Taxonomy

Phylogeny

evolutionary relationships among species

cladistics

classification based on common ancestry

clade

group of species that share a common ancestor

species

basic unit of classification and a taxonomic rank of an organism

1. organisms that are reproductively isolated

2. members of a species have a shared gene pool

3. gene flow does not occur between populations of different species

cladogenesis

Establishing a phylogenetic tree is difficult because of incomplete data from fossils and modern organisms, misleading similarities (homoplasy), different evolutionary rates, genetic recombination (especially in prokaryotes with horizontal gene transfer), and the abstract nature of interpreting evolutionary relationships, requiring complex analyses to distinguish true ancestry from convergence. 

parallelism

two closely related kinds of animals pursue similar modes of life and evolve similar structural adaptions

convergence

species not closely related develop similar adaptations to similar or even identical styles of life

selective pressure

any factor in an organism's environment disease—that influences its ability to survive and reproduce, favoring individuals with advantageous traits and driving evolutionary change over time

vestigial eyes function

detecting light/dark or influencing development