EvolutionIB Biology Topic 5.4

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.

5.4.4 Population size and survival

•  It should be noted that the 'struggle for survival' in this model is a consequence of over-population.

• The struggle takes the form of individuals in the population being 'selected for' or 'selected against' .

•Survivors form the new breeding population.• The frequency of advantageous alleles has

increased.• The change in the heritable characteristics is by

definition evolution.  

5.4.5 Variation in a species.

• Individuals in a species are not all identical, but show variations in their characteristics

5.4.6 Sexual reproduction and variation

• The sources of genetic variation in a populations :▫Meiosis and the independent assortment of

chromosomes creates 2n new combinations of chromosome in the next generation n = haploid number of chromosomes

▫Random fertilisation increases the variation in the population to 22n again where n = haploid number of chromosomes

▫The number of different genetic variations is increased further by cross-over in meiosis by an estimated 23 in addition to the two above.