Speciation and Evolution Unit 3.3 Year 10 Biology

Speciation Definition: Members of the SAME species can interbreed to produce fertile offspring in their natural surroundings. So how do organisms become a species? Sometimes, members of a species can become isolated from each other for long enough to GENTICALLY change so much that they become a new species. i.e. they can no longer interbreed. When this happens we say SPECIATION has occurred. See Table 27.2 (page 557) for a list of reproductive isolating mechanisms.

Process of Speciation Allopatric speciation occurs when a geographic barrier isolates two sub-populations from each other; when the barrier is removed, the two groups are no longer able to reproduce.

Process of Speciation Sympatric speciation occurs when a single population suddenly becomes two reproductively isolated groups without geographic separation.

Allopatric speciation

Allopatric speciation Divergence occurs in geographic isolation

Divergence occurs despite lack of geographic isolation Sympatric speciation Divergence occurs despite lack of geographic isolation

Speciation in steps 1 - VARIATION! There must be variation in a population or speciation cannot occur Natural selection is involved and can only act on variation in the population 2 – ISOLATION Preventing interbreeding of two separate populations of the same species. This limits gene flow between the two populations. Done by a physical barrier (GEOGRAPHICAL, CLIMATIC) or a reproductive barrier (SOCIO CULTURAL) 3 – SELECTION Natural selection affects the separated populations and causes changes in the genotypes. This will prevent the groups interbreeding to produce fertile offspring, should the two groups meet again.

What prevents breeding? Courtship displays Breeding seasons Sterility Chemical barriers

What is the Evidence? Fossil evidence Transitional forms like Archaeopteryx sp and the lobe finned fish show how species have evolved to increase complexity in the species. Comparative anatomy Homologous structures show a divergence from a common ancestor. Comparative embryology Embryos with similar anatomy share some of the same genes for development of the embryo DNA Species that have similar structures have many common genes Protein structure Proteins have similar sequences of Amino Acids

WHICH OF THESE IS HUMAN? There are various animals here. Pig, chicken, fish, axolotl, tortoise, dog and one other The difficulty is to be able to recognise which is which. All the embryos in this first line are similar. this indicates that they have similar backgrounds. Notice that they all have pharyngeal gill slits, yet only two are truly aquatic when fully developed. Once again this reinforces the idea that evolution has taken place, with all these organisms having a history in water.