subject guide notes

A4.1.1—Evolution as change in the heritable characteristics of a population

helps distinguish between Darwinian theory of evolution & Lamarckian evolution

Darwinian theory is based on Darwin’s observations on Galapagos Islands

• he realized that species change/evolve, based on natural selection

Lamarck’s theory of evolution proposed that changes in the environment meant that certain organs were used more than others, and these physical changes were passed on to offspring

*acquired changes that aren’t genetic in origin, aren’t part of evolution

A4.1.2—Evidence for evolution from base sequences in DNA or RNA and amino acid sequences in proteins

sequence data provides info of common ancestry

molecular phylogeny conducts comparative analysis of DNA, RNA, & amino acids in proteins

A4.1.3—Evidence for evolution from selective breeding of domesticated animals and crop plants

Variation between different domesticated animal breeds and varieties of crop plant, and between them and the original wild species, shows how rapidly evolutionary changes can occur.

A4.1.4—Evidence for evolution from homologous structures

the pentadactyl limbs of vertebrates

• the limbs are used for different functions, but there is similarity in their arrangement of digits, ulna, humerus, carpus, radius

homologous structures indicate divergent evolution

A4.1.5—Convergent evolution as the origin of analogous structures

body parts that hv the same function, but hv different structures/forms/different evolutionary origins

• dont indicate the organisms originated from same ancestor

• ex: wings in birds & butterflies

analogous structures indicate convergent evolution

A4.1.6—Speciation by splitting of pre-existing species

new species can only appear through the splitting of an ancestral species, into 2 or more descendant species

• descendant species are genetically different from each other = cannot interbreed

  • due to the original gene pool being separated into different gene pools

speciation increases total number of species on Earth & extinction decreases it

the gradual evolutionary changes in a species is not speciation

• genetic make-up of gene pool has to change sufficiently, that the species can no longer interbreed with each other

A4.1.7—Roles of reproductive isolation and differential selection in speciation

geographical isolation is a way of achieving reproductive isolation

case study - Chimpanzees and Bonobos

chimpanzees & bonobos are African apes that reside in close proximity with each other

chimpanzees are larger, more aggressive & reside in male-dominated groups

bonobos are slender, more playful & reside in female-dominated groups

chimpanzees & bonobos belonged to a common ancestral species living in the rainforests of the Democratic Republic of Congo

Congo River was formed, dividing the ancestral species into 2 groups

• group North of the River - faced intense competition for resources (competed with each other & gorillas)

  • developed aggressive behaviour in order to obtain resources & survive

  • over time, evolved into present day chimpanzees

• group South of the River - had lots of resources

  • evolved to become cooperative & slender

  • eventually evolved into present-day bonobos

A4.1.8—Differences and similarities between sympatric and allopatric speciation

reproductive isolation can be geographic, temporal, or behavioural

sympatric speciation is result of behavioural or temporal isolation

allopatric speciation is result of geographical isolation & can lead to reproductive isolation

A4.1.9—Adaptive radiation as a source of biodiversity

allows closely related species to coexist in harmony & without competition, which increases the biodiversity in an ecosystem that has vacant niches

ex: Darwin’s finches

• ancestral finch species that arrived on the Galapagos Islands were seed-eating birds that had a thick, short beak

• islands were inhabited, so there were abundant resources of seeds, fruits, rotting wood, insects, etc

• variations in beaks meant these food resources could be exploited

• eventually, selection of favourable beaks led to rapid diversification of the finches into diverse ecological niches

A4.1.10—Barriers to hybridization and sterility of interspecific hybrids as mechanisms for of preventing the mixing of alleles between species

prezygotic & postzygotic mechanisms prevent hybridization

• prezygotic mechanisms - can be variation in courtship behaviour

• postzygotic mechanisms - reduce viability of zygote & decrease fertility of hybrid & offspring

hybrid organisms are infertile & arent reproductively isolated from parent species

• mule is example of sterile species

A4.1.11—Abrupt speciation in plants by hybridization and polyploidy

polyploidy means the organism has more than 2 sets of chromosomes

• can be triploid (3 sets of chromosomes)

• can be tetraploid (4 sets of chromosomes)

• etc.

Allopolyploidy/alloploidy - special case of polyploidy

• seen in numerous crops, such as knotweed

  • has contributed to formation of at least 6 other allopolyploid hybrids

• another example is Japanese knotweed

  • is an octoploid with 88 chromosomes

  • female can be fertilized by pollen from other species of knotweed (such as giant knotweed)

  • is indicative of the weed’s pervasive nature & capacity to rapidly speciate