Evolution of Populations

1. Evolution of Populations

2. 23.1 – Mutation & sexual reproduction produce genetic variation that makes evolution possible • 1) Microevolution • Change in the allele frequencies of a population over generations • Evolution on the smallest scale

3. 2) Mutations • The only source of NEW genes & NEW alleles • Only mutations in cell lines that produce gametes can be passed on to offspring

4. Types of Mutations • A) Point Mutation • Change in one base in a gene • Can impact phenotype • Sickle cell anemia • B) Chromosomal Mutation • Delete, disrupt, duplicate, or rearrange many loci at once • Most are harmful, but not always

5. 3) Variations due to sexual reproduction • Rearranges alleles into new combinations in every generation • 3 mechanisms for this shuffling: • Next slide

6. 1) Crossing over • During Prophase I of meiosis • 2) Independent assortment • During meiosis (223 different combinations possible) • 3) Fertilization • 223 x 223 for sperm and egg

7. 23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving • Population genetics • Study of how populations change genetically over time • Population • Group of individuals of the same species that live in the same area • Interbreed & produce fertile offspring

9. Gene pool • All of the alleles at all loci in all the members of a population • In diploids, each individual has 2 alleles for a gene & the individual can be heterozygous or homozygous • If all are homozygous for an allele, the allele is FIXED – only one allele exists at the locus in the population • The greater the # of FIXED alleles, the lower the species’ diversity

10. Hardy-Weinberg • Used to describe a population that is NOT evolving • Frequencies of alleles & genes in a gene pool will remain constant over generations

12. 5 Conditions for Hardy-Weinberg • 1) No mutations • 2) Random mating • 3) No natural selection • 4) The population size must be large (no genetic drift) • 5) No gene flow (Emigration, immigration, transfer of pollen, etc.)

13. If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then • p2 + 2pq + q2 = 1 • where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

14. Practice • Suppose in a plant population that red flowers (R) are dominant to white flowers (r). In a population of 500 individuals, 25% show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous for this trait?

15. 23.3 – Natural Selection, genetic drift, & gene flow can alter allele frequencies in a population • Mutations can alter gene frequency, but are rare • 3 major factors alter allelic frequencies • 1) Natural selection • Alleles are passed to the next generation in proportions different from their frequencies to the present generation • Those that are better suited produce more offspring than those that are not

16. 2) Genetic Drift • Unpredictable fluctuation in frequencies from one generation to the next • The smaller the population, the greater chance • Random & nonadaptive • A) Founder effect = individuals are isolated and establish a new population – gene pool is not reflective of the source population • B) Bottleneck effect = a sudden change in the environment reduces population size – survivors have a gene pool that no longer reflects original

18. Genetic drift is significant in small populations • Genetic drift causes allele frequencies to change at random • Genetic drift can lead to a loss of genetic variation within populations • Genetic drift can cause harmful alleles to become fixed

19. 3) Gene Flow • Populations loses or gains alleles by genetic additions or subtractions • Results from movement of fertile individuals or gametes • Reduces the genetic differences between populations, makes populations more similar

21. 23.4 Natural Selection is the only mechanism that consistently causes adaptive evolution • Relative fitness • The contribution an organism makes to the gene pool of the next generation relative to the contributions of the other members • Does NOT indicate strength or size • Measured by reproductive success

22. Natural selection acts more directly on the phenotype and indirectly on the genotype • Can alter the frequency distribution of heritable traits in 3 ways: • 1) Directional selection • 2) Disruptive selection • 3) Stabilizing selection

23. 1) Directional selection • Individuals with one extreme of a phenotypic range are favored, shifting the curve toward this extreme • Example: Large black bears survived periods of extreme cold better than small ones, so they became more common during glacial periods

24. 2) Disruptive Selection • Occurs when conditions favor individuals on both extremes of a phenotypic range rather than individuals with intermediate phenotypes • Example: A population has individuals with either large beaks or small beaks, but few with intermediate – apparently the intermediate beak size is not efficient in cracking either the large or small seeds that are available

25. 3) Stabilizing Selection • Acts against both extreme phenotypes and favors intermediate variations • Example: Birth weights of most humans lie in a narrow range, as those babies who are very large or very small have higher mortality rates

27. How is genetic variation preserved in a population? • Diploidy • Capable of hiding genetic variation (recessive alleles) from selection • Heterozygote advantage • Individuals that are heterozygous at a certain locus have an advantage for survival • Sickle cell anemia – homozygous for normal hemoglobin are more susceptible to malaria, homozygous recessive have sickle-cell, but those that are heterozygotes are protected from malaria and sickle-cell

29. Why Natural Selection cannot produce perfect organisms: • 1) Selection can only edit existing variations • 2) Evolution is limited by historical constraints • 3) Adaptations are often compromises • 4) Chance, natural selection, & the environment interact