POPULATION GENETICS, SELECTION AND SPECIATION

CAPE BIOLOGYFEBRUARY 18, 2013MRS. S. HAUGHTON

• A population is a group of organisms of the same species usually found in a clearly defined geographical area.

• Genes, sometimes working together with environmental factors, determine the phenotypes of organisms and are responsible for variations within populations.

• Theory of natural selection suggests that phenotypes adapted to the environmental conditions are selected for.

• Non-adapted phenotypes are selected against and eventually eliminated.

• Long term effects of natural selection are at the population level and not individual level.

GENE POOL

• This is the total variety of genes and alleles present in a sexually reproducing population.

• New gene combinationsnew phenotypes

• May constantly change from generation to generationevolutionary change.

• Static gene poolnon-evolving.

ALLELE FREQUENCY

• The number of organisms in a population carrying a particular allele determines the allele frequency.

• E.g. frequency of dominant alleles for skin pigmentation is 99% in population while that for lack of pigment (albinism) is 1%.

HARDY-WEINBERG EQUILIBRIUM

• This theory states that the frequency of dominant and recessive alleles in a population will remain constant from generation to generation provided to following factors exist:

1. Population is large2. Mating is random3. No mutations occur

FACTORS PRODUCING CHANGES IN POPULATIONS

1. Crossing over (meiosis)2. Independent segregation (meiosis)3. Random fertilization4. Mutation5. Non-random breeding6. Small population leading to genetic drift7. Genetic load8. Gene flow occurring between populations

NON-RANDOM BREEDING• If individuals (usually females) are choosy in their

selection of mates (leading to non-randomness), the gene frequencies may become altered.

• Darwin called this sexual selection.• Influenced by breeding territories, courtship displays,

"pecking orders“.• In each case certain individuals do not get to make their

proportionate contribution to the next generation.• Organisms with less favourable characteristics have a

decreased reproductive potential and the frequency of their alleles being passed on to subsequent generations is reduced.

GENETIC DRIFT• An allele may increase — or decrease — in frequency

simply through chance. • Not every member of the population will become a

parent and not every set of parents will produce the same number of offspring.

• The effect, called random genetic drift, is particularly strong in small populations (e.g., 100 breeding pairs or fewer).

• Eventually the entire population may become homozygous for the allele or — equally likely — the allele may disappear.

• Before either of these fates occurs, the allele represents a polymorphism.

GENETIC LOAD• This is the existence within the population of

disadvantageous alleles in heterozygous phenotypes.

• Any increase in recessive alleles in a population as a result of harmful mutations will increase the genetic load.

• Genetic load is a measure of the cost of lost alleles due to selection (selectional load) or mutation (mutational load). It is a value in the range 0 < L < 1, where 0 represents no load.

GENE FLOW

• The movement of alleles from one population to another as a result of interbreeding between members of the two populations.

• Random introduction of new alleles into the recipient population from the donor population changes the allele frequency in both populations.

SELECTION AND TYPES OF SELECTION

SELECTION• The process by which those organisms which

seem physically, physiologically, and behaviorally better adapted to the environment survive and reproduce.

• Those not well adapted fail and die out.• Advantageous alleles kept, disadvantageous not• Selection depends on the existence of

phenotypic variation within the population.• There are three types of selection process

occurring in natural and artificial selection: stabilising, directional and disruptive.

STABILISING SELECTION• Form of natural selection responsible for preserving

adaptive characteristics of organisms under constant environmental conditions.

• Operates by means of the removal, or elimination, of individuals who deviate from the established norm.

• Therefore, a population remains unchanged for a given characteristic, despite the continuous process of mutagenesis.

• Favours "average" organisms best adapted to that environment

• Selects against organisms with extreme forms of characteristics/mutations 

• E.g. Heaviest and lightest babies have highest mortality

STABILISING SELECTION

DIRECTIONAL SELECTION• Natural selection favours organisms with one

extreme form of a characteristic while removing those from the other extreme and averge.

• Pesticide resistance (warfarin - poison used to kill rats)– Resistant rats / need a lot of vitamin K / stabilising selection– New environmental effect: warfarin / kills normal rats– Resistant rats survived, reproduced, pass on resistance gene– New population forms by directional selection

• Antibiotic resistance (penicillin resistance)– Resistant bacteria / unnecessary enzymes / selected against– New environmental factor: penicillin / kills normal bacteria– Resistant bacteria survived, reproduced, passed on resistance

gene

DIRECTIONAL SELECTION

DISRUPTIVE SELECTION• Natural selection favours organism with two extreme forms of a

characteristic• A population is divided into subgroups that are still connected. They

can still reproduce, but the flow of genes is reduced. Directional selection takes place within each of the subgroups.

• Sickle-cell anaemia– Abnormal Hb makes red blood cells sickle-shaped / stick in capillaries– People homozygous for this recessive allele die before reproducing– People heterozygous for the allele should be at a disadvantage / red

blood cells can sickle during exercise / allele should be selected against and rare

– Where malaria is found, people heterozygous for sickle-cell have an advantage (resistant) and are likely to survive, reproduce and pass on the allele; people without the allele also have an advantage, because their red cells behave normally

– Balanced polymorphism is produced / carrier is heterozygous for sickle cell

DISRUPTIVE SELECTION

SELECTION PRESSURE

• This is due to factors which may decrease or increase the spread of an allele between the gene pools.

• Both the environment and population size operate together to produce selection pressure which can vary in intensity.

INTENSITY OF SELECTION PRESSURE

• The intensity on a population varies at different times.

• External factors include increase in predators, pathogens and competition from other species (interspecific competition).

• Internal factors include rapid increase in population size increasing competition for resources (intraspecific competition).

• Selection pressure may cause organisms to take up narrower and narrower niches.

SPECIES CONCEPT AND SPECIATION

SPECIES CONCEPT

• A species represents the lowest taxonomic group capable of being defined with any degree of precision.

• Members of a species usually exist as a small interbreeding population with their own gene pool.

• Spatial separation causes each group to come under different types and degrees of selection pressure.

DEFINITIONS OF SPECIES

1. A group of organisms capable of interbreeding to produce fertile offspring.

2. A group of organisms sharing the same ecological niche.

3. A group of organisms showing close similarity in genotype.

4. A group of organisms sharing a unique collection of structural and functional characteristics.

SPECIATION

• The process by which one or more species arise from previously existing species.

INTRASPECIFIC AND INTERSPECIFIC SPECIATION

INTRASPECIFIC SPECIATION• If a single species gives rise to a new species

this is intraspecific speciation.• Gene flow within populations in interrupted.• Each subpopulation becomes genetically

isolated.• If interbreeding between the species is

successful they still belong to the same species. If not speciation has occurred.

• Speciation will only occur as a result of the formation of barriers which lead to reproductive isolation between the populationsisolating mechanisms.

ISOLATING MECHANISMS• Prezygotic mechanisms (barriers to the

formation of hybrids)1. Seasonal isolation-two species mate at

different times of the year2. Ecological isolation-species in same region but

prefer different habitats.3. Behavioral isolation- species with different

courtship patterns and styles.4. Mechanical isolation-different genitalia so

copulation compromised; or related plants pollinated by different animals

ISOLATING MECHANISMS

• Postzygotic mechanisms (barriers affecting hybrids)

1. Hybrid inviability- hybrids are produced by fail to develop to maturity.

2. Hybrid sterility – hybrids fail to produce functional gametes

3. Hybrid breakdown- F1 hybrids are fertile but F2 hybrids are not

• Intrapsecific speciation is of two types:1. Allopatric speciation 2. Sympatric speciation

ALLOPATRIC SPECIATION

• If intraspecific speciation occurs while species are separated.

• Geographical barriers such as mountain ranges, seas, rivers pr habitat preference may produce a barrier to gene flow because is spatial separation.

• Adaptations to new conditions or random genetic drift in small populations leads to changes in allele and genotypic frequency.

SYMPATRIC SPECIATION

• If speciation occurs while the populations are occupying the same geographical area.

• If populations that were once separated are brought together and interbreed hybrids may form.

• These hybrids have reduced fertility and serve as a reproductive barrier to gene flow between to two original populations.

ALLOPATRIC VS. SYMPATRIC

INTERSPECIFIC HYBRIDIZATION

• If two different species give rise to a new species this is interspecific hybridisation.

• This is a form of sympatric speciation.• Fertile hybrids only occur usually in the

case of interspecific hybridisation as result of a form of chromosomal mutation called allopolyploidy.