Population Genetics, Natural Selection & Speciation 2015-2016

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CONCEPT OF GENE POOL

Population genetics study of genetic comparison in a POPULATION changes and how they are inherited.

What do we mean by a population?

A GROUP OF INDIVIDUALS OF THE SAME SPECIES IN AN ECOSYSTEM/HABITAT.

Genetic make-up of population is known as GENE POOL.

Gene pool is defined as : ALL THE ALLELES OF ALL THE GENE LOCI IN A POPULATION AT A PARTICULAR TIME

Example:

THE HARDY- WEINBERG EQUATION-CALCULATING EXPECTED FREQUENCIES

ALLELE FREQUENCY : certain proportion of alleles in the population

General Formula , represents the Dominant and Recessive forms of allele

p + q = 1

p= frequency of dominant allele, q =recessive allele (Nb when alleles are co dominant, randomly assigned as either p or q ).

HARDY - WEINBERG EQUATION enables us to estimate the EXPECTED allele frequency.

key

p Is frequency of the DOMINANT ALLELE in gene pool

q is frequency of RECESSIVE ALLELE in gene pool

Frequency of HOMOZYGOUS DOMINANT GENOTYPE is P^2

Frequency of HOMOZYGOUS RECESSIVE GENOTYPE (or phenotype) is q^2

Frequency of HETEROZYGOUS GENOTYPE is 2pq

CALCULATING ACTUAL ALLELE FREQUENCIES -eg. CODOMINANCE

THE HARDY- WEINBERG PRINCIPLE

This states :

THE FREQUENCIES OF THE ALLELES OF A PARTICULAR GENE IN A POPULATION WILL STAY CONSTANT FROM GENERATION TO GENERATION.

Will only occur under these conditions :

THE POPULATIONS MUST BE LARGE- small pop., chance events can cause large swings in frequencies

MATING BETWEEN INDIVIDUALS MUST BE RANDOM- ensures equal chance of each allele being passed on to the next generation.

NO MUTATIONS must occur - this will change the allele frequency

ALL GENOTYPES MUST EQUAL LIKELY TO REPRODUCE, I.E. NO SELECTION OCCURS - this ensures equal chance of allele being passed on to the next generation.

THERE MUST BE NO MIGRATION INTO THE POPULATION (IMMIGRATION) OR OUT OF THE POPULATION (EMIGRATION)- migration adds new alleles to or removes alleles from pop., this is know as GENE FLOW

Dependent variable may be subjective - you may taste something as strong, someone else may taste it as weak

when conditions are met & frequency doesn't change, population said to be in GENETIC EQUILIBRIUM

NATURAL SELECTION

Individuals are more likely to reproduce than others i.e. show DIFFERENTIAL REPRODUCTIVE SYSTEMS.

The size of a population is limited by a number of environmental factors eg. Competition for food and breeding sites in animals and for water, space light in plants .

There is VARIATION in the population. some organisms inherit characteristics that mean they are better competitors for the scarce resources. they are said to be 'biologically fit'. May happen due to MUTATIONS (chance events) may result in new, beneficial alleles that giver an advantage.

These organisms are MORE LIKELY TO SURVIVE TO Reproduce and have MORE offspring and will PASS ON THEIR ALLELES to next generation

This causes a change in the ALLELE FREQUENCY of population

Selection can be triggered by an event or change in environment of population, can change phenotype and Genotype. Environmental change creates a SELECTION PRESSURE.

DIRECTIONAL AND STABILISING SELECTION

Directional selection

Results in increase of allele frequency of one phenotype relative to another. acts against

ONE of the extreme phenotypes. Arises when environmental change is sustained over many generations, including

HUMAN INDUCED CHANGES.

EXAMPLE :

Rats spread diseases. Warfarin added as a pesticide. Rats died of haemorrhages. CHANCE MUTATION occurred meaning , new allele resistant to warfarin. This rat would have a SELECTIVE ADVANTAGE over other rats. This rat more likely to survive and reproduce passing on the allele on to OFFSPRING. Making allele more common INCREASING THE ALLELE FREQUENCY. Eventually most of rat population becomes resistant to warfarin.

STABILISING SELECTION

Acts against BOTH extremes in a range of phenotypes ie. favours the middle phenotype and so acts to PREVENT CHANGE .

Stabilising selection occurs when there is NO CHANGE IN THE RANGE OF ENVIRONMENTS successive generation experience.

SPECIATION

Selection changes gene pools. Greatest change when .

EXISTING SPECIES GIVES RISE TO TWO OR MORE SPECIES process called SPECIATION.

SPECIES = A GROUP OF ORGANISM THAT HAVE SIMILAR CHARACTERISTICS THAT CAN REPRODUCE T PRODUCE FERTILE OFFSPRING.

SPECIATION = IS A PROCESS WHEREBY ONE GENE POOL GIVES RISE TO MORE THAN ONE GENE POOL.

Part of population becomes ISOLATED, most usually by GEOGRAPHICAL LOCATION i.e.) Mountain range or river separates two pop. that were originally from the same gene pool.

No interbreeding occurs between the 2 pop.

Different MUTATIONS occur in each pop. giving rise to beneficial alleles.

2 separate pop. experience different SELECTION PRESSURES.

Diff. features and alleles will give ADVANTAGE in each environment, so NATURAL SELECTION occurs separately in both pop

Each pop. with advantageous features ARE MORE LIKELY TO SURVIVE AND REPRODUCE, PASSING ON ALLELE TO OFFSPRING. Therefore ALLELE FREQUENCY INCREASES.

Eventually, genotype of both pop. changes over time becoming so diff. that they CAN NO LONGER INTERBREED TO PRODUCE FERTILE OFFSPRING.

Means they are NOW TWO DIFF. SPECIES