Introduction to Natural Selection: Study Guide (Biology 7.1)

What evolution is

Unit 6 showed how DNA is copied and how a changed base can change a protein. Unit 7 asks what happens to those changes over many generations.

Evolution is a change in the heritable characteristics of a population over generations. Three words in that definition matter:

• Heritable: the change must be in features that can be passed on through genes. A scar or a muscle built up by training is not inherited.
• Population: a group of the same species living in the same place and able to interbreed. Populations evolve; individuals do not.
• Generations: evolution is measured by comparing parents with offspring, then their offspring, and so on.

Natural selection is a major mechanism of evolution, and it is the one that explains adaptations: features that suit organisms to their surroundings. It is not the only mechanism. Random events, such as new mutations and chance changes in small populations, also change populations. Those are the subject of Topic 7.4. This topic is about how selection works and how to measure it.

Darwin’s argument, step by step

Charles Darwin and Alfred Russel Wallace each reached the idea of natural selection independently. Their ideas were presented together in 1858, and Darwin published On the Origin of Species in 1859. The argument rests on a few observations and the conclusions that follow from them.

Step	Observation or inference	Example in a population of seed-eating birds
1. Overproduction	Organisms produce more offspring than the environment can support.	Each pair raises several chicks a year, but the population stays about the same size.
2. Competition	Resources such as food, water, space and mates are limited, so individuals compete for them.	In a dry year, seeds run short.
3. Variation	Individuals in a population differ in their traits (phenotypes).	Beak depth differs from bird to bird.
4. Inheritance	Much of this variation is heritable: offspring tend to resemble their parents.	Deep-beaked parents tend to have deep-beaked chicks.
5. Differential survival and reproduction	Individuals with more favourable phenotypes are more likely to survive and to produce more offspring.	Deep-beaked birds can crack the large, hard seeds that remain, so more of them survive to breed.
6. Change over generations	Favourable heritable traits become more common in the next generation.	Mean beak depth rises in the chicks hatched after the dry year.

Steps 1 and 2 together explain why survival is not random: there is not enough for everyone. Steps 3 and 4 explain why the result carries over to the next generation. Take away any one of variation, inheritance or differences in reproduction, and natural selection cannot change the population.

Describing Figure 1. A flowchart shows a process as linked stages. Each box is one stage; each arrow means “leads to”. The dashed arrow closes the loop, which tells you the process repeats every generation. The box at the bottom is outside the loop: it is the input that sets the direction of selection. When you describe a model like this in an answer, name what the boxes and arrows represent, and say what the model leaves out. For example, Figure 1 does not show where new variation comes from (mutation) or any random effects.

Fitness means reproductive success

In everyday English, “fit” means strong or healthy. In biology, evolutionary fitness measures reproductive success: how many offspring an individual leaves that themselves survive to reproduce. This is the only kind of success that passes traits on.

• A long-lived animal that never breeds has a fitness of zero.
• A short-lived plant that sets thousands of seeds that grow into adults has high fitness.
• Survival matters only because an individual must survive long enough to reproduce.

Because each individual’s success depends on chance as well as on its traits, biologists usually measure fitness for a phenotype (a group of individuals with the same trait), using averages:

mean offspring per individual = total offspring of the phenotype ÷ number of individuals with that phenotype

relative fitness (w) = mean offspring of a phenotype ÷ mean offspring of the most successful phenotype

The most successful phenotype has w = 1. A phenotype with w = 0.70 leaves 70% as many offspring per individual. Relative fitness has no units, because it is a ratio.

Changing environments change the direction of selection

Which phenotype is “favourable” depends on the environment. The environment has two parts:

• Biotic factors are living: predators, prey, parasites, disease-causing organisms, competitors, mates.
• Abiotic factors are non-living: temperature, rainfall, salinity, light, soil type, pH.

Both kinds change: a wet year follows a dry one, a new predator arrives, a disease spreads. When conditions change, a different variant may have the highest fitness. So:

• the direction of evolution can reverse (a trait that was favoured last year may be selected against this year);
• the rate of evolution can speed up or slow down (strong differences in fitness change a population quickly; small differences change it slowly);
• different genetic variants can be selected in different generations, which helps keep variation in the population.

A real example comes from the long-term study of medium ground finches on the Galápagos island of Daphne Major by Peter and Rosemary Grant. After a severe drought in 1977, mostly large, hard seeds were left, and birds with deeper beaks survived better. The next generation had deeper beaks on average. After very wet years in the early 1980s, small, soft seeds were plentiful again, and selection then favoured smaller beaks. The direction of selection reversed as the environment changed.

Worked example 1: survival is not the same as fitness

Question. A fictional population of ground beetles contains 300 large and 200 small adults. Birds catch large beetles more easily, but large females lay more eggs. The table shows what happened in one breeding season.

Phenotype	Adults at start	Fraction surviving to breed	Offspring per surviving adult (that reach adulthood)
Large	300	0.50	4.0
Small	200	0.70	2.0

(a) Calculate the mean number of offspring per starting adult for each phenotype. (b) Calculate the relative fitness of each phenotype. (c) Assuming body size is inherited exactly, calculate the fraction of large beetles in the next generation. (d) A student says: “Small beetles are fitter, because more of them survive.” Evaluate this claim.

(a)

1. Large: survivors = 300 × 0.50 = 150. Offspring = 150 × 4.0 = 600. Mean per starting adult = 600 ÷ 300 = 2.0.
2. Small: survivors = 200 × 0.70 = 140. Offspring = 140 × 2.0 = 280. Mean per starting adult = 280 ÷ 200 = 1.4.
3. Shortcut and check: mean offspring = fraction surviving × offspring per survivor. Large: 0.50 × 4.0 = 2.0. Small: 0.70 × 2.0 = 1.4. Same answers.

(b) The most successful phenotype is large (2.0).

• w(large) = 2.0 ÷ 2.0 = 1.00
• w(small) = 1.4 ÷ 2.0 = 0.70

(c) Total offspring = 600 + 280 = 880. Fraction large = 600 ÷ 880 = 0.68 (to 2 decimal places). The parents were 300 ÷ 500 = 0.60 large, so large beetles became more common.

(d) The claim confuses survival with fitness. Small beetles do survive better (0.70 against 0.50). But fitness is measured by offspring, and large beetles leave more offspring per individual (2.0 against 1.4). So large beetles have the higher fitness, and the population shifts towards large size. The claim is not supported.

Assumptions. The calculation treats size as fully inherited and ignores mutation, migration and chance. In real populations, offspring only partly resemble their parents, so the change would be smaller.

Worked example 2: reading a before-and-after histogram

Question. Figure 2 shows beak depth in a fictional island population of seed-eating birds, before a drought and among the birds that survived it.

Beak depth / mm	8	9	10	11	12	Total
Birds before drought	40	120	180	110	50	500
Survivors after drought	4	18	45	44	29	140

(a) Describe the characteristics of the figure and the main pattern it shows. (b) Calculate the percentage survival in the 8 mm and 12 mm classes, and the mean beak depth before and after. (c) Explain the pattern using natural selection, and predict what happens if heavy rains return and small seeds become common.

(a) Characteristics and pattern.

1. It is a grouped bar chart (a histogram of beak-depth classes). The x-axis is beak depth in mm; the y-axis is the number of birds.
2. Each class has two bars: open for the whole population before the drought and solid for the survivors. The pattern, not the colour, tells them apart.
3. The before distribution peaks at 10 mm. Far fewer birds survived (140 of 500, which is 28%), and among the survivors the deeper classes are relatively better represented.

(b) Calculations.

1. Survival at 8 mm = 4 ÷ 40 × 100 = 10%. Survival at 12 mm = 29 ÷ 50 × 100 = 58%. Birds with the deepest beaks were 5.8 times as likely to survive as those with the shallowest.
2. Mean before = (8×40 + 9×120 + 10×180 + 11×110 + 12×50) ÷ 500 = 5010 ÷ 500 = 10.02 mm.
3. Mean of survivors = (8×4 + 9×18 + 10×45 + 11×44 + 12×29) ÷ 140 = 1476 ÷ 140 = 10.54 mm.
4. Shift = 10.54 − 10.02 = about 0.52 mm towards deeper beaks.

(c) Explanation and prediction.

During the drought, seeds were limited and the small, soft ones ran out first. Birds competed for the remaining large, hard seeds. Deeper beaks can crack these, so deep-beaked birds were more likely to survive and then to breed. If beak depth is heritable, the chicks hatched after the drought will have a higher mean beak depth than the population before it. This is evolution: a change in the population, not in any one bird’s beak.

If heavy rains return and small seeds become common, small-beaked birds may handle them more efficiently and leave more offspring. Selection would then favour shallower beaks, and the mean would move back down. The direction of evolution follows the environment.

Limits of the data. Survival is not yet fitness. To be sure the trait spreads, you would also need to count how many chicks the survivors raise, and to show that beak depth is inherited.

Common misconceptions

• “Individuals evolve during their lifetime.” An individual’s genes do not change to suit the environment. Populations evolve as the mix of heritable traits shifts across generations.
• “Organisms develop traits because they need them.” Need does not create variation. Selection can only act on variation that already exists; new variants arise by random mutation.
• “Survival of the fittest means the strongest or longest-lived wins.” Fitness is reproductive success. A weaker individual that leaves more surviving offspring is fitter.
• “Natural selection is the only cause of evolution.” It is a major one, but random processes such as genetic drift also change populations (Topic 7.4).
• “Evolution always moves in the same direction, towards perfection.” Selection favours whatever works in the current environment. When the environment changes, the direction can reverse.
• “Every trait that helps survival will spread.” Only heritable traits spread. A bird that gains weight by finding a good feeding spot does not pass on the extra weight.
• “A higher survival rate in one group proves that its trait will become more common.” You also need its offspring numbers and evidence that the trait is inherited.

Where this leads

Topic 7.2 looks more closely at the raw material of selection: phenotypic variation, the selective pressures that act on it, and how differences in molecules inside cells affect fitness. Go to Natural Selection next. First, test yourself with the practice questions, then use the revision notes and the checklist to consolidate. The previous topic was Biotechnology.