chapter 5 Flashcards
what are two different population management models?
population management:
- generational model
> each individual survives for one generation
> entire set of parents is replaced by offspring
- steady state model
> few new individuals are generated by gen
> few old members are replaced
whats the generational gap?
generational gap: proportion of the population replaced
> 1 for generational EA
what is fitness proportional selection (FPS)?
> name 3 problems
FPS: p(i) =fi / sum(fi)
problems:
- premature convergence: highly fit member takes over >>> converge to local optimum
- loss of selection pressure: in the end when fitness values are similar >>> random search
- highly sensitive to function transposition
what are two types of scaling approaches to parent selection?
scaling:
- windowing f’(i) = f(i) - (worst fitness in the last n gens)
- sigma scaling
> tries to flatten selection pressure over the course of evolution
how does rank based selection work?
rank based selection: select based on relative fitness rather than absolute fitness
why use tournament selection?
how?
tournament selection
> other methods rely on global population stats
> might be bottleneck in large or structured populations
>>> use only local fitness info, pick k members at random then select best
what does tournament selection with replacement do?
tournament selection with replacement increases selection pressure
what are two approaches to survivor selection?
- age based
- fitness based
what are two fitness based survivor selection approaches?
fitness based approaches:
- elitism
> alwaus keep at least one copy of the fittest solution (or keep the N best)
- delete worst aka GENITOR
> pff keine ahnung
whats the difference between the comma and the plus strategy in survivor selection?
comma: based on set of children only
plus: based on set of children and parents
comma or plus, which one is often preferred? why?
often comma:
- better in leaving local optima
- better in following moving optima
- using the plus strategy bad sigma vals can survive long if the host is fit
how to quantify selection pressure?
tau* = is the number of generations it takes until the application of selection completely fills the population with copies of the best fit individual
what are two ways of preserving diversity?
> with examples
preserving diversity:
- explicit
> make similar individual compete for resources
> make similar individuals compete for survival
- implicit
> impose the equivalent of geographical separation
> impose the equivalent of speciation
how does fitness sharing work? what does it do?
fitness sharing: restrict the number of individuals in a given niche
> keep more individuals in niche with generally higher fitness
>>> preserves diversity
what does crowding do?
crowding: distribute individuals evenly amongst niches
