1 of 9© Boardworks Ltd 2009. 2 of 9© Boardworks Ltd 2009 Review Natural Selection & Evolution act...

1 of 9 © Boardworks Ltd 2009


Review

• Natural Selection & Evolution act on natural variation of the population

• This natural variation comes from differences in DNA sequences


What is a gene pool?

The gene pool can be defined as:

The gene pool’s composition changes from one generation to the next as the relative proportions of alleles vary.

The total information from all the genes and alleles of the breeding individuals in a population at a particular time.

If there is a consistent change in allele frequency (the proportion of organisms in the population carrying a particular allele) then a population is evolving.


Genetic drift


Population bottlenecks

A population bottleneck occurs when a large, genetically diverse population is drastically reduced by a catastrophic, non-selective event, such as a volcanic eruption.

The total genetic diversity of the few survivors is likely to be much lower than that of the original population. As the population reestablishes itself, this low level of diversity will be maintained.

The cheetah population has an exceptionally low genetic diversity. This is thought to be due to a very narrow bottleneck, where only a single family group survived the last ice age.


The founder effect


Introducing the Hardy-Weinberg principle

The Hardy-Weinberg principle is a mathematical model used to calculate the allele frequencies of traits with dominant and recessive alleles.

The model assumes that the population:

is large

has random mating

is experiencing no selection

has no mutation, emigration or immigration.

If these assumptions are met then the allele frequencies of the population will remain stable over time.


Factors affecting the gene pool


Types of selection

Selection causes some traits to survive and spread, while others are lost. A selection pressure determines which traits are successful.

Starting population has a normal distribution

of traits.

mode

There are three types of selection: disruptive, stabilizing and directional.

Selection can be represented using graphs showing the distribution of individuals with a particular trait.

trait

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mode mode

Types of selection

stabilizingdisruptive directional

Selection can cause the mode and/or distribution to change.

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trait

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trait trait

Selection pressure toward

the extremes creates two

modal values.

Selection pressure toward the center

increases the number of

individuals at the modal value.

Selection pressure toward

one extreme moves the mode in this direction.

mode


Selection pressures


Humanity as a selection pressure

Human activity provides some of the strongest selection pressures in the world today.

The widespread use of antibiotics has exerted a very strong selection pressure on bacteria. Any that can survive exposure to antibiotics can rapidly divide and produce a resistant population. MRSA is an example of an antibiotic resistant bacteria.

What are the implications of the evolution of antibiotic resistance for humans?


Inbreeding and artificial selection

In artificial selection humans decide which members of a population will breed.

This allows the alleles for desirable characteristics to be maintained in the population and others eliminated. This technique is used by farmers to produce animals and plants with a high yield.

Artificial selection can lead to inbreeding, which reduces genetic diversity. This can increase the risk of a disease affecting the whole population.


Convergent Evolution

• unrelated species become more and more similar in appearance as they adapt to the same kind of environment


Divergent Evolution

• the process of two or more related species becoming more and more dissimilar.


Coevolution

• See Clip


Coevolution

• joint change of two or more species in close interaction– Predators and their prey sometimes coevolve– parasites and their hosts often coevolve– plant-eating animals and the plants upon

which they feed also coevolve– plants and the animals that pollinate them

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