9.1-Mechanisms of Evolution & Their Effect on Populations SBI3U1

Evolution & Population Individuals organisms do not evolve POPULATIONS DO! Therefore when we study evolutionary processes we study the changes in population

What is a gene pool? A A A A a a A a A a A A a A population that consists of all the alleles of all genes of each individual in that population= GENE POOL. This provides the population’s genetic variation. A A A a A a A a A a A A a

MICROEVOLUTION Allele Frequency: the number of copies of a particular allele compared to the total number of alleles in a population Microevolution = change in % or frequency of alleles within a population Therefore, small changes to allele frequency can lead to evolution

5 Factors that Affect Allele Frequencies Mutation Gene Flow ( migration) Non-Random Mating Genetic Drift *Natural Selection *Note: natural selection is the most significant factor

1) Mutations Recall: Mutation is a change in DNA of an individual A heritable mutations may affect an entire gene pool More genetic variation in a population = greater diversity = greater chance of selective advantage (in changing environment) (ie. Norway rats resistant to poison- see next slide)

Example: Norway Rat Population It is likely that an allele already existed in the Norway rat population that gave some individuals resistance to the poison warfarin. When warfarin was applied, those individuals survived, reproduced, and passed on the allele. This selective advantage changed the allele frequency of this characteristic. 

2) Gene Flow The net movement of alleles from one population to another due to the migration of individuals Through gene flow, modelled here, genetic information is exchanged between individuals of different populations.

Example: Grey Wolves -Grey wolves will travel great distances to find a neighbouring pack & mate -This brings new alleles into the distant pack -This will change the allele frequencies & increase genetic diversity

3)Non-Random Mating Non-random mating is mating among individuals on the basis of a particular phenotype or due to inbreeding In animals, individuals may choose mates based on their physical or behavioural characteristics. This will affect the alleles in the gene pool of the next generation

Non-Random Mating Examples Peacocks choose mates based on the tail/dance Male Caribou spar for a mate Humans choose mates based on certain characteristics/values

3)Non-Random Mating Cont’d… Inbreeding: When closely related individuals breed This increases the frequency of homozygous genotypes. Therefore, as homozygous genotypes become more common, harmful recessive alleles are more likely to be expressed. Ex: Self fertilization in plants Purebred dogs

4)Genetic Drift Genetic Drift= the frequency of certain alleles can be changed due to chance ( in small popl’n) The smaller the popl’n the less likely the parent gene pool will be reflected in certain populations.

In each generation, only some of the plants in this population reproduce. When the light pink (aa) and heterozygous roses (Aa) in the second generation did not reproduce, the allele for light pink petals was lost from the gene pool.

4)Genetic Drift Cont’d… Large populations are less susceptible to the effects of genetic drift, However…. 2 situations that can lead to significant genetic drift in large populations are called: Founder Effect Bottleneck Effect

4)Genetic Drift: The Founder Effect Founder Effect: a new population is started by a few members of the original population Reduces genetic variation/ Gene pool is reduced Common in islands New “founders” carry some but not all of the alleles from t he original population’s gene pool. Higher health concerns-> Polydactylism has a high frequency in the Amish founder population in Philadelphia

4)Genetic Drift: The Bottleneck Effect When a popl’n is quickly reduced by starvation, disease, or a natural catastrophe, the surviving population likely has only a fraction of the alleles that were present before the population declined. The gene pool will have lost its diversity, and allele frequencies will have changed

5)Natural Selection Recall: Selective forces such as competition and predation affect populations. Some individuals are more likely to survive and reproduce than others. Alleles that are selected for by environmental conditions will increase in frequency over time. 3 Types of natural selection : Stabilizing selection Directional selection Disruptive selection

5) Natural Selection: Stabilizing Selection Stabilizing Selection: favours intermediate phenotypes and acts against extreme variants of the phenotype Reduces variation E.g. Birth weight in humans

5) Natural Selection: Directional Selection Directional Selection: favours phenotypes at one extreme over the other. Common during environmental change E.g. Peppered moths

5) Natural Selection Disruptive Selection Disruptive Selection: favours the extremes of a range of phenotypes rather than intermediate are favoured Intermediate phenotypes can be eliminated E.g. Male coho salmon (ranges from 500- 4500g)

5)Natural Selection Cont’d… Sex Selection: competition for mates between males through combat or through visusal displays. Combat (caribou) Visual display (Peacocks) Sexual dimorphism: difference in appearance Btw. males and females Ie. Green head in male mallard ducks

Evolution Simulations http://glencoe.mcgraw- hill.com/sites/9834092339/student_view0/chapter20/ani mation_-_mechanisms_of_evolution.html Genetic Drift Simulation: http://highered.mcgraw-hill.com/sites/dl/free/0072835125/126997/animation45.html

Homework: Read and make notes 9.1 Genetic Drift Dry Lab- TBA Complete pg. 352 Q#1-4 Pg.356 Q #7-12