evolution

Evolution is the change over time in the proportion of individuals in a population differing in one or more inherited traits. Evolution can occur through the random processes of genetic drift or the non-random processes of natural selection and sexual selection.

drift

natural selection

natural selectionNatural selection is the non-random increase in frequency of DNA sequences that increase survival.

natural selectionAs organisms produce more offspring than the environment can support, those individuals with variations that best fit their environment are the ones most likely to survive and breed. Through inheritance, these favoured traits are therefore likely to become more frequent in subsequent generations.

mutationVariation in traits arises as a result of mutation. Mutation is the original source of new sequences of DNA. These new sequences can be novel alleles.

mutation

Most mutations are harmful or neutral but in rare cases they may be beneficial to the fitness of an individual.

modes of selection

modes of selectionThere are three different modes of selection:

• directional selection favours a single extreme phenotype;• stabilising selection favours the average phenotype, giving a decrease in variation as more extreme traits do not survive;• disruptive selection favours two extreme forms within a population.

activity 1 – modes of selection

Collect a Modes of selection worksheet.1.Cut out the three examples and the three graphs.2.Pair up each example with a graph.3.Stick these into your notes naming the mode of selection shown.

genetic driftGenetic drift refers to the changes in the frequencies of alleles in a population that occur by chance, rather than because of natural selection.Neutral traits do not seem to have any obvious selection pressures.

Modelling Genetic Drift – The Jar of Marbles AnalogyImagine a jar containing 20 marbles – 10 blue and 10 red - representing 20 organisms

The random reproduction of the organisms can be modelled as follows:

• Select one marble from the first jar, at random

• Place a marble of the same colour in a second jar

Different colours of marble represent different alleles

• Replace the first marble in the original jar and repeat until the second jar holds 20 marbles

Random sampling error means that the next generation is unlikely to have exactly the same proportions of red and blue marbles

With each subsequent generation, the red allele is less likely to be picked for

reproduction

Eventually, the red allele may go ‘extinct’

genetic driftGenetic drift has a greater impact in small, isolated populations, as alleles are more likely to be lost from the gene pool. The random loss of rare alleles, without respect to the survival or reproductive value, reduces the genetic diversity of a population.

activity 2 – founder and bottleneck effectsThe founder and bottleneck effects are examples of genetic drift.

Collect a Founder and Bottleneck Effects worksheet.

Research one example of each effect.Be prepared to share your findings at the next lesson.

founder effectIn population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population.

bottleneck effectThe bottleneck effect occurs when there is a disaster of some sort that reduces a population to a small handful, which rarely represents the actual genetic makeup of the initial population. This leaves smaller variation among the surviving individuals.

prairie chicken PPQ

fitnessFitness, from a selection point of view, is a term used to measure how successful a particular genotype has been in surviving from one generation to the next. There are two forms; absolute fitness and relative fitness.

absolute and relative fitness

Absolute fitness is the ratio of frequencies of a particular genotype from one generation to the next. Relative fitness is the ratio of surviving offspring of one genotype compared with other genotypes.

Two hikers are walking in the woods. They round a bend and see a bear on the trail ahead of them. The bear charges towards them. They begin to run, but the bear is catching up. Suddenly, the first hiker sits down and starts changing his hiking boots for sneakers. The second hiker says, “What are you doing?! You’ll never outrun that bear, sneakers or no sneakers!” The first hiker replies, “I don’t have to outrun the bear, I just have to outrun you.”

The joke is about the difference between absolute and relative fitness, and the importance of not mixing them up. In evolution, it’s usually relative fitness that’s of interest, as that’s all that natural selection cares about. For instance, even if two alleles at the same locus are both unfit in some absolute sense, the fitter of the two will still increase in frequency at the expense of the other. If a species survives and reproduces well in some absolute sense, it will be excluded if a competing species survives and reproduces even better.

activity 3 – absolute and relative fitnessCollect a Absolute and relative fitness worksheet. Your teacher will explain how to calculate absolute and relative fitness.Now calculate the absolute and relative fitness for the other generations.

Original (P) population

F1 population F2 population F3 population

 AA Aa aa AA Aa aa AA Aa aa AA Aa Aa

 

20 20 20 10 30 15 5 40 15 7 42 7 

Absolute Fitness 

 0.5

 1.5

 0.75

            

Relative Fitness  0.33

 1

 0.5