10.3 Flashcards
what is meant by genetic equilibrium
genetic equilibrium exists when all members of a population have an equal chance of contributing to the future gene pool
what happens to proportion of alleles in a gene pool in genetic equilibrium
random matings perpetuate the proportions of alleles in the population thus the frequency of alleles does not change
what happens to a population in genetic equilibrium
populations in genetic equilibrium will not evolve
what factors can change the allele frequency in a gene pool
genetic drift: the change in composition of a gene pool as a result of random or chance events
sexual reproduction: sex can introduce new gene combinations and alter allele frequencies if mating is non-random
mutations: a change in the genetic composition of an organism due to changes of the DNA base sequence
gene flow: the movement of new alleles into or out of a population due to emigration or immigration
natural selection: the change in composition of a gene pool due to differentials selective environmental pressures
what is meant by selection pressures
- individuals produce more offspring than their environment can support and some die because of factors known as selection pressures
- these factors include predation, food shortages or disease
- they determine which individuals will do best at surviving and reproducing
what is stabilising selection and give an example
- when an intermediate phenotype is favoured at the expense of both phenotypic extremes
- results in removal of phenotypic extremes (becomes centrally clustered)
- operates when environmental conditions are stable & competition is low
eg. human birth weights:
too large= birthing complications
too small= high infant mortality rate
what is directional selection and give an example
- where one phenotypic extreme is selected at the cost of the other
- causes phenotypic distribution to shift in one direction
- operates in response to gradual or sustained changes in environmental conditions
(- might be followed by stabilising selection once optimal phenotype is stable)
eg. antibiotic resistant in bacteria
what is disruptive selection and give an example
- when both phenotypic extremes are favoured at the expense of the intermediate
- cause phenotypic distribution to deviate from the centre and results in bimodal spread
- operates when environmental conditions fluctuate (eg seasons)
- continued separation of the phenotypic variant may eventually split into 2 sub populations
eg. coho salmon males which are either ‘small sneaker’ or ‘large fighters’
what is and describe temporal isolation
a type of reproductive isolation
- when 2 populations have different periods of activity or reproductive cycles
eg. leopard frogs & wood frogs reach sexual maturity at different times thus can’t interbreed
what is and describe behavioural isolation
a type of reproductive isolation
- when 2 populations respond to or exhibit different courtship patterns
eg. different crickets respond to specific mating calls
what is and describe geographical isolation
a type of reproductive isolation
- when 2 populations occupy different habitats or specific niches in a common area
eg. lions & tigers occupy different habitats and don’t interbreed
what is allopatric speciation
- occurs when 2 populations of an ancestral species become physically isolated by a geographical barrier
- the 2 populations evolve separately due to cumulative mutations, genetic drift, and natural selection
- eventually the 2 populations reach a degree of divergence whereby they can no longer interbreed (speciation)
what is sympatric speciation
- it is divergence of a species within the same geographical location
- may result from the reproductive isolation a 2 populations due to genetic abnormalities
- typically, a chromosomal error occurs, preventing successful reproduction with any organism lacking the same error
explain punctuated equilibrium
- a theory that holds that long periods of relative stability in a species are ‘punctuated’ by periods of rapid evolution
how does punctuated equilibrium explain gaps in the fossil record
- theory proposes that gaps may not be because there were no long sequences of intermediate forms
