General Biology: Evolution, Natural Selection, and Speciation Study Guide

Evolutionary Theory and Models

Understanding Models of Evolution

Evolutionary biology uses various models to explain how species change over time. These models help us interpret patterns in nature and the mechanisms driving evolutionary change.

• Typological Model: Assumes species are fixed and unchanging types. Variation is considered unimportant.

• Aristotelian Model: Based on a hierarchical "Great Chain of Being" with species ranked by complexity.

• Lamarckian Model: Proposes that organisms can acquire traits during their lifetime and pass them to offspring (inheritance of acquired characteristics).

• Darwin-Wallace Model: Emphasizes natural selection acting on heritable variation within populations, leading to evolution.

• Example: Darwin's finches in the Galápagos Islands show variation in beak shape due to natural selection.

Fossil Record and Uniformitarianism

The fossil record provides evidence for evolution by showing changes in species over time. Uniformitarianism is the idea that geological processes observed today have operated similarly in the past.

• Relative vs. Absolute Dating: Relative dating compares fossil positions in layers; absolute dating uses radioactive decay to estimate age.

• Uniformitarianism: Supports the concept that the Earth is ancient and has changed gradually.

Lamarck's Theory vs. Darwin's Theory

Lamarck proposed that individuals change during their lifetime and pass these changes to offspring. Darwin argued that variation exists in populations and natural selection acts on this variation.

• Lamarckian Transformation: Change occurs within individuals.

• Darwinian Evolution: Change occurs across generations due to differential survival and reproduction.

• Additional info: Modern genetics supports Darwin's view; acquired traits are generally not inherited.

Key Concepts in Evolution

Wallace and Darwin

Alfred Russel Wallace independently conceived the theory of natural selection. Darwin is credited for his extensive evidence and synthesis.

• Wallace's Influence: Wallace's work prompted Darwin to publish his own findings.

• Summary of Influence: Both contributed to the acceptance of natural selection as the main mechanism of evolution.

Common Descent and Natural Selection

Common descent is the principle that all living organisms share a common ancestor. Natural selection explains how advantageous traits become more common in populations.

• Transition Feature: A trait that is intermediate between ancestral and derived forms, supporting evolutionary change.

• Phylogenetic Trees: Diagrams showing evolutionary relationships based on common descent.

• Homology: Similarity due to shared ancestry (e.g., vertebrate limb bones).

Fitness and Selection

Fitness measures an organism's ability to survive and reproduce. Natural selection increases the frequency of traits that enhance fitness.

• Fitness: Often defined as the number of offspring an individual contributes to the next generation.

• Artificial Selection: Humans select traits in domesticated species; differs from natural selection in its intentionality.

• Natural Selection: Acts on heritable variation; populations, not individuals, evolve.

• Adaptation: A trait that increases fitness in a particular environment.

Population Genetics

Hardy-Weinberg Principle

The Hardy-Weinberg equilibrium describes a non-evolving population where allele and genotype frequencies remain constant.

• Equation:

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

• p: Frequency of dominant allele

• q: Frequency of recessive allele

• Assumptions: No mutation, migration, selection, or genetic drift; random mating.

• Application: Used to predict genotype frequencies and test for evolution.

Genetic Drift and Gene Flow

Genetic drift is random change in allele frequencies, especially in small populations. Gene flow is the movement of alleles between populations.

• Genetic Drift: Can lead to loss of genetic diversity; includes bottleneck and founder effects.

• Gene Flow: Increases genetic diversity by introducing new alleles.

• Example: Drift can cause fixation of alleles; gene flow can counteract drift.

Inbreeding and Inbreeding Depression

Inbreeding increases homozygosity and can lead to inbreeding depression, a reduction in fitness due to expression of deleterious alleles.

• Non-random Mating: Changes genotype frequencies but not allele frequencies.

• Inbreeding Depression: Reduced survival and reproduction due to increased homozygosity.

Speciation

Concepts and Mechanisms

Speciation is the process by which new species arise. It can occur through various mechanisms and is central to evolutionary biology.

• Allopatric Speciation: Occurs when populations are geographically separated.

• Sympatric Speciation: Occurs without geographic separation, often via polyploidy or ecological niche differentiation.

• Reproductive Barriers: Prevent gene flow between species (prezygotic and postzygotic barriers).

• Hybridization: Can result in new species or gene flow between species.

• Meiosis Errors: Can lead to instant speciation, especially in plants.

Comparative Table: Allopatric vs. Sympatric Speciation

Type	Geographic Separation	Mechanism	Example
Allopatric	Yes	Physical barrier divides population	Darwin's finches on different islands
Sympatric	No	Genetic changes, polyploidy, niche differentiation	Polyploid plants

Additional Evolutionary Concepts

Selection, Adaptation, and Progress

Natural selection does not always lead to "perfect" organisms. Evolution is not goal-directed, and progress is not guaranteed.

• Selection: Acts on existing variation; does not anticipate future needs.

• Adaptation: Traits that increase fitness in current environment.

• Progress: Evolution can result in loss or gain of complexity; not always "improvement".

Deleterious and Beneficial Mutations

Mutations can be harmful, neutral, or beneficial. Their effects on genetic diversity depend on selection and drift.

• Deleterious Mutations: Reduce fitness; may be eliminated by selection.

• Beneficial Mutations: Increase fitness; may spread in population.

• Genetic Diversity: Maintained by mutation, gene flow, and balancing selection.