POPULATIONS AND THE ENVIRONMENT - inheritance

These revision cards aim to look at:

  • key definitions
  • different types of natural selection
  • natural selection
  • speciation
  • monohybrid inheritance
  • the hardy-weinburg principle
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KEY DEFINITIONS

Dominant Allele: allele that is always expressed in the phenotype.

Recessive Allele: allele that is expressed in the phenotype in the absence of a dominant allele.

Codominance: both alleles are dominant and are expressed in the phenotype.

Genotype: constitution of an organism comprising all the genes possessed by an individual.

Phenotype: characteristics of an organism, often visible, resulting from the genotype and the effects of the environment.

Heterozygous: having two different alleles for a given gene.

Homozygous: having two dominant/recessive alleles present for a given gene.

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NATURAL SELECTION

Causes a change in the frequency of a particular genotype and phenotypes in a population, resulting in a change in the proportion of alleles within any population of species, causing variation to exist.

Example of Natual Selection:

  • resistance to pesticides
  • antibiotics as an example of the effect of the human body
  • sickle cell aneamia in relation to maleria
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DIFFERENT TYPES OF NATURAL SELECTION

Directional Selection:

Where individuals at one extreme many have an advantage whereas there are other extremes which are at disadvantages, causing the mean and range to shift towards the favoured extreme.

Stabilising Selection:

Whereby selection operates against both extremes of a range, which causes the population to become more uniform, and environmental stresses weed out unsuitable phenotypes.

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SPECIATION

Geographical Isolation (ALLOPATRIC)

When two populations are physically sperated.

Interbreeding becomes impossible and speciation may result.

  • abiotic
  • biotic

This explains how populations of a species become adapted to their environment and change in a changing environment.

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