Mechanisms of Evolution and Their Effect on Populations Section 9.1

Summation of Natural Selection  Species that reproduce sexually, each individual inherits a new combination of alleles from parents  New mutations occur randomly in each generation (potential for new traits to develop)  Genetic variation in a population is random  Individuals with genes that help them survive and reproduce pass them along to offspring

Why look at populations when studying evolution?  Individual organisms do not evolve, but populations of organisms do.  The gene pool of a population consists of all the alleles of all genes of each individual in that population.  The percentage of each allele in any given gene present in the population determines the genetic characteristics of the population.

Factors that Change Allele Frequencies in Populations  Allele frequencies are the number of copies of an allele compared to the total number of alleles in a population.  The changing percentages or frequencies of alleles within populations are the small events that lead to evolution within a population.  When the frequency of an allele in a population changes, microevolution has occurred.

Factors that Cause Microevolution 1. Mutation 2. Gene Flow (migration) 3. Non-Random Mating 4. Genetic Drift 5. Natural Selection

1. Mutations  A permanent change to the DNA of an individual.  If it is heritable, it can be passed onto offspring and can add to the population’s gene pool and genetic diversity.

Example  Warfarin is a blood thinner (prevents blood from clotting) and has also been used a rat poison since the 1950’s.  Norway rats are resistant to warfarin. It is likely that a few rats had a mutation that made them resistant to warfarin’s effects and they survived the applications, mated, and passed on the resistance to their offspring.

2. Gene Flow (migration)  Describes the net movement of alleles from one population to another as a result of the migration of individuals.

Examples  Example – Grey Wolf often breeds with member of a nearby population

3. Non-Random Mating  Mating among individuals on the basis of mate selection for a particular phenotype or due to inbreeding.  In random mating, partners are randomly selected (pulled from a hat). There is no way to predict which males will mate with which females.

Non-Random Mating – Preferred Phenotypes  In animal populations, individuals may choose mates based on physical or behavioural traits (phenotypes).  Caribou males lock horns and fight for the right to mate.  It’s non-random mating because it prevents individuals with particular traits from breeding. Only the ones that mate will pass on their gene pool to the next generation.

Non-Random Mating - Inbreeding  Inbreeding occurs when closely related individuals breed together.  Close relatives share similar genotypes so inbreeding increases the frequency of homozygous genotypes.  Negative effects are evident in purebred animals which have a higher incidence of deformities and health problems, low fertility rates, and short life expectancies.

4. Genetic Drift  The change in frequencies of alleles due to chance alone.  The smaller the population, the less likely the parent generation will be reflected in the offspring (traits may disappear).

 Most populations are large enough to overcome the effects of genetic drift.  Two situations can lead to it in any population.  The Founder Effect  The Bottleneck Effect

Genetic Drift and The Founder Effect  New populations are formed by a few individuals or founders.  Founders carry some but not all of the alleles from the original population’s gene pool and the new population’s diversity is therefore limited.  If founders are rare, their alleles will increase in frequency.  Founder Effect – a change in a gene pool that occurs when a few individuals start a new isolated population.

Example  The Amish population of Philadelphia Pennsylvania was founded in the 1700’s by a few families.  The current population has an unusually high frequency of polydactylism – the presence of a sixth finger or toe.

Genetic Drift – Bottleneck Effect  Starvation, disease, human activities, natural disasters can quickly reduce the size of a population.  Survivors only have a fraction of the alleles present in the original population and therefore gene pool loses diversity.  Bottleneck Effect – gene pool changes resulting from a rapid decrease in population size.

Example  In 1775 a typhoon devastated Pingelap Island (in the Pacific Ocean) and there were fewer than 30 survivors from the original  One of the survivors had a genetic mutation causing colour vision deficiency and now 10% of the current population does. This condition is more rare in the general population.  Bottleneck effect is usually evident in nearly extinct species that have regenerated.

Natural Selection  Selective forces such as competition and predation affect populations.  If having a single allele gives even a slight, yet consistent, selective advantage, the frequency of the allele in the population will increase from one generation to the next at the expense of less favourable alleles.  Natural selection affects the frequency of a heritable trait in a population which can lead to evolutionary change.

Stabilizing Selection  Favours an intermediate phenotype and acts against extreme variants of the phenotype.  The most common phenotype (the intermediate form) is made more common in the population by removing the extreme forms.

Directional Selection  Favours the phenotypes at one extreme over the other.  Common during times of environmental change or when a population migrates to a new habitat with different environmental conditions and niches to exploit.  Example – English Peppered Moth colouring or antibiotic resistant bacteria.

Disruptive Selection  Takes place when the extremes over a range of phenotypes are favoured over intermediate phenotypes.  Intermediate phenotypes can be eliminated from the population.  Example – male coho salmon size, small are good for sneak-attack fertilization and large is good for fighting for access to female’s eggs.

Sexual Selection  Involves competition between males through combat or through visual displays (such as showy feathers).  Involves choices females make for mates.  Sexual dimorphism – the different physical characteristics between males and females.

Sexual Dimorphism

Males Impressing Females

Homework  Mutation, Gene Flow, Non-random Mating  #1, 2, 10, 11 on page 359.  Genetic Drift  #5, 6, 7, 8, 9 on page 359.  Natural Selection  #4 on page 359 and Activity 9.1 on page 358.