3: Evolution

Key Concepts

Key concepts pertaining to natural selection.

• Variation: Variation refers to the diversity of traits within a population. Individuals within a species can exhibit differences in physical characteristics, behaviors, genetic makeup, and so on.
• Inheritance: Varying traits are inherited. They are passed from one generation to another. Today we know this inheritance happens through DNA. There are DNA copy errors that result in random mutations. Some mutations are better suited for survival (depending on the environment) compared to other random mutations.
• Differential Reproductive Success: Individuals with advantageous traits are more likely to survive and reproduce, passing those traits to their offspring, while less advantageous traits may diminish over time.
• Adaptation: Organisms change over time in response to their environment, leading to traits that enhance survival and reproduction.

Human Adaptations

We will study modern human biology more closely in a later chapter, but for now we can explore two examples of modern human adaptation to the environment.

Molecules of Life

All organisms are made up of four essential molecules: proteins, lipids, carbohydrates, and nucleic acids. Proteins shape cells and perform most cellular tasks, from receiving signals to initiating responses. Lipids, including fats, oils, and hormones, form the cell membrane’s phospholipid bilayer. Carbohydrates, such as glucose, provide energy for cells. Nucleic acids, like DNA, carry an organism's genetic information.

Characteristics of each molecule:

• Proteins
  • Composed of long chains of amino acids and often folded into complex 3D shapes related to their functions.
  • Interact with other proteins and molecules.
  • Categories include:
    • Structural proteins (e.g., collagen, keratin, lactase, hemoglobin, cell membrane proteins)
    • Defense proteins (e.g., antibodies)
    • Enzymes (e.g., lactase)
    • Hormones (e.g., insulin)
    • Motor proteins (e.g., actin)
• Lipids
  • Insoluble in water, with a hydrophilic head and a hydrophobic tail.
  • Includes fats, like triglycerides, which store energy.
  • Steroid hormones (e.g., estrogen, testosterone) act as chemical messengers, aiding in cellular communication and pathways.
• Carbohydrates
  • Large group of organic molecules made of carbon and hydrogen.
  • Starches and sugars (e.g., blood glucose) provide energy to cells.
• Nucleic Acids
  • Carry the genetic information of an organism.
  • Examples include deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

Cells

In 1665, Robert Hooke observed plant cork under a microscope, noting structures resembling “cella,” or “small rooms” in Latin. Nearly 200 years later, biologists recognized the cell as the basic unit of life, with all organisms composed of either prokaryotic cells, which lack a nucleus, or eukaryotic cells, which contain a nucleus.

Prokaryotes, such as bacteria and archaea, are single-celled and lack membrane-bound DNA or organelles—specialized cell structures that perform distinct functions. While some bacteria, like E. coli and Salmonella, can cause illness, others in the human microbiome aid in digestion, immunity, and vitamin production.

Eukaryotes, in contrast, have membrane-bound DNA and organelles and can be single- or multi-celled. Examples include the single-celled phytoplankton, which produces oxygen, and multicellular organisms like plants and animals. Plant and animal cells share similarities but differ in some structures, like the cellulose-based cell wall in plants, which helps prevent water loss, while animals have diverse tissues including skin, cartilage, and brain.

Animal Cell Organelles

An animal cell is enclosed by a phospholipid bilayer—a double membrane made of lipids and proteins that regulates molecule and ion passage. Inside, the jelly-like cytoplasm houses organelles, specialized structures like the nucleus, which contains DNA, and mitochondria, which generate energy (ATP) for the cell. Multicellular eukaryotes can have numerous mitochondria per cell, each with its own DNA (mtDNA), as mitochondria evolved from symbiotic bacteria. Proper organelle function is essential for cell health, and dysfunction can lead to disease.

• Centrioles
  • Assist with organizing mitotic spindles, which extend and contract to facilitate cellular movement during mitosis and meiosis.
• Cytoplasm
  • Gelatinous fluid inside the cell membrane that contains organelles.
• Endoplasmic Reticulum (ER)
  • Continuous with the nucleus, this membrane network helps transport, synthesize, modify, and fold proteins.
  • Rough ER has embedded ribosomes, while Smooth ER lacks ribosomes.
• Golgi Body
  • Layers of flattened sacs that receive messages from the ER to process, secrete, and transport proteins within the cell.
• Lysosome
  • Located in the cytoplasm; contains enzymes to break down cellular components.
• Microtubule
  • Involved in cellular movement, including intracellular transport and cell division.
• Mitochondrion
  • Responsible for cellular respiration, producing energy by converting nutrients into ATP.
• Nucleolus
  • Located inside the nucleus, it is the site of ribosomal RNA (rRNA) transcription, processing, and assembly.
• Nucleopore
  • Selectively permeable pores in the nuclear envelope.
• Nucleus
  • Contains the cell's DNA and is surrounded by the nuclear envelope, a double membrane with pores that regulate molecule exchange with the cytoplasm.
• Ribosome
  • Located in the cytoplasm and on the rough ER, where messenger RNA (mRNA) binds to synthesize proteins.

DNA Structure

The 1953 discovery of the molecular structure of deoxyribonucleic acid (DNA) is considered one of the greatest scientific achievements. Rosalind Franklin used X-ray crystallography to produce an image that revealed DNA's double helix shape. Despite her significant contribution, Franklin received less recognition than her colleagues, James Watson, Francis Crick, and Maurice Wilkins, who were awarded the Nobel Prize in 1962 for their biochemical model of DNA. Franklin passed away in 1958, but her contributions now are widely acknowledged.

DNA can be described as a twisted ladder, consisting of two antiparallel strands made of nucleotides with a sugar-phosphate backbone. There are four types of DNA nucleotides: adenine (A), thymine (T), cytosine (C), and guanine (G). The strands are held together by base pairs, following complementary pairing rules: A bonds with T, and C bonds with G. These bonds are formed by weak hydrogen interactions, allowing the strands to separate easily. A DNA sequence refers to the order of nucleotide bases along one strand. For example, if one strand has the sequence CATGCT, the complementary strand will be GTACGA. In total, human cells contain approximately three billion DNA base pairs.

Directional Natural Selection

Directional natural selection favors individuals with traits at one extreme of the range, leading to a shift in the average phenotype of a population over time. Prior to the Industrial Revolution, light, speckled moths predominated while darker moths and lighter moths were on the extremes. During the industrial revolution, the dark moths became more common because they had the advantage of blending in with buildings covered in soot from factory smoke. They had an easier time hiding from predators (Cook & Saccheri, 2013).

Stabilizing Natural Selection

Whereas directional natural selection favored traits on an extreme, stabilizing natural selection favors traits in the middle. This reduces the extremes and maintains the average phenotype, promoting stability within a population. A good example here is birthweight. When human babies are born, complications can occur if babies are too small or too big. The neonatal mortality rate is lowest for babies in the middle of the spectrum (Jeon et al., 2019).

Disruptive Natural Selection

Disruptive natural selection favors individuals with extreme traits at both ends of the range. This potentially leads to the formation of distinct phenotypic groups within a population. In Chinook salmon, disruptive natural selection helps maintain two successful male reproductive strategies: large dominant “hooknose” males that compete directly for mates and smaller “sneaker” “jack” males that can get closer to the eggs and produce higher-quality sperm, allowing both forms to persist evolutionarily (Young et al., 2013).

To review, directional natural selection favors one extreme, stabilizing natural selection favors the middle between extremes, and disruptive natural selection favors both extremes.

Character Displacement

Character displacement occurs when closely related species sharing the same environment and ecological roles evolve differences to reduce competition for resources. There are multiple species of Galapagos finches. Each of these finch species varies in beak size and depth, which enables their coexistence due to their consumption of specific seeds to match their beak characteristics. Finches with larger, stronger beaks consume tougher, larger seeds, while those with smaller beaks feed on softer, smaller seeds. In areas where species have less overlap and competition for resources, the features that differentiate them are less exaggerated (Grant & Grant, 2006).

Gene Flow

Gene flow occurs with the transfer of genetic material between populations, enhancing genetic diversity and countering the effects of genetic isolation. Gene flows occur during migrations and when different groups of the same species come into contact with one another.

Genetic Drift in Small Populations

Genetic drift in small populations occurs when chance events lead to changes in allele frequencies, which potentially results in the loss of genetic diversity over generations. Chance events can be related to the bottleneck effect and the founder effect. The bottleneck effect occurs when a sudden reduction in population size due to a catastrophic event, disease outbreak, or natural disaster leads to a random loss of diversity. According to Gibbons (1993), the eruption of Mt. Toba about 75,000 years ago killed off most people leaving only a few thousand. Since all humans today are descended from these same few thousand individuals, we don't have much genetic diversity compared to other animals. While some anthropologists entertained this idea, later genetic and archaeological evidence suggested that the human population decline associated with the Toba supereruption may not have been as severe or globally devastating as originally proposed (Ambrose, 2003).

The founder effect occurs when a small group of individuals establishes a new population. The genetic makeup of the founders becomes the basis for the new population's genetic diversity.

Sexual Selection

Traits that enhance an individual's mating success often lead to the development of striking and diverse characteristics. Examples include peacocks' vibrant tail feathers, elk's impressive antlers, and lions' manes.

Convergent Evolution

Convergent evolution is the process by which unrelated species independently evolve similar traits or adaptations in response to comparable environmental pressures or ecological roles. This occurs not because the species share a recent common ancestor with those traits, but because natural selection favors similar solutions to similar challenges. For example, penguins in the Southern Hemisphere and the now-extinct great auks of the Northern Hemisphere both developed streamlined bodies, short wings adapted for swimming, and similar coloration. Although these birds are not closely related, both evolved similar features for efficient movement through cold ocean waters.