11. Analogous Structures are features of different species that are similar in function but not necessarily in structure and which do not derive from a common ancestral feature (compare to homologous structures) and which evolved in response to a similar environmental challenge. Example: Insects and birds both have wings to fly, although their wing structure is very different this is an example of an analogous structure. The fat-insulated, streamlines shapes of seals (mammals) and of penguins (birds) is another example.
13. 5.4.4 Population size and survival <ul><li> It should be noted that the 'struggle for survival' in this model is a consequence of over-population . </li></ul><ul><li>The struggle takes the form of individuals in the population being 'selected for ' or 'selected against ' . </li></ul><ul><li>Survivors form the new breeding population . </li></ul><ul><li>The frequency of advantageous alleles has increased. </li></ul><ul><li>The change in the heritable characteristics is by definition evolution . </li></ul>
14. 5.4.5 Variation in a species. <ul><li>Individuals in a species are not all identical, but show variations in their characteristics </li></ul>
15. 5.4.6 Sexual reproduction and variation <ul><li>The sources of genetic variation in a populations : </li></ul><ul><ul><li>Meiosis and the independent assortment of chromosomes creates 2 n new combinations of chromosome in the next generation n = haploid number of chromosomes </li></ul></ul><ul><ul><li>Random fertilisation increases the variation in the population to 2 2n again where n = haploid number of chromosomes </li></ul></ul><ul><ul><li>The number of different genetic variations is increased further by cross-over in meiosis by an estimated 2 3 in addition to the two above. </li></ul></ul>