Table of Contents
Overview
Sexual reproduction is a way of producing offspring in which genetic material from (usually) two different cells is combined to form a new individual. Unlike asexual reproduction, which produces genetically almost identical copies, sexual reproduction generates new combinations of genes in every generation and is therefore a major source of genetic diversity.
This chapter focuses on what is specific to sexual reproduction as a biological process. Details that are specific to particular groups (bacteria, plants, animals, humans) are covered in their respective later chapters.
Basic Principles of Sexual Reproduction
Fusion of Gametes (Syngamy)
The defining step of sexual reproduction is the fusion of two specialized sex cells, called gametes:
- A male gamete (e.g., sperm cell, pollen grain’s sperm cells)
- A female gamete (e.g., egg cell, ovum)
Their fusion is called fertilization or syngamy and produces a zygote:
- The zygote contains a new combination of genetic material from both parents.
- It is the first cell of the new individual.
Haploid and Diploid Phases
Sexual reproduction alternates between two nuclear states:
- Diploid (2n): cells have two sets of chromosomes (one from each parent).
- Haploid (n): cells have one set of chromosomes.
Key points:
- In most animals, the adult body is diploid.
- Gametes are haploid.
- Fertilization restores the diploid state:
$$ n + n \rightarrow 2n $$
The reduction from diploid to haploid occurs during meiosis (treated in detail elsewhere).
Meiosis and Genetic Recombination (Conceptual)
Specific to sexual reproduction is that gametes are formed by meiosis, not by simple cell division.
Conceptually, meiosis:
- Reduces chromosome number (2n → n) so that fusion of gametes does not endlessly increase chromosome sets.
- Creates new combinations of alleles via:
- Crossing-over (exchange of corresponding chromosome segments between homologous chromosomes).
- Independent assortment (random distribution of maternal and paternal chromosomes into gametes).
These mechanisms produce genetically unique gametes and thus contribute to variation among offspring.
Forms of Sexual Reproduction
Although the core concepts (gametes, fertilization, alternation of haploid and diploid phases) are universal, the details and timing differ among organisms.
Isogamy, Anisogamy, and Oogamy
Isogamy
- Gametes of similar size and shape.
- Often both gametes are motile.
- Typically no distinct “male” and “female” individuals, but different mating types (+ and −, or similar).
- Common in some algae and fungi.
Anisogamy
- Gametes differ in size or form.
- Both are still regarded as gametes, but usually one is larger.
- Transition form between isogamy and oogamy.
- Found in some algae and simple multicellular organisms.
Oogamy
- Extreme form of anisogamy, characteristic of most animals and many plants:
- Egg cell: large, non-motile, nutrient-rich.
- Sperm cell: small, usually motile (flagellated) and specialized for seeking out and fertilizing the egg.
- Sets the stage for the evolution of distinct male and female sexes (discussed in more detail in later chapters on plants, animals, and humans).
External vs. Internal Fertilization
How and where gametes meet is a central aspect of sexual reproduction.
External Fertilization
- Gametes are released into the external environment (often water).
- Fertilization occurs outside the bodies of the parents.
- Typical in many aquatic organisms:
- Many algae
- Numerous aquatic invertebrates
- Many bony fishes and amphibians
Characteristics:
- Large numbers of gametes to compensate for losses.
- Often synchronized by environmental cues (temperature, light, tides) or by mating behavior (spawning).
- Vulnerable to environmental changes and predation.
- Little or no parental care in many species.
Internal Fertilization
- Sperm are transferred directly into or near the reproductive tract of the female.
- Fertilization occurs inside the female body (or, more rarely, inside specialized structures of males or hermaphrodites).
Typical of:
- Most land animals (e.g., reptiles, birds, mammals, many insects).
- Some aquatic animals with specialized mating behavior.
Characteristics:
- Fewer gametes needed because encounter is more controlled.
- Allows reproduction in terrestrial (dry) environments.
- Often associated with:
- Complex mating behaviors.
- Courtship and mate choice.
- Greater potential for parental care and investment.
Plants have analogous distinctions:
- Many aquatic or simple plants may have gametes that meet in water.
- Seed plants use pollen to deliver immobile sperm to the egg within the ovule (functional equivalent of internal fertilization), treated in detail in plant chapters.
Sexual Reproduction and Life Cycles
The alternation between haploid and diploid phases is arranged differently in various groups. These are patterns of sexual life cycles, not separate reproduction types.
Diploid-Dominant Life Cycle
- The diploid phase forms the main body of the organism.
- Haploid cells are limited to gametes.
- Meiosis directly produces gametes.
Common in:
- Most animals
- Some algae and protists
Sequence:
- Diploid adult (2n) produces haploid gametes (n) by meiosis.
- Gametes fuse (fertilization) to form a zygote (2n).
- Zygote develops by mitosis into new diploid adult.
Haploid-Dominant Life Cycle
- The main body is haploid.
- The diploid phase is usually just the zygote, which quickly undergoes meiosis.
Common in:
- Many fungi
- Many unicellular algae
Sequence:
- Haploid adult (n) produces haploid gametes by mitosis (no reduction needed).
- Gametes fuse to form diploid zygote (2n).
- Zygote undergoes meiosis, producing haploid cells.
- These cells grow into new haploid individuals.
Alternation of Generations
- More balanced alternation of multicellular haploid and diploid stages.
- Found in many algae and all land plants.
Key feature:
- Gametophyte: haploid, forms gametes by mitosis.
- Sporophyte: diploid, forms haploid spores by meiosis; spores grow into gametophytes.
Details and examples (mosses, ferns, seed plants) are discussed in plant-specific chapters.
Sexual Reproduction and Genetic Diversity
Sexual reproduction has specific evolutionary consequences that set it apart from asexual reproduction.
Sources of Genetic Variation in Sexual Reproduction
In sexual organisms, multiple processes contribute to variation among offspring:
- Recombination in meiosis
- Crossing-over reshuffles alleles between homologous chromosomes.
- Independent assortment rearranges combinations of maternal and paternal chromosomes across gametes.
- Random fusion of gametes
- Any sperm can fertilize any egg (within compatibility limits).
- The combination of gametes is thus highly unpredictable.
Combined, these processes ensure that even siblings with the same parents are genetically different (except for rare identical twins or clones).
Long-Term Advantages and Costs
Advantages
- Increased genetic diversity:
- Enhances ability of populations to adapt to changing environments.
- Helps populations eliminate harmful mutations over generations.
- Facilitates evolution of complex traits by bringing beneficial mutations together into the same genetic background.
Costs
- Two-parent requirement: Typically, two individuals must find each other.
- Energy and risk:
- Investment in mating behaviors, display structures, or chemical signals.
- Risk of predation, injury, or infection during mating.
- “Cost of males” in many species:
- Males may invest little in offspring directly, yet consume resources.
- In purely numerical terms, asexual populations can theoretically grow faster, because every individual can reproduce.
The fact that sexual reproduction is still so widespread suggests that its long-term benefits (especially in changing or challenging environments) often outweigh its short-term costs.
Hermaphroditism and Sex Systems
Hermaphroditism
Hermaphrodites possess both male and female reproductive organs, either:
- Simultaneously (at the same time), or
- Sequentially (male first, then female, or vice versa).
Simultaneous Hermaphrodites
- Each individual can produce both sperm and eggs.
- Common in many plants and some animals (e.g., earthworms, certain snails).
- Mating often still involves two individuals that exchange sperm, preserving the recombination benefits of sexual reproduction.
Sequential Hermaphrodites
- Individuals change sex during their lifetime:
- Protandry: male → female.
- Protogyny: female → male.
- Found in various fish and invertebrates.
- Enables flexibility in reproduction depending on social structure or environmental context (e.g., size-based advantages of being male or female).
Details of particular systems and examples are discussed in later chapters on plants and animals.
Separate Sexes (Gonochorism/Dioecy)
- Individuals are either male or female throughout life.
- Common in:
- Most animals (gonochorism).
- Many plants (dioecy).
Consequences:
- Requires mechanisms to ensure successful meeting of sperm and eggs (mate-finding, pollinators, water currents, etc.).
- Often leads to sexual dimorphism (differences between males and females), which is connected to sexual selection and reproductive strategies (covered in behavioral biology and reproduction chapters for animals and humans).
Sexual Reproduction and Reproductive Strategies
Sexual reproduction can be combined with different reproductive strategies to maximize success under given ecological conditions.
Mating Systems (Conceptual Overview)
These are relevant especially in animals and some plants (described in detail elsewhere):
- Monogamy: one male, one female for at least one breeding season.
- Polygyny: one male, multiple females.
- Polyandry: one female, multiple males.
- Promiscuity/polygynandry: multiple mating partners for both sexes.
These patterns influence:
- How often and with whom gametes are combined.
- Genetic diversity within populations.
- Distribution of parental care.
Self-Fertilization vs. Outcrossing
Some sexually reproducing organisms can fertilize themselves, others require a partner.
Self-Fertilization (Selfing)
- Gametes from the same individual fuse to form a zygote.
- Common in many hermaphroditic plants and some animals.
- Advantages:
- Reproduction possible when partners are rare.
- Rapid colonization of new habitats.
- Disadvantages:
- Reduced genetic diversity.
- Increased risk of expressing harmful recessive alleles (inbreeding).
Outcrossing (Cross-Fertilization)
- Gametes from different individuals fuse.
- Maintains or increases genetic variation.
- Many species have mechanisms that promote outcrossing and prevent selfing (e.g., self-incompatibility systems in plants, mate choice in animals).
Mixed Strategies: Combining Sexual and Asexual Reproduction
Many organisms are capable of both sexual and asexual reproduction and switch depending on conditions.
Environmental Triggers
- Stable, favorable conditions:
- Asexual reproduction may dominate (rapid multiplication of well-adapted genotypes).
- Stressful or changing conditions (e.g., crowding, nutrient shortage, seasonal changes):
- Sexual reproduction may be induced to generate variation or form resistant stages (e.g., resting eggs, spores).
Examples (elaborated in group-specific chapters):
- Many protists alternate between asexual divisions and a sexual phase.
- Some plants reproduce vegetatively and also by seeds.
- Some invertebrates, such as certain crustaceans and insects, show alternating generations of sexual and asexual reproduction.
Summary
- Sexual reproduction is defined by the fusion of haploid gametes to form a diploid zygote, leading to alternation of haploid and diploid phases in the life cycle.
- Meiosis and fertilization are the key processes that distinguish sexual from asexual reproduction by generating and recombining genetic variation.
- Sexual reproduction can take multiple forms:
- Isogamy vs. anisogamy vs. oogamy.
- External vs. internal fertilization.
- Diploid-, haploid-, or generation-alternating life cycles.
- Sexual systems vary from separate sexes to various forms of hermaphroditism, with numerous mating systems and strategies that influence how often and with whom genes are combined.
- Despite its energetic and ecological costs, sexual reproduction is widespread because it enhances genetic diversity, supporting long-term adaptability and evolutionary potential of populations.