Gap filling exercise on Speciation

Gap-fill exercise

Fill in all the gaps, then press "Check" to check your answers. Use the "Hint" button to get a free letter if an answer is giving you trouble. You can also click on the "[?]" button to get a clue. Note that you will lose points if you ask for hints or clues!
A group of organisms that can together and produce offspring is called a . The members of the group are said to share a common . This means they share a limited set of which enables us to distinguish them from other such groups.
It therefore follows that for to occur a new gene pool has to be formed by the original one. Through the processes of and , the gene pools diverge until they are genetically . This is the ultimate in speciation and if individuals from the two gene pools try to cross the result will be no offspring or sterile hybrids. Even if this state has not been fully achieved the two pools may be sufficiently different so that betwen the two show fertility. In that case there will be strong selection against resulting in the evolution of and isolating mechanisms to complete the process.
The most obvious method of speciation is splitting the gene pools geographically so that between the two is extremely rare. This is called speciation. Classic examples are sinking of land bridges, formation of mountain ranges and ice ages. Darwin was aware that animals in South America looked as though they evolved in isolation due to what we now know as the sinking of the Panamanian land bridge.
However what mostly impressed him was the flora and fauna of . He noted that they showed similarities to adjacent continental species which he reasoned were the ancestors of these island species. Today we call this the . Speciation is thout to be because the founders will be few and carry only a small amount of the in the gene pool. This means that selection works in a different way on the island and causes divergence.
Speciation can also take place almost instantaneously. This seems to be true of grasses like . Two species may hybridise, but the resulting plant will be infertile unless the whole chromosome set to produce a , which is fertile. These are often very successful as they show hybrid .