Evolution and SpeciationMechanisms of

Evolution

Factors that affect Populations• Genetic variation of individuals within a

population makes evolution possible.• Each new generation is made up of

genetically unique individuals.• Also, new mutations occur randomly

new each generation• These lead to creation of new alleles

inherited from parents

Populations not individuals evolve• PopulationPopulation:A localized

group of individualsindividuals belonging to the same same speciesspecies.

• Species:Species: A group of populationspopulations whose individualsindividuals have the potential to interbreedinterbreed and produce viableviable offspring.

• Gene pool: The total collection of genescollection of genes in a population at any one time.

Microevolution

• Changing percentages, or frequencies, of alleles within populations are the small events that lead to evolution within a population = microevolution

Hardy-Weinberg PrincipleHardy-Weinberg Principle• The conceptconcept that the shuffling of genesshuffling of genes that

occur during sexual reproduction, by itself, cannot changecannot change the overall genetic makeup of a population.

Hardy-Weinberg PrincipleHardy-Weinberg Principle

• This principleprinciple will be maintained in nature only if all fivefive of the following conditions are met:

1.1. Very large populationVery large population

2.2. Isolation from other populationsIsolation from other populations

3.3. No net mutationsNo net mutations

4.4. Random matingRandom mating

5.5. No natural selectionNo natural selection

Hardy-Weinberg PrincipleHardy-Weinberg Principle• Remember:Remember:

If these conditions are met, the population is at equilibriumequilibrium.

• This means “No Change” or “No “No Change” or “No Evolution”.Evolution”.

MicroevolutionMicroevolution

• The origin of taxonomic groups higher than higher than the species levelthe species level.

• A change in a population’s gene poolpopulation’s gene pool over a secession of generations.

• Evolutionary changesEvolutionary changes in species over relatively brief periods of geological timegeological time.

Factors that Cause Evolutionary Change

Five Mechanisms of Microevolution

• Mutation

• Gene flow

1. Non-random mating

2. Genetic drift

3. Natural selection

1. MutationMutation: Change in

an organism’s DNA that creates a new allele.

Randomly introduces new alleles into a population and can change allele frequencies in a population

2. Gene flow• Net movement of alleles from one population

to another due to the migration of individuals• Occurs between two different interbreeding

populations that have different allele frequencies

• Hamadryas baboons in Saudi Arabia. At some point in their lives, female Hamadryas baboons leave their birth group and migrate to a different one. This promotes gene flow between different groups, helping all populations

maintain healthy and diverse gene pools.

3. Non-random mating

• Selection of mates other than by chance• Individuals in a population select mates often

based on their phenotypes• Increases the population of particular

characteristics• Based on:

– Preferred Phenotypes– Inbreeding

Preferred Phenotypes• Choosing of mates

based on their physical and behavioural traits

• Prevents individuals with particular phenotypes form breeding

• Only the individuals that mate will contribute to the gene pool of the next generation

Inbreeding• Occurs when closely related individuals breed

together• E.g. self-fertilization of flowers• Purebred animals tend to have a higher

incidence of deformities and health problems compare with non-purebred animals. (lower fertility rates)

• E.g. Shar-pei dog -- hyaluronic acid and mucinosis

4. Genetic Drift• The change in frequencies of alleles due to chance

events in a breeding population.• Sample size can greatly affect the gene pool of a

population.• The smaller the population, the less likely it is that the

parent gene pool will be reflected in the next generation.

• In a large population there is a better chance that the parent gene pool will be reflected in future generations.

• Occurs by:– Bottleneck Effect -Founder Effect

Bottleneck Effect• Changes in gene distribution that result

from a rapid decrease in population size

• Colonization of a new location by a small number of individuals, results in random change of the gene pool

• E.g. Darwin’s finches

http://www.associatedcontent.com/video/492512/microevolution_by_genetic_drift.html?cat=7

BOTTLENECK EFFECT• Northern elephant seals have reduced

genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000 — but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.

Don’t kill me

FOUNDER EFFECTFOUNDER EFFECT• Genetic driftGenetic drift resulting from the

colonizationcolonization of a new location by a small number of individuals.

• Results in random changerandom change of the gene pool.

• Example:Example:1.1. Islands (first Darwin finch)Islands (first Darwin finch)

Darwin’s finches

FOUNDER EFFECTFOUNDER EFFECTFounder effectFounder effect• A founder effect occurs when a new colony is A founder effect occurs when a new colony is

started by a few members of the original population. started by a few members of the original population. This small population size means that the colony This small population size means that the colony may have:may have:a. reduced genetic variation from the original a. reduced genetic variation from the original population. population. b. a non-random sample of the genes in the original b. a non-random sample of the genes in the original population. population.

• Computer Simulation

FOUNDER EFFECT• Example: Amish community

• Ellis-van Creveld syndrome, involves not only short stature but polydactyly (extra fingers or toes), abnormalities of the nails and teeth, and, in about half of individuals, a hole between the two upper chambers of the heart. The syndrome is common in the Amish because of the founder effect

5. Natural SelectionThe result of the environment

selecting for individuals in a population with certain traits that make them better suited to survive and reproduce than other in the population

Over many generations, frequencies of alleles of many different genes may change, resulting in significant changes in the characteristics of a population

MODES OF ACTIONMODES OF ACTION

• Natural selectionNatural selection has three modesthree modes of action:

1.1. Stabilizing selectionStabilizing selection

2.2. Directional selectionDirectional selection

3.3. Diversifying selectionDiversifying selection Number ofIndividuals

Size of individualsSmall Large

1.1. STABILIZING SELECTIONSTABILIZING SELECTION• ActsActs upon extremesextremes

and favorsfavors the intermediateintermediate

• Reduces variation, improves the adaptation of the population to aspects of the environment that remain constant

2.2. DIRECTIONAL SELECTIONDIRECTIONAL SELECTION

• FavorsFavors variants of one extremeone extreme

• Common during times of environmental change or during migration to a new habitat

• Peppered moths

3.3. DIVERSIFYING SELECTIONDIVERSIFYING SELECTION• FavorsFavors variants of opposite opposite

extremesextremes

• Intermediate phenotypes can be eliminated

Animations

• http://zoology.okstate.edu/zoo_lrc/biol1114/tutorials/Flash/life4e_15-6-OSU.swf

Sexual Selection• Natural selection for mating based, in

general, on competition between males and choices made by females– Combat– Visual displays– Sexual dimorphism