The theory of natural selection was posited by Charles Darwin (and also Alfred Wallace) who described it as ‘survival of the fittest’. According to this theory, it is not necessarily the strongest or most intelligent that survives, but the ones most responsive to change. Within a species, different individuals of that species show genetic variation.
Individuals that are best suited for their environment will survive and reproduce. If there was no variation within a species, then all individuals would be the same and no individual would be favoured over the other and natural selection would not take place.
Natural selection requires variation among members of a species in order to differentiate survival (variation needed for selection)
Individuals that are best suited for their environment will survive and reproduce. If there was no variation within a species, then all individuals would be the same and no individual would be favoured over the other and natural selection would not take place.
Natural selection requires variation among members of a species in order to differentiate survival (variation needed for selection)
- This variation can manifest as either discontinuous (distinct classes) or continuous (range across a characteristic spectrum)
Understanding:
5.2.U2: Mutation, meiosis and sexual reproduction cause variation between individuals in a species.
Objective:
5.2.U2: Mutation, meiosis and sexual reproduction cause variation between individuals in a species.
Objective:
- List sources of genetic variation.
There are three main mechanisms by which genetic variation between individuals in a species may occur:
Mutations
A gene mutation is a change in the nucleotide sequence of a section of DNA coding for a specific trait
Gene mutations can be beneficial, detrimental or neutral
- 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
As a consequence of this recombination, all four chromatids that comprise the bivalent will be genetically different
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
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
As meiosis results in genetically distinct gametes, random fertilisation by egg and sperm will always generate different zygotes
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