IB DP Biology- A4.1 Evolution and speciation - IB Style Questions For SL Paper 2 - FA 2025

(a)
Mitosis ensures genetic identity through two key steps:
1. Replication: Before mitosis begins (during the S-phase of interphase), the cell’s DNA is replicated. This creates chromosomes, each consisting of two identical sister chromatids.
2. Separation: During anaphase of mitosis, these sister chromatids are separated and pulled to opposite poles of the cell.
This results in two genetically identical daughter cells, each receiving a complete and identical copy of the original chromosomes, maintaining the chromosome number.

(b)
Speciation is the splitting of one species into two or more species. DNA is fundamental to this process:
• Organisms of the same species share a common genome (DNA).
• Variations in DNA (alleles) arise through processes like mutation.
• These variations are passed to offspring through sexual reproduction.
• If two populations become reproductively or geographically isolated, they stop sharing DNA. Different selection pressures in their respective environments will favor different alleles, causing the allele frequencies (the DNA) in the two populations to change (evolve) independently. Over time, the DNA differences become so significant that the two populations can no longer interbreed, and they become new species.

(c)
In sexually reproducing organisms, offspring inherit traits via gametes. The genetic basis involves several key principles:
• Alleles: Organisms are diploid, meaning they have two copies (alleles) of each gene. These alleles are inherited, one from each parent.
• Dominance: Alleles can be dominant or recessive. A dominant allele will be expressed in the phenotype if present (in both homozygous and heterozygous states). A recessive phenotype is only expressed if the individual is homozygous (has two copies) of the recessive allele.
• Codominance: Sometimes, both alleles are expressed equally in the phenotype (e.g., ABO blood groups).
• Sex-Linkage: Some genes are located on the sex chromosomes (e.g., the X chromosome). This results in different inheritance patterns for males and females.
• Multiple Alleles: While an individual has two alleles, many different alleles for a single gene can exist within the population, leading to complex traits (e.g., blood groups).