BIOLOGY · Stage 9 · Chapter 13: Variation and Natural Selection
Biology 13.2 Natural selection in action — Adaptation, Survival and Evolution
5 min read 6 parts Lesson 2 of 8
Lesson summary
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
Section 1 of 6Variation and adaptation
- 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 = 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
- structural (body shape), behavioural and physiological (such as producing concentrated urine).
- (such as migration)
- structural (body shape), behavioural and physiological (such as producing concentrated urine).
Section 2 of 6Overproduction and survival
- 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
- the total number of each species stays approximately the same.
- Struggle
- individuals compete for food, space and mates, and face predators and disease.
Section 3 of 6The process of 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
- five stages make up the theory.
- Variation: individuals in a population differ because of their genes.
- Overproduction: more offspring are born than the environment can support.
- Struggle for survival: individuals compete and face predators and disease.
- Survival of the best adapted: individuals with advantageous features are more likely to survive and breed.
- Inheritance: they pass the advantageous alleles to offspring, so these alleles become more common over generations.
- Whole populations
- natural selection does not change individuals, but whole populations or even species change.
- New species
- repeated over a long period it may lead to the development of a new species.
Section 4 of 6Examples of natural selection
- 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
- 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
- the one that attracts the most insects has the best chance of making many fertile seeds.
- 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
- 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
- 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.
Section 5 of 6The giraffe and Lamarck
- 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
- 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 |
Section 6 of 6Evolution and evidence
- Evolution = the gradual change in the inherited characteristics of a population over many generations.
- New species
- if populations become isolated and change differently, they may eventually become new species.
- Evidence
- the fossil record, the similarity of DNA between related species, and observed examples such as antibiotic-resistant bacteria and the peppered moth.
Last updated: