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Q2:
Which is a better mode of reproduction: sexual or asexual? Why?
Solution :
Step 1: Determining the Superior Reproductive Strategy
In the overarching context of evolutionary biology and long-term species survival, sexual reproduction is considered a fundamentally better mode of reproduction compared to asexual reproduction. While asexual reproduction is energetically efficient and allows for rapid population expansion in stable environments, sexual reproduction provides the critical genetic diversity required for populations to adapt, survive, and evolve in dynamic and unpredictable ecosystems.
Step 2: Generation of Genetic Variation (The Primary Advantage)
The foremost evolutionary advantage of sexual reproduction is the introduction of vast genetic variation among offspring. This variation is mathematically and biologically generated through two primary cytological mechanisms during gametogenesis and fertilization:
- Crossing Over (Homologous Recombination): [Per the mechanics of Meiosis I], during the pachytene stage of prophase I, non-sister chromatids of homologous chromosomes exchange genetic material. This breaks genetic linkage and creates novel allele combinations.
- Independent Assortment: [By Mendel’s Law of Independent Assortment], homologous chromosomes align randomly at the metaphase plate, yielding $2^n$ possible gametic combinations, where $n$ is the haploid number of chromosomes. For humans ($n=23$), this alone creates $2^{23}$ (over 8.3 million) distinct gametes.
- Syngamy (Random Fertilization): The fusion of two distinct, highly varied haploid gametes ($n + n \rightarrow 2n$) produces a zygote with a wholly unique genotype, significantly increasing the heterozygosity of a population.
Step 3: Evolutionary Adaptability and The Red Queen Hypothesis
Genetic variation translates directly into phenotypic variation. If an environmental pressure alters (e.g., climate change, resource scarcity, or the introduction of a new predator), a genetically diverse population is statistically far more likely to contain individuals possessing pre-adaptive traits that confer survival advantages.
[Per the Red Queen Hypothesis], organisms are in a continuous evolutionary arms race against pathogens and parasites. Asexual clones provide a static, easily exploitable genetic target for pathogens. Sexual reproduction acts as a moving evolutionary target by constantly shuffling the genetic deck, ensuring that offspring have novel combinations of immune defense alleles that pathogens have not yet adapted to overcome.
Step 4: Purging Deleterious Mutations (Muller’s Ratchet)
In strict asexual reproduction, species are subjected to a phenomenon known as Muller’s Ratchet. Because offspring are direct genetic clones (mitotic divisions), any deleterious (harmful) mutation that arises is permanently passed on to the progeny. Over thousands of generations, these mutations irreversibly accumulate, leading to a steady decline in population fitness (mutational meltdown).
Sexual reproduction effectively breaks Muller's Ratchet. Through genetic recombination and independent assortment, it is possible for two parents carrying distinct deleterious mutations to produce offspring completely free of those mutations (reconstituting the "wild-type" pristine chromosome).
Step 5: Visualizing the Divergence in Genetic Outcomes
Step 6: Comprehensive Comparative Analysis
| Parameter | Asexual Reproduction | Sexual Reproduction |
|---|---|---|
| Genetic Variation | Absent (Offspring are exact clones, except for rare spontaneous mutations). | Extremely High (Due to meiosis and random fertilization). |
| Evolutionary Potential | Low. Lineages often go extinct during sudden environmental shifts. | High. Serves as the primary driver for speciation and macroevolution. |
| Vigor and Vitality | Decreases over time due to the unchecked accumulation of deleterious mutations. | Maintained or increased through hybrid vigor (heterosis) and purging of harmful mutations. |
Final Solution: Sexual reproduction is the superior mode of reproduction because it generates immense genetic variation through meiosis and syngamy. This variation provides populations with the necessary phenotypic plasticity to survive changing environments, effectively combat pathogenic pressures, prevent the accumulation of deleterious mutations, and drive evolutionary progress over millions of years.
More Questions from Class 12 Biology Reproduction in Organisms EXERCISES
- Q1: Why is reproduction essential for organisms?
- Q1: Why is reproduction essential for organisms?
- Q2: Which is a better mode of reproduction: sexual or asexual? Why?
- Q3: Why is the offspring formed by asexual reproduction referred to as clone?
- Q3: Why is the offspring formed by asexual reproduction referred to as clone?
- Q4: Offspring formed due to sexual reproduction have better chances of survival. Why? Is this statement always true?
- Q4: Offspring formed due to sexual reproduction have better chances of survival. Why? Is this statement always true?
- Q5: How does the progeny formed from asexual reproduction differ from those formed by sexual reproduction?
- Q5: How does the progeny formed from asexual reproduction differ from those formed by sexual reproduction?
- Q6: Distinguish between asexual and sexual reproduction. Why is vegetative reproduction also considered as a type of asexual reproduction?
- Q6: Distinguish between asexual and sexual reproduction. Why is vegetative reproduction also considered as a type of asexual reproduction?
- Q7: What is vegetative propagation? Give two suitable examples.
- Q7: What is vegetative propagation? Give two suitable examples.
- Q8(a): Define (a) Juvenile phase,
- Q8(a): Define (a) Juvenile phase,
- Q8(b): Define (b) Reproductive phase,
- Q8(b): Define (b) Reproductive phase,
- Q8(c): Define (c) Senescent phase.
- Q8(c): Define (c) Senescent phase.
- Q9: Higher organisms have resorted to sexual reproduction in spite of its complexity. Why?
- Q9: Higher organisms have resorted to sexual reproduction in spite of its complexity. Why?
- Q10: Explain why meiosis and gametogenesis are always interlinked?
- Q10: Explain why meiosis and gametogenesis are always interlinked?
- Q11(a): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (a) Ovary ———————————
- Q11(a): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (a) Ovary ———————————
- Q11(b): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (b) Anther ———————————
- Q11(b): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (b) Anther ———————————
- Q11(c): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (c) Egg ———————————
- Q11(c): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (c) Egg ———————————
- Q11(d): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (d) Pollen ———————————
- Q11(d): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (d) Pollen ———————————
- Q11(e): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (e) Male gamete ———————————
- Q11(e): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (e) Male gamete ———————————
- Q11(f): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (f) Zygote ———————————
- Q11(f): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (f ) Zygote ———————————
- Q12: Define external fertilisation. Mention its disadvantages.
- Q12: Define external fertilisation. Mention its disadvantages.
- Q13: Differentiate between a zoospore and a zygote.
- Q13: Differentiate between a zoospore and a zygote.
- Q14: Differentiate between gametogenesis from embryogenesis.
- Q14: Differentiate between gametogenesis from embryogenesis.
- Q15: Describe the post-fertilisation changes in a flower.
- Q15: Describe the post-fertilisation changes in a flower.
- Q16: What is a bisexual flower? Collect five bisexual flowers from your neighbourhood and with the help of your teacher find out their common and scientific names.
- Q16: What is a bisexual flower? Collect five bisexual flowers from your neighbourhood and with the help of your teacher find out their common and scientific names.
- Q17: Examine a few flowers of any cucurbit plant and try to identify the staminate and pistillate flowers. Do you know any other plant that bears unisexual flowers?
- Q17: Examine a few flowers of any cucurbit plant and try to identify the staminate and pistillate flowers. Do you know any other plant that bears unisexual flowers?
- Q18: Why are offspring of oviparous animals at a greater risk as compared to offspring of viviparous animals?
- Q18: Why are offspring of oviparous animals at a greater risk as compared to offspring of viviparous animals?
CBSE Solutions for Class 12 Biology Reproduction in Organisms
Chapters in CBSE - Class 12 Biology
- Human Reproduction
- Reproductive Health
- Principles of Inheritance and Variation
- Molecular Basis of Inheritance
- Evolution
- Human Health and Disease
- Strategies for Enhancement in Food Production
- Microbes in Human Welfare
- Biotechnology: Principles and Processes
- Biotechnology and Its Applications
- Organisms and Populations
- Ecosystem
- Biodiversity and Conservation
- Environmental Issues
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