Editing Molecular evolution (section)

===Genome size===
{{Main|Genome size}}
Genome size is influenced by the amount of repetitive DNA as well as number of genes in an organism.  Some organisms, such as most bacteria, ''Drosophila'', and ''Arabidopsis'' have particularly compact genomes with little repetitive content or non-coding DNA.  Other organisms, like mammals or maize, have large amounts of repetitive DNA, long [[introns]], and substantial spacing between genes. The [[C-value paradox]]  refers to the lack of correlation between organism 'complexity' and genome size.  Explanations for the so-called paradox are two-fold.  First, repetitive genetic elements can comprise large portions of the genome for many organisms, thereby inflating DNA content of the haploid genome. Repetitive genetic elements are often descended from [[transposable elements]].

Secondly, the number of genes is not necessarily indicative of the number of developmental stages or tissue types in an organism.  An organism with few developmental stages or tissue types may have large numbers of genes that influence non-developmental phenotypes, inflating gene content relative to developmental gene families.

Neutral explanations for genome size suggest that when population sizes are small, many mutations become nearly neutral.  Hence, in small populations repetitive content and other [[junk DNA|'junk' DNA]] can accumulate without placing the organism at a competitive disadvantage.  There is little evidence to suggest that genome size is under strong widespread selection in multicellular eukaryotes.  Genome size, independent of gene content, correlates poorly with most physiological traits and many eukaryotes, including mammals, harbor very large amounts of repetitive DNA.

However, [[birds]] likely have experienced strong selection for reduced genome size, in response to changing energetic needs for flight.  Birds, unlike humans, produce nucleated red blood cells, and larger nuclei lead to lower levels of oxygen transport.  Bird metabolism is far higher than that of mammals, due largely to flight, and oxygen needs are high.  Hence, most birds have small, compact genomes with few repetitive elements.  Indirect evidence suggests that non-avian theropod dinosaur ancestors of modern birds<ref>{{cite journal | vauthors = Organ CL, Shedlock AM, Meade A, Pagel M, Edwards SV | title = Origin of avian genome size and structure in non-avian dinosaurs | journal = Nature | volume = 446 | issue = 7132 | pages = 180–184 | date = March 2007 | pmid = 17344851 | doi = 10.1038/nature05621 | s2cid = 3031794 | bibcode = 2007Natur.446..180O }}</ref> also had reduced genome sizes, consistent with endothermy and high energetic needs for running speed.  Many bacteria have also experienced selection for small genome size, as time of replication and energy consumption are so tightly correlated with fitness.