Unit 4.6

Genetic variation within a species and geographical isolation leads to accumulated different genetic information in a population and the potential to form new species.

Know the terms genotype, phenotype, gene, allele and locus.Understand alleles can be dominant, recessive or co-dominantKnow that there may be multiple alleles of a gene e.g. blood groupsPredict the result of monohybrid crosses involving dominant/recessive/co-dominant alleles and compare with observed results using X2.Understand the behaviour of chromosomes during meiosis (reduction division) explain the ratios and combinations of alleles seen in monohybrid crosses.Predict the results of monohybrid crosses involving multiple alleles and sex linked characteristics.Understand the concept of gene pool and allele frequency in a population.Calculate allele/genotype/phenotype frequencies using Hardy-Weinburg equations(p2 +2pq + q2 = 1.0 and p + q = 1.0)Appreciate the conditions needed before Hardy-Weinburg equations can be used.Understand this is a mathematical model which predicts allele frequencies will not change from one generation to the next.Appreciate selection results from differential reproductive success and that this does alter allele frequencies within a gene pool.Interpret data which shows selection occurring.Know selection can be directional and stabilising.Understand that when populations are separated geographically selection for different characteristics results in differences in their gene pool.Know if individuals from these populations try to breed together they may not now be physically capable or their offspring are infertile and so they are now separate species.

Definitions:

Genotype: the genetic composition of an organism

Phenotype: the characteristics of an organism – results from interaction of genes and the environment.

Gene: length of DNA coding for a polypeptide

Allele: a version or form of a gene.

Dominant: allele which is always able to express itself in the phenotype.

Recessive: An individual must have 2 copies of this allele for it to be expressed in the phenotype.

Locus: the position of a gene on a chromosome.

Homologous: a pair of chromosomes which have genes controlling the same characteristics at the same loci. They pair up then separate during meiosis.

Homozygous: both alleles of a pair of genes are identical.

Heterozygous: both alleles of a pair of genes are different.

Co-Dominance and multiple alleles:

Co-dominance – in which both alleles are equally dominanto Both alleles are expressed in the phenotype.

Multiple alleles – where there are more than two alleles, of which only two may be present at the loci of an individual’s homologous chromosomes.

o E.g. blood type.

Allele frequencies and population genetics

The Hardy-Weinburg principle o No mutations ariseo The population is isolated, that is, there is no flow of alleles into or out of the

population.o There is no selection, that is, all alleles are equally likely to be passed to the next

generation.o The population is large.o Mating within the population is random.

(q2 + 2pq + p2 = 1.0 AND p + q = 1.0)

Selection

Reproductive success and allele frequencyo All organisms produce more offspring that can be supported by the supply of food,

light, space etc.o Despite overpopulation – the population remains relatively constant in size.o This means there is competition between members of a species to be the ones to

survive.o Within any population of a species there will be a gene pool containing a wide

variety of alleles.o Some individuals will possess combinations of alleles which make them better able

to survive in their competition to others.o These individuals have a higher chance of surviving and therefore can successfully

reproduce producing more offspring.o Only the successful breeders will pass on their alleles.o Therefore advantageous alleles are passed on to the next generationo The number of advantageous alleles will increase in the gene pool as they are

successfully allowing the species to survive as well as reproduce.o Advantageous alleles are depended on the current environmental conditions – these

could change and the alleles could be no longer advantageous. Types of selection

o Directional selection: selection may favour individuals that vary in one direction from the mean of the population.

If the environment changes – so will the phenotype of a species. Some individuals left or right of the mean will possess a phenotype more

suited to the new conditions in the environment.

Directional selection therefore results in phenotypes at one extreme of the population being selected for and those at the other extreme being selected against.

o Stabilising selection: selection may favour average individuals. This preserves characteristics of a population.

If environmental conditions stay stable, the individuals with phenotypes closest to the mean are favoured. This tends to eliminate phenotypes at the extremes.

Speciationo Speciation is the evolution of a new species from existing species. A species is a

group of individuals that share similar genes and are capable of breeding with one another to produce fertile offspring. In other words they belong to the same gene pool.

o Geographical isolation Occurs when a physical barrier prevents two populations from breeding with

one another. E.g. oceans, rivers, mountains and deserts.

The individuals of the species form a single gene pool and freely interbreed within the population.

Climate changes over the centuries lead to drier conditions which reduce the area of forest and separate into 2 regions that are many hundreds of kilometres apart.

Further climate changes cause forest region A to become much colder and wetter and the other forest region B to become warmer and drier.

In region A phenotypes are selected that are better able to survive in colder and wetter conditions.

And vice versa for region B The type and frequency of the alleles in the gene pools of each group of

species X become increasingly different. In time, the differences between the two gene pools become so great that

they are in effect separate species. If they were to reunite – they would be unable to breed and produce fertile

offspring and are in fact separate species.

Pedigrees

And sex linkage – determined by recessive alleles on the X chromosome.

Much more common in males cause they only need one recessive whereas women need double recessive.

Affected males inherit from the mother. Affected female inherit from both parents. Females can be carriers of the disease.

5 mark question:

Changes in ecosystems can lead to speciation; in southern California 10000 years ago a number of interconnecting lakes contained a single species of pupfish. Increasing temperatures caused evaporation and the formation of separate, smaller lakes and streams. This led to the formation of a number of different species of pupfish. Explain how these different species evolved.

Geographical isolation Separate gene pools/no interbreeding between the two populations. Different environmental/abiotic/biotic conditions/ which therefore mean that there are

different selection pressures. Selection for different or advantageous features/characteristics/specific alleles. Differential reproductive success – the selected organisms survive and reproduce. This leads to a changes in the allele frequency Which occurs over a long period of time.