Evolution
Evolution by Natural Selection
Species Change
Definition: Species do not remain static; they evolve over millions of years.
Evolution Defined: Change in heritable characteristics of organisms across generations.
Heritable Characteristics: Traits that can be inherited by the next generation.
Non-heritable Changes: Changes acquired during an organism's life (e.g., being eaten or gaining weight) do not lead to evolution.
Role of Alleles: Heritable characteristics are influenced by alleles (gene variants) that can change through random mutations.
Advantageous Alleles: Traits that improve survival chances are more likely to be passed on to offspring, leading to gradual species change.
Natural Selection Defined
Explanation: The process where organisms better adapted to their environment survive, reproduce, and pass on advantageous alleles.
Stages of Natural Selection:
Variation: Exists among individuals in a population.
Selection Pressures: Environmental factors (e.g., predation, competition, disease) that affect survival.
Survival and Reproduction: Well-adapted individuals are more likely to survive, reproduce, and pass on advantageous traits.
Frequency Changes: Alleles for favorable traits increase in frequency while unfavorable ones decrease.
Example of Natural Selection: Rabbits
Variation in Fur Color: Brown fur allele vs. white fur allele.
Selection Pressure: Predation by foxes acts against the white-furred rabbits, making brown rabbits more likely to survive and reproduce.
Result: Over generations, brown fur becomes more common in the population while white fur decreases in frequency.
Examiner Tips
Evolution occurs as a result of natural selection acting on random variations; avoid wording implying purposeful evolution (e.g., "evolution occurs 'so that' an organism can survive").
Key Stages to Remember:
Variation present in a population.
Selection pressures affect the population.
Individuals with advantageous alleles are more likely to survive and reproduce.
These alleles are passed to offspring, increasing in population frequency.
Speciation
Definition of Speciation: The development of new species from pre-existing species over time due to isolation and evolution.
Isolation Mechanisms:
Geographical Isolation: Populations divided by physical barriers (e.g., mountains, rivers) leading to allopatric speciation.
Random Mutations leading to changes that prevent interbreeding, resulting in sympatric speciation.
Effects of Isolation: Isolated populations experience different selection pressures resulting in divergent allele frequencies.
Allopatric Speciation
Occurs when populations are separated by geological barriers, preventing interbreeding (e.g., mountains forming a divide).
Changes in allele frequencies arise due to different selection pressures and genetic drift.
Over time, populations evolve into distinct species incapable of interbreeding.
Example: A mountain range divides a tree population into two, leading to the development of new species over thousands of years.
Sympatric Speciation
Occurs without geographical barriers; involves reproductive isolation through random allele changes.
Examples of changes leading to isolation:
Seasonal Changes: Different mating or flowering seasons.
Mechanical Changes: Anatomical differences prevent successful mating.
Behavioral Changes: Altered courtship behaviors affect mate attraction.
In fruit flies, random mutations in a lab lead to differing food preferences, preventing interbreeding.
Examiner Tips for Speciation
Understand that natural selection acts differently on isolated populations, leading to unique evolutionary paths.
Stages to remember in speciation:
Variation present.
Selection pressures act differently on populations.
Advantageous alleles vary between populations.
Allele frequencies change, leading to speciation.
Evidence of Evolution & the Scientific Community
Support for Evolution Theory: Evolution by natural selection is supported by substantial evidence.
Types of Evidence:
Fossil Record: Fossils show changes in organisms over millions of years and provide transitional forms.
Real-Life Observation: Example: Antibiotic resistance in bacteria acts as a selection pressure, exemplifying natural selection.
Molecular Evidence: Analysis of DNA and proteins shows similarity across species indicating common ancestry.
Gene Sequence Analysis: DNA sequencing reveals genetic similarities; closely related species have similar gene base sequences.
Measure of Divergence: Similar base sequences indicate recent common ancestry, while distinct sequences suggest long divergence times.
Enables the establishment of evolutionary relationships.
Protein Sequence Evidence:
Proteomics: Examines the order of amino acids in proteins to determine evolutionary links based on similarities.
Similar amino acid sequences indicate common ancestry, establishing relationships between species.
Building Evolutionary Trees: DNA and protein sequencing results allow scientists to construct evolutionary trees representing relationships among species.
Scientific Community Assurance:
The scientific community critically evaluates and validates theories through peer review and replication of studies.
Methods of Assessment:
Scientific Journals: Research assessed by experts; peer-reviewed before publication, ensuring validity.
Conferences: Forums for scientists to present and discuss findings, fostering collaborative scrutiny and idea sharing.
Role of Peer Review: Ensures studies are rigorous and methodologies sound; faulty studies can be retracted if significant issues are discovered.
Conclusion: Evolutionary theories are robust due to extensive verification and dialogue among the scientific community, emphasizing the scientific method's integrity and reliability.