What does each node of a phylogenetic tree represent?

According to this phylogenetic tree, which pair of organisms is the most closely related?

During the last ice age, an ice sheet separated the meadowlarks. This ice sheet allowed the two groups of meadowlarks to evolve separately. This is an example of a:

Diane Dodd, a research scientist, completed an experiment breeding fruit flies. She divided one group of fruit flies into two different containers and fed them different food. After eight generations, the fruit flies were placed back in the same container. The fruit flies from the different groups no longer interbred. By placing the fruit flies in different containers, Dr. Dodd created a:

This type of reproductive isolation is due to structural differences between organisms.

This type of reproductive isolation is due to differences in behavior that prevent breeding.

The American Toad breeds in the spring. The Fowler's Toad breeds in the fall. This difference keeps them reproductively isolated from each other. This is an example of:

Two species of snail cannot physically mate because their shells coil in different directions, preventing the possibility of mating. This is an example of:

A mule is a cross between a female horse and a male donkey. The mule is sterile. This is an example of:

__________ can introduce new alleles into a gene pool, changing the frequency of alleles in that pool.

There are several different versions of a gene that codes for hair color. These different versions are referred to as:

This little bird just built his best nest. A female came by to check it out. She decided this nest wasn't good enough. She knocked it to the ground, showing an unwillingness to mate with the nest-builder. This is an example of:

Individuals often select mates based on heritable traits. Because of this, genes that are more often selected become more common in the population. This describes:

When individuals immigrate or emigrate from a population, this occurs.

_________ doesn't usually have major effects on large populations, but a small population can change drastically due to this.