Table 2.

Some reasons why evolution routinely yields suboptimal biological outcomes

1	Natural selection is a nonsentient process of nature, as uncaring and dispassionate as gravity.
2	Natural selection is not all-powerful. Instead, it is just one in a nexus of evolutionary forces, others of which can override the adaptation-promoting power of natural selection in particular instances, and thereby yield products that fall far short of designer perfection.
3	Random mutations, most of which are either deleterious or fitness-neutral, continually arise.
4	Harmful mutations (especially those that are only slightly deleterious individually) often fly below the radar screen of purifying natural selection, especially in small populations.
5	Genetic drift can alter the genetic composition of populations in ways that are uncorrelated with adaptive benefits.
6	Sexual selection on particular traits often operates in direct opposition to natural selection.
7	Genetic correlations and conflicts are common. In such cases, deleterious alleles linked to host-beneficial alleles at other loci can hitchhike with the favorable alleles, and thereby escape eradication by purifying natural selection, at least temporarily.
8	Pleiotropy and fitness tradeoffs are common. Thus, a particular genotype often has multiple phenotypic consequences, some of which benefit and others of which may harm the organism.
9	In sexual species, natural selection acts not only at the organismal level but at the level of genes. Thus, “selfish DNAs” (e.g., many mobile elements) can persist and proliferate in a genome without enhancing the well-being of a host population.
10	Phylogenetic constraints are ubiquitous. At any point in geological time, natural selection going forward can only work with the genetic diversity presented by lineages that have survived from the past. This places severe constraints on what evolution can achieve.