Natural Selection and Evolutionary Biology Concepts
Unit 7: Natural Selection in Biology
Chapter 19 – Descent with Modification
Overview: This chapter discusses early ideas about the evolution of life, earlier hypotheses, and the disciplines that influenced Darwin's thoughts, including geology, paleontology, sociology, and economics. It highlights the overwhelming scientific evidence supporting Darwin’s theory of evolution by natural selection.
Key Concepts:
Definition of Evolution:
Evolution is both a pattern and a process. It refers to the change in the genetic composition of populations over successive generations. As a pattern, it signifies observable changes in biological characteristics over time, while as a process, it describes the mechanisms leading to these changes like natural selection, mutation, gene flow, and genetic drift.
Darwin vs. Lamarck:
Lamarck posited that organisms evolve through the inheritance of acquired characteristics, suggesting that traits developed during an organism's lifetime can be passed to its offspring.
Darwin's Theory: Natural selection is the mechanism by which evolution occurs, emphasizing the survival of individuals with advantageous traits.
Natural Selection:
Natural selection is defined as the differential survival and reproduction of individuals due to differences in phenotype. It leads to the adaptation of organisms to their environment.
Topic 7.1 Introduction to Natural Selection
Learning Objectives:
Describe the causes of natural selection.
Explain how natural selection affects populations.
Adaptations:
Adaptations are inherited characteristics that enhance an organism's ability to survive and reproduce in specific environments. These can be structural, behavioral, or physiological.
Descent with Modification:
This concept explains the branching pattern of evolution, suggesting that all living organisms are related through common descent but have diverged over time, resulting in varied forms.
Topic 7.3 Artificial Selection
Learning Objectives:
Explain how humans can affect diversity within a population.
Explain the relationship between changes in the environment and evolutionary changes in populations.
Artificial Selection:
Artificial selection is the intentional breeding for certain traits in organisms. It demonstrates how certain traits can become more prevalent in a population due to human preferences rather than natural processes.
Individuals Do Not Evolve:
Evolution occurs at the population level; individuals do not evolve. Instead, the allele frequencies within the population change over generations through natural selection.
Topic 7.6 Evidence of Evolution
Learning Objectives:
Describe types of data evidencing evolution.
Explain morphological, biochemical, and geological data as evidence for evolution.
Describe shared molecular and cellular features that provide evidence of common ancestry.
Homologous Structures:
Homologous structures are anatomical features in different organisms that share a common ancestry but may serve different functions. These often relate to vestigial structures, which are residual forms that serve little or no purpose in the modern organism.
Convergent Evolution:
Convergent evolution occurs when organisms from different evolutionary backgrounds develop similar traits. This is contrasted with homologous structures, which arise from common ancestry, while analogous structures arise independently under similar environmental pressures.
Fossil Record:
The fossil record provides chronological evidence of organisms that existed in the past, showcasing the changes in life forms over time and supports the theory of evolution by demonstrating transitional forms.
Chapter 20 – Phylogeny
Overview: This chapter explains how phylogeny traces the evolutionary history of species, allowing biologists to classify organisms and discern their evolutionary relationships.
Topic 7.9 Phylogeny
Learning Objectives:
Describe evidence to infer evolutionary relationships.
Explain phylogenetic trees and cladograms.
Phylogeny Definition:
Phylogeny refers to the evolutionary history of a species or group of species.
Phylogenetic Trees:
Phylogenetic trees are diagrams representing evolutionary relationships, depicting lineages diverging from common ancestors. The key features include branch points (where lineages diverge) and the length of branches, which can represent time or genetic change.
Understanding Phylogenetic Trees:
Important points about phylogenetic trees include:
They represent hypotheses about evolutionary relationships.
They may be revised as new evidence is discovered.
They cannot depict the exact timing of evolutionary events.
Homologies vs. Analogies:
It is critical to distinguish between homologous (shared ancestry) and analogous (similar function, different ancestry) traits when constructing phylogenies. This can be done by comparing anatomical structures, developmental pathways, and molecular data.
Key Vocabulary in Cladistics:
a. Cladistics: A method of classifying organisms based on common ancestry.
b. Clade: A group of organisms consisting of a common ancestor and all its descendants.
c. Derived Characteristics: Traits that have evolved more recently in a clade.
d. Outgroup vs. Ingroup: An outgroup is a closely related species or group used to compare with the ingroup (the species of study).
Shared Ancestral vs. Shared Derived Characters:
A shared ancestral character is a trait that originated in an ancestor of the taxon, while a shared derived character is a trait that evolved within a taxon and is not found in its ancestor.
Topic 7.7 Common Ancestry
Learning Objective:
Describe evidence for the common ancestry of all eukaryotes at structural and functional levels.
DNA Sequences:
Some DNA sequences change slowly, making them useful for studying ancient evolutionary events, while others change rapidly and are better for examining more recent evolutionary events.
HIV Evolution:
Evidence reveals when HIV first entered the human population by tracing mutations.
Three-Domain System Evidence:
The three-domain system categorizes life into Bacteria, Archaea, and Eukarya, indicating relationships based on genetic and biochemical data. Archaea is more closely related to Eukarya than to Bacteria.
Chapter 21 – The Evolution of Populations
Overview: This chapter focuses on microevolution, defined as changes in allele frequencies in populations across generations, emphasizing the importance of genetic variation for adaptation.
Topic 7.5 Hardy-Weinberg Equilibrium
Learning Objectives:
Describe conditions under which allele and genotype frequencies change in populations.
Explain impacts when Hardy-Weinberg conditions are unmet.
Key Terms:
a. Population: A localized group of individuals of the same species.
b. Gene Pool: All alleles in a population.
c. Fixed Allele: An allele that is the only variant in a population.
d. Allele Frequency: Proportion of a specific allele in the gene pool.
Hardy-Weinberg Equation:
The equation is p^2 + 2pq + q^2 = 1, where p is the frequency of the dominant allele and q is the frequency of the recessive allele.
Hardy-Weinberg Equilibrium Conditions:
The five conditions are:
Large population size (no genetic drift).
No migrations (gene flow).
No mutations.
Random mating.
No natural selection.
Effect of Natural Selection on Allele Frequencies:
Natural selection can increase the frequency of advantageous alleles, leading to adaptation over time.
Genetic Drift:
Genetic drift is the change in allele frequencies due to chance events, often detrimental in small populations.
Example: Greater prairie chicken's population decline led to decreased genetic diversity.
Bottleneck Effect vs. Founder Effect:
Bottleneck Effect: Sudden reduction in population size reduces genetic variation.
Founder Effect: When a small group establishes a new population, leading to reduced genetic variation compared to the original population.
Gene Flow:
Gene flow is the transfer of alleles between populations and can increase or decrease adaptability.
Topic 7.12 Variations in Populations
Learning Objective:
Explain how genetic diversity impacts a population’s ability to adapt to environmental pressures.
Natural Selection and Allele Frequencies:
Natural selection alters allele frequencies! A population's genetic variations lead to differential survival and reproduction.
Relative Fitness:
Relative fitness is the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals.
Types of Selection:
Directional Selection: Favors one extreme phenotype.
Disruptive Selection: Favors both extremes, leading to two or more contrasting forms.
Stabilizing Selection: Favors intermediate phenotypes, reducing variation.
Key Role of Natural Selection in Adaptive Evolution:
Factors maintaining genetic variation include:
a. Heterozygote Advantage: Individuals heterozygous for a trait have a fitness advantage over homozygous individuals, maintaining both alleles in the population.
Chapter 22 – The Origin of Species
Overview: This chapter focuses on the biological species concept, mechanisms leading to the evolution of new species, reproductive isolation, and the differences between microevolution and macroevolution.
Topic 7.10 Speciation
Learning Objectives:
Describe conditions leading to the evolution of new species.
Explain rates of evolution and speciation under varying ecological conditions.
Describe mechanisms driving speciation.
Speciation Definition:
Speciation is the process by which one species splits into two or more distinct species.
Microevolution vs. Macroevolution:
Microevolution involves changes within a population, while macroevolution encompasses larger-scale changes that lead to new species and major evolutionary trends.
Reproductive Barriers:
Reproductive barriers are mechanisms that prevent species from interbreeding. Examples include geographical, temporal, behavioral, mechanical, and gametic isolation. Cases where barriers break down can lead to hybrid species.
Prezygotic vs. Postzygotic Isolating Mechanisms:
Prezygotic mechanisms prevent mating or fertilization between species, while postzygotic mechanisms occur after fertilization, affecting the viability or fertility of hybrids.
Allopatric Speciation:
Allopatric speciation occurs when populations are geographically isolated, leading to divergence and new species formation. Evidence includes the Galápagos finches.
Evidence for Allopatric Species Model:
The geographic separation can accumulate genetic differences over time, leading to speciation.
Sympatric Speciation:
Sympatric speciation occurs without geographical isolation, often through polyploidy in plants.
Chromosomal Changes in Sympatric Speciation:
Polyploidy, the condition of having more than two complete sets of chromosomes, can result in new species formation through hybridization events.
Gradualism vs. Punctuated Equilibrium:
Gradualism: Evolution occurs slowly over time.
Punctuated Equilibrium: Long periods of stasis interrupted by brief, rapid changes, supported by fossil evidence.
Genetic Changes in Speciation:
Changes at the genetic level, such as mutations and chromosomal alterations, drive the speciation process.
Chapter 23 – Broad Patterns of Evolution
Overview: This chapter examines macroevolution through the fossil record, geology, and the historical forces shaping life on Earth.
Topic 7.6 Evidence of Evolution
Learning Objectives:
Explain how various data types provide evolution evidence.
Describe fundamental features shared across life domains as evidence for common ancestry.
Macro Evolution Definition:
Macroevolution involves large-scale evolutionary changes over long periods, leading to the emergence of new species and groups.
Fossil Dating:
Fossils are dated using relative dating (comparing with other layers) and absolute dating (using radioactive isotopes).
Emergence of Life:
Based on the fossil record, life first emerged approximately 3.5 billion years ago, with evidence from stromatolites.
Eukaryotes Emergence:
Eukaryotes are estimated to have first appeared around 1.8 billion years ago, transitioning from prokaryotic life.
Topic 7.11 Extinction
Learning Objectives:
Describe factors leading to population extinction.
Explain how environmental changes influence extinction risk and how extinction can pave the way for adaptive radiation.
Mass Extinctions:
Mass extinctions are events in which large numbers of species go extinct in a relatively short period, greatly impacting biodiversity and the evolutionary trajectory of life on Earth.
Adaptive Radiation:
Adaptive radiation is the burst of genetic Divergence from a common ancestor into varied forms adapting to different environmental challenges. Factors influencing it include ecological opportunity and morphological innovations.