Overview
This lecture covers evolution and speciation for A-level Biology, including types of selection, mechanisms of speciation, genetic drift, and related exam-style questions with mark schemes.
Variation in Organisms
- Phenotypic variation in populations comes from genetic and environmental factors.
- Mutation is the main source of genetic variation, creating new alleles.
- Meiosis (crossing over, independent segregation) and random fertilization also generate variation.
- Environmental variation can be caused by disease, predation, and competition.
Natural Selection and Allele Frequencies
- Not all individuals survive and reproduce due to disease, predation, and competition.
- Individuals with advantageous phenotypes are more likely to reproduce and pass on beneficial alleles.
- Evolution is the change in allele frequencies over time.
Types of Natural Selection
- Directional selection: favors one extreme phenotype, shifting the population mean.
- Stabilizing selection: favors the average phenotype, reducing variation.
- Disruptive selection: favors both extremes, leading to increased frequency of both extreme phenotypes.
Speciation
- Speciation is the evolution of new species due to genetic differences that prevent interbreeding.
- Allopatric speciation occurs when populations are geographically isolated, leading to genetic divergence and reproductive isolation.
- Sympatric speciation happens without geographical isolation, often due to behavioral differences caused by mutations.
Genetic Drift
- Genetic drift is a random change in allele frequencies between generations, especially significant in small populations.
- It is not driven by selection but by chance events, leading to rapid evolution in small populations.
Exam Questions and Mark Schemes
- Allopatric speciation: requires reference to geographical isolation, separate gene pools, variation via mutation, different selection pressures, differential reproductive success, and change in allele frequency.
- Twin studies on schizophrenia: use both types to distinguish genetic (identical twins) and environmental (non-identical twins) influences; genes have a greater effect than the environment if identical twin concordance is higher.
- Factors for twin studies: age and ethnicity are important to control for potential confounding variables.
Key Terms & Definitions
- Phenotypic variation — visible differences among individuals due to genes and environment.
- Allele frequency — the proportion of a specific allele among all alleles in a population.
- Natural selection — process where advantageous traits increase in frequency over generations.
- Speciation — formation of new species from existing populations.
- Genetic drift — random changes in allele frequencies, most significant in small populations.
- Allopatric speciation — speciation due to geographical isolation.
- Sympatric speciation — speciation without geographical isolation, often behavioral.
Action Items / Next Steps
- Review types of natural selection and speciation mechanisms.
- Practice exam questions on evolution and speciation.
- Revise key terms and definitions for upcoming assessments.