T19 Flashcards
Describe the stages of succession
Initial colonisation by pioneer communities - These pioneer species can cope with the extreme initial conditions (e.g. lichen or algae).
Alteration of conditions by pioneer species - They contribute organic matter, helping in soil formation and making the environment more hospitable for other species.
Settlement by intermediate communities - Intermediate species (sometimes called secondary colonisers) move in when the soil improves and continue to modify the environment (e.g. fern plants).
Diversification of intermediate communities - Over time, the variety of species and biomass grows, leading to the arrival of larger and more complex organisms.
Development of climax communities - These are stable ecosystems with a balanced equilibrium of species, characterised by a small number of dominant plant and animal species that are rarely replaced by new species.
Animal succession occurs alongside plant succession. Primary consumers such as insects are the first to arrive, with secondary consumers following once a reliable food source is present.
What are typical adaptations of pioneer communities?
Ability to reproduce asexually.
Seeds that are dispersed by wind.
Rapid germination.
The ability to photosynthesise.
Nitrogen fixation to enrich soil.
Extreme condition tolerance.
Describe allopatric speciation
Some members of a population are geographically separated from the rest by a physical barrier like mountains, rivers, or seas.
Geographical separation exposes distinct parts of the population to different environmental pressures.
These prezygotic reproductive barriers lead to reproductive isolation.
Reproductive isolation prevents gene flow, and the physical separation leads to genetic divergence.
This causes the populations to evolve separately and form separate species.
Describe sympatric speciation
Speciation takes place within the same geographical location.
Ecological or behavioural separation mechanisms like different habitat preference, mate selection, or chromosomal changes lead to the groups becoming reproductively isolated.
Reproductive isolation prevents gene flow, and leads to genetic divergence.
This causes the populations to evolve separately and form separate species.
What are the three types of selection and how do they affect phenotypes?
Directional selection - Selects for one extreme phenotype over other phenotypes.
Stabilising selection - Selects for the average phenotype and selects against extreme phenotypes.
Disruptive selection - Selects for extreme phenotypes and selects against the intermediate phenotype, especially when an environmental factor takes two or more distinct forms.
