenogen should be

“frozen” by the hybridization event.• Parthenogenesis should be disadvantageous.• But at Macho Draw the parthenogen appears to have a

greater advantage.• General-purpose genotype hypothesis (Lynch)• Genomes from two ecologically successful sexual species

give parthenogens greater ecological breadth than either progenitor species. Seen in breadth of habitat use.

• General purpose genotypes would be conserved by parthenogenesis (not broken up by recombination).

Why is sex so prevalent?

• Linkage disequilibrium is established under conditions of strong selection, drift, and asexual reproduction.

• Tends to counter linkage equilibrium

• The “Red queen” hypothesis

• Host/parasite relationships

• Available evidence: genotypic variability reduces parasitic loads.