Evolution, Classification, Body Plans and the Sequence of Life

Classification Levels review• Domain: Eukaryota, Archea and Bacteria• Kingdom

– Animals, Plants, Fungi, Monera (bacteria), Protista

• Phylum• Class• Order• Family• Genus• Species

Humans• Kingdom: Animalia

• Phylum:Chordata

• Class:Mammalia

• Order:Primates

• Family:Hominidae– Subfamily:Homininae

• Tribe:Hominini

• Genus:Homo

• Species:H. sapiens

Body PlansSymmetry

• With some exceptions organisms (tend to show some sort of symmetry. It may be:

• Radial Symmetry– These organisms resemble a pie where

several cutting planes produce roughly identical pieces. An organism with radial symmetry exhibits no left or right sides. They have a top and a bottom (dorsal and ventral surface) only.

– e.g. Starfish, anemones, coral polyps.

Body Plans

• Bilateral Symmetry– In bilateral symmetry (also called plane

symmetry), only one plane, called the sagittal plane, will divide an organism into roughly mirror image halves (with respect to external appearance only).

– E.g. humans

Why do most organism exhibit symmetry?

• Developmental biology reasons:– Body plans are often set by the diffusion of certain

proteins that determine the left, right, back, front, up and down axis of an organism. Since molecules have no direction preference they get distributed symmetrically.

Evolutionary reasons:

Symmetry is selected for when it is an advantage. For example it is better to have two legs the same length because you can walk more efficiently.

Body plans

• Cephalisation– Bilaterally symmetrical animals tend to

concentrate sense organs in a head region.– The mouth tends to be a this end too.

Body Plans

• Segmentation– Body divided into segment, structural units

are repeated.– E.g. Segmented worms, insects, muscles

groups/vertabrae in humans