This was the first scientific approach to classifying organisms.

In the Linnaean classification, organisms are placed in hierarchical groups based on appearance.

Organisms in lower groups bear more similarity in appearance.

Each organisms is identified by a binomial scientific name: its Genus and species.

For example, Equus quagga for the Zebra, which also belongs in:

• Family Equidae,
• Order Perissodactyla,
• Class Mammalia,
• Phylum Chordata,
• Kingdom Animalia,
• Domain Eukaryota.

The binomial name for humans is Homo sapiens.

Organisms are related to each other by descent from a common ancestor.

The more recent the common ancestor, the more related 2 organisms are.

Thus a rat is evolutionarily closer to a mouse than it is to a human.

Note there is no left-right relationships in this tree.

exercise

• In this diagram, the closest group to humans is
  • both rat and mouse

For example, all organisms descended from an ancestor that had jaws constitute a large clade; smaller clades later developed 4 limbs, milk, etc.

Note:

• All animals with milk have jaws (and 4 limbs, vertebrae).

• Only animals that have jaws can produce milk.

• Thus:
  1. you cannot find an animal with milk but no jaws
  2. an animal without jaws cannot produce milk

Birds and crocodiles compose a clade because they share a recent common ancestor (A).

Lizards and crocodiles do not compose a clade; their common ancestor (B) is also shared by birds.

Birds, crocodiles, and lizards do compose a larger clade.

An even larger clade includes all 4 organisms: terrestrial vertebrates with 4 limbs (tetrapods).

The wings of bats and insects are superficially similar but evolved independently from different anatomical bases and inherited from unrelated ancestors.

Note that bats and insects also do not constitute a clade: they did not inherit wings from a common ancestor.

Since organisms inherit DNA from their ancestors, DNA analysis can verify evolutionary relationships.

As species diverge, their DNA sequences also diverge, due to accumulated mutations.

Thus, the more related two organisms are:

• the more recent their common ancestor is

• the more similar their DNA sequences (and amino acids) are

DNA evidence is revealing phylogenetic relationships that are otherwise difficult to determine.

The African golden mole looks superficially similar to a shrew.

DNA analysis shows that it belongs in a clade called Afrotheria with elephants.

• Is a hyena more related to a lion or to a wolf?
  • Lion

All living organisms descended from a common ancestor, rooted in domain Bacteria.

Archaea and Eukarya diverged later and are more related to each other than either is to Bacteria.

Groups like animals form smaller clades with their own lines of descent.

• What's wrong with this picture?
  • There are 3 groups called "Protists". Protists are not a monophyletic clade, even though some scientists place them in one kingdom.

Geographic isolation of different populations of a species can lead to differential adaptations to their environments.

Over time, allele frequencies among the populations diverge, leading to speciation: the evolution from an ancestral species.

The finches of the Galapagos Islands provide an example.

• Descendants of an ancestral finch from Ecuador occupied separate islands.

• Geographic isolation led to diversification of the descendants, which developed different beaks that specialize in eating different food sources.

Speciation can also occur among populations in the same geographic area.

An example is polyploidy in plants.

1. Abnormal meiosis may yield a diploid gamete.

2. Self-fertilization among diploid gametes can yield polyploid offspring with twice the number of chromosomes as the parent.

Many crop plants, including wheat, bananas, potatoes, and coffee, are polyploid.