Evolution: Descent with Modification and the Foundations of Modern Evolutionary Theory

Evolution: Descent with Modification

Introduction to Evolutionary Thought

Evolution is the central unifying concept in biology, explaining both the unity and diversity of life. The modern understanding of evolution began with Charles Darwin, but the idea that species change over time predates him. Early thinkers recognized change but did not understand the mechanisms involved.

• Charles Darwin (1859): Published The Origin of Species, proposing natural selection as the primary mechanism of evolution.

• Earlier contributors: James Hutton (gradualism), Jean-Baptiste Lamarck (inheritance of acquired traits), Thomas Malthus (population limits), and others.

The History of Evolutionary Thought

Timeline of Key Events

• 1795: Hutton proposes gradual geological change.

• 1809: Lamarck publishes his hypothesis of evolution.

• 1831–1836: Darwin travels on HMS Beagle, gathering evidence.

• 1859: Darwin publishes The Origin of Species.

Biological Evolution

Descent with Modification

Biological evolution explains the similarities and differences among Earth's many species. All species share a common ancestor and accumulate differences as they adapt to different environments over generations.

• Descent with modification: Shared ancestry results in shared characteristics; accumulation of differences leads to diversity.

Three Key Observations About Life

1. Organisms are Well Suited for Their Environment

• Adaptations allow organisms to survive and reproduce in specific environments (e.g., coral and orchids).

2. Unity of Life

• Shared characteristics among diverse organisms (e.g., all Mantodea have three pairs of legs, bulging eyes, triangular heads, and flexible necks).

3. Diversity of Life

• There are approximately 2,300 species of Mantodea, illustrating how species can be different despite similarities.

Darwin's Major Ideas

Descent with Modification from Common Ancestors

• Gradual accumulation of adaptations leads to new species.

• Adaptations: Inherited characteristics that enhance survival and reproduction in specific environments.

• Example: Darwin's finches (cactus-eater, insect-eater, seed-eater) show adaptations to different diets.

Natural Selection

Mechanism of Evolution

• Natural selection acts on individual variation in inherited traits.

• Results in differential survival and reproductive success.

Darwin's Tree of Life

Branching Evolution

• History of life resembles a tree with multiple branches from a common trunk.

• Labeled branches: extant (living) organisms; unlabeled: extinct organisms.

• Forks represent most recent common ancestors.

Artificial Selection

Human Influence on Evolution

• Humans select for desirable traits in plants and animals (e.g., wild mustard bred into cabbage, broccoli, kale, etc.).

Darwin's Observations and Inferences

Key Observations

• Observation 1: Members of a population often vary in their inherited traits.

• Observation 2: All species produce more offspring than the environment can support; many fail to survive and reproduce.

Key Inferences

• Inference 1: Individuals with traits that increase survival and reproduction tend to produce more offspring.

• Inference 2: Favorable traits accumulate in the population over generations due to unequal survival and reproduction.

Modern Evolutionary Theory

Beyond Darwin

• Incorporates genetics, behavior, geology, atmospheric science, and mathematical modeling.

• Recognizes additional mechanisms of evolutionary change beyond natural selection.

Microevolution vs. Macroevolution

• Microevolution: Changes in allele frequencies within populations (natural selection, genetic drift, gene flow).

• Macroevolution: Broad patterns of evolutionary change above the species level (origins of new structures, trends, mass extinctions).

Mechanisms of Genetic Change

Sources of Genetic Variation

• Mutations: Formation of new alleles; introduces variation, usually detrimental but can spread.

• Gene duplication: Duplication of DNA segments; large duplications are harmful, small duplications can be beneficial (e.g., olfactory genes in mammals).

• Rapid reproduction: Short generation times allow mutations to accumulate quickly (e.g., Clostridium bacteria).

• Sexual reproduction: Shuffles alleles, creating new combinations.

Genetic Terminology and Population Genetics

Key Definitions

• Locus: Location of a gene on a chromosome.

• Diploid vs. Haploid: Diploid cells have two sets of chromosomes; haploid have one.

• Genotype: Genetic makeup; Phenotype: Observable traits.

• Alleles: Different versions of a gene.

• Population: All individuals of a species in a specific area.

• Gene pool: All alleles at all loci in a population.

• Allele frequency: Proportion of a specific allele in the gene pool.

Dominance

• Dominant alleles mask the effect of recessive alleles in heterozygotes.

• Example: BB (homozygous dominant), Bb (heterozygous), bb (homozygous recessive).

What is Biological Evolution?

Biological evolution is defined as a change in allelic frequencies in populations across generations.