1. The Evolution of Populations Chapter 23 Biology – Campbell Reece

2. Population What is a population? Species Gene pool

3. Genetic Variation & Evolution Variations within a population AND Geographic variation How does variation occur? What is the ultimate source of new alleles? Where must mutations occur in order to be passed to the next generation?

4. Alleles in a Population Allele frequency example… ◦Red flower (R) is dominant over white flower (r) ◦In a population of 500, 20 have white flowers (rr) ◦The other 480 have red flowers (RR or Rr)  320 are RR, 160 are Rr ◦The dominant allele (R) accounts for 800 or 80% of the total (1000) number of genes ◦The recessive allele (r) accounts for 200 or 20%

5. Hardy-Weinberg Theorem Describes a nonevolving population The frequencies of alleles and genotypes in a population’s gene pool remain constant over generations ◦Chance of RR – 0.8 x 0.8 =.64 ◦Chance of Rr – 0.8 x 0.2 =.16 +.16 (for rR) =.32 ◦Chance of rr – 0.2 x 0.2 = 0.04 ◦The allele frequency does not change

6. Hardy-Weinberg Theorem

8. Hardy-Weinberg Equilibrium p = one allele, q = other allele p + q = 1 Frequency of RR = p 2 Frequency of Rr/rR = 2pq Frequency of rr = q 2 Hardy-Weinberg Equation: ◦p 2 + 2pq + q 2 = 1

9. 5 Conditions for H-W Equilibrium 1. Very large population size. 2. No migration. 3. No net mutations. 4. Random mating. 5. No natural selection. We do not really expect a natural population to be in H-W equilibrium

10. Altering Allele Frequencies What might cause the allele frequencies to change?

11. Genetic Drift What is genetic drift? ◦What size population is most likely to be affected? ◦Founder effect ◦Bottleneck effect

12. Genetic Drift

13. Bottleneck Effect

15. Genetic Drift 4 key points: Genetic drift… 1.is significant in small populations 2.can cause allele frequencies to change at random 3.can lead to a loss of genetic variation within populations 4.can cause harmful alleles to become fixed

16. Gene Flow What is gene flow? What results from gene flow?

17. Directional Selection

18. Disruptive Selection

19. Stabilizing Selection

20. No ‘Perfect’ Organisms Selection can act only on existing variations Evolution is limited by historical constraints Adaptations are often compromises Chance, natural selection, and the environment interact