sources of genetic variation

Sources of Genetic Variation

Two important criteria for evolution to proceed include:

• Heritable genetic variation• Continuous origin of new genetic

variation

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

Blending versus Mendelian Heredity

Blending versus

Mendelian Heredity

Sources of Genetic VariationI. Point Mutations

A. Base pair substitutionsB. Insertions and deletions

II. Chromosomal MutationsA. Changes in the structure of a chromosome1. Deletions2. Duplications3. Inversions4. Translocations

B. Changes in the number of chromosomes1. Centric fusion

2. Centric fission3. Aneuploidy4. Polyploidy: . Autopolyploidy and Allopolyploidy

Point Mutations

Chromosomal Mutations: Change

in Structure

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

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

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

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

Formation of an Allopolyploid

• The union of gametes from this hybrid may give rise to a new species of interbreeding plants, reproductively isolated from both parent species

• 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

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 10-5 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.

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

Maintaining Genetic Variation

• Meiosis

– Independent assortment

– Chromosomal crossing over

• Syngamy

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)