Mutations, Evolution, and Genetics

Types of Mutations

Point Mutations

A change in a single nucleotide.

• Substitution: One nucleotide is incorrectly added in place of the correct complementary nucleotide.
• Insertion: One or more extra nucleotides are added to the growing DNA strand; this can lead to a frameshift.
• Deletion: One or more nucleotides are eliminated from the growing DNA strand; this can lead to a frameshift.

Chromosome Mutations

Changes in the structure or number of chromosomes.

Structural Mutations

These are due to two simultaneous breakages in nuclear chromosomes. This can cause:

• Deletion: The loss of a chromosomal segment.
• Duplication: A region of the chromosome is repeated.
• Inversion: The change in direction of the chromosomal segment.
• Translocation: A segment splits off from a chromosome and exchanges with a segment of a non-homologous chromosome.

Aneuploidies

An abnormal number of chromosomes in the karyotype.

• Nullisomy: A pair of homologous chromosomes is missing.
• Monosomy: A chromosome is missing from each homologous pair.
• Trisomy: The existence of an extra chromosome in a pair of homologous chromosomes.

Mechanisms of Evolution

Genetic Drift

Some individuals cannot reproduce due to random causes. E.g., rabbits with a specific allele are caught in a trap. Natural disasters can shrink populations and change allele frequencies.

Non-Random Mating

Most species do not mate randomly but look for specific characteristics in partners. Female birds choose males with vivid colors and beautiful songs. Humans select partners who resemble them. Fish, birds, mice, and primates select mates with different HLA genes.

Mutations

Changes in DNA genes produce new alleles.

Gene Flow

Migration in and out of a population changes allele frequencies.

Natural Selection

Nature acts on random variations, selecting the most successful (best adapted). The fittest survive and reproduce, passing on their features.

Definitions

• Evolution: Any change in heritable traits within a population, allowing organisms to change over time and give rise to new species.
• Homologous Organs: Physical features shared due to evolutionary history. Some look different externally due to adaptation to different environments (adaptive divergence). Example: Homology of vertebrate forelimb.
• Evolutionism: Advocates that all living things have a common ancestor and that present-day species come from previous ones through successive transformations.
• Fixist Theory: Defends the immutability of species, based on Greek philosophers and the Old Testament.
• mRNA: Carries DNA messages to ribosomes, which transform them into proteins.
• tRNA: Carries amino acids to the ribosome for polypeptide inclusion.
• Codon: mRNA nucleotide triplet corresponding to one amino acid.
• Anticodon: Three tRNA nucleotides complementary to the codon.