Describe and example of how natral selection impacted the evolution of a particular species?

Answer 1

Natural selection works on the genetic variability within a species. Darwin's finches in the Galapagos show the way the bird's beak is selected by climate variation. The trade winds bring in rain seasonally. In good years there is enough rain for plants that are not as drought resistant to dominate especially on the wet side of the islands. These plants have thin walled seeds easily crushed by bird's with smaller beaks but harder to handle with larger beaks. In a series of years with good rain birds that breed smaller beaks survive well and produce then rear more offspring. The climate is selecting for small beaked bird and wet tolerant plants. However the climate runs in cycles so after a few years the drought returns so plants that are drought resistant are selected for. These plants have thick hard to crack seeds that can survive long periods until conditions are right to germinate. The finches with heavy beaks are the favored birds now since they can eat they hard seeds they find and the softer seeds. Small beaked finches only eat the softer now rarer seeds. This constant push to the two ends covers more than beaks size because as the flora shift to drought tolerant the birds nesting site options change as does their water resources. Now if something happens to isolate a portion of the population such as migration to a new island or habitat fragmentation they could easily be assorted with unequal allelic division. One population has more small beaks the other large beaks. This could be the last push to cause speciation into a heavy beaked drought tolerant species that lives on drier islands or dry sides away from the trade winds and a small beaked water dependent species on the islands with more regular trade winds. Islands or small populations are more open to fragmentation producing new species.

A common disadvantage to speciation is the presence of vestigal organs but they give us they story of the divergence.
Slugs seem simpler than snails, but snails are the ancestors of slugs. All animal evolution uses the basic torus form; mouth at one end anus at the other. Snails still need to excrete, but this is not good inside a shell. So snails contort or twist in their growth. With the left half dominating the right half, they put in a 180 so the anus and mouth both face out of the shell. Snails are stuck with the same basic body tube we have with a slight reorganization just so they can wear a shell as protection.
Now the slug. They quit the shell but are stuck with a tiny vestigial shell and contorting of body form. They still contort but then they de-tort as they grow. A result of this contorting/de-contorting is that the right-hand-side organs are reduced. They have to grow then shed the shell as they mature. Their vestigial process is very costly and not simple yet slugs appear simpler than snails. They are often called snails without shells with no understanding of the cost slugs paid to remove their shell.
This is all observable through DNA sequence, developmental observation and fossil evidence. It makes a fine list of facts but how did it happen. What benefit did the slug get to offset the cost of going through all those additional growth steps.
Shells are better defensive structures in water where the snail evolved. They are buoyancy balanced so are easier to move there. Once the snail moved onto land the shell offered less advantage. Initially the shell worked against drying allowing the snail out od the water. Animals evolved that eat snails (many rodents, birds, reptiles, insects & even other snails). They simply crush, break or drill thru snails' shells. Now the shell was a handicap, heavy and hard to maneuver.

So slugs evolved thicker mucus that coats their bodies for defense and to block evaporation. The thicker slime was a modification of the mucus snails use to seal the shell in dry spells while they estivate. Slugs with smaller shells but also slime protected were able to avoid more predators. Slugs can squeeze into crevasses to escape predatory beetles or, if bitten, the beetles get gummed mouth parts. Slugs are nocturnal and hide in the soil or under the plants they eat to avoid drying and predators. They are still land dwelling, more maneuverable and with an effective defense system.

Answer 2

Man was superior to the dodo bird and so killed them all off through survival of the fittest. Man's evolution was impacted by the fact that now he goes around scratching the seeds of some tree that the dodo ate of fruit of to cause it to germinate. So killing off the dodo led to man developing a new action.