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The organisms in this group all have the same first three taxonomy groups. The starfish, sea urchin, and sand dollar all have Animalia, Echinoderm, and Asteroidea as their first three taxonomy groups. This shows that these organisms came from a common ancestor. The more taxonomy groups they have in common the more similar characteristics the organisms will have

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Descent with Modification
The theory of descent with modification is shown with echinoderms because over time the animals have reproduced and their offspring have  gotten better traits allowing the species to evolve and further survive.
Stabilizing Selection
In phylum Echinoderm all of the animals have tube feet. Therefore this group is stabilizing selection because they all share one common trait.

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All echinoderms are involved in reproductive isolation. These animals only reproduce with their own species. Although they experience random mating they only can reproduce with their own species. This is prevented by different mating times and habitats.

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Echinoderm reproduction is random. The male and the female of starfish, sand dollar and sea urchin all release their sperm and egg into the water. The reproduction rate is very low because the eggs and sperm of these animals don't always meet. When the egg an the sperm meet is a random mating.

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The DNA sequences in sand dollars, starfish and sea urchin are all the same. The DNA sequences are all repetitive and single copy DNA sequences. This shows that the these animals came from a common ancestor.

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The animals in Phylum Echinoderm are involved in divergent evolution. The starfish, sea urchin and sand dollar all have very similar traits. Although they have the same traits they are used for different purposes. This is shown with the tube feet and spines on the body. Sometimes the animals will use tube feet moving, eating, or for grasping. Not all the organisms will use them for the same purpose. The spines are also used for protection on some animals and as a way for transportation on another.

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Kingdom: Animalia
Phylum: Echinoderm
Class: Asteroidea
Order: Forcipulatida
Family: Asteriidae Genus
Genus: Asterias

Characteristics
Sand dollars are small round flat animals. They also have small spines on the hard body that help them move. These spines also help them to butty themselves.

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Kingdom: Animalia
Phylum: Echinoderm
Class: Asteroidea
Order: Echinoida
Family: Strongylocentrotidae
Genus: Strongylocentrotus

Characteristics
Sea Urchins are round animals that have spines around most of their body. They also have tube feet that are used to move. The spines around the body are used as a form of protection and sensing.

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Echinoderms are going through gradual equilibrium. This is shown through the fossil evidence. The fossils that have been found are all very similar. Over millions of years the animals have only changed in colors and sizes

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Kingdom: Animalia
Phylum: Echinoderm
Class: Asteroidea
Order: Echinoida
Family: Strongylocentrotidae
Genus: Strongylocentrotus

Characteristics
Starfish are star shaped animals that have 5 or more arms. The starfish has a mouth on the underside of the body. The starfish also uses tube feet for moving around.

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The Echinodermata, (from the Greek meaning spiny skin), is a phylum containing some 13,000 extinct and 7,000 extant species. Living representatives are only found in marine environment, making the phylum the largest lacking terrestrial and fresh water forms. Echinoderms evolved from bilaterally symmetric animals exhibiting fivefold radial symmetry in portions of their body at some stage of life. This pentameral symmetry is readily apparent in familiar adult starfish and sand dollars. Other echinoderms both extant and extinct lack the five point morphology because it was lost somewhere during development
http://www.fossilmuseum.net/Tree_of_Life/Phylum-Echinodermata.htm

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The star fish has 36 chromosomes, the sand dollar has 52 chromosomes, and the sea urchin has 44 chromosomes. This shows that the organisms from this taxonomic group do not have the same number of organisms. In conclusion these organisms have evolved in the number of chromosomes over time.

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Pseudocidaris
This fossil of a sea urchin shows that they have changed a lot since they were first formed. The spines on this sea urchin were round and now they are very sharp and pointy. Although the sea urchin is very different it has gradually evolved over time.

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Urasterella verruculosa
This fossil also shows that the starfish has not changed very much over time further showing it has gone through gradual evolution.

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Australaster giganteus
This fossil shows that there is very slow evolution in this species. It shows gradual evolution because the starfish from 250 mya are very similar to the starfish today

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All of the organisms are very similar. They are similar because they all have spines and spikes on their outer bodies. They also all have rough skin. The organisms all have tube feet which allow them to move. This shows that although the organisms don't look the same they still all have similar structures and functions.

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Phymosoma
This fossil  shows that there has been no change to this species of sea urchin. This shows it is undergoing gradual evolution.

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Irregular Echinoid
This fossil of an irregular sand dollar is the exact same as the irregular sand stars today. This shows that the sand dollars have had little to no changes over millions of years. This is also another example of gradual evolution.