1. Sources of Genetic Variation

2. Two important criteria for evolution to proceed include:
Heritable genetic variation
Continuous origin of new genetic variation

3. Heritable Genetic Variation and an Acceptable Theory of Inheritance
Blending Theory
Heredity “stuff” of parents blend together to produce characteristics observed in the offspring
Particulate Theory
Heredity factors from the parents (=genes) will remain unchanged in the hybrid

4. Blending versus Mendelian Heredity

5. Blending versus Mendelian Heredity

6. Sources of Genetic Variation
I. Point Mutations
A. Base pair substitutions
B. Insertions and deletions
II. Chromosomal Mutations
A. Changes in the structure of a chromosome
1. Deletions
2. Duplications
3. Inversions
4. Translocations
B. Changes in the number of chromosomes
1. Centric fusion
2. Centric fission
3. Aneuploidy
4. Polyploidy: . Autopolyploidy and Allopolyploidy

7. Point Mutations

8. Chromosomal Mutations: Change in Structure

9. Aneuploidy
One or more chromosomes of a normal set are lacking or present in excess
Due to meiotic nondisjunction - members of pairs of homologous chromosomes do not separate during meiosis I or sister chromatids fail to separate during anaphase of meiosis II

10. POLYPLOIDY - the duplication of chromosome sets such that individuals have more than 2 of each chromosome.
DIPLOIDY is the normal state (2 of each chromosome, 2N), but some animals are TRIPLOID (3N) and TETRAPLOID (4N).
There are 2 types of polyploidy: autopolyploidy and allopolyploidy

11. Autopolyploidy
The multiplication of chromosomes sets within a species
For example, a failure of meiosis during gamete production can double chromosome number from the diploid count (2N) to a tetraploid number
The tetraploids formed can mate with themselves (self-pollinate) or with other tetraploids

12. Allopolyploidy
It refers to the contribution of two different species to the polyploid hybrid
It begins with 2 different species interbreeding and combining their chromosomes
Interspecific hybrids are usually sterile because the haploid set of chromosomes from one species cannot pair during meiosis with the haploid set from the other species

13. Formation of an Allopolyploid
During the history of the clone, a mitotic error affecting the reproductive tissue occurs, doubling chromosome number
The hybrid will be able to make gametes because each chromosome has a homologue with which to synapse during meiosis
The union of gametes from this hybrid may give rise to a new species of interbreeding plants, reproductively isolated from both parent species

14. Rates of Mutation
Most mutations are either neutral or deleterious; few are advantageous
There are only approximately 1 X 10-5 mutations per locus/individual/generation (i.e. 1/100,000); However, when summed over all loci, over all gametes, over all individuals, the rates of mutation are rather high
Example:
Assume that there are at least 100,000 pairs of genes in humans, and that the average mutation rate/gene/generation is 10-5
The average number of mutations arising per generation would then be estimated as at least: 2 X 105 X mutations/gene = 2 mutations for a human zygote
There are about 6 X 109 humans in the world; with 2 mutations/individual, the total number of mutations newly arisen in the human population is 12 X 109
Clearly, the number of mutations in a population or species in any given generation can be quite large
Even if only a very small fraction of these were advantageous, there are clearly enough mutations to provide the raw material for evolutionary change.

15. Random versus Non-random Mutations
The probability of a mutation is not random with respect to sites within the genome: some locations and base pair sequences have very high mutation rates while others have very low mutation rates
Also, the mutation rates for organisms can be increased by exposing organisms to certain chemicals called mutagens
Mutations are, however, random with respect to the direction of adaptation
There appears to be no evidence for the notion of directed mutations

16. Maintaining Genetic Variation
Meiosis
Independent assortment
Chromosomal crossing over
Syngamy

17. Crossing Over the Chromosomal Material
If the 2 genes were not linked, the hybrid AaBb would yield 4 different gametes
A hybrid with 2 pairs of linked genes can also yield 4 different gametes due to crossing over
However, new recombinants (Ab and aB) will arise less frequently, than original combination types (AB and ab)