Lesson 2: Natural Selection Flashcards
how does natural selection affect evolution?
evolution occurs when natural selection acts on the genetic variability of a population
- this happens when the environment favours a certain trait over another
- EX: higher rates of sickle cell anemia in countries where Malaria is common
types of natural selection
- directional
- stabilizing
- disruptive
- sexual
directional selection + example
this occurs when selection favours individuals with a more extreme variation of a trait
- the result is a shift away from the average condition
- this is very common in breeding
- EX: tall giraffe necks or dark coloured pepper moths being favoured
stabilizing selection + example
occurs when the average phenotype within a population is favoured
- ex: in hummingbirds, medium sized beaks are favoured
- the long beak is a disadvantage because it requires too much energy to carry around
- the short beak is a disadvantage because it cannot reach the food within the flower effectively
disruptive selection + example
this selection favours individuals with variation at opposite extremes of a trait over individuals with intermediate variations
- ex: if you are a flower which hummingbirds feed upon, a medium sized flower would be a disadvantage
- large flowers are favoured because the beak is too small to get into the flower well
- small flowers are also favoured because the beak would be too large to get into the flower well
DIRECTIONAL SELECTION
selection against one extreme
- FOR: one extreme trait
- AGAINST: the other extreme
EX: long wiggly tails look like a snake and scare predators. the longer the tail, the more it looks like a snake
STABILIZING SELECTION
selection against both extremes
- FOR: moderate traits
- AGAINST: both extremes
EX: short tails mess up the cat’s balance. long tails drag on the ground. medium tails are best
DISRUPTIVE SELECTION
selection aganist the mean
- FOR: both extremes
- AGAINST: moderate traits
EX: short tails help keep predators from catching you on the ground. long tails are good for balance in the trees. medium tails don’t help
sexual selection
this selection favours a specific trait because it enhances the mating success of an individual
- this selection often leads to males or females within a population evolving appearances or behaviours which are quite different from each other
- you may see this pressure being given by female mate choice or male-versus-male competition
change without selection
- genetic drift
- genetic bottleneck
- the founder effect
genetic drift
the genetic makeup of a population can also change simply by chance
- when individuals produce offspring, any particular allele is passed on by chance
- the smaller the number of individuals in a population, the greater the impact of random chance
what does genetic drift cause
can cause a change in the frequency of an allele in the genetic pool, without any selection pressures (it may even lead to the disappearance of particular allele all together)
EX: bottleneck and founder effect
genetic bottleneck + what might it cause
results in a loss of genetic diversity following an extreme reduction in the size of a population
- based on which individuals survive the population reduction, there may be a huge change in allele frequency
the founder effect + what might it cause
occurs when a small number of individuals establish a new population
- there may be a huge change in allele frequency dependent on which individuals establish the new population
check w slides, videos and gizmo for more info
