Understanding Darwin's Five Theories of Natural Selection

1. Lecture 3: Natural Selection EEES 3050

2. Review • Darwin’s Five Theories • 1 – The non-constancy of species • The idea of a changing world can be considered the fact of evolution. • 2 – The descent of all organisms from common ancestors • 3 – Gradualism (no saltations, no discontinuities) • 4 – Speciation by populations • 5 – Natural selection • This is the “theory” of the process of evolution.

3. Review • Evidence • Fossil Record, Biogeography, Morphology, Vestigial Structures, Molecular Evidence • Evolutionary Ecology: • World changes and so do the phenomena studied in ecology • Sex ratios, feeding preference, life cycles • Human impacts • Antibiotic resistance, epidemiology, conservation biology

4. Darwin’s Model of Natural Selection: 5 facts, 3 inferences • Fact 1: Every population has such high fertility that its size would increase exponentially if not constrained. (Malthus) • Fact 2: The size of populations, except for temporary annual fluctuations, remains stable over time. (Natural History) • Fact 3: The resources available to every species are limited. (Malthus) • Inference 1: There is intense competition among the members of a species.

5. Darwin’s Model – 5 facts, 3 inferences • Fact 4: No two individuals of a population are exactly the same. (Animal breeders and taxonomists) • Inference 2: Individuals of a population differ from each other in the probability of survival. (Darwin) • Fact 5: Many of the differences among the individuals of a population are, at least in part, heritable. (Animal breeders) • Inference 3: Natural selection, continued over many generations results in evolution. (Darwin)

6. Two criticisms of natural selection • Selection is a process of chance. • Selection is deterministic. Natural selection is both!

7. The 2 steps of Natural Selection • Random production of variation • Mutation • Recombination • Random mate choice • Non-random aspects of survival and reproduction • Superior success of certain phenotypes • Non-random mate choice.

8. Natural Selection • What drives selection? • Usually an emphasis on survival or struggle for existence. • Natural selection is actually a process of elimination • “Survival of the fittest” – coined by Sociologist Herbert Spencer. • What else drives selection? • All factors leading to an increase in reproductive success. • Sexual Selection or “Selection for reproductive success”

9. Fitness • What is fitness? • “…a measure of the contribution of an individual to future generations and can also be called adaptive value”. • A relative term • Not absolute • Cannot compare across species • Not only reproductive success • Not a short-term measure • Not about individual traits.

10. Three types of selection • Directional Selection • Stabilizing Selection • Disruptive Selection

11. Lower rate of survival

12. Lower rate of survival

13. Lower rate of survival

14. Types of Natural Selection • Directional selection • Industrial Melanism • Galapagos ground finch. • Other cases of anthropogenic selection • Stabilizing Selection • Infant Mortality • Hatch date in lesser snow geese • Clutch size in birds • Disruptive Selection • Black-bellied seed crackers in Africa • Species has two sizes of beak – no intermediates.

15. Planned Tangent • Throughout this semester, there will be a common framework of how to think about conducting ecology. • I will ask (again and again and again) • What are the stated observations or theory in question? • Develop an hypothesis based on the observations? • How would you develop an experiment to test your hypothesis? • Summarize the reported results.

16. Industrial Melanism Background: • The species: Peppered Moth (Biston betularia) • The place: Industrial Europe (also USA) • The time: last 100 years • Scenario: adults rest on tree trunks • natural state: light colored, lichen covered trunks • industrial state: soot-darkened trunks • Observation: proportions of light:dark moths • natural state: mostly light colored (>90%) • industrial state: increasingly dark (>90%)

17. Industrial Melanism • Natural state: light colored, left • Industrial state: dark colored, right • Observation: proportions of light:dark moths • natural state: mostly light colored (>90%) • industrial state: increasingly dark (>90%)

18. Industrial Melanism • Hypothesis: • Birds eat moths they can see. • birds are visual predators • Experiment: • release marked adults of both color • Results • rural: more light adults survived • urban: more dark adults survived • Visual verification of bird predation

19. Footnote However ... other factors also are important • differences in physiology • differences in dispersal rates But, melanism is decreasing with cleaner air!

20. Results: Pepper Moth Frequencies

21. Darwin’s ground finch • Change in percent survival as related to bill depth during a drought. • The Beagle visited 4 islands, Darwin merely labeled all specimens as “Galapagos Islands”. Recommended reading: Darwin’s Finches by D. Lack 1947

22. Anthropogenic Selection • Every domesticated plant and animal vs.

23. Anthropogenic Selection • Pest species • Antibiotic resistance.

24. Stabilizing Selection • Human infant mortality

25. Stabilizing Selection: Clutch Size in Birds • Theory: • Natural selection should favor birds with most descendants. • Hypothesis: • Birds should lay as many eggs as possible. • Determinate vs. Indeterminate • Indeterminate – continue to lay eggs. • Experiment • remove eggs • Results • Most birds under normal circumstances do not lay their physiological limit of eggs. • One mallard female laid an egg a day for 100 days. • Lack – 1947: clutch size in birds is determined ultimately by the number of young that parents can provide with food.

26. Stabilizing Selection • Cost-benefit Analysis • No organism has an infinite amount of energy to spend on its activities

27. Stabilizing Selection • Blue tit experiment • Observation: • Normal brood size is 9-11 eggs. • Hypothesis: • Changing number of eggs will reduce fitness, i.e. offspring survival • Experiment: • Add and remove chicks

29. Coevolution: the evolutionary “Arms Race” • Reciprocal evolutionary influences. • Occurs when a trait of species A has evolved in response to a trait of species B. • Example: • Cowbirds (See Essay 2.1 in book) • Flowers and pollinators • Defense chemicals of plants: • Plants develop toxins that protect against herbivory • Herbivores develop detoxifying enzymes to enable them to eat the plant.

30. Red Queen Hypothesis ‘Now! Now!’ cried the Queen. ‘Faster! Faster!’ And they went so fast that at last they seemed to skim through the air, hardly touching the ground with their feet, till suddenly, just as Alice was getting quite exhausted, they stopped, and she found herself sitting on the ground, breathless and giddy. The Queen propped her up against a tree, and said kindly, ‘You may rest a little now.’ Alice looked round her in great surprise. ‘Why, I do believe we've been under this tree the whole time! Everything's just as it was!’

31. The species problem…What is a species? • Mayr (2001) “Even at present there is not yet unanimity on the definition of the species”. • Major problem: • Species concept vs. species as taxon. • Species concept = the meaning of species in nature and to their role in the household of nature. • Species taxon = a zoological/botanical object.

32. Types of speciation • Allopatric Speciation • Sympatric Speciation • Instantaneous Speciation • Polyploidy • Species by hybridization • Only 8 cases known • Speciation by distance (Circular overlap)

33. Allopatric Speciation • Geographic/Reproductive Isolation • Dichopatric speciation • New geologic barrier • plate techtonics (e.g.nothofagus, ratites) • uplift (e.g., Hawaiian land snails) b b a a

34. Peripatric speciation • Founder populations beyond the periphery of the current range.

35. Distribution of Nothofagus • Genus of about 35 species the “southern” beeches.

36. Ratite Distribution Brown & Lomolino, 1998

37. Ratites

38. Sympatric Speciation • Speciation occurring without geographic separation • More difficult to explain • Insects • Plant specificity • Fishes • Simultaneous habitat preference among certain males and females.

39. Speciation by distance (Circular overlap): Ensatina salamanders

40. Rates of Speciation • Highly variable – • The less gene flow between populations the faster the rate of speciation • Opposite extremes • Skunk Cabbage in Eastern U.S. and Asia • Isolated for 6-8 million years. • Lake Victoria – 400 species of cichlids • Basin was dry 12,000 years ago.

41. Extinction • 5 major extinctions • End of Ordovician - 444 Million years ago • Main theory – onset of a long ice age • Late Devonian -364 mya • Main theory - multiple causes and a series of distinct extinction pulses • Permian (The Great Dying) – 251 mya • ~96 % of all marine species and 70 % of terrestrial vertebrate species becoming extinct • Many theories - plate tectonics, an impact event, a supernova, extreme volcanism… • Triassic-Jurassic Extinction – 200 mya • Opened the door for the dinosaurs • Main theories – climate change, asteroid, volcanoes • Cretaceous-tertiary Extinction – 65.5 mya • 50–80% of all plant and animal families • Main theory – asteroid impact.

42. Extinction rates • Background rate (poorly understood): • Mammals: 1 species in 400 years • Birds: 1 species in 200 years • The process is natural, the current rate is not! • Generalists tend to survive better than specialists. Rats, coyotes, cockroaches and humans

43. Relationship to Life History • Rarity: geographic range, habitat breadth, local density • Dispersal Ability ~ isolation • Specialization: especially nutritional requirements • Population Variability: boom and bust? • Trophic Status: Eltonian Pyramid • Longevity: long-lived may survive variations • Intrinsic Rate of Increase: quick recovery

44. Threats (mostly human) • Habitat loss or modification • Overexploitation • Introduced species • Persecution, such as predator "control" • Incidental take: fishing "by-catch" • Introduced disease • Combinations of the above: multiple threats Recommended readings: Guns, Germs and Steel and Collapse by Jared Diamond