Question

Humans and many other organisms are diploid, possessing two sets of genes, one from the mother and the other from the father. However, a number of eukaryotic organisms spend most of their life cycles in a haploid state. Ex. Neurospora undergoes meiosis and sexual reproduction, but most of the cells comprising Neurospora are haploid. Considering that haploid organisms are fully capable of sexual reproduction and generating genetic variation, why are most complex eukaryotes diploid? What might be the evolutionary advantage of existing in a diploid state instead of a haploid state? Discuss three advantages. What is the advantage of being haploid? Discuss three advantages.

Answer 1

It has to be understood that whether haploid or diploid eukaryotes exist in an ever-changing environment. Also, there is microevolution taking place in their genome, largely driven by the mutations. We do understand that mutations are the reason for the variance.

These mutations are also related directly to the ploidy of an organism. Henceforth a diploid organism has more targets available for the mutations. Also, mutations are either beneficial or harmful and thus determine the fitness of an organism. For a diploid organism since there are two copies of the chromosomes present in them, there are more than one targets available for the mutation to acquire and thus they can better mask a harmful mutation (as they will be a mutation free copy of the gene available to compensate)
Now imagine if a harmful mutation takes place due to the environment that complex organism is, if it is diploid it has better chances of survival, and thus it is more fit. Henceforth, these complex eukaryotes are diploid. Since evolution always drives survival of the fittest, imagine a haploid organism with a harmful mutation (with no compensatory copy of the gene that will harbor the mutation) and a diploid organism with the same harmful mutation but with a compensatory copy of the gene. Which one will have more survival chances or I must say which one will be more fit to survive. Undoubtedly, it will be a diploid organism. So from an evolutionary perspective too, the diploid state favors the organism. So in the nutshell the diploid state has following advantages a) Fitness advantages over haploids in terms of the exposure to the mutations b) Better adaptability for masking the mutation as they have a compensatory copy of the gene and c) they can better adapt to the toxic environments as compared to their haploid counterparts.

Now this does not mean that being in a haploid state has disadvantages, that state has its own perks and they are as follows: a) a larger surface to volume ratio b) a minimal mutational load and c) certain adaptability advantages in a low nutrient environment setup. Since the haploid organisms are smaller in the size as compared to the diploid counterparts they are better at absorbing nutrients from their microenvironment as their surface to volume ratio is higher and that explains point a) and c). Since they have a haploid genome they bear less load off the mutations win to the less availability of the targets, remember the mutations always happen in the base pairs of the DNA that constitute a gene.