subject guide notes

D4.1.1—Natural selection as the mechanism driving evolutionary change

natural selection occurs over billions of years on Earth

natural selection results in heritable, favourable traits being passed onto to next generations

• frequency of unfavourable traits decreases

• so only certain traits can be seen in a population, which is evolution

D4.1.2—Roles of mutation and sexual reproduction in generating the variation on which natural selection acts

mutations, which are errors in copying genetic info, result in alleles

• these alleles introduce variation into a population

• *but note only mutations in the cells that produce gametes, will be inherited

sexual reproduction introduces genetic variation through:

• gametes - formation of gametes involve crossing over & independent assortment of homologous chromosomes

  • this results in unique allele combinations

• random fertilisation - leads to unique combos of alleles from both parents

so in a population that reproduces through sexual reproduction, alleles are mixed again & again, introducing variation

so mutation creates new alleles & sexual reproduction creates new combos of these alleles

D4.1.3—Overproduction of offspring and competition for resources as factors that promote natural selection

in nature, there’s a tendency to overproduce

• plants & animals produce more offspring than the environment can handle, in order to increase the survival of their offspring

  • but most of the offspring die as the environment cannot support them

overproduction of organisms = increased competition for resources

better-adapted organisms with favourable traits will obtain these resources

less-adapted organisms may not obtain these resources & die

this keeps population size relatively stable & natural selection would favour these heritable alleles

eventually offspring of survivors may look & behave differently than the ancestral species = evolution of new species

D4.1.4—Abiotic factors as selection pressures

density-independent factors, such as high or low temps, can impact the survival of a population

density-dependent factors affect a population depending on its size

so, i guess whichever organism is able to survive these abiotic factors, will survive & their alleles will be passed on

D4.1.5—Differences between individuals in adaptation, survival and reproduction as the basis for natural selection

intraspecific competition is density dependent, and is when individuals of the same species compete for same resources

organisms with high fitness hv higher chance of acquiring resources

these heritable traits are favored by natural selection & become prominent in the population

so intraspecific evolution leads to natural selection?

D4.1.6—Requirement that traits are heritable for evolutionary change to occur

traits favoured by natural selection hv to be:

1. beneficial to organism & increase chances of survival

2. heritable - characteristics that’re encoded in genetic material will be passed on to offspring

D4.1.7—Sexual selection as a selection pressure in animal species

mates selected based on physical & behavioural characteristics, which are often an indicator of overall fitness

these characteristics can drive the evolution of an animal population

• ex: peacocks advertise their fitness by spreading their tail feathers & strutting back and forth

  • peahens would consider males with ornate plumage

  • this selective pressure ensures that males with larger & ornate feathers would pass on their genes

birds of paradise are good example of intersexual selection

• ribbon-tailed Astrapia hv elaborate courtship dances, which often consist of bird songs, spreading their feathers, and dancing

• the female will mate with the male that is most impressive

  • ensures that his genes pass onto next generation

  • so again females exert selective pressures, which influences how the males evolve?

D4.1.8—Modelling of sexual and natural selection based on experimental control of selection pressures

John Endler was fascinated with the range of colouration in guppies residing in different streams & different parts of same streams

wanted to see the rate at which evolution happened??

conducted experiment to answer this question

Stage 1

Endler calculated mean number of spots on male guppies in pool.

guppies bred for 6 months

after, Endler found there was an increase in the mean number of spots on males

suggests sexual selection - males with more spots were more attractive to females & produced more offspring

Stage 2

guppies were either placed in pond A, B, or C

in A - didn’t add other fish

in B - added Rivulus, a genus of fish that is least dangerous guppy predator

in C - added pike sichlids, a dangerous guppy predator

Endler found that in ponds A & B, the mean number of spots increase (sexual selection favored males with lots of spots)

but in pond C, mean number of spots decreased

• so natural selection favored individuals with drab colors, as this trait ensured their survival

D4.1.9—Concept of the gene pool

a gene pool consists of all the genes & their different alleles, which’re present in a population

large gene pools indicate extensive genetic variation

• this is good because it means the population has the genes needed to adapt to different environments, and survive

D4.1.10—Allele frequencies of geographically isolated populations

formula to calculate allele frequency of allele X = # of copies of allele X in population/total # of copies of gene in population

D4.1.11—Changes in allele frequency in the gene pool as a consequence of natural selection between individuals according to differences in their heritable traits

natural selection can occur due to directional, stabilizing, or disruptive selection

• all 3 result in changes in allele frequency

Neo-Darwinism is the combined work of many scientists & tries to explain natural selection on foundation of Mendelian genetics

• the theory states that:

  • phenotype is largely a result of the genotype

  • evolution is change in the genetic composition of a population’s gene pool

  • natural selection increases frequency of useful alleles

  • genetic drift & gene flow can bring changes in gene frequency of population

D4.1.12—Differences between directional, disruptive and stabilizing selection

directional, disruptive, stabilizing selection all result in a change in allele frequency

D4.1.13—Hardy–Weinberg equation and calculations of allele or genotype frequencies

Hardy-Weinberg principle states that in a stable population, frequency of alleles would stay constant generation after generation

• but certain conditions hv to be met in order for this to happen

• is also used to calculate allele or genotype frequencies in population

p = dominant allele & q = recessive allele

since p & q represent 100% of the alleles of a specific gene in a gene pool, p+q=1

genotype frequencies can be predicated by: $p^2+2pq+q^2=1$

p² = frequency of individuals homozygous for one allele

2pq = frequency of heterozygous individuals

q² = frequency of individuals homozygous for one allele

D4.1.14—Hardy–Weinberg conditions that must be maintained for a population to be in genetic

equilibrium

the equation stands true if:

• there’s random mating

• natural selection doesn’t occur

• no genes enter or leave population

• no mutations

• population remains large

if genotype frequencies in population don’t fit Hardy-Weinberg, indicates that one or more of the conditions aren’t being met

D4.1.15—Artificial selection by deliberate choice of traits

selection of certain traits over others, leads to change in allele frequency

causes a directional selection towards one trait, leading to changes in allele frequencies

unintended consequences of human action (such as evolution of resistance in bacteria when antibody is used) are a result of natural selection and not artificial selection