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TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Biology > Populations, Natural Selection and Evolution

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Outline how population size can be affected by natality, immigration, mortality and emigration

Natality +

Immigration +

Mortality -

Emigration -

Draw a graph showing the sigmoid shaped population curve

Explain reasons for the exponential growth phase, the plateau phase and the transitional phase between these two phases

• Lag Phase
  • Only a few individuals to breed
• Exponential Growth Phase
  • Numbers increase. More individuals are available for reproduction.
  • Population grows at an every increasing rate, with no inhibitors
• Plateau Phase
  • There is a limit to the number of individuals that a community can support.
  • The population has reached its environment's carrying capacity.
  • Factors limit population growth
• Death Phase
  • The high population causes carrying capacity to decline.
  • Pollution builds up. Population rapidly declines.

Define carrying capacity

The maximum number of a species that can be sustainably supported by the environment.

List three factors which set limits to population increase

• Pollution
• Amount of food available
• Predators

State that population trends tend to produce more offspring than the environment can support

Explain that the consequences of the potential overproduction of offspring is a struggle for survival

• Overpopulation
• Scarce resources
• Survival of the fittest

State that the members of a species show variation

Explain how, by natural selection, the best adapted will survive to breed

• Natural selection keeps the disorganizing effects of mutation and other processes in check because it multiplies beneficial mutations and eliminates harmful ones.
• Natural selection may occur not only because of competition but also because of some aspect of the physical environment, such as inclement weather. Moreover, natural selection would occur even if all the members of a population died at the same age, simply because some of them would have produced more offspring than others.

Discuss the theory that species evolve by natural selection

Natural selection can be defined as the differential reproduction of alternative hereditary variants, determined by the fact that some variants increase the likelihood that the organisms having them will survive and reproduce more successfully than will organisms carrying alternative variants. Natural selection is quantified by a measure called Darwinian fitness, or relative fitness. Fitness in this sense is the relative probability that a hereditary characteristic will be reproduced; that is, the degree of fitness is a measure of the reproductive efficiency of the characteristic.

Evidence

• Fossil Record
• Structural Similarities
• Embryonic Development
• Molecular Biology

Discuss the need for evolution in response to environmental change

Darwin maintained that competition for limited resources results in the survival of the most effective competitors.

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