Variation and Evolution Flashcards
three types of variation (+2 more)
- discontinuous (discrete)
- continuous
- phenotypic
and intra/inter specific variation
link between genes and discontinuous variation
controlled by a single gene (monogenic) OR multiple genes in epistasis
link between genes and continuous variation
- controlled by several genes (polygenic)
- genes must not be linked/carried on the same chromosome so can be shuffled during meiosis (crossing over could however happen if genes are on the same chromosome)
what two things contribute to phenotypic variation?
- genotype
- environment
which type of phenotypic variation is more affected by environmental factors?
continuous variation
three types of selection
- stabilising
- directional
- disruptive
stabilising vs directional vs disruptive selection
stabilising - selects for the norm (extremes most affected by selective pressure)
directional - selection in response to environmental change (some characteristics may have a selective advantage)
disruptive - selects for the extremes (norms most affected by selective pressure)
example of stabilising selection
- clutch size in birds (number of eggs)
- too few will increase the probability that all will die before reproductive age
- too many will cause food availability to become a selective pressure
example of directional selection
- peppered moth more abundant before industrial revolution
- melanated moths more abundant after industrial revolution as they have a selective advantage (camoflauge for the soot selective pressure)
OR antibacterial resistance
genetic drift
change in allele frequency caused by chance (often seen in small populations)
two causes of genetic drift
- founder effect
- bottlenecks
example of disruptive selection
- young male birds with either dull or bright blue feathers are selected for
- dull are seen as non-threatening
- bright are seen as threatening
- both are left alone (not attacked/less competition)
founder effect
few individuals of a species colonise a new area leading to a loss in genetic variation and rare alleles can become more common in the population
two types of limiting factors in population size
- density dependent factors (affect different sized populations differently): competition, predation, parasitism, communicable disease
- density independent factors (affect populations of all sizes the same): climate change, natural disaster, seasonal change, human activity
bottleneck
catastrophic events significantly reduce the population size (lasting for at least one generation) which reduced genetic variation
speciation
formation of new species through evolution
three types of isolation mechanisms
- ecological (geographical) isolation
- seasonal (temporal) isolation
- reproductive isolation
allopatric speciation
- caused by ecological/geographical isolation
- members of a population are separated by a physical barrier (rivers, mountain ranges, sea)
- different areas have different selection pressures
- different genes will provide different selective advantages leading to different adaptations
- differences accumulate until it is impossible for successful interbreeding
what other form of genetic drift can occur with ecological isolation?
founder effect
sympatric speciation (seasonal and reproductive)
- caused by seasonal and reproductive isolation
- occurs within a population sharing the same habitat
- less frequent than allopatric speciation
- more frequent in plants than animals
two types of reproductive isolation
- prezygotic
- postzygotic
prezygotic reproductive barriers (3)
- courtship behaviour: unrecognisable courtship rituals can prevent mating
- mechanical barriers: different sized genitalia
- physiological barriers: pollen tubes may not grow at the same time, preventing fertilisarion
postzygotic reproductive barriers (2)
- sterile hybrid: mules have odd numbers of chromosomes so cannot pair in meiosis to produce gametes/plants can double chromosomes (polyploidy) to enable gamete production
- hybrid not viable: fertilisation occurs but embryo fails to develop and onaborts
polyploidy
mutation which doubles chromosomes so that they can pair up to produce gametes (usually to make infertile plant hybrids fertile)
