Understanding Population Genetics and Evolution: Principles, Strategies, and Isolation Mechanisms

1. Section 15.2 Summary– pages 404-413 Population Genetics and Evolution The principles of evolution are rooted in population genetics

2. Population Genetics and Evolution • A populationconsists of all the individuals of a species that live together in one place at one time andcan successfully breed.

3. Population characteristics • Population size~ number of individuals in a population • Density~number of individuals per unit of area • Dispersion~pattern of spacing •random~ unpredictable, patternless spacing (a) •clumped~patchy spacing (b) •uniform~ even spacing (c)

4. Factors that affect Populations • Birthrate (natality)~ number of offspring produced

5. Factors that affect Populations • Death rate (mortality)-how many people die

6. Factors that affectPopulations • Age structure~ relative number of individuals of each age

7. Factors that affect Populations • Survivorship curve~ plot of numbers still alive at each age

8. Carrying capacity is maximum number of individuals a habitat can support

10. Population limiting factors • Density-dependent factors • Competition • Predation • stress/crowding • waste accumulation • Density-independentfactors •weather/climate •periodic disturbances

11. r-selected opportunistic Short lifespan Many small offspring reproduces once no parental care High death rate Population “strategies”

12. Population “strategies” • K-selected • Long lifespan • Few large offspring • several reproductions late in life • extensive parental care • Low death rate

13. Population Growth Models • Exponential model (blue) • (J-curve) • r-selected • Logistic model (red) • (S-curve) • K-selected species

14. Section 15.2 Summary– pages 404-413 Changes in genetic equilibrium Another mechanism that disrupts a population’s genetic equilibrium is genetic drift—the alteration of allelic frequencies by chance events.

15. Section 15.2 Summary– pages 404-413 Changes in genetic equilibrium • The movement of genes by migration is called gene flow. • When an individual leaves a population, its genes are lost from the gene pool. • When individuals enter a population, their genes are added to the pool.

16. Section 15.2 Summary– pages 404-413 Natural selection acts on variations • Some variations increase or decrease an organism’s chance of survival in an environment. • There are three different types of natural selection that act on variation: • stabilizing selection • directional selection • disruptive selection

17. Section 15.2 Summary– pages 404-413 Natural selection acts on variations • Stabilizingselection is a natural selection that favors average individuals in a population. Normal variation Selection for average size spiders

18. Section 15.2 Summary– pages 404-413 Natural selection acts on variations • Directional selectionoccurs when natural selection favors one of the extreme variations of a trait. Selection for longer beaks Normal variation

19. Section 15.2 Summary– pages 404-413 Natural selection acts on variations • In disruptive selection, individuals with either extreme of a trait’s variation are selected for. Selection for light limpets Normal variation Selection for dark limpets

20. Section 15.2 Summary– pages 404-413 Natural selection acts on variations • Natural selection can alter the genetic equilibrium of a population’s gene pool over time. • Significant changes in the gene pool could lead to the evolution of anew species over time.

21. Section 15.2 Summary– pages 404-413 • In nature, physical barriers can break large populations into smaller ones. • Geographic isolation occurs whenever a physical barrier divides a population. • A new species can evolve when a population has been geographically isolated.

22. Section 15.2 Summary– pages 404-413 The Evolution of Species • When geographic isolation divides a population of tree frogs, the individuals no longer mate across populations. • Tree frogs are a single population.

23. Section 15.2 Summary– pages 404-413 The Evolution of Species • The formation of a river may divide the frogs into two populations.

24. Section 15.2 Summary– pages 404-413 The Evolution of Species • Over time, the divided populations may become two species that may no longer interbreed, even if reunited.

25. Section 15.2 Summary– pages 404-413 • As populations become increasingly distinct, reproductive isolation can arise. • Reproductive isolation occurs when formerly interbreeding organisms can no longer mate and produce fertile offspring.

26. Section 15.2 Summary– pages 404-413 Reproductive isolation can result in speciation • Reproductive Isolation can occur when there are changes in the species: • Genetic material (DNA) • Behavior

27. The Western meadowlark (left) and the Eastern meadowlark (right) appear to be identical, and their ranges overlap, but their distinct songs prevent interbreeding