Understanding Evolution: Darwin's Theory and Natural Selection

1. Ch. 15: Mechanisms of Evolution 15.1 – Evolution is both Factual and the Basis of Broader Theory 15.2 – Mutation, Selection, Gene flow, Genetic Drift, and Nonrandom Mating result in Evolution 15.3 – Evolution can be measured by changes in allele frequencies 15.4 – Selection can be stabilizing, Directional, and Disruptive 15.5 – 15.6 Skip AP Biology C 2013

2. Charles Darwin What do we know about him?

3. Charles Darwin 1809-1882 • 1830’s – took a 5 year trip around the world on a ship called the HMS Beagle • He went to the Galapago’s Islands, and was intrigued by unique organisms such as giant tortoises and finches • He noticed similarities and differences among the many organisms he saw as he traveled • He noticed striking differences between the species he saw in South America and those of Europe • He noted that most of the animals on the Galapago’s Islands were endemic – unique to that location and nowhere else

5. He concluded that the finches must have adapted to their habitat and food available on that continent.

6. Jean Baptiste Lamarck (1744-1829) • Thought that simple organisms could arise from nonliving matter • Also thought that simple forms of life develop into more complex forms • Proposed that individuals could acquire traits during their lifetime as a result of experience or behavior – inheritance of acquired characteristics • His ideas were rejected by scientists

7. 3 Major Propositions of Darwin • Species change over time (evolve) • Divergent species share a common ancestor • Natural Selection • Mechanism that produces change in species • The differential survival and reproduction of individuals in a population based on variation in their traits Darwin published his ideas in a book called, Origin of Species, published in 1859. It stirred considerable interest (and controversy) among scientists and the public.

8. Theory of Natural Selection (4 parts) • Overproduction • Genetic Variation • Struggle to survive • Differential Reproduction

9. #1 - Overproduction • More offspring can be produced that can survive to maturity • Darwin used Malthus’s book as part of his reasoning • Human populations can increase more quickly- that food supplies and that populations are often limited by conditions such as war, disease, or lack of food • Darwin realized that the environment limits the population of ALL organisms by causing deaths or by limiting births

10. #2 – Genetic Variation • Within a population, individuals have different traits • Ex – some deer have thicker fur or longer legs than others. The deer with thicker fur will have offspring with thicker fur • Occasionally, new traits may appear in a population

11. #3 – Struggle to Survive • Individuals must compete with each other in what Darwin called a “struggle for existence” • Some variations improve an individual’s chance to survive and reproduce, but some variations reduce this chance • Ex – thicker furred deer may survive better in the cold • A trait that makes an individual successful in its environment, such as thick fur, is called an adaptation

12. #4 – Differential Reproduction • Organisms with the best adaptations are most likely to survive and reproduce • And through inheritance, the adaptations will become more frequent in the population, so populations may begin to differ

13. Chapter 15 Natural Selection

14. “Survival of the fittest” • Term Darwin sometimes used • Describes natural selection • Fitness – is a measure of an individual’s hereditary contribution to the next generation • A fit individual is one that has offspring that also live long enough to reproduce

15. 3 Patterns of Natural Selection • Stabilizing Selection • Individuals with the average form of a trait have the highest fitness • Reduces variation in populations • Example – Predators can easily catch larger visible lizards and the slower smaller lizards. The average ones best survive! • Example – human birth weight

16. 3 Patterns of Natural Selection Disruptive Selection • Individuals with either extreme variation of a trait have greater fitness than individuals with the average form of the trait • Increases variation in population • Example – bill sizes in black-bellied finches • Food source is two different kinds of seeds from marsh plants (soft and hard) • The birds with large bills can easily crack the hard seeds • Birds with the small bills can crack the soft seeds • Birds with medium bills do not survive well because they are less efficient that the other birds at cracking either seed.

17. 3 Patterns of Natural Selection Directional Selection • Individuals that display a more extreme form of a trait have greater fitness than individuals with an average form of the trait • Average shifts in one direction • Example – Hunters hunt for the biggest animals, therefore the smaller ones survive!

18. Evidence of Evolution • Fossil Record • Biogeography • Anatomy and Embryology • Biological Molecules

19. Fossils • Remains or traces of an organism that died long ago • Show that different types of organisms appeared at different times and places on Earth • Some fossils are of organisms that have become extinct • Most powerful evidence of evolution

20. Transitional Species • Fossil record shows that species have differed in a gradual sequence of forms over time • Transitional species – have features that are intermediate between those of hypothesized ancestors and later descendent species

21. Section 2 Evidence of Evolution Chapter 15 Evidence of Whale Evolution Transitional (mammal) – lived in coastal waters; could swim by kicking its legs and using tail for balance. It could waddle on land with its short legs Ancestor – land dwelling mammal Transitional (mammal) – lived in ocean; resembled a dolphin, propelled itself with massive tail; had forelimbs that were flippers and tiny hind limbs that could not have been used for walking or swimming Modern whales – have forelimbs that are flippers; They have tiny non-functioning hip bones at the rear of their bodies

22. Evidence #2 - Biogeography • Biogeography – study of the locations of organisms around the world • Darwin and another scientist, Wallace, observed animals that seemed closely related yet were adapted to different environments in nearby regions • They also observed animals that seemed unrelated but that had similar adaptations to similar environments in regions that were far apart • Mammals of Australia provide the evidence of biogeography • There are native Australian animals that resemble wolves, cats, mice, moles or anteaters • However, most Australian mammals are marsupials (pouches for young); • Possible explanation is that these animals evolved in isolation on the Australian continent

23. Evidence #3 – Anatomy and Embryology • Anatomy – the study of the body structure of organisms • Embryology – study of how organisms develop

24. The forelimbs are used in different ways in each animal; yet each limb has a similar bone structure • One explanation is that an early ancestor shared by all of these vertebrates had a forelimb with a similar bone structure • As generations passed, different populations of descendents adapted to different environments • Bones inherited from ancestors may have become modified for different tasks.

25. Body Structures • Homologous Structures – anatomical structures that occur in different species and that originated by heredity from a structure in the most recent common ancestor of the species • Might have different function • Example – forelimbs on birds, dolphins, humans • Analagous structures – have closely related functions but do not derive from the same ancestral structure • Example – wings on bats, birds and moths – all have different underlying structure

26. Vestigial Structures • Structures that serve no function but that resemble structures with functional roles in related organisms • Example – human tail bone, coccyx is made up of 4 fused vertebrae that resemble the bones in an animal’s tail • Another Example – human appendix and pelvic bones of modern whales

27. Development of Animal Embryos • Stages of vertebrate embryo development are very alike • One possible explanation for these similarities is that vertebrates share a common ancestor and have inherited similar stages of development

28. Evidence #3 – Biological Molecules • In all species, DNA and RNA are the molecular basis for inheritance of traits • Biologists can compare the DNA, RNA, proteins, and other biological molecules from many different organisms