Population Genetics Chapter 13 – Part 1

Variation Variation means differences between individuals within a species or population. A population is considered to be all the organisms of the same species found within a specific geographic region at a particular time. A group of organisms consisting of similar individuals capable of exchanging genes or interbreeding and producing viable offspring.

Types of Variation There are many different types of variation that can exist within a population, including: Structural Biochemical Physiological Behavioural Developmental Geographic

Structural Variation Structural variation is seen in many different species. For example: the picture on the right shows four types of Galapagos finches that Charles Darwin studied. Each of these finches have a distinct beak shape and come from different islands. These are structural variants.

Biochemical Variation An example of biochemical variation is in the colouring of fur in cats. Colouring is determined by whether a pigment is produced or not. If a pigment is not produced, then the cat will be white. However if a pigment is produced then there will be colour present. The production of pigment is by specific biochemical pathways involving enzymes.

Physiological Variation Physiological variations may include any of the following: Ability to taste chemicals Ability to detect odours Distinguishing between colours Differences in hormones

Behavioural Variation Behavioural variations are differences in the behaviours expressed by the members of a species. For example: there has been research into the behaviours of various populations of chimpanzees. Researchers have found distinctly different behaviours in each of the populations that they studied. This was especially seen in their nesting behaviours.

Developmental Variation Developmental variations refer to the changes between young immature members of a population/species and the older mature individuals.

Geographic Variation Geographic barriers can stop members of a species from moving in between regions. This isolation can cause some species to differentiate over time. Isolation may be caused by: Mountains Rivers Oceans Human development E.g.: Chimpanzees and Bonobo Chimpanzees live in the Congo Basin in Central Africa. Separation between these species occurred about one million years ago by the formation of the Congo River.

Monomorphic vs Polymorphic Traits within a populations may be monomorphic or polymorphic. Monomorphic refers to traits were there is no variation in a population. E.g.: the colouring on galahs. Polymorphic refers to traits which have two or more variants. E.g.: human eye colour, height, etc.

Discontinuous or Continuous Variation Discontinuous variation refers to traits which are discrete. E.g.: a person may be A, B, AB or O blood type Continuous variation refers to traits which occur on a continuum. E.g.: a person’s height, or their skin colour.

Discontinuous Variation Blood TypePercentage in Australia A38 B10 O49 AB3 (Red Cross Australia, 2012)

Continuous or Discontinuous Variation Discontinuous variation refers to traits which are discrete. E.g.: a person may be A, B, AB or O blood type Continuous variation refers to traits which occur on a continuum. E.g.: a person’s height, or their skin colour. Continuous Variation

Causes of Phenotypic Variation There are many causes of phenotypic variation. Some of these include: Genetic factors including mutation Environmental factors (including food sources and predators)

Causes of Phenotypic Variation Phenotypic Variation = Genetic Variation + Environmental Variation V P = V G + V E

Causes of Phenotypic Variation Phenotypic variation may be caused by many different factors, not just genetics. The environment can have a large impact on whether certain traits are displayed or not and to what degree.

The External Environment The external environment can have an impact on the phenotype of individuals in a population. E.g.: the flower colour of hydrangeas is determined by the acidity of the soil they are growing in.

The Internal Environment The internal environment of organisms can also cause variation. This may involve hormones which causes distinct variation seen between the sexes.

Inherited Factors Cause Variation The most obvious causes of variation are those traits that are inherited from the parents of an individual (i.e. genetic). These inherited traits distinguish individuals from others, as they have a distinct combination of traits.

Variation Due to One Gene (Monogenic) Monogenic inheritance refers to the case where only one gene determines a phenotype, usually with two alleles, one being dominant and the other recessive. E.g.: ABO Blood Groups

Variation Due to Polygenes Polygenic inheritance refers to characteristics that are determined by the interaction of genes at several different loci This can be on different chromosomes or at different places on a single chromosome. Polygenic traits usually show continuous variation. E.g.: height, eye colour

Mutations: Source of New Genetic Variation Gene mutation allows the introduction of new alleles into a population. This means that the population will have increased genetic variation.

Genes in Populations

Gene Pool The entire genetic information found in a population is called the gene pool.

Allele Frequencies in a Gene Pool The proportion of alleles in a gene pool is referred to as the ‘allele frequency’. Allele frequencies tend to stay the same within a population. Over time allele frequencies may change depending on other factors, including environmental influences, selection, migration and chance events. The allele frequencies are: p = most common allele, and q = least common allele. The allele frequencies add up to 1.0

Selection Survival of the Fittest Individuals with an advantageous phenotype tend to be more successful than their peers reproductively. This means that they contribute more offspring to the next generation than others. When these traits have a genetic basis, selection can increase the prevalence of those traits, because offspring will inherit those traits from their parents.

Selection Acts on Phenotypes Selection acts on phenotypes. This is because the phenotype is what an individual expresses. If the traits they are displaying are advantageous, then they are selected for.

Genetic Fitness Varies in Different Environments A phenotype which may be advantageous in one environment may not be as advantageous in another. An example from your text book discusses individuals that carry the sickle-cell anaemia allele in malaria prone regions of Africa compared with North America.