Understanding Genetic Variation and Evolution in Populations

1. Species – Discrete groups differing in appearance, behavior, ecology, genetics, etc. Population – a localized group of individuals Gene – a sequence of nucleotides producing a protein Allele – alternate form of a gene Gene pool – all alleles at all gene loci in all individuals in a population Some Useful Terms

2. Species, population, gene pool, allele, gene

3. Why is genetic variation important? Darwin and the ingredients for evolution in response to natural selection 1. Individuals within species vary (phenotypic variation) 2. Some of this variation is heritable (genetic variation) 3. Survival and/or reproduction are non-random (natural selection) The individuals that survive & reproduce the most are MORE LIKELY TO BE those with variations most suited to their environment Genetic variation is essential for evolution to occur; and population dynamics is about likelyhood (normal distribution)

4. Microevolution vs. Macroevolution Microevolution: change in allele frequencies within a population Macroevolution: evolutionary change at the species level or higher

5. Microevolution • A generation to generation change a populations allele or genotype frequency • Causes include: • Genetic drift • Gene flow • Mutations • Non-random mating • Natural selection

6. Change in gene frequencies of a population due to chance Genetic Drift

7. Bottleneck Effect Disaster reduces population size so gene pool is reduced and unlikely to represent the original population By chance, some alleles will be over- or under-represented and in some cases eliminated Genetic Drift

8. Founder Effect A few individuals colonize an isolated island, lake etc. Small gene pool unlikely to reflect variety of original population Genetic Drift

9. Genetic exchange between populations Gene Flow

10. Gene flow Less likely when populations are isolated Movement of genes across the landscape Increases genetic diversity within populations Populations become similar Genetic drift More likely when populations are small and isolated Random change in allele frequencies = direction is unpredictable Decreases genetic diversity within populations Populations can become different Compare & Contrast

11. A change in DNA Point – single base pair Chromosomal Polyploidy - The doubling of the whole sets of chromosmes in the nucleus of a developing egg or seed Mutations

12. Non-Random Mating

13. Differential reproductive success Alleles are passed on to the next generation in numbers disproportionate to current generation Only mechanism likely to adapt a population to its environment Natural selection

14. Heritable Variation – idea is central to Darwin’s theory Within a population Polymorphism – variation in a discrete characteristic Among populations Geographic clines – graded change in some trait along a geographic axis Natural selection

15. Evolutionary (Darwinian) Fitness – the contribution an individual makes to the gene pool of the next generation Selection acts on phenotypes – indirectly on genotypes Natural Selection

16. Trait variation Difference in fitness among traits Inheritance Sources of variation Polymorphism – occurrence of more than one form of a species Heterozygosity – the fraction of loci that have multiple alleles Prerequisites for natural selection Hamlet

17. Stabilizing Average individuals have highest fitness Directional Shift in mean value or frequency Disruptive Average individuals have lowest fitness Modes of natural selection

18. Effect of Selection: Stabilizing

19. Most common during periods of environmental change Shifts the frequency curve in one direction or the other by favoring what are initially rare types E.g. size of black bears during glacial and interglacial periods Effect of Selection Directional

20. Selection favors individuals on both extremes Size of prey species Small fit in smaller holes than predator Large – too big to swallow Effect of Selection Disruptive

21. Constraints to adaptations • Genetic forces prevent “perfect” adaptations • Mutation • Gene flow • Environments are constantly changing

22. Constraints to adaptations • Historical constraints • Adaptations involve trade-offs

23. Macroevolution - A Basic Observation • The living world is wildly diverse. • Bacteria to whales (size) • Antarctic ice to oceanic thermal vents (habitat) • All organisms now alive shared ancestors 3500 mya. • Basic unit of classification for life forms is the species. • Discrete groups differing in appearance, behavior, ecology, genetics, etc. • Bridge between micro- and macroevolution is speciation. Result = diversity of life.

24. Barriers that prevent interbreeding Prezygotic – impede mating or prevent fertilization Postzygotic – mating and fertilization occur, but hybrid has low viability and fertility Mechanisms of Speciation

25. Habitat Isolation Species in different habitats in the same area may not encounter each other Garter snakes – thamnophis – same area but 1 aquatic, 1 terrestrial Mechanisms of Speciation Prezygotic

26. Behavioral Isolation Fireflies blinking pattern Bird songs Courtship rituals Mechanisms of Speciation Prezygotic

27. Temporal Isolation Breed at different times of day, seasons Frogs Mechanisms of Speciation Prezygotic

28. Mechanical Anatomically incompatible Pollination of flowering plants Mechanisms of Speciation Prezygotic

29. Species Separation by Pollinator Recognition Columbines Hawkmoth- pollinated Hummingbird- pollinated

30. Gametic Isolation Gametes may meet but fail to form a zygote External fertilization of many marine species Mechanisms of Speciation Prezygotic

31. Reduced hybrid viability a zygote is formed, but is never born Reduced hybrid fertility hybrid dies before reaching reproductive age, or is infertile Hybrid breakdown hybrids can mate and produce offspring, but offspring are weak and/or sterile Mechanisms of Speciation Postzygotic

32. Geographic Modes of Speciation Allopatry - vicariance Ratites Ratites = rhea, ostrich, emu

33. Geographic Modes of Speciation Allopatry- dispersal Galapagos finches

34. Parapatric Evolution of reproductive isolation between geographically contiguous (adjacent) populations Geographic patterns

35. Geographic patterns Sympatry Sympatric speciation in animals can work when disruptive selection coevolves with assortative mating. 3 spine sticklebacks