Mutation and Genetic Drift:Mechanisms of Evolution

Terminologies applied:Speciation: the process by which populations become different speciesMicroevolution: results from evolutionary mechanisms (mutation, drift, selection, etc) acting within a populationMacroevolution: refers to evolutionary change above the species level

MUTATION - Any heritable change in the DNA of a cell or an alteration in a DNA sequence

Main features of molecular mutation:• Germ line mutation – Affects tissues that produces the sperm and egg• Somatic mutation – Affects other body tissues (e.g. cancer)• Errors during cell division brought about by endogenous or exogenous factors• Mutation happens at random – whether a particular mutation happens or not is unrelated

to how useful that mutation would be- Mutations do not try to supply what the organism needs

• The ultimate source of all genetic variation - it creates the raw materials for which natural selection acts upon

• Mutations can be deleterious, beneficial, lethal, and neutral

***Existing variation can be reshuffled by a variety of mechanisms that we don’t always consider as mutations leading to increases or decreases in variation and thus altering the potential for evolution

GENETIC DRIFT - Changes in allele frequencies from one generation to the next that occurs by chance events

- First introduced by Sewall Wright

Two types:1. The Bottleneck Effect2. The Founder Effect

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Main Features of Genetic Drift:• Small populations are much more likely to experience genetic drift than larger

populations because sampling error is greatest in small samples and smallest in large samples

• It’s a random process that can lead to large changes in populations over a short period of time

• May cause gene variants to disappear completely and thereby reduce genetic variation

• Increases the inbreeding coefficient and increases homozygosity as a result of removing alleles

A. The Bottleneck Effect

A bottleneck event (e.g. earthquakes, fires, over-hunting) decimates a population and results in only a small number of individuals surviving

The remaining, random survivors may not have the same allele and genotype frequencies as the original population

***In the new population, some alleles may be found at higher frequencies whereas others may be found at lower frequencies or even lost altogether***The rapid and radical decline in population size has reduced the population’s genetic variation.

Example:Gypaetus barbatus – a type of bearded vulture that was hunted to extinction in the 1900s in the European Alps. Today, there are only about 120 bearded vultures being kept in zoos across Europe, Asia, and US. Without sufficient genetic variability, there is always a risk that a population will not be able to respond very well to new selective pressures caused by environmental change

Loss in genes to a small population means that certain combinations cannot arise and will never be tested by natural selection. Overtime, the small population of flowers will be Red=1.0 and white=0.0

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Inbreeding - After a bottleneck (e.g.catastrophe), inbreeding increases thereby increases the damage done by recessive deleterious mutations, in a process called inbreeding depression – leading to the loss of other alleles

As a consequence, the loss of variation leaves the surviving population vulnerable to any new selection pressures such as disease, climate change, or shift in the available food source, because adapting in response to environmental changes requires sufficient genetic variation in the population for natural selection to take place…

B. The Founder Effect

***For a period after the foundation, the small population experiences intensive drift.

Example:- Amish migration to Pennsylvania – 1700s- Ellis-van Creveld syndrome (allele frequency is 7% = 0.1% in general population) –

(Ellis-van Creveld syndrome is a rare form of dwarfism causing short stature, extra fingers, abnormal teeth and nails, and heart defects)

• Genetic isolation and group interbreeding allows the frequency of the allele for Ellis-van Creveld syndrome to not only persist but increase over time

Occurs when a few individuals representing a fraction of the original allele pool, invade a new area and establish a new population

There is isolation from the original population which prevents breeding between the two populations

By random chance alone, the allelic frequencies of one or more genes in the new population can be quite different than those of the original population