Adaptation and starvation resistance — Grade 12 Life Sciences

Adaptations

An adaptation is an inherited characteristic that increases an organism’s chance of surviving and reproducing in its environment. Adaptations arise because individuals in a population vary, and those with favourable variations survive and reproduce more often (natural selection).

Structural — body form: long legs for running, thick fur, a long beak.
Physiological — body chemistry: storing fat, concentrating urine, tolerating low oxygen.
Behavioural — actions: migrating, burrowing, feeding at night.

Key idea: individuals do not adapt during their lives in response to need. Populations change over generations because some inherited variations are passed on more often than others.

Starvation resistance

Starvation resistance is the length of time an organism can survive without food. It depends on:

Energy reserves — mainly fat (lipid) and glycogen stores.
Metabolic rate — a lower rate uses reserves more slowly.
Body size — larger animals store more energy relative to their energy use per unit mass.
Behaviour — reducing activity saves energy.

Because starvation resistance varies between individuals and is partly inherited, it can change by natural selection when food is repeatedly scarce.

Worked example: an investigation

Scientists can test whether starvation resistance evolves by keeping a population of fruit flies (Drosophila) under repeated food shortage. In each generation only the flies that survive the shortage breed. Survival time without food is measured every 10 generations. (The data below are illustrative.)

Generation	0	10	20	30
Mean survival without food (hours)	40	52	63	71

✏️ Analysing the investigation

1Hypothesis: repeated food shortage will increase the starvation resistance of the fruit fly population over generations.

2Independent variable: the number of generations under food shortage. Dependent variable: mean survival time without food.

3Controlled variables: temperature, humidity, access to water, age and sex of the flies tested.

4Trend: survival time increased from 40 h to 71 h over 30 generations; the increase per 10 generations became smaller.

5Explanation: flies with larger fat stores survived the shortage and bred, so alleles for greater fat storage became more common in the population.

A good investigation also uses a control population kept with normal food, so that the change can be linked to the food shortage and not to something else.

Ratites: flightless birds as evidence

Ratites are large, flightless birds: the ostrich (Africa), rhea (South America), emu and cassowary (Australia and New Guinea) and kiwi (New Zealand). They share features such as a flat breastbone with no keel for flight muscles, and small wings.

Their distribution on southern continents that were once joined as Gondwana has been used as biogeographical evidence for evolution: related species are found on landmasses that were once connected. DNA studies show the story is more complex — some ratite lineages appear to have lost flight independently after their ancestors spread — which is itself an example of convergent evolution under similar conditions.

Exam tip: when asked how distribution is evidence for evolution, link three ideas: species share a common ancestor, the ancestor lived on a connected landmass, and populations evolved separately after the landmasses split.

Writing about an adaptation

A full-mark answer to “explain how this adaptation helps the organism survive” has three parts:

Name the adaptation (for example large fat stores).
State its advantage (energy is available when food is scarce).
Link it to survival and reproduction (the organism survives food shortage and can still breed, passing on the allele).

Answer in your exercise book, then open each memo to mark your work.

Question 1 · (3 marks)

Define the term adaptation. Give ONE structural and ONE behavioural adaptation that helps an animal survive in a dry habitat.

Show memo

An inherited characteristic that increases an organism’s chance of survival and reproduction in its environment ✓.
Structural: e.g. long legs for travelling far to find water, or thick skin that reduces water loss ✓.
Behavioural: e.g. resting in shade or being active at night ✓.

Question 2 · (6 marks)

Use the fruit-fly data in the lesson. (a) State a suitable hypothesis. (b) Identify the independent and dependent variables. (c) Name TWO variables that must be controlled. (d) Describe the trend in the data.

Show memo

(a) Repeated food shortage increases the starvation resistance of fruit flies over generations ✓ (accept a testable statement linking the two variables).
(b) Independent: number of generations under food shortage ✓. Dependent: mean survival time without food ✓.
(c) Any TWO: temperature, humidity, water supply, age of flies, sex of flies ✓✓.
(d) Survival time increased from 40 h to 71 h over 30 generations ✓ (accept: increased, but more slowly in later generations).

Question 3 · (4 marks)

Explain, in terms of natural selection, how the mean starvation resistance of the population increased.

Show memo

There was variation in starvation resistance in the original population ✓.
During food shortage, flies with larger energy (fat) reserves survived; others died ✓.
The survivors reproduced and passed on their alleles ✓.
Over many generations the alleles for greater starvation resistance became more frequent, so the mean increased ✓.

Question 4 · (3 marks)

Ratites are found on several southern continents. Explain how this distribution is used as evidence for evolution.

Show memo

The ratites share a common ancestor ✓ that lived when the southern continents were joined (Gondwana) ✓. After the continents separated, the populations evolved separately into different species ✓.