Study Guide: Evolution, Natural Selection, and Genetics

Evolution and Natural Selection

Definition and Mechanisms of Evolution

Evolution is the process by which populations of organisms change over generations through alterations in gene frequencies. This process is driven by several mechanisms, including natural selection, genetic drift, mutation, and gene flow.

• Evolution: Change in gene frequencies within a population over time.

• Requirements for Evolution:

  • Genetic variation within the population.

  • Inheritance of traits from one generation to the next.

• Natural Selection: The process by which individuals with advantageous traits survive and reproduce more successfully, leading to an increase in those traits in the population.

• Overproduction: Most species produce more offspring than the environment can support, leading to competition for resources.

• Survival: Individuals best suited to their environment are more likely to survive and reproduce ("survival of the fittest").

Example: In a population of beetles, those with coloration that camouflages them from predators are more likely to survive and pass on their genes.

Positive vs. Negative Selection

Selection can act in different ways on genes:

• Positive Selection: Favors beneficial alleles, increasing their frequency in the population.

• Negative (Purifying) Selection: Removes harmful alleles from the population.

• Most genes are thought to be under negative selection, maintaining functional integrity by removing deleterious mutations.

Human Adaptation and Skin Color

Melanin and Geographic Variation

Melanin is a pigment in human skin that provides protection against ultraviolet (UV) radiation. Its distribution in human populations is influenced by geographic location and evolutionary pressures.

• High Melanin Levels: Favored in populations near the equator due to intense UV radiation, which can damage DNA and deplete folate.

• Low Melanin Levels: Favored in populations at higher latitudes (further from the equator) to facilitate vitamin D synthesis in conditions of lower UV exposure.

• Selection Today: For humans living near the equator, the melanin gene may still be under positive selection due to ongoing UV exposure.

• Skin Cancer and Selection: While skin cancer is a risk of low melanin, it typically affects individuals after reproductive age, so it may not be the primary driver of selection for high melanin levels.

• Inuit Skin Color: Inuit people, despite living at high latitudes, have darker skin than expected. This is likely due to a diet rich in vitamin D, reducing the selective pressure for lighter skin.

Example: People of European descent generally have lighter skin, an adaptation to lower UV environments, while people of African descent have darker skin, an adaptation to high UV environments.

Homologous Traits and Comparative Anatomy

Homologous Traits

Homologous traits are anatomical features in different species that are similar due to shared ancestry.

• Example: The forelimbs of tetrapods (vertebrates with four limbs) all share a common structure: one bone (humerus), two bones (radius and ulna), many bones (carpals), and digits (fingers/toes).

Evolutionary Trees and Relationships

Evolutionary trees (phylogenies) depict relationships among species based on shared traits and genetic data.

• Common Ancestry: Species that share a more recent common ancestor are more closely related.

• Crocodiles and Birds: According to modern phylogenies, crocodiles are more closely related to birds than to lizards, as they share a more recent common ancestor.

Genetics: Telomeres and Chromosomes

Telomeres

Telomeres are repetitive DNA sequences at the ends of chromosomes that protect them from deterioration.

• Function: Prevent loss of genetic information during cell division.

• Human Chromosome 2: Humans have a telomere sequence in the middle of chromosome 2, evidence of a fusion event between two ancestral ape chromosomes.

• Chromosome Numbers:

  • Humans: 46 chromosomes (23 pairs)

  • Chimpanzees, gorillas, orangutans: 48 chromosomes (24 pairs)

Population Genetics and Hardy-Weinberg Equilibrium

Hardy-Weinberg Principle

The Hardy-Weinberg equilibrium describes a non-evolving population where allele and genotype frequencies remain constant from generation to generation, provided certain conditions are met.

• Conditions: No mutation, random mating, no gene flow, infinite population size, and no selection.

• Equation:

$p^2 + 2pq + q^2 = 1$

where p and q are the frequencies of two alleles in the population.

• If a population is in Hardy-Weinberg equilibrium, the gene is not evolving.

• Populations, not individuals, evolve over time.

Summary Table: Chromosome Numbers in Primates

Species	Number of Chromosomes
Human	46
Chimpanzee	48
Gorilla	48
Orangutan	48

Additional info: The presence of a telomere in the middle of human chromosome 2 supports the hypothesis of a chromosomal fusion event in human evolution.