Patterns of Evolution Year 12 Biology Revision of

Patterns of Evolution Year 12 Biology

Revision of Yr 12 Work o Darwin’s Theory o Sources of Variation o Agents that change gene frequencies

Darwin’s Theories o Believed that species evolved from ancient forms o Species did not remain unchanged throughout time

Darwin’s theories o REASONING n Species have high reproductive rates but selection pressures act to create a struggle for existence n There is variation in offspring, some better adapted than other. n Those best adapted will survive longer and therefore have an increased chance of passing on their favourable characteristics n Fittest will survive n Natural Selection

Sources of Variation o Three main sources of variation n Crossing over Sexual reproduction/meiosis Mutations o The actions of these sources create variation by ‘mixing’ alleles into new combination or creating new alleles

Agents that change allele frequencies o Genetic Drift o Mutations o Gene flow o Founder Effect o Natural Selection

Genetic Drift o A change in allele frequencies of a population as a result of chance – random events o This happens in small populations where chance alone may play a considerable role o Bottleneck and founder effects are two situations where genetic drift can be significant.

Mutations o The ultimate source of variation o Can change equilibrium in populations by adding or taking away alleles therefore changing possible combinations o Also provides new material for Natural Selection

Gene Flow o Immigration/emigration o Introduce (or take away) alleles into a gene pool o When immigrants arrive from another population possessing a different gene pool, new alleles are introduced. o Can both increase or decrease genetic variation

Natural Selection o Sorts genetic variability and accumulates and maintains favourable genotypes in a population o Reduces genetic diversity within the gene pool but increases differences between populations o There are three types of Natural Selection n Stabilising, directional and disruptive

Types of Selection o Stabilising selection - favours the average o Directional selection – one extreme favoured over the other and average o Disruptive selection – favours both extremes