IB biology HL: Natural selection - D4.1

Overproduction of offspring/competition for resources

promotes natural selection because competition. There is a carrying capacity of for many resources, promoting natural selection

natural selection

The process by which organisms that are better adapted to their environment survive, reproduce, and pass on their advantageous alleles, causing advantageous characteristics to increase in frequency within a population

outline theory of evolution

• excess of offspring

• but number of organism is a species remains constant

• so there must be a high mortality rate

• organisms show variation in their characteristics

• so, some variants will survive better than the others

• they will be able to breed and reproduce and pass in genes

• natural selection occured

• hereditary resemblance between parents and children is a fact

• next generation, greater frequency of successful alleles

• future generations will better adapted → frequency of desired trait increases

evolution

gradual change of characteristics over a long period of time

how can variation emerge from sexual reproduction

• Random assortment of maternal and paternal chromosomes during gamete formation 

• Crossing over of segments of homologous chromosomes during meiosis 

• Random fusion of male and female gametes during fertilization 

artificial selection

when humans pick traits they prefer and mate them together.

endler guppy experiment set up

• take members of the same species.

• put them in different enclosures

• put them in different colours of environment (different types of background) → pebbly gravel vs sand gravel

• in one, put a strong predator, another weak predator, another no predator

• (weak predator more realistic)

• 6 months of free breeding, and then predators put

endler guppy experiment results

• no/weak predators = more contrasts, sexual selection

• strong predators = less contrast

gene pool

all the alleles of all the genes available in the general population

change in gene pool

consequence of natural selection between individuals according to differences in their heritable traits. best adapted ones survive. evolution by natural selection

types of selection

• directional

• disruptive

• stabilizing

stabilising selection

increase in frequency of average traits. maintains favorable characteristics and the alleles responsible for it, and eliminates useless/harmful variants

directional selection

results from a change in environmental conditions. Alternative phenotypes might have competitive advantage, majority is unsuited

disruptive selection

increase in frequency of extreme phenotypes.

Hardy–Weinberg equation

 p + q =1 ( the values if the dominant and recessive expression values are equal to one)

p2 + 2pq + q2 = 1 ( this is the genotype frequency; pq is heterozygous)

Hardy–Weinberg equation conditions

only true if:

• no natural selection

• no mutation

• no migration

• large population

• random mating occurs

why is Hardy–Weinberg equation still useful despite conditions?

because they can provide a frame of reference for reference populations