Selection

Biology · Inheritance. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Natural selection explains how populations change. Individuals in a population vary. They produce many more offspring than can survive, so there is a struggle for survival and competition for resources. Those better adapted to the environment are more likely to survive and reproduce, and they pass their alleles to the next generation. Over many generations the helpful feature becomes more common. This is how a population evolves.

Antibiotic-resistant bacteria such as MRSA are an example. A mutation makes a few bacteria resistant. The antibiotic kills the others, and the resistant ones reproduce.

In artificial selection (selective breeding) humans choose the parents with desirable features.

Rules to remember

Natural selection in order: variation → many offspring → struggle for survival → the better adapted survive and reproduce → their alleles are passed on.
Evolution: the change in the inherited features of a population over time as a result of natural selection.
Resistance: the mutation happens by chance first; the antibiotic only selects the bacteria that are already resistant.
Artificial selection: humans select individuals with desirable features, cross them, select the best offspring, and repeat over many generations.
Artificial selection gives economically important organisms, such as cows with a high milk yield and wheat with a high grain yield.
Difference: in natural selection the environment selects; in artificial selection humans select.

Common mistake

Do not write that bacteria "become immune" or "learn to resist" the antibiotic. Resistant bacteria already exist because of a random mutation; the antibiotic kills the rest.

Worked example

In a cold region some hares have white fur and some have brown fur. The winters become warmer and there is little snow. Over 50 years the proportion of brown hares rises. Explain why.

There is variation in fur colour, and it is inherited.
With no snow, white hares are easy for predators to see, so more of them are eaten.
Brown hares are better camouflaged, so more of them survive and reproduce.
They pass the allele for brown fur to their offspring, so it becomes more common each generation.

Answer: Natural selection: brown hares are better adapted to the new environment, so their alleles spread through the population.

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