Costs and benefits of sexual reproduction The disadvantages

Costs and benefits of sexual reproduction: The disadvantages and advantages of sexual reproduction Cf. E Advanced Higher Biology Unit 2: Organisms and Evolution

SQA mandatory key information • Disadvantages of sexual reproduction – males unable to produce offspring, only half of each parent’s genome passed onto offspring. Benefits outweigh disadvantages due to increase in genetic variation in the population. • Successful asexual reproduction strategies – to include examples of vegetative cloning, parthenogenesis and horizontal gene transfer.

Key concepts 1 • Compared to asexual reproduction, sexual reproduction appears to have two disadvantages. First, half of the population is unable to produce offspring – this is known as the paradox of the existence of males. Second, by mixing the genetic information between two individuals, each parent disrupts a successful genome and only passes on half to each offspring. • Given that sexual reproduction is so widespread, the benefits must outweigh these disadvantages. The benefit lies in the greater genetic variation within sexually reproducing organisms. This genetic variation provides the raw material required to keep running in the Red Queen’s arms race between parasites and their hosts.

Key concepts 2 • Asexual reproduction can be a successful reproductive strategy, particularly in very narrow stable niches or when recolonising disturbed habitats. In eukaryotes, examples of asexual reproduction include vegetative cloning in plants and parthenogenic animals that lack fertilisation. Parthenogensis is more common in cooler climates with low parasite diversity. For organisms that reproduce principally by asexual reproduction, many have mechanisms for horizontal gene transfer between individuals, such as the plasmids of bacteria and yeast.

The problem “Sex is the queen of problems in evolutionary biology. Perhaps no other natural phenomenon has aroused so much interest; certainly none has sowed as much confusion. The insights of Darwin and Mendel, which have illuminated so many mysteries, have so far failed to shed more than a dim and wavering light on the central mystery of sexuality. . . ” Graham Bell (1982) ‘The Masterpiece of Nature: The Evolution and Genetics of Sexuality’.

What are the disadvantages of sexual reproduction? There are two main disadvantages: 1. In any population where sexual reproduction is the reproductive strategy, only half of the population are able to produce offspring. 2. Sexual reproduction involves the mixing of genetic information between two individuals with each parent disrupting a successful genome and only passing half onto each offspring.

The paradox of the existence of males In any sexually reproducing species, half of any offspring produced will be male and half female. However, it is only the females who are able to reproduce the next generation. In an asexually reproducing species, all offspring are able to reproduce and would be expected to increase in number at twice the rate of a sexually reproducing species.

The paradox of the existence of males The fact that the males in a sexually reproducing organism are not able to reproduce would appear to put sexually reproducing species at a disadvantage. In terms of the number of offspring they are able to produce and, they would be outcompeted by asexual species. This is known as the paradox of the existence of males.

Genome disruption Sexual reproduction requires that the genomes of both parents (male and female) are disrupted in order to half the number of chromosomes present in each gamete. In all sexually reproducing eukaryotes, this involves the production of gametes through the process of meiosis. Human male genome In order to produce offspring, sexual reproduction requires the successful genomes of both parents to be halved and then combines these halves during fertilisation. By its very nature, only half of each of these genomes will be passed on and which alleles are present in each gamete is random as a result of crossing over and independent assortment.

The benefits of sexual reproduction As sexual reproduction is so widespread across the eukaryotic kingdoms, the benefits must outweigh the disadvantages. The benefits: • Greater genetic variation which allows species to maintain a large gene pool. • Genetic variation allows organisms to cope with selection pressures and evolve to changing environmental situations. • It provides the raw material required to keep running in the Red Queen’s arms race between parasites and their hosts. It is important to remember that evolution itself does not have a goal or a direction.

Sexual and asexual reproduction Asexual reproduction Sexual reproduction Asexual populations rapidly outgrow sexual ones as males in BLUE do not reproduce offspring.

Asexual reproduction • Asexual reproduction, unlike sexual reproduction does not involve the fusion of gametes, the process of meiosis or any disruption of a successful parental genome in order to produce offspring. • It can be a very successful reproductive strategy, producing a large number of offspring, much more rapidly than through sexual reproduction. • Asexual reproduction uses the process of mitotic cell division. • It is a useful strategy particularly in very narrow, stable niches or when recolonising disturbed habitats. Sea anemone with asexually reproduced offspring

Asexual reproduction – Eukaryotes Mechanisms of asexual reproduction in Eukaryotes Budding – eg certain species of hydra and coral. Offspring grow out from the parent’s body. Hydra Vegetative cloning – in plants. eg. production of rhizomes (ginger) or runners (strawberry) or bulbs (daffodils) or tubers (dahlia) Parthenogenesis – a type of asexual reproduction in which an unfertilised egg develops into a new individual eg. fire ants.

Parthenogenesis – continued… • The progeny of parthenogenesis can be either haploid or diploid. • If haploid, the offspring develop into adults that produce eggs or sperm without meiosis. • It occurs in certain species of bees, wasps, and ants. • In vertebrates, parthenogenesis is observed in roughly one in a thousand species. • It has been observed in certain species of rock lizard, geckos and a Komodo dragon. • Parthenogenesis is more common in cooler climates with low parasitic diversity. Other examples of parthenogenesis are stick insects and Komodo dragons

Asexual reproduction unicellular organisms Many organisms reproduce principally by asexual reproduction, such as the unicellular organisms in the archaea, bacteria and the protists. Mechanism of asexual reproduction – Fission In the process of fission, the parent cell is replaced by two daughter cells as it literally splits into two cells of approximately equal size. Example of Archaea domain - Halobacter Example of the Protist Kingdom - Acathocystis turfacea Example of bacteria domain – Staphylococcus aureus

Finally. . . For the many organisms for whom asexual reproduction is the principal method of reproduction, many have mechanisms for horizontal gene transfer between individuals, such as the plasmids of bacteria and yeast. Main mechanisms of horizontal gene transfer: • Transformation • Transduction • Conjugation