Question
▶️Answer/Explanation
Markscheme
a. sex linked/gene is on the X chromosome ✔
b. allele «for red-green colour blindness» is recessive/colour blindness is recessive trait/disorder ✔
c. heterozygous females are unaffected/carriers ✔
d. XB denotes normal allele and Xb denotes colour blindness allele ✔
e. more frequent in males because they only have one X chromosome ✔
f. 50% chance of colour blindness in sons whose mother is heterozygous/XBXb ✔
a. height/skin colour/other valid example ✔
b. with continuous variation any level of the variable/phenotype is possible/OWTTE ✔
c. polygenic inheritance/combined effect of more than one gene on the trait ✔
d. additive effects on the trait of alleles of multiple genes/OWTTE ✔
e. histogram showing effects of alleles of multiple genes ✔
f. environment «may» also affect the trait/sunlight affects skin colour/other example ✔
g. normal distribution curve drawn or described to show typical pattern with continuous variation ✔
a. evolution is a change in the heritable characteristics «of a species» ✔
b. natural selection «causes evolution» ✔
c. overpopulation/over-reproduction/more offspring «than the environment can support» ✔
d. competition «for resources/mates» ✔
e. variation in population/species ✔
f. mutation/meiosis/sexual reproduction contributes to variation ✔
g. adaptation increases chance of survival ✔
h. reproduction/offspring produced «by the better adapted/by those that survive» ✔
i. characteristics passed to offspring by reproduction/variation is heritable ✔
j. allele frequencies/number of organisms carrying a gene changes/gene pool changes ✔
k. environmental change stimulates/triggers/speeds up natural selection/evolution ✔
l. increase in rainfall/introduction of antibiotic/pollution on tree trunks/other valid example of environmental change/new selection pressure ✔
m. artificial selection/selective breeding can speed up evolution ✔
Question
Evolution causes gene pools to change over time and new species to be formed.
(a) Outline how adaptive radiation provides evidence for evolution. [3]
(b) Describe polyploidy and how it can lead to speciation. [5]
(c) Explain how a newly discovered plant species would be classified and named. [7]
Answer/Explanation
a a. diversification/ different species produced from a common/shared ancestor;
b. homologous features have similarities of structure
c. despite different functions;
d. (different) adaptation to different environments/different selective pressures;
e. pentadactyl limbs/Darwin’s finches/other example of adaptive radiation described correctly;
b a. polyploidy is having more than two (complete) sets of chromosomes/3n/4n/other specific example of polyploidy;
b. can be due to errors in meiosis/production of diploid gametes;
c. can be due to DNA replication without mitosis/cytokinesis;
d. polyploidy causes reproductive isolation;
e. diploids crossed with tetraploids produce infertile (triploid) offspring / triploid offspring are infertile;
f. tetraploids are therefore a new species/failure to interbreed/reproductive isolation leads to speciation;
g. (many) examples in the onion family/Allium/other valid example of speciation by polyploidy;
h. infertile interspecific hybrids can become fertile by becoming polyploid;
c Naming:
a. binomial nomenclature/(plant is) given a binomial/double name;
b. first name is the genus and second name is the species/genus initial upper case and species lower case;
c. names (of plant species) are international/are universally understood/are published in journals;
Classification:
d. study the characteristics/structure/reproduction/chemical properties/DNA (of the plant);
e. put/classify (the plant) in a group/genus with other similar species;
f. natural classification corresponds with evolution/natural classification is based on many features
g. analogous features/features due to convergent evolution should not be used;
h. hierarchy of groups/taxa (in traditional classification/3 or more taxa in correct sequence (kingdom-phylum-class);
i. two or more of bryophyta, filicinophyta, coniferophyta and angiospermophyta named;
j. a clade is a group of organisms evolved from a common ancestor;
k. base sequences/amino acid sequences used to group organisms into clades/deduce evolutionary relationships;
l. cladograms show the relationships between clades/likely evolutionary divergence of clades;
m. each branch point/node represents where species are formed via divergent evolution;
n. species are now classified into a sequence of clades (rather than a rigid hierarchy of taxa);