Spright

As defined in the wiktionary: haploid = Of a cell having a single set of unpaired chromosomes; vs diploid which has a pair of each chromosome i.e. if you have 2 chromosomes of each, if you get a mutation that is deleterious in one chromosome it happens that this condenses (epigenetical deactivation) while the other chromosome becomes activated and the gene is used from the active/healthy chromosome. If an organism is haploid, it doesn't have a spare chromosome so if a deleterious mutation happens, it is more or less screwed --> natural selection.

Think in terms of X-linked diseases being abundant in males. Males have one X and one Y; if the X chromosome is screwed, he doesn't have a spare one so disease occurs; while in females: easy peasy, just activate the other X chromosome and deactivate the sick one. In females, to get a x-linked disease, both X's must be affected, which is unlikely.

Andreea