IGCSE Biology Theory Unit 5 Full Class Notes

VARIATION, NATURAL SELECTION, AND EVIOLUTION

VARIATION

 Difference btwn individuals of same species
 Beneficial cos allows natural selection + reduces risk of extinction

CAUSES

 Genetic variation
o Each org has different DNA cos of genetic variation in meiosis
o Different alleles = new set of genes made
 Environmental variation
o Characteristics affected by climate, diet, accidents, culture, lifestyle
 Genetic + environmental variation
o Phenotype of org based on genetic + environmental characteristic
o Potential height decided by genes but due to lack of nutrient from environment it wont
o

CAUSES OF GENETIC VARIATION IN SEXUAL REPRODUCTION

VARIATION IN GAMETES COS MEIOSIS

 Meiosis creates haploid no. of chromosomes from diploid no.
 Gametes formed by meiosis + don’t have same combo of alleles = genetic variation
 Crossing over btwn homologous pairs of chromosomes introduce variation
 Separation of homologous pairs in daughter cells causes variation

RANDOM FERTILISATION DURING SEXUAL REPRODUCTION

 Sperm could fertilize w ovum = zygote = genetic variation of offspring
 Every living thing is genetically different
 Cos gametes genetically different + fusion of gametes completely random

MUTATIONS

 Random changes in DNA/genetic material coding for different protein

EVOLUTION

 Change in inherited characteristics w in ppltn over time thru natural selection
 New species arise from existing species + others become extinct
 Mechanism of evolution proposed by Darwin
 Based on 3 main observations
o w/in ppltn organisms have varying characteristics + inherited by offspring
o org produce ^ offspring than needed to replace parents
o on ave, ppltn size remains relatively constant
 2 conclusions
o Struggle for existence – many offspring but ppltn constant = compete for available
resources = many die

, o Survival of fittest – best adapted org get selective advantage + survive to reproduce
 Advantageous allele passed to offspring = frequency of allele ^

NATURAL SELECTION

 Natural process where best adapted individuals survive longer, have ^ offspring + spread
characteristics
 Mechanism of evolving of new species
 Allows different forms of species to survive in different environments
 Theory of natural selection proposes factor in env selects which form of species survives to
reproduce = not well adapted will not survive
 Evidence for natural selection must show that
o There is variation w in species
o Selection pressure – environmental conditions change
o Selective advantage – favors on particular form of individual in species
o Frequency of favored allele ^
o Frequency of disadvantageous allele decreases
o Allele frequency of ppltn changes

EXAMPLES OF NATURAL SELECTION

SNAIL SHELL COLOUR

 Variation of shell color in ppltn of snails
 Normal varieties = black/grey
 Chance mutation = white shelled which survive longer cos better camouflaged
 ^ reproduction oppo = pass allele to offspring
 Majority of snails have white shells = change in frequency of phenotype in ppltn over time

PEPPERED MOTH

 Variation w in peppered moth ppltn w light + dark moth
 Natural selection favored dark colored moths in areas of heavy pollution
 Light colored favored in less polluted areas

NATURAL SELECTION + ANTIBIOTIC RESISTANCE

 Antibiotic kills/inhibits growth + reproduction of bacteria
 Bacteria reproduce every 20 min = evolution occurs in shorter time span
 Variation caused by mutation
 Chance mutation causes bacteria = ^ resistant to antibiotic
 When treated w antibiotic = resistant bacteria wont die
 Reproduce w less competition from non-resistant bacteria
 Genes for resistance passed at ^ frequency to next gen
 Whole bacteria ppltn eventually resistant = natural selection
 ^ in antibiotic resistance = harder to control infections + diseases

OTHER EXAMPLES OF NATURAL SELECTION