Biology · Evolutionary Processes & Adaptation

Natural selection, reproductive success and biological fitness

Two inherited variants begin at different frequencies. If their carriers contribute different numbers of descendants, the next population can have different frequencies. No individual has to decide to improve, and no useful variant has to be created because it is needed.

The introductory inheritance chapter distinguishes alleles, genotypes and phenotypes. Meiosis explains segregation and crossing over; the DNA chapter explains sequence changes. Fractions and simple multiplication are enough for the constructed examples here. No example represents a measured natural population unless explicitly identified as published research.

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By the end, you can

Explain selection through inherited variation and differential reproduction, with fitness defined for a specified context.

Follow a causal chain through generations

For evolutionary change by natural selection, relevant variation must be inherited and associated with differences in reproductive contribution. A feature may influence survival, obtaining mates, fertility or the survival of descendants. Those effects can change how much inherited material reaches the next generation.

Selection acts through the actual organisms and biological interactions in a setting. A visible difference caused only by temporary conditions can affect an individual, but its appearance does not establish an inherited variant. Researchers must connect the character, inheritance and reproductive outcome rather than assume that every noticeable difference has a genetic basis.

An individual can grow, learn or acclimate during its life. Population evolution concerns inherited differences across reproduction. Saying that a population became better suited to a setting does not mean its members individually rewrote their DNA to match a goal.

Weight starting numbers by reproductive contribution

Consider an original haploid clonal model with 20 X parents and 80 Y parents. Each parent transmits its variant faithfully. In environment 1, assign three counted offspring per X parent and one per Y parent. This gives 60 X and 80 Y offspring. X rises from 20/100 = 0.20 to 60/140 ≈ 0.429.

The increase follows from differential contribution in this model because mutation, gene flow and random sampling have been excluded by assumption. The same before-and-after frequencies in a real population would not establish selection without evidence about those alternatives and the reproductive differences.

The diagram retains each starting group, its contribution multiplier, the resulting offspring counts and the final denominator. A high per-parent contribution is different from being the largest group: X contributes more per parent, but Y still contributes more offspring in total in this single step.

Reproductive contributions can reverse between environments

Original haploid clonal model with exact assigned group totals, faithful inheritance and no mutation, movement or random sampling. Panels are alternative contexts, not consecutive generations.

Read the diagram in words

The same X/Y starting group splits into two alternative environmental comparisons. Within each panel, weighted reproduction leads to different offspring pools.

Environment 1

Variant	Starting parents	Counted offspring per parent	Offspring count	Relative reproductive contribution
X	20	3	60	1
Y	80	1	80	≈ 0.333

X: 60 / 140 ≈ 0.429; relative contributions 1 and 1/3

Environment 2

Variant	Starting parents	Counted offspring per parent	Offspring count	Relative reproductive contribution
X	20	1	20	0.5
Y	80	2	160	1

X: 20 / 180 ≈ 0.111; relative contributions 0.5 and 1

Fitness is a comparison with conditions and a census stage

Biological fitness concerns reproductive success or expected reproductive contribution in a defined setting. It is not a general score for strength, intelligence, health or moral worth. A long-lived organism leaving no descendants can contribute less directly than a shorter-lived organism that reproduces successfully. Which descendant stage is counted must be stated.

In the model, absolute contributions are three and one in environment 1. One useful relative scale divides each by the largest contribution: X has relative fitness 1 and Y has 1/3. Other normalisations are possible, so a relative value should come with its definition.

Now change the assigned context: in environment 2, X contributes one offspring per parent and Y contributes two. From the same starting counts, there are 20 X and 160 Y offspring. X becomes 20/180 ≈ 0.111. Relative to the maximum contribution, X is 0.5 and Y is 1. The ranking reverses because the environment and reproductive outcomes changed.

Fitness can also depend on the frequencies of other variants, the genetic background and interactions with other organisms. Selection can favour an intermediate character, one extreme or different variants in different circumstances. There is no universal requirement for every generation to move towards a larger or more elaborate organism.

Worked example

A favourable contribution does not guarantee immediate dominance

In environment 1, each X parent contributes three times as much as each Y parent. Nevertheless, X offspring number 60 while Y offspring number 80, because Y began with four times as many parents. X increased in proportion without becoming the majority.

The next step cannot be calculated without deciding whether the same contribution values and other assumptions continue. These assigned values are an explanation of a model step, not measurements of a species or a promise that X will eventually fix in a changing, finite population.

Check your understanding

If both variants instead contribute two offspring per parent in the model, what happens to X frequency?
Show reasoning
X contributes 40 offspring and Y contributes 160. X remains 40/200 = 0.20. Population size changes, but with faithful inheritance and the other stated exclusions the variant proportion does not change.
Does observing that X rose from 0.20 to about 0.429 prove natural selection in a field sample?
Show reasoning
No. Sampling, migration, chance transmission and other processes can contribute. Evidence linking an inherited difference to different reproductive outcomes, alongside checks of alternatives, is needed to support the selection explanation.
Why must fitness specify an environment and the stage of descendants counted?
Show reasoning
Conditions can reverse reproductive advantages, and counting eggs, surviving juveniles or reproducing adults can give different comparisons. A defined measure makes the claim interpretable instead of treating fitness as an all-purpose rank.

What to carry forward

Selection changes inherited representation through differential reproduction; fitness is a contextual comparison, not a universal ranking.

References

Resources supporting this lesson’s explanations and examples.