D3.1 Reproduction

Reproduction is the one biological process no individual needs to survive but every species needs to continue. The IB asks you to weigh up two contrasting strategies: asexual reproduction, which is fast and produces genetically identical offspring, and sexual reproduction, which is slower and costlier but generates the variation on which evolution depends. The engine of that variation is meiosis, the special nuclear division that halves the chromosome number and shuffles genes, while fertilisation restores the full number. Understanding how these fit together — in animals and in flowering plants — is the core of this topic.

Asexual versus sexual reproduction

Asexual reproduction involves a single parent and produces offspring that are genetically identical to it — a clone. It relies on mitosis, so there is no fusion of gametes and no genetic variation among the offspring. It is rapid and efficient and works well in a stable, favourable environment, but because every individual is identical, a single disease or change in conditions can wipe out the whole population.

Sexual reproduction involves two parents (or two gametes) and the fusion of a male and a female gamete. It produces offspring that are genetically different from each other and from their parents. The advantage is variation: when the environment changes, some varied offspring are more likely to survive, which is the raw material for natural selection. The trade-off is that it is slower and requires finding a mate. The syllabus wants you to evaluate this trade-off, not just list the differences.

Meiosis and the production of gametes

Sexual reproduction depends on gametes — sex cells — that are haploid (n), carrying one set of chromosomes. They are made by meiosis, a division that takes one diploid (2n) cell through two divisions to produce four haploid cells. Halving the chromosome number is essential: it means that when two gametes fuse at fertilisation, the normal diploid number is restored rather than doubling each generation.

Meiosis is also the main source of genetic variation, through three mechanisms the syllabus highlights:

• Crossing over: homologous chromosomes exchange segments of DNA, producing new combinations of alleles on a chromosome.
• Independent assortment: the random orientation of homologous pairs at the equator means maternal and paternal chromosomes are mixed differently in each gamete.
• Random fertilisation: which of the many varied gametes happen to fuse is also a matter of chance.

Together these ensure that essentially no two offspring (other than identical twins) are genetically the same.

Hormones and reproduction in humans

In humans, reproduction is coordinated by hormones. At puberty, sex hormones drive the development of secondary sexual characteristics — for example, testosterone in males and oestrogen in females. In females the menstrual cycle is controlled by an interacting set of hormones (including FSH, LH, oestrogen and progesterone) that regulate the development and release of an egg and prepare the lining of the uterus for a possible pregnancy.

The syllabus also expects awareness of how an understanding of these hormones is applied, for example in in vitro fertilisation (IVF). In IVF, hormones are used to stimulate the ovaries to release several eggs, which are collected and fertilised outside the body before an embryo is placed in the uterus — a direct application of reproductive biology that also raises ethical discussion the IB encourages you to consider.

Reproduction in flowering plants

Flowering plants reproduce sexually using flowers. The male parts (the stamens, made of anther and filament) produce pollen, which contains the male gametes; the female parts (the carpel, made of stigma, style and ovary) contain the ovules with the female gametes. The sequence the IB wants is:

• Pollination: the transfer of pollen from an anther to a stigma, often by insects or wind. Transfer between different plants (cross-pollination) promotes variation.
• Fertilisation: a pollen grain grows a tube down the style so a male gamete can fuse with the egg cell in the ovule.
• Seed and fruit formation: the fertilised ovule develops into a seed and the surrounding ovary becomes the fruit, which aids seed dispersal.

Many flowers have features that encourage cross-pollination and discourage self-pollination, again favouring genetic variation — the same evolutionary logic that underlies sexual reproduction in animals.