Evolution and the Evidence for Evolution

Evolution

Definition and Overview

Evolution is the process by which living species are thought to have descended from ancestral species, resulting in the diversity of life observed today. It explains both the unity and diversity of organisms.

• Descent with modification: Living species are descendants of ancestral species, with changes accumulating over generations.

• Microevolution: Evolutionary change in the genetic composition of a population.

• Macroevolution: Evolutionary change above the species level, such as the emergence of new species.

Darwin’s Influences

Historical Context and Pre-Darwinian Ideas

Before Darwin, most people believed in the fixity of species, often based on religious doctrine. However, several scientists began to challenge these views:

• James Hutton: Proposed the principle of gradualism, suggesting that Earth's geological features are the result of slow, continuous processes such as valleys formed by rivers.

• Georges Cuvier: Observed that older fossils were different from current life forms and noted that some species disappeared while others appeared in different strata, supporting the idea of extinction and change over time.

• Jean-Baptiste de Lamarck: Compared living species to fossils and proposed that species evolve through the use and disuse of body parts and inheritance of acquired characteristics. (This mechanism was later disproven, but he was among the first to suggest evolution.)

• Charles Lyell: Proposed that geological processes are still operating today as in the past, influencing Darwin’s thinking about gradual change over time.

• Thomas Malthus: Argued that human populations grow faster than resources, leading to competition and struggle for existence.

Additional info: These early ideas set the stage for Darwin’s theory by introducing the concepts of gradual change and the possibility of species evolving over time.

Charles Darwin

Voyage and Observations

Darwin traveled on the HMS Beagle to South America and the Galápagos Islands, where he observed a variety of adaptations among plants and animals.

• He noted that birds on the Galápagos Islands resembled those on the mainland but had unique features adapted to their specific environments.

Darwin’s Conclusions

• All species share a common ancestor, but species can change due to different environmental pressures (descent with modification).

• Adaptations are due to natural selection, where individuals with advantageous traits survive and reproduce at higher rates.

• Neo-Darwinism: Modern synthesis of Darwinian evolution, incorporating Mendelian genetics and random mutations as sources of genetic variation.

Mechanisms of Natural Selection

How Natural Selection Operates

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

• Struggle for existence: Individuals compete for limited resources, and only some survive to reproduce.

• Genetic variation: Individuals in a population vary in their traits, some of which are heritable.

• Differential reproductive success: Individuals with advantageous traits are more likely to survive and reproduce, passing those traits to the next generation.

Genetic Variation

• Organisms can reproduce sexually or asexually, leading to genetic variation.

• In humans, there are two major types of cells: somatic cells (non-sex cells) and gametes (sex cells).

• Humans have 23 pairs of chromosomes (n = 23), with one set from each parent.

• Genes are segments of DNA that code for proteins, and different versions of a gene are called alleles.

• Genotype: The genetic makeup of an organism.

• Phenotype: The observable characteristics of an organism.

• For sexual reproduction, gametes are created through meiosis, resulting in unique combinations of alleles.

Evidence of Evolution

1. Fossils

• Fossils are found in sedimentary rock and provide a record of past life forms.

• Older fossils are found in deeper layers, while newer fossils are found closer to the surface.

• Fossil records are often incomplete, but they show a progression of life forms over time.

• Dating of fossils can be done using relative dating (position in rock layers) or absolute dating (radioactive isotopes).

2. Direct Observation of Evolution

• Examples include the evolution of soapberry bugs in response to introduced plants and the development of antibiotic resistance in bacteria such as Staphylococcus aureus.

3. Homologous Structures

• Homologous structures are anatomical features that are similar due to shared ancestry, even if they serve different functions.

• Comparative anatomy shows similarities in the bone structure of mammals, birds, and other vertebrates.

• Vestigial structures are remnants of features that served a function in ancestors (e.g., human tailbone).

• Molecular biology reveals that more closely related organisms have more similar DNA and proteins.

4. Biogeography

• Species tend to be more closely related to species from the same area than to species from different areas.

• Continental drift and plate tectonics have led to the isolation of populations, resulting in unique species on islands and continents.

• Islands often have endemic species not found elsewhere.

• Islands with similar environments in different parts of the world are populated by species related to those on the nearest mainland, not by similar species from other islands.

Summary Table: Types of Evolutionary Evidence

Type of Evidence	Description	Example
Fossils	Remains or traces of ancient organisms preserved in rock	Transitional fossils showing evolution of whales from land mammals
Direct Observation	Observable evolutionary changes in living populations	Antibiotic resistance in bacteria
Homologous Structures	Similar anatomical features due to shared ancestry	Forelimb bones in humans, cats, whales, and bats
Biogeography	Geographic distribution of species	Unique species of finches on the Galápagos Islands

Key Terms and Definitions

• Natural Selection: The process by which individuals with advantageous traits survive and reproduce more successfully.

• Adaptation: A heritable trait that increases an organism’s fitness in a particular environment.

• Homologous Structures: Anatomical features that are similar due to shared ancestry.

• Analogous Structures: Features that are similar in function but not in structure or ancestry (e.g., wings of bats and insects).

• Endemic Species: Species found only in a specific geographic area.

Important Equations

• Hardy-Weinberg Equation: Describes genetic equilibrium in a population. $p^2 + 2pq + q^2 = 1$ Where:

  • $p$ = frequency of dominant allele

  • $q$ = frequency of recessive allele