87 How Natural Selection Works

Key Idea: Evolution by natural selection describes how organisms that are better adapted to their environment survive to produce a greater number of offspring.

What is evolution?

Evolution is the change in the genetic makeup of a population (the allele frequencies) over time. Evolution is the consequence of the interaction between four factors: (1) The potential for populations to increase in numbers,

(2) genetic variation as a result of mutation and sexual reproduction, (3) competition for resources, and (4) proliferation of individuals with better survival and reproduction.

Natural selection is the term for the mechanism by which better adapted organisms survive to produce a greater number of viable offspring. This has the effect of increasing their proportion in the population so that they become more common. This is the basis of Darwin's theory of evolution by natural selection.

We can demonstrate the basic principles of evolution using the analogy of a 'population' of M&M's sweets.

#1

In a bag of M&M's, there are many colours, which represents the variation in a population. As you and a friend eat through the bag of sweets, you both leave the blue ones, which you both dislike, and return them to bag.

The blue sweets become more common...

Eventually, you are left with a bag of blue M&M's. Your selective preference for the other colours changed the make-up of the M&M's population. This is the basic principle of selection that drives evolution in natural populations.

Darwin's theory of evolution by natural selection

Darwin's theory of evolution by natural selection is outlined below. It is widely accepted by the scientific community and it is one of founding principles of modern science.

Overproduction

Populations produce too many young: many must die

Populations generally produce more offspring than are needed to replace the parents. Natural populations normally maintain constant numbers. A certain number will die without reproducing.

Variation

Individuals show variation: some variations more favourable than others

Individuals in a population have different phenotypes and therefore, genotypes. Some traits are better suited to the environment, and individuals with these have better survival and reproductive success.

#2 Natural selection

Natural selection favours the individuals best suited to the environment at the time

Individuals in the population compete for limited resources. Those with favourable variations will be more likely to survive. Relatively more of those without favourable variations will die.

#3 Inherited

Variations are inherited: the best suited variants leave more offspring

The variations (both favourable and unfavourable) are passed on to offspring. Each generation will contain proportionally more descendents of individuals with favourable characters.

Natural populations, like the ladybird population above, show genetic variation. This is a result of mutation (which creates new alleles) and sexual reproduction (which produces new combinations of alleles). Some variants are more suited to the environment of the time than others. These variants will leave more offspring, as described for the hypothetical population (right). 1. Variation through mutation and sexual reproduction: In a population of brown beetles, mutations independently produce red colouration and 2 spot marking on the wings. The individuals in the population compete for limited resources.

Red Brown mottled Red 2 spot

2. Selective predation: Brown mottled beetles are eaten by birds but red ones are avoided.

3. Change in the genetics of the population: Red beetles have better survival and fitness and become more numerous with each generation. Brown beetles have poor fitness and become rare.

1. What produces the genetic variation in populations?

2. (a) Define evolution:

(b) Identify the four principles of evolution by natural selection as proposed by Darwin:

3. Using your answer 2(b) as a basis, explain how the genetic make-up of a population can change over time: