Heredity · Lesson 1 of 7
Accumulation of Variation During Reproduction
“Every generation keeps the family recipe and adds a few tiny edits.”
• Explain how inherited and newly created variations accumulate over generations. • Compare variation produced during asexual and sexual reproduction. • Relate DNA-copying inaccuracies to minor differences among offspring. • Explain why environmental conditions favour some variants over others. • Use trait frequency cautiously to infer the probable age of a variation.
Imagine following a family of bacteria through several rounds of division. The descendants retain the basic bacterial design, yet DNA copying is not absolutely error-free. A small difference that appears in one cell may be passed to its descendants, while fresh differences arise in later generations. Diversity therefore builds gradually rather than appearing all at once.
A variation is a difference in a characteristic among individuals of the same species.
Creation Of Diversity Over Succeeding Generations
Every new generation receives hereditary information from the preceding generation. Most of that information is copied accurately, preserving the common body design. Small workable inaccuracies may also be inherited. When the next generation reproduces, its members transmit some existing differences and may produce additional ones. Descendants therefore contain a mixture of shared features, inherited variations and newly created variations.
An inherited variation need not appear independently in every generation. If a DNA difference arose in one first-generation individual, only the branch descending from that individual would initially carry it. A later variation might occur within that branch, producing descendants that carry both differences. This branching pattern explains why some members of a population share a variation while others possess unique combinations.
Variation During Asexual Reproduction
Asexual reproduction usually involves one parent. The offspring receive hereditary information from that single source, so they are highly similar. Minor differences still arise because DNA copying and cell division are chemical processes rather than perfect photocopying. In a rapidly dividing bacterial population, these small changes can accumulate over many generations even though each individual division produces only limited variation.
Variation During Sexual Reproduction
Sexual reproduction combines genetic material from two parents. Each parent already carries variations inherited from earlier generations. Germ-cell formation rearranges which parental chromosomes enter each gamete, and fertilisation brings two independently formed gametes together. DNA-copying differences may also occur. These processes create more possible combinations than reproduction from one parent, so sexually produced offspring generally show greater diversity.
| Feature | Asexual reproduction | Sexual reproduction |
|---|---|---|
| Number of parents contributing DNA | One | Usually two |
| Main source of differences | Mostly copying inaccuracies | Copying differences plus recombination of parental information |
| Similarity among offspring | Usually very high | Greater diversity is common |
| Basic body design | Preserved | Preserved |
Survival Advantages Produced By Variation
Variations do not carry fixed labels such as useful or harmful. Their effect depends on the surroundings. Suppose a heat wave affects a bacterial population. A heat-resistant variation that allows a bacterium to tolerate higher temperature gives an advantage under that condition. The tolerant bacteria survive and reproduce more successfully, so the variation becomes more common. Under a different condition, the same variation might offer no benefit.
Environmental factors therefore select among existing variants; they do not deliberately create the variation that an organism needs. Some variants die, some survive without special advantage, and some leave more descendants. This unequal survival and reproduction is a foundation of long-term biological change.
A Strategy For Reasoning From Trait Frequency
When two traits occur in an asexually reproducing population, a more common trait may have arisen earlier because it has had more generations in which to be inherited. Use this as a probability-based inference, not an absolute rule. A newer beneficial trait can spread rapidly, and an older harmful trait may remain rare. First identify the reproductive mode, then compare frequencies, and finally state the assumptions about survival and reproduction.
Problem
Trait A occurs in 10% of an asexual population and trait B in 60%. Which probably arose earlier?
- 1.Given: frequencies of 10% and 60% in an asexually reproducing population.
- 2.Find: the variation with the longer probable inheritance history.
- 3.Method: if other conditions are similar, the variation found in more descendants probably arose earlier.
- 4.Trait B occurs in a larger fraction, so it has probably been transmitted through more generations.
- 5.Conclusion: trait B is likely to have arisen earlier, but frequency alone cannot prove this.
Problem
A rare heat-tolerant bacterial variant is present before a heat wave. Predict what may happen.
- 1.The population already contains variation; the heat wave does not create the useful change on demand.
- 2.Heat-sensitive bacteria are more likely to die or reproduce slowly.
- 3.Heat-tolerant bacteria survive and leave more descendants.
- 4.The inherited tolerance becomes more frequent in later generations.
- 5.The example shows environmental selection of a pre-existing variation.
Problem
A recently arisen variation becomes common very quickly. Does high frequency prove that it is old?
- 1.Identify the claim: high frequency is being treated as certain evidence of age.
- 2.A strongly advantageous new variation can spread rapidly because its carriers reproduce more successfully.
- 3.Population movement and chance can also affect frequency.
- 4.Therefore frequency supports an inference only when other influences are similar.
- 5.Conclusion: the trait may be new despite being common.
The environment selects among variations already present; it does not instruct organisms to produce exactly the variation they need.
Quiz
What is a variation?
Why are asexual offspring usually highly similar?
Why does sexual reproduction usually produce more diversity?
What happens to heat-tolerant bacteria during a heat wave?
Why can trait frequency not prove when a variation arose?
Practice Problems
- Problem: Explain how diversity increases over three asexual generations. Solution: Each generation inherits earlier workable differences, while fresh DNA-copying inaccuracies can add new differences. Branches of descendants therefore become progressively less identical.
- Problem: Compare the main sources of variation in asexual and sexual reproduction. Solution: Asexual variation comes mainly from copying inaccuracies; sexual reproduction adds the reshuffling and combination of genetic information from two parents.
- Problem: A pesticide kills most insects, but a few survive and reproduce. Explain. Solution: The original population contained variation. A resistant variant survived the pesticide and transmitted resistance to more descendants; the pesticide selected the variant rather than creating it.
- Problem: Trait P occurs in 70% and trait Q in 15% of an asexual population. Infer their probable relative ages. Solution: If survival and reproduction were similar, P probably arose earlier because it occurs in more descendants; this remains an inference, not proof.
- Problem: Explain why a variation useful today may not remain useful. Solution: Advantage depends on environmental conditions. If temperature, food, predators or disease change, the effect of the variation may become neutral or harmful.
Key Takeaways
• Reproduction preserves a common body design while also producing variation. • Inherited variations and newly created variations accumulate across generations. • Asexual offspring are highly similar but not perfectly identical. • Sexual reproduction creates more combinations of hereditary information. • The effect of a variation depends on environmental conditions. • Environmental factors select variants; they do not create needed changes on demand. • Trait frequency can support, but cannot prove, an inference about when a variation arose.
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Heredity