Variation Learning Objectives: · Define the term variation. ·   Discuss the fact that variation occurs within as well as between species. ·   Describe the differences between continuous and discontinuous variation, using examples of a range of characteristics found in plants, animals and micro-organisms. Explain both genetic and environmental causes of variation.

Causes of Variation GENE MUTATION INDEPENDENT ASSORTMENT CROSSING OVER Cause of variation How variation is produced An error occurs during replication of the DNA in a gene. A change in the order of base alters the amino acid sequence in the protein coded for by the gene. During meiosis maternal and paternal chromosomes are reshuffled. The chromosomes and therefore alleles of genes can combine in new ways. During metaphase 1 sections of chromatids in the bivalents are interchanged. Blocks of genes are moved and linked alleles may separate and rejoin in new combinations. During cell division sections of chromosomes are displaced, e.g. during anaphase. This can result in genes being deleted, duplicated or inversion of a sequence. Each parent is genetically different and can produce huge numbers of gametes. Which gametes fuse at fertilisation is a matter of chance. The expression of genes may be affected by diet, disease or temperature during development. Mutagens may cause gene mutations in somatic cells. Causes of Variation GENE MUTATION INDEPENDENT ASSORTMENT CROSSING OVER CHROMOSOME MUTATION RANDOM FERTILISATION ENVIRONMENTAL FACTORS

Variation Variation is the existence of variety and differences within and between species On the surface organisms may look identical but on closer inspection show a great deal of variation in their genetics, behaviour and lifestyles

Types of Variation - Continuous The full range of variables between 2 extreme values Examples in nature include : height, weight, length, size Data is normally presented in a Normal Distribution bell-shaped curve

Types of Variation - Discontinuous Also known as Discrete variables Variables are assigned to clearly defined categories or distinct groups Examples include : sex, blood groups, colour Data usually presented as a bar graph

Variation within species This relies upon mutations within a species This variation is acted upon by selection pressures Only the fittest individuals will survive to breed This is the way that Evolution via Natural Selection operates The Beetle Game

Genetic Variation Every person on Earth has their own unique genetic make-up (unless they are an identical twin) Most of our 25,000 genes are controlled by more than 1 allele A few traits are determined by a single allele e.g. sex, eye colour, blood group, inherited diseases Caused by miscopying of the DNA of gametes resulting in changes in (or loss of) proteins

Environmental Variation Most variables are affected by the environment Skin colour, weight, height, dialects, accents etc are all influenced by this There is a recognised relationship between genes & environment Most phenotypic traits are affected by this

Advantages of Variation Scenario: Imagine a small island on which just shrubs and grasses grow. A group of small rodents reaches the island, perhaps floating to it on tree trunks blown out to sea by a storm. The rodents feed on the seeds of the grasses and shrubs. The climate of the island is somewhat colder than the mainland from which the rodents originated. What would happen to the size of the rodent population during the first few years?

With no predators and plenty of food the rodents do well With no predators and plenty of food the rodents do well. There is however, a limit to the number of rodents that can survive on the island. Many of the young rodents die before they can breed. What might cause the rodents to die?

Some might die from sheer bad luck, others may inherit a pair of recessive genes that cause a fatal disorder. As a result of continuous variation some will be smaller than others, some will have thicker layers of fat under their skin, some will have longer fur, some may be better at finding seeds, others may be better at reaching seeds high on the shrubs. Which of these features will help the rodents to compete more successfully for food? Which features help them to survive the colder climate?

The rodents have two alleles for a hair length gene - long (H) and short (h). On the mainland the two phenotypes were present in equal numbers in the population. The longer hair length gives better protection from the cold so the shorter phenotype becomes less favourable. What will happen to the rodents with genes that favour survival? What happens in the population to the frequency of alleles that favour survival?

It seems a reasonable prediction that rodents with genes that help them to survive are more likely to be successful. They are therefore more likely to breed and pass on these genes to their offspring. Alleles of genes for unfavourable features can be expected to disappear from the population.

In practice however, things change In practice however, things change. The climate may warm up or get colder. New predators may reach the island. Other species which compete with rodents for food might reach the island. The plant population may change as a result of the rats eating them