5.2 natural selection

variation (sources)

makes evolution possible

1. environmental factors: temp, resources

2. genetic variation:

   1. inheritable mutations → new alleles

   2. crossing over (prophase 1), independent assortment/segregation (metaphase, anaphase 1 & 2) of homologous chroms / chromatids → new combis of alleles

   3. random fertilisation of gametes

types of evo

macro

micro: within a popn → change in allele freq

features of natural selection

differential survival

1. variation

2. struggle for survival: overprod / survival of the fittest

3. selective adv

4. microevolution

struggle for survival (overprod)

organisms prod a much larger no of offspring than mature indvs but popn size remains mostly constant due to selection pressures exerted by environment.

• eg. predation, overcrowding, spread of disease, competition for resources.

• many indvs die, will not reach reproductive stage.

• only those showing variations best adapted to the environment will survive → survival of the fittest

selective adv: adaptations

adaptations are random, not deliberately developed. indvs alr possess them due to variation → dev by natural selection.

adaptations (inherited) ≠ acquired characteristics.

better adapted to survive & prod more offspring → indvs pass on alleles coding for the favourable characteristics to their offspring.

eg. structure of a bird’s beak correlates w its diet & method of feeding

eg. evo of giraffe: ancestors had long & short necks, but during dry season only remaining leaves were on v high branches → indvs w longer necks became better adapted.

natural selection

the differential success of indvs allows them to survive & pass favourable alleles to offspring. natural selection increases the freq of alleles that make indvs better adapted.

galapagos islands finches

all finches descended from the ancestral finch. selection pressure: food.

geospiza fortis: medium ground finch → feeds on small & large seeds.
geospiza fulignosa: small ground finch → feeds on small seeds.

drought: small supply of small seeds → G. fortis selective adv → evolutionary change in beak size as those w smaller beaks died.
el nino: more small seeds, less large seeds → G. fortis bred rapidly.
dry weather conditions: less small seeds → v few bred, G. fortis now have longer & narrower beaks.

antibiotic resistant bacteria

selection pressure: presence of of antibodies (inhibit bacteria).

some cells may have undergone random mutations (supplies variation) → have an allele coding for antibiotic resistance → have slective adv over susceptible bacteria → survive & reprod → pass on resistance gene to offspring.

→ a strain of antibiotic resistant bacteria has dev through natural selection → the strain that is sensitive to the antibiotic is eliminated → characteristics of the bacteria has changed → evolution.