Higher Biology Unit 1 1 7 Evolution Speciation

Higher Biology Unit 1: 1. 7 Evolution - Speciation

Species • A species is defined as a group of organisms with similar characteristics that are able to interbreed to produce fertile offspring • They are genetically isolated from other organisms • The numbers and kinds of species are always changing • Some will be in a stable relationship with the environment • Some are moving towards extinction • Some are undergoing speciation

Speciation • Speciation is the formation of a new species • It is brought about by evolution as a result of isolation, mutation, and selection • There are 2 types of speciation • Allopatric • Sympatric

Allopatric speciation • Allopatric speciation occurs when interbreeding is restricted by a geographical barrier such as river, sea, mountain or desert

1. Large population occupies same habitat. Interbreed freely. 2. Population becomes split into 2 by geographical barrier 3. Mutations occur at random. New variation occurs within each group 4. Natural selection affects each group in different ways favouring alleles that promote survival in each habitat 5. Natural selection continues to occur over many generation until the 2 groups are genetically distinct 6. If the barrier is removed the 2 population cannot interbreed. They are separate species

Sympatric speciation • Sympatric speciation occurs when two or more populations live in close proximity but become genetically isolated • Reproduction between them is prevented by behaviours, ecological barriers or changes in sets of chromosomes in plants (polyploidy) • Sympatric speciation is promoted by disruptive selection

1. Large population occupies same habitat. Interbreed freely 2. Alternative ecological niche becomes available 3. Some members of the population exploit new niche 4. 2 populations are formed each exploiting different niche and no longer interbreed 5. Mutations occur to allow better exploitation of different niches 6. Natural selection occurs. 2 distinct populations are formed with different genetic sequences and different niches

Polyploidy • If plants undergo a mutation that occurs in polyploidy they will no longer be able to interbreed with the plants that produced it due to differences in chromosome numbers • This plant is genetically isolated and distinct from other plants • A new species has formed “overnight”

Darwin’s finches are believed to have undergone both types of speciation. Allopatric speciation occurred when populations of birds were carried to different islands by weather conditions. The sea acted as the barrier in this case. Sympatric speciation occurred when a population of birds occupied the same island but started to exploit different ecological niches, for example the types of food eaten.

Hybrid Zones • A region may be occupied by a number of populations that vary in their ability to interbreed • The areas in which interbreeding between populations may occur are known as hybrid zones

C Hybrid zones B A D E Genes can transfer from population A to E through B, C and D. If population B, C or D were to disappear this gene flow would stop.