Two ways to reproduce

1. Sexual reproduction combines genetic information when male and female gamete nuclei fuse.
2. The gametes usually come from two parents, although self-fertilisation can occur in some organisms.
3. The first cell formed is a zygote, which divides and develops into an offspring.
4. In animals, the gametes are sperm and eggs; in flowering plants, pollen delivers male gametes to an ovule containing an egg cell.
5. Asexual reproduction involves one parent and no fusion of gametes, usually producing offspring genetically identical to that parent.
6. In plants and animals, asexual reproduction involves mitosis, whereas bacteria reproduce by binary fission rather than mitosis.

Asexual reproduction can be rapid

1. One parent can reproduce without finding a mate, allowing an isolated organism to establish a population.
2. Less time and energy are required for mate attraction and mating, so offspring can often be produced rapidly in favourable conditions.
3. Repeated asexual reproduction can produce many offspring in a short period, allowing rapid use of available food or space.
4. Successful combinations of alleles are retained in clones, which is useful when the environment remains suitable.
5. Bacteria copy their DNA before binary fission, then divide to form two cells that can each reproduce again.
6. Strawberry plants produce runners that root and grow into new plants, while potato tubers can develop into new plants from their buds.
7. Yeast can reproduce asexually by budding, in which a small outgrowth receives a copied nucleus and separates from the parent cell.
8. A Hydra can form a bud that grows and separates into a new animal.

Clones share vulnerabilities

1. Asexual offspring usually have little genetic variation, apart from differences introduced by mutations.
2. If a disease can infect the parent, many genetically similar offspring may also be susceptible.
3. A change in temperature, food availability or other conditions may affect most members of a clone population in a similar way.
4. Rapid production of offspring is therefore less helpful if few can survive the changed conditions.
5. Clones can still develop different phenotypes when they experience different environments.

Sexual reproduction increases variation

1. Sexual reproduction usually produces offspring with different combinations of alleles.
2. This genetic variation makes it more likely that at least some offspring can survive a new disease or an environmental change.
3. Variation does not guarantee survival and does not mean that all sexually produced offspring are better suited to their surroundings.
4. Finding a mate, producing gametes and transferring them can require time and energy.
5. Fertilisation may fail, particularly when gametes are released into the environment or pollen does not reach a suitable flower.
6. Reproduction may therefore be slower or less reliable when population density is low.

Offspring number and survival

1. The number produced per reproductive event varies greatly among organisms and cannot be predicted from reproductive mode alone.
2. Many mammals produce relatively few young and invest substantial resources in pregnancy and parental care.
3. Many fish release large numbers of gametes, but only a fraction produce offspring that survive to adulthood.
4. Many flowering plants produce numerous seeds sexually, while runners may produce fewer new plants per event but establish them rapidly.
5. To compare reproductive success, consider offspring per event, frequency of reproduction, time to maturity and survival to reproductive age.
6. An organism that produces fewer young per event can still produce more surviving offspring over time if it reproduces frequently or its young survive well.