An Expensive Habit That Persists
Reproduction comes in two forms with very different economics.
Asexual reproduction is fast, needs no partner, and passes on 100 % of the parent's genes. Every offspring is a clone, so a well-adapted genotype is preserved intact. In a stable environment it is hard to beat.
Sexual reproduction is expensive. It needs a partner, costs time and risk in finding one, and each parent passes on only 50 % of their genes to each offspring. That last figure is the twofold cost of sex: an asexual mutant in a sexual population should, all else equal, double its representation each generation. Yet sex is nearly universal among eukaryotes, so something must be paying for it.
The answer is variation. Sex generates it three ways: independent assortment of chromosomes in meiosis, crossing over between homologues, and random fertilisation combining two gametes. Independent assortment alone gives distinct gametes for chromosome pairs, which for humans () is over 8 million before crossing over is counted at all.
Variation pays in a changing environment, and especially against parasites, the Red Queen hypothesis, where host and parasite must keep evolving simply to hold their position. A clonal host presents the same target every generation; a sexual one does not.
The prediction is testable and holds: asexual reproduction dominates in stable environments and among colonising species, while sexual reproduction dominates where conditions vary and parasites are abundant. Many organisms do both, switching to sex under stress.
Fisher's principle explains why sex ratios sit near 1:1. If males became rare, each male would have more offspring on average, so parents producing sons would do better, until the advantage disappeared at parity. The ratio is an equilibrium maintained by frequency-dependent selection, not a fixed constant of biology.
Common pitfall: describing sex as "good for the species". Selection acts on individuals and genes, not on species. Sex persists because sexual individuals leave more surviving descendants under the right conditions, an explanation that has to work at the individual level or it does not work at all.