Speciation chapter 4
1.
D
2.
(a) positive relationship/species diversity increases as island area increases; as island size increases, rate of increase in number of plant species decreases/steepness of curve decreases;
(b) number of habitats increases with island area/larger islands more complex ecosystems; larger islands have longer shores, therefore more likely to receive drifting seeds, fruits, etc.
; larger islands more likely to be nesting sites for seabirds and therefore to have seeds, fruits brought from mainland; larger islands more likely to be conserved; larger islands can support larger populations of a given organism and so extinction is less likely;
Any reasonable alternative explanation.
(c) reduction in plant biomass; reduction in number of plant species; increase in very low “ground-living” plant species; competition with and decline of any other herbivorous species; trampling and soil erosion; accelerated turnover of nutrients (especially N, P) though herbivore excretion;
Any reasonable alternative explanation.
Do not expect sophisticated ideas on island biogeography in this question, but reward appropriately if they appear.
October 2013
2 max
1 max
2 max
3.
(a) (i) species diversity [1 max] : the variety of species per unit area (this includes both the number of species present and their relative abundance)/
OWTTE ; (Glossary definition)
The response must have concept of unit area and/or relative abundance for the mark. “Number of species” is insufficient. habitat diversity : the range of different habitats in an ecosystem; often associated with the variety of ecological niches/ OWTTE ; 2 max
[1]
[5]
1

(ii) description [1 max] : city has by far the highest biomass per square km; city has highest density of population; city has lowest biodiversity as absolute number of species and expressed as index; i.e
. a small number of common species are very abundant; forest has lowest biomass; and population; forest biodiversity is high, but not as high as farmland; i.e. a wide variety of species, none of which is very abundant; farmland has much lower biomass than city, higher than forest; farmland has lower population than city, higher than forest; explanation : city is a specialized environment that a few species can exploit very well; food from gardens, rubbish (garbage) etc . may be available to support some species; city has low habitat diversity/low number of ecological niches; forest is a multi-layered habitat, with a variety of plant species, so habitat diversity is high; farmland may have highest habitat diversity of all, having both natural and artificial habitats; some food from human sources may be available in farmland (crops); the diversity index of the three environments takes account of the species diversity and the relative abundance of the species, hence farmland highest;
Any other appropriate point.
At least two “explain” points needed to achieve full marks.
(b) (i) the process through which new species form/ OWTTE
(Glossary)/ the process by which change in the frequency of genetic trait occurs (in response to environmental pressures);
(ii) individual organisms in a population vary; natural selection = the tendency of those organisms most adapted to/fittest for environment to survive; and therefore to pass their characteristics to their progeny; thus organisms become increasingly adapted to their environment; a changing environment may affect speciation/evolution; mutations may affect rate of speciation; appropriate example;
Reward any two of the above points or any other reasonable points.
4 max
1
2 max
2

(iii) isolation of a small sub-group of the original population may encourage/accelerate speciation; through impossibility of interbreeding/exchange of genetic material with original population; and adaptations to new environment; entirely new endemic species/unique species not found elsewhere may appear; e.g. unique finches/tortoises/iguanas on Galapagos Islands;
Any other reasonable point .
2 max
4.
(a) habitat diversity; because different habitats tend to have different species; so more habitats will generally include greater variety of species; similarly, different species tend to have different genes; so more species will generally include greater variety of genes;
Award [1 mark] each for any two explanations.
If “species diversity” is identified, no mark should be awarded for it, but either of the last two marking points may be credited for [1].
(b) Natural selection : some genetic types will be better adapted and thus contribute to next generation more than others; hence gene frequencies/genetic types in a species will change over time;
Isolation : plate tectonics (or other environmental events) may cause part of a population to become isolated; this isolated population may be exposed to different agents/pressures of natural selection;
Speciation : these isolated populations may undergo genetic change to a point that they can no longer breed with those of their ancestors’ type; by definition these will then be a new species;
Environmental change : changes in environment lead to changes in direction of natural selection; leading to new adaptations and possible further speciation (as above);
Any other reasonable points or valid examples can be credited.
Must mention at least two of the processes for full marks.
3 max
4 max
[11]
3

(c) Advantages : [1 max] often it can generate great public appeal ( e.g. pandas, tigers, etc.
); natural habitats may be so diminished/deteriorated that the species is unlikely to survive; it may support detailed scientific research/reintroduction programmes;
Disadvantages : [1 max] maintenance of species may be difficult in captivity; population and therefore gene pool of species will be very small; it ignores the value of other species in the habitat/and the role it would normally play in that habitat for other species; difficulties of re-introduction to wild; 2 max
[9]
4