15. Natural selection and extinction GapFill

Natural selection

When a species of organism has plenty of everything it needs in its habitat and not too many predators or diseases, most members of the species will survive and  hunteathibernatediereproducelive.

Other times, only some members of a species live long enough to pass on their genes, and these are the ones who are most adapted to survive in that environment.

This is called natural selection. It is sometimes known as the  life cycledivine plansurvival of the biggestmeaning of lifecircle of lifesurvival of the fittest.

Natural selection can bring about changes to a species, which is called evolution. It works like this:

• In any species there is a variation in  allelescolourmineralsheightstructureshape, leading to a variation in characteristics.
• Predators, diseases and   competitionsurvivalinterdependencefood sourcesfossilisationadaptation mean that many individuals die.
• Individuals with characteristics most suited to the environment they live in are more likely to survive and breed successfully. They will pass on their alleles to the next generation.

Most alleles are inherited, and a few new alleles are formed by changes in a gene called  mutationschromosomesabnormalitiesvarietiesalterationsdiversions. 

If a characteristic which is a result of one of these changes happens to be beneficial, because it increases the organism’s chances of survival, then individuals with the alleles for this characteristic are more likely to  survive and breedbe mutantssurvive but be infertilekill the othersdie outlive forever.

The new characteristic will then be passed down through the generations.

If the new characteristic gives a much better chance of survival, then eventually most of the surviving population will have it, and the species will have changed permanently, or evolved. 

Over millions of years, this process produces not just changes within species, but entirely new species as well. 

The theory of evolution by natural selection states that all species of living things which exist today, and many more which are now extinct, evolved from  amoebassimple life formsgermsfungianimalsplants which first developed between 3 and 4   trillionbilliondozenmillionhundredthousand years ago.

A scientist called Charles  CrickMendelLamarckRussel WallaceWatsonDarwin proposed this theory in 1858 after studying hundreds of animal and plant species. The theory is supported by evidence from fossils, and by the rapid changes that we have seen in microorganisms such as antibiotic-resistant bacteria. Advances in the science of  geologychemistryphysicsarchaeologybotanygenetics mean that scientists can compare DNA from different organisms to look for similarities.