Mechanisms of Evolution As knowledge expands, particularly in the field of genetics 
we know have the most convincing evidence relating to 
the mechanisms of natural selection.

Genetic Variation Geneticists study changes in the inheritable traits of 
organisms. Any trait, which distinguishes individuals from one 
another, represents GENETIC DIVERSITY. Traits are coded for on DNA in units called GENES. 
Genes are located at specific LOCI (Positions on a 
chromosome)

Most eukaryotes are DIPLOID (2 sets of 
homologous chromosomes) Genes may come in 2 or more forms or ALLELES

All individuals of the same species share a common 
GENOME (Complete set of chromosomes) However, each will have a different GENOTYPE 
(Combination of alleles at specific loci). Differences in genotype and environmental 
influences account for different PHENOTYPES 
(observable traits).

New alleles result from mutations. Mutations are changes in the genetic code of genes Can have neutral, harmful or beneficial mutations. Only mutations within sex cells can be passed on.

Variation within a species is a result of the variety and 
combinations of alleles possessed by individuals. Genetic Diversity within a POPULATION increases 
enormously through sexual reproduction as the various 
alleles from 2 parents recombine in each offspring. Imagine an individual of a species with 10000 genes 
of which 10% come in different "flavours". Sexually, this individual could produce 21000 different genetic recombinations in their gametes. That’s more atoms then there are in the universe!

Key Factors Leading to Evolution 1. When populations are small, chance fluctuations 
 can cause changes in allele frequencies. 2. When mating is nonrandom, individuals that are 
 preferred as mates will pass on more alleles 3. Mutations may cause new alleles to be created or 
 changed, causing a change in allele frequency

4. Migration will remove alleles from one pop. and add 
 them to another. 5. Natural selection causes certain alleles to be passed 
 on increasing the relative frequency of alleles in the 
 next generation.

Types of Selection Many factors influence how selection can operate on individual 
phenotypes in a population. 1. Stabilizing selection occurs when the most common phenotypes within a population are most favoured by 
the environment.

2. Directional selection occurs when the environment favours individuals with more extreme variations of a 
trait.

3. Disruptive selection favours individuals with variations at opposite extremes of a trait over individuals with 
intermediate variations.

4. Sexual selection favours the selection of any trait that influences the mating success of the individual. Examples included: female mate choice and from 
male-versus-male competition

Cumulative Selection Challenge - Read and Try Remember natural selection is not a matter of chance. Furthermore, it is a stepwise constructive process which selectively builds new functional complex systems piece by piece, often just modifying previous systems to perform new functions. Cumulative Selection Challenge - Read and Try

Evolutionary Change without Selection Sometimes changes in genetic makeup of populations are not the result of traits of individuals.

Genetic Drift Changes to allele frequency as a result of chance. Very important is small populations

a. Bottleneck Effect When a severe event results in a drastic reduction in 
numbers, a population may experience a bottleneck 
effect. When this form of genetic drift occurs, a very 
small sample of alleles survives to establish a new 
population.

b. Founder Effect Genetic drift that results when a small # of 
individuals separate form their original 
population. Amish community in Pennsylvania are all descendants of about 30 people

3. Gene Flow the movement of alleles from one 
population to another through the 
movement of individuals.

Speciation A biological species, includes the members of groups or populations that interbreed or have the ability to 
interbreed with each other under natural conditions. Species can be differentiated on a biological basis by 
identifying their reproductive isolating mechanisms

Any series of events that results in 
the reproductive isolation of two 
populations may also lead to the 
formation of new species.