Genetic Variation and Evolutionary Mechanisms

Metaphase I and Independent Assortment

Independent assortment is the process where the chromosomes move randomly to separate poles during meiosis. A gamete will end up with 23 chromosomes after meiosis, but independent assortment means that each gamete will have many different combinations of chromosomes. During this phase, paired chromosomes line up along the middle of the cell.

Spermatogenesis and Oogenesis

Spermatogenesis

• Transformation of spermatogonia into spermatozoa (sperm).
• Meiosis does not begin until puberty.
• This is the production of the male gamete.

Oogenesis

• This is the production of the female gamete.
• These develop prenatally in the fetus.
• Mitosis of primordial germ cells produces oogonia.

Mechanisms of Allelic Variation

• Gene mutations: These create new alleles.
• Crossing over: Occurs at Meiosis I and results in novel combinations of alleles in chromosomes.
• Independent assortment: The assortment of chromosomes in meiosis mixes maternal and paternal chromosomes in gametes.
• Fertilization: Interaction between genetically varied parents produces "new" combinations of genes.
• Chromosomal changes: Changes in chromosome structure or number lead to the loss, duplication, or repositioning of genes.

Darwin's Postulates of Natural Selection

1. Individuals within species are variable.
2. Some of these variations are passed on to offspring.
3. In every generation, more offspring are produced than can survive.
4. The survival and reproduction of individuals are not random; those individuals who survive and go on to reproduce the most are those with the most favorable variations, and they are naturally selected.

Types of Speciation and the Founder Effect

• Sympatric speciation: Occurs when populations of a species in the same habitat become reproductively isolated from each other.
• Allopatric speciation: The most common form of speciation; occurs when populations of a species become geographically isolated. When populations become separated, gene flow between them ceases.
• Parapatric speciation: Extremely rare; occurs when populations are separated not by a geographical barrier, but by an extreme change in habitat.
• The founder effect: The loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population.