13.2Chapter 13: Variation and Natural Selection

Natural selection in action — Alhussan Biology - Stage 9 Revision Notes & Topical Questions

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📚 Alhussan Biology - Stage 9 Hub / Topical Revision

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13.2 Natural selection in action — Adaptation, Survival and Evolution — Alhussan International Academy — Biology (Stages 7–9) notes: Describe the steps of natural selection and explain how it leads to adaptation and evolution.Variation — individuals of a species differ from each …

Population size
Whole populations
New species
Wildebeest
Early spider orchid
Rats and warfarin

Part 1 of 6Variation and adaptation

Variation and adaptation

Variation

Variation individuals of a species differ from each other, and some differences are inherited through genes.
Each young animal gets a different combination of genes from its parents.

Adaptation

Adaptation a feature that helps an organism survive and reproduce in its environment.

Examples are the thick fur of polar bears, the long neck of a giraffe, camouflage in the peppered moth and long roots in desert plants.

Types

Types structural (body shape), behavioural and physiological (such as producing concentrated urine).
- (such as migration)

Part 2 of 6Overproduction and survival

Offspring

Offspring living organisms produce many offspring, such as the seeds in a pomegranate or baby birds in a nest, and most do not survive.

Population size

Population size the total number of each species stays approximately the same.

Struggle

Struggle individuals compete for food, space and mates, and face predators and disease.

Part 3 of 6The process of natural selection

Natural selection

Natural selection the process by which the organisms with the characteristics best adapted to their environment live and reproduce, passing useful characteristics to their offspring.

Charles Darwin and Alfred Russel Wallace independently proposed it in the 19th century.

Steps

Steps five stages make up the theory.
Variationindividuals in a population differ because of their genes.
Overproductionmore offspring are born than the environment can support.
Struggle for survivalindividuals compete and face predators and disease.
Survival of the best adaptedindividuals with advantageous features are more likely to survive and breed.
Inheritancethey pass the advantageous alleles to offspring, so these alleles become more common over generations.

Whole populations

Whole populations natural selection does not change individuals, but whole populations or even species change.

New species

New species repeated over a long period it may lead to the development of a new species.

Part 4 of 6Examples of natural selection

Wildebeest

Wildebeest those with the longest legs, strongest hearts and muscles or best eyesight are most likely to escape lions or hyenas.
Females that make the most milk increase the chances that their calves survive.

Puffins

Puffins those with the brightest colours attract a mate and those with the biggest beaks catch more fish,
so eventually all puffins will have bigger beaks or brighter colours.

Early spider orchid

Early spider orchid the one that attracts the most insects has the best chance of making many fertile seeds.

Rats and warfarin

Rats and warfarin warfarin has poisoned rats since 1950, and some rats inherit a gene that makes them resistant.
Whenever the poison is used only the resistant rats survive and breed, so in some populations all rats are resistant.

Pitcher plants

Pitcher plants variations that let ancestors trap insects and use their digested bodies gave an advantage in mineral-poor places,
- so now all get nitrates this way.

Lizards

Lizards a variation that matches the colour of the environment made them hard for predators to find,
- so lizards of this species are now all leaf green.

Part 5 of 6The giraffe and Lamarck

Giraffe

Giraffe early giraffes varied in neck length, and where food was high in trees those with longer necks reached more leaves and reproduced more.
Their offspring inherited the genes for long necks, so the average neck length increased over many generations.

Lamarck's idea

Lamarck's idea that giraffes stretched their necks and passed on the stretch is not supported,
- because characteristics acquired in life are not inherited.

Lamarck's idea	Darwin's idea
Features acquired during life are passed on	Inherited variation; the best adapted survive
Use and disuse	Natural selection

Part 6 of 6Evolution and evidence

Evolution

Evolution the gradual change in the inherited characteristics of a population over many generations.

New species

New species if populations become isolated and change differently, they may eventually become new species.

Evidence

Evidence the fossil record, the similarity of DNA between related species,
- and observed examples such as antibiotic-resistant bacteria and the peppered moth.

Key terms

Term	Meaning
Natural selection	The process by which best adapted organisms survive and pass on their genes.
Adaptation	A feature that helps an organism survive in its environment.
Evolution	Gradual change in the inherited features of a population.
Variation	Differences between individuals of a species.
Allele	A version of a gene.
Population	All organisms of one species in an area.