Why Sex/Evolutionary Constraint Flashcards
Two-fold cost of sex (bigger picture)
Asexual - produce offspring genetically identical
Sexual - only passes on half genes to offspring
Creates “cost” of producing males that can’t reproduce themselves. asexual populations grow much faster (should dominate populations, but they don’t)
Passing on genes = increases fitness; wouldn’t it be better to be asexual?
Diploid
Having two copies of each chromosome in a nucleus (all cells in body; except gametes)
Halploid
one copy of each chromosome in a nucleus; ex: gametes (egg/sperm)
Muller’s ratchet (cost of sex)
asexual being - passing on all mutations; including bad ones…could accumulate = leading to genetic load (reducation in avg. fitness of population)
How is Muller’s ratchet broken?
Sex. Recombination of DNA = allows for purging of deleterious alleles
A reproductive process in aphids that allows them to alternate between sexual and asexual reproduction
Facultative parthenogenesis
- All female in summer; eggs in winter (sexual)
- Snail example: male production more common when parasite loads are higher
What are evolutionary constraints?
- slows rate of adaptive evolution
- prevents population from evolving the optimal traits
- not all possible beneficial traits can evolve bc the variation needed to produce them isn’t present due to these constraints (biology/genetic makeup)
Constraints related to the properties of biological material
Physical evolutionary constraints
Ex: insects move CO2 and O2 – diffusion rate sets limits on insect body size
Inescapable compromises between traits that limit their evolution
Trade-off constraints
Ex: male flowers give bees pollen; females have no pollen
Bee/Flower trade off example
Bees select for larger flowers; female flowers aren’t huge
trade-off between flower size and # of flowers produced in an inflorescence
if plant can’t produce large flowers due to resource limits, it evolves to produce larger quantity of smaller flowers to max pollination chances
Genetic variation for a particular phenotype may not be present within a population
Genetic Constraints
Ex: Leaf beetles (brown and green) brown more likely to survive/produce offspring, green spotted more easily by birds; pass down genes for brown coloration to offspring
A bias on the production of various phenotypes caused by the structure, character, composition, or dynamics of the developmental system.
Developmental Constraints
Ex: butterfly species (restrict range of wing patterns/sizes that evolve) due to the way the wings develop in larval stage
How does population survive fitness landscape?
population evolves on fitness landscape by moving “uphill” through genetic mutations and natural selection, those with higher fitness genotypes are more prevalent, shifting the population towards the peak of landscape (highest possible fitness in current environment)
What is correlated response to selection driven by? (the change observed in one trait when selection is applied to another trait)
driven by genetic correlations between traits, caused by…
- pleiotropy (a single gene affecting multiple traits)
- linkage disequilibrium (non-random association between alleles at different loci due to their physical proximity on a chromosome)
Correlated response to selection and genetic correlations
change observed in a trait that is not directly selected for, but occurs as a consequence of selecting for another trait that is genetically correlated with it
Ex: Inc back fat in pigs –> decrease body length
