Brent Cornell

    
Natural selection requires variation among members of a species in order to differentiate survival (variation needed for selection)

• This variation can manifest as either discontinous (distinct classes) or continuous (range across a characteristic spectrum)

There are three main mechanisms by which genetic variation between individuals in a species may occur:

• Mutations – Changing the genetic composition of gametes (germline mutation) leads to changed characteristics in offspring
• Meiosis – Via either crossing over (prophase I) or independent assortment (metaphase I)
• Sexual reproduction – The combination of genetic material from two distinct sources creates new gene combinations in offspring

Mutations

A gene mutation is a change in the nucleotide sequence of a section of DNA coding for a specific trait

• New alleles are formed by mutation

Gene mutations can be beneficial, detrimental or neutral

• Beneficial mutations change the gene sequence (missense mutations) to create new variations of a trait
• Detrimental mutations truncate the gene sequence (nonsense mutations) to abrogate the normal function of a trait
• Neutral mutations have no effect on the functioning of the specific feature (silent mutations)

Variation via Mutation

Meiosis

Meiosis promotes variation by creating new gene combinations via either crossing over or independent assortment

1.  Crossing Over

Crossing over involves the exchange of segments of DNA between homologous chromosomes during prophase I

• The exchange of genetic material occurs between non-sister chromatids at points called chiasmata

As a consequence of this recombination, all four chromatids that comprise the bivalent will be genetically different

• Chromatids that consist of a combination of DNA derived from both homologous chromosomes are called recombinants
• Offspring with recombinant chromosomes will have unique gene combinations that are not present in either parent 

2.  Independent Assortment

When homologous chromosomes line up in metaphase I, their orientation towards the opposing poles is random

The orientation of each bivalent occurs independently, meaning different combinations of maternal / paternal chromosomes can be inherited when bivalents separate in anaphase I

• The total number of combinations that can occur in gametes is 2ⁿ – where n = haploid number of chromosomes
• Humans have 46 chromosomes (n = 23) and thus can produce 8,388,608 different gametes (2²³) by random orientation 
• If crossing over also occurs, the number of different gamete combinations becomes immeasurable

Sexual Reproduction

The fusion of two haploid gametes results in the formation of a diploid zygote

• This zygote can then divide by mitosis and differentiate to form a developing embryo

As meiosis results in genetically distinct gametes, random fertilisation by egg and sperm will always generate different zygotes

• This means that individual offspring will typically show variation despite shared parentage
• Identical twins are formed after fertilisation, by the complete fission of the zygote into two separate cell masses

Summary of Human Fertilisation