Question

Match the specific benefit of sexual reproduction on the left with its explanation on the right. 

10. Red Queen Hypothesis 

11. Muller's Ratchet 

12. Combining of Adaptive traits

a. Sexually reproducing population have less conflict between tissue types due to the evolution of cellular fusion mechanisms. 

b. Genetic load, the build up of slightly deleterious alleles in a population, is lower in sexually reproducing populations. 

c. Two beneficial mutations that appear simultaneously in different individuals cannot be passed on to a single offspring via mitosis. 

d. Organisms that reproduce sexually are better able to coevolve with their parasites.  

Answer 1

ANSWER 10

Red queen hypothesis - correct answer is d. - organisms that reproduce sexually are better able co-evolve with their parasites.

The Red Queen hypothesis in evolution is related to the co-evolution of species.

It was first proposed by Leigh Van Valen of the University of Chicago in 1973 and states that species must continuously adapt and evolve to pass on genes to the next generation and also to keep from going extinct when other species within a symbiotic relationship are evolving.

It refers to a line in Lewis Carroll’s Through the Looking Glass in which the Red Queen tells Alice – “it takes all the running you can do to keep in the same place”.

As species that live at each other’s expense co-evolve, they are engaged in a constant struggle for survival (they need all the running because the environment is constantly changing)

This theory becomes very important for understanding a parasitic relationship.

The Red Queen hypothesis in sexual selection –

Species which choose a mate (sexual reproduction) can identify characteristics in that partner that are desirable and will produce the more fit offspring for the environment.

Let us consider a host parasite situation -

Individuals wanting to mate in an area with an abundance of parasitic relationships may be looking out for a mate that seems to be immune to the parasite.

Since most parasites are asexual or not able to undergo sexual selection, then the species that can choose an immune mate has an evolutionary advantage.

The goal would be to produce offspring that have the trait that makes them immune to the parasite.

This would make the offspring more fit for the environment and more likely to live long enough to reproduce themselves and pass down the genes.

ANSWER 11

Muller's Ratchet - correct answer b. Genetic load, the build up of slightly deleterious alleles in a population is lower in sexually reproducing organisms

Muller's ratchet is named after Hermann Joseph Muller, the Nobel-prize winning geneticist who discovered the mutagenic effects of high levels of radiation.

It is a process in which absence of recombination (like in an asexual population) results in accumulation of deleterious mutations which are harmful in an irreversible manner.

This happens due to the fact that in absence of recombination, offspring bear the same mutational load as their parents (assuming reverse mutations are rare).

Extinction occurs if -

• Recombination is absent,
• Population size is finite,
• Almost no back mutations occur,
• Slightly deleterious mutation rates are high and
• Purifying selection is too weak to remove all new deleterious mutations.

This according to Muller was one of the reasons why sexual reproduction is favored.

Although he discussed the advantages of sexual reproduction in 1932, the word Ratchet does not appear in the talk.

The term Ratchet was introduced in 1964 by Muller and the phrase "Muller's Ratchet" was coined by Joe Felsenstein in his 1974 paper, "The Evolutionary Advantage of Recombination”

Asexually dividing or populations exhibit a continual loss of fitness and eventual extinction

Since mutations are occurring in every lineage, selection is not sufficient to remove harmful mutations from the population, and the deleterious mutations accumulate in the genome, reducing fitness.

Sexual reproduction allows recombination to shuffle the genomes of individuals, providing an opportunity to remove deleterious mutations from offspring, thereby restoring fitness and avoiding Muller’s ratchet.

ANSWER 12

Combining of adaptive traits - c. - 2 beneficial mutations appearing simultaneously in different individuals cannot be passed on to a single offspring via mitosis.

Sexual reproduction involves the production of haploid gametes by meiosis.

Meiosis is a process by which germ cells divide to produce gametes.

Since the chromosome number of a species remains the same from one generation to the next, the chromosome number of germ cells must be reduced by half during meiosis.

To accomplish this feat, meiosis, unlike mitosis, involves a single round of DNA replication followed by two rounds of cell division.

Two divisions, meiosis I and meiosis II, are required to produce gametes.

Meiosis I is a unique cell division that occurs only in germ cells.

Meiosis II is similar to a mitotic division.

Before germ cells enter meiosis, they are generally diploid, meaning that they have two homologous copies of each chromosome.

Then, just before a germ cell enters meiosis, it duplicates its DNA so that the cell contains four DNA copies distributed between two pairs of homologous chromosomes.

Meiosis also differs from mitosis in that it involves a process known as recombination, during which chromosomes exchange segments with one another.

As a result, the gametes produced during meiosis are genetically unique.