BY2 Section 1 Revision

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  • Phylogenic - reflected revolutionary relatedness
  • A phylogenic classification system is where organisms that are closely related are placed into groups. Organisms of the same group have a more recent common ancestor with each other than with those in another group.
  • Classification is hierarchial: larger groups divide into smaller groups.
  • In classification any group is called a taxon.
  • Taxa are discrete: at any level of classification organisms can belong to one taxon only.
  • A classification system is universal and so helps us to communicate.
  • It allows us to infere the relatedness of two species, and if a new species is discovered, we can predict some of its features.
  • It is also beneficial when determining the health of an ecosystem, as we count the families rather than individual species.
  • Classification is tentative: it is not fixed or certain.



Characteristics of the five kingdoms:

1. Prokaryota

  • Unicellular
  • Cell wall comprised of murein
  • No membrane-bound organelles

2. Protoctista

  • Membrane-bound organelles
  • No tissue differentiation

3. Fungi

  • They have hyphae
  • They are saprotrophs
  • Cell walls made of chitin
  • Reproduce by spores

4. Plantae

  • They have membrane-bound organelles, such as chloroplasts
  • Cell walls made of cellulose
  • They are photoautotrophic: they undergo photosynthesis

5. Animalia

  • Heterotrophic
  • Show nervous coordination
  • Lack cell walls

Assessing the relatedness of organisms

1. Using physical features

You must look for a homologous structure - these are structures that have a similar structure and same developmental origin but a different function.

For example, the pentadactyl limb of the vertebrate. Its common structure is present in all four terrestrial organisms: birds, reptiles, mammals and amphibians. However, they all have different functions. Therefore, this provides an example of divergent evolution: a common ancestrial feature has evovled. 

Analogous structures - structures of the same shape and function but a different developmental origin.

For example, a bird, a butterfly and a bat. These all have wings, except one is an insect, one is a bird and the other is a mammal. Therefore, they do not share a common ancestor, but instead their ancestors have adapted to a similar environment and so evolved wings. This is an example of convergent evolution: a structure of similar function has evolved from different developmental origins.

2. Using genetic evidence

When a species evolves over time to form a new one, the new species will have slight differences in its DNA base sequences. Over time, this species would accumulate…


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