Monthly Archives: January 2009

5- Selection & Evolution

Darwin’s Origin of the Species & Natural Selection

One of the key points of selection and evolution is that of Natural Selection – that some organisms have a better chance of survival than others.

Let’s look at what might happen:

  • Organisms within a population are not identical, varying in both genotypes and phenotypes.
  • While some organisms in the population survive, others die (removing themselves from the gene pool).
  • The surviving organisms then breed, passing on their genes to their offspring. (As the parent organisms survived, it is likely they hold some advantageous genetic characteristics that the organisms which perished did not).
  • This increases the frequency of the advantageous genes in the population, and the cycle can begin again.

Over time, the population is filtered to leave only organisms with characteristics suited to their environment – helping them survive.

5- Classification & Taxonomy

The Five Kingdoms

There are 5 kingdoms in the classification system. Organisms are classified according to their evolutionary relationships (their phylogeny).

Phylogeny is the study of the evolutionary history of organisms, and gives us an insight as to how to group them and their extinct relatives. The base hierarchy in the classification system is the Kindom.

Generally, we can order the Kingdoms by increasing complexity. To help remember the names of the kingdoms, I was taught:

Pretty Polly Finds Parrots Attractive – Prokaryote, Protoctista, Fungi, Plantae, Animalia.

Prokaryotes Protoctista Fungi Plantae Animalia
Cell Structure Unicellular; no membrane bound organelles Eukaryotes, Unicellular & Multicellular Eukaryotes, Unicellular & Multicellular (Yeast) Eukaryotic, Multicellular; Large Vacuoles Eukaryotic, Multicellular
Cell Wall Murein Sometimes Polysaccharide Chitin Cellulose None
Nutrition Autotrophic, Aerobic Heterotrophic Autotrophic, Hetrotrophic Heterotrophic Autotrophic (Photosynthetic) Heterotrophic, Digestive System
Reproduction Binary Fission Fission Spores Seeds/Spores, Some asexual while some sexual Develop from embryo
Example Bacteria Algae, Protozoa Penicillin Mosses, Ferns Humans, Animals


Q. What’s a photosynthetic organism?

A. An organism that gets its energy by absorbing light.

Q. What’s a autotrophic organism?

A. An organism which gets it’s energy from light (photosynthesis) or from chemical interaction (chemosynthesis).

Q. What’s a heterotrophic organism?

A. An organism that relies on complex organic matter for food.

Remember that 4 of the 5 kingdoms feature Eukaryotes! Only Prokaryotae contains Prokaryotes (no surprise there!).

Taxonomy (Breaking it down)

We break down organisms into a total of 7 hierarchical classes (including Kingdom above). That’s a lot of possible choices for organisms, and is know as Taxonomy, or Alpha Taxonomy.

The 7 levels are Kingdom, Phylum, Class, Order, Family, Genus and Species. You could remember this as:

King Penguins Climb Over Frozen Grassy Slopes

Here’s an example of two organisms and their taxonomy:

Humans Large White Butterfly
Kingdom Animalia Animalia
Phylum Chordata Arthropoda
Class Mammalia Insecta
Order Primates Lepidoptera
Family Hominidae Pieridae
Genus Homo Pieris
Species sapiens brassica

As you can see, humans are sapiens of the Genus Homo. AKA Homo sapiens (I bet you’ve heard that before!).

The only similarity between these two examples is that they are both in the Animalia kingdom. This means they share a great number of common traits, and so actually tells us a lot about the organisms.

It is also worth bearing in mind that Protoctista is often the ‘Other’ category where organisms who have no clear Kingdom are put. For example, Slime Moulds have fungi characteristics, yet are not quite suitable for classification in the Fungi Kingdom.

The Species

Species is the final tier on the taxonomy hierarchy; and is a group of organisms with similar traits. These include:

  • Morphology (The outside appearance of an organism, including shape, colour, structure and pattern)
  • Physiology (The way in which an organisms works, by looking at it’s biochemical, mechanical and physics functions)
  • Behaviour

BUT most importantly, we can class two organisms as the same species if they can naturally breed together and produce fertile offspring.

The fertility point is an important one, as there are several organisms that can breed together, but produce a sterile offspring which cannot breed any further – such as a horse and a zebra which can produce a hybrid. This hybrid is sterile, so we know what the horse and the zebra are different species.