Chapter 11: Evidence for evolution

1. Chapter 11: Evidence for evolution Part 1

2. Reflections of a distant past • You are about to make a leap through time: from what is known to what can only be inferred. • How do we know about events that happened in Earth’s past, millions of years ago or more? • First of all, we must remember that the same physical, chemical, and biological processes that are going on today were going on back then also. • Next, we can look for evidences of things that happened back then that are present still today. • For example, the mile-wide Barringer Crater in Arizona was most probably caused by a 300,000 ton asteroid that slammed into Earth more than 50,000 years ago. • The impact was probably 150 times more powerful than the bomb that was dropped on Hiroshima!!!! • Scientists have determined this by analyzing meteorites, melted sand, and other clues in the rocks at that site.

3. Reflections of a distant past • What about events that happened even further back in Earth’s history? • An even larger asteroid impacted Earth in the more distant past. • A mass extinction (or permanent loss of major groups of organisms) occurred around 65.5 million years ago. • How do we know? • There is an unusual, worldwide layer of rock called the K-T boundary. • Below this rock layer, there are many dinosaur fossils. • But, above this boundary (in rock layers deposited more recently), there are NO dinosaur fossils ANYWHERE. • There is an impact crater off the coast of the Yucatan Peninsula that dates to about this same time. • Scientists have inferred from similar evidence as was used to investigate the Barringer Crater, that an asteroid over 12 miles wide hit the Earth and wiped out the dinosaurs at about this time.

4. Reflections of a distant past: the Barringer Crater

5. Reflections of a distant past: illustration of the chicxulub crater in the yucatan peninsula

6. Early beliefs • Naturalists: people who observe life from a scientific perspective • Aristotle: believed in a “great chain of being” that extended from the lowest form (snakes) to humans to spiritual beings. Each link in the chain is a species and all were created at the same time in a perfect state. Species could not change after being created because any variation from its original, perfect state would result in imperfection. • Alfred Wallace: noticed patterns in where species lived and how they survive. Began to think about the natural forces that shape life.

7. Early beliefs • Naturalists were pioneers in biogeography: the study of patterns in the geographic distribution of species. • They noticed that species living in very isolated places, very far away from each other looked suspiciously similar and survived in similar ways. • They wondered if these different species could be related? • If so, how did they end up geographically so far apart from one another?

8. Early beliefs • Consider the three flightless birds pictured below. Each lives on a different continent but all three run on long, muscular legs in flat, open grasslands in similar environments. • All three raise their long, thin necks to look for predators. • Wallace thought these similarities might mean that these three types of birds are descendants of an ancient common ancestor. African ostrich South American rhea Astralian emu

9. Early beliefs • Yet, Wallace had no idea how these similar, apparently related species ended up on different continents. • To add to the confusion, naturalists had trouble classifying species that are similar in some species but different in others, such as the cacti below. • Though both live in hot deserts where water is scarce for most of the year, they both have sharp spines that deter predators, and thick, fleshy stems that store water, they live on different continents and their reproductive parts are very different. American spiny cactus African spiny spurge

10. Early beliefs • In addition, as naturalists began their study of comparative morphology (the study of body plans and structures among groups of organisms), they began to find body parts with no obvious function. • If every species were created in a perfect state, why would there be leg bones in a snake or remnants of a tail in humans?

11. Early beliefs • Puzzling also were the geological studies of sequential rock layers. • Geologists found identical rock layers in different parts of the world. • Fossils in lower layers were more simple. • Fossils in higher layers were more complex and detailed. • Fossils of gigantic animals that no longer survived were also found. • If animals had been created perfect, why did fossil evidence show changes in sequential rock layers? • If animals had been created perfect, why would they become extinct? • All of these findings taken together caused scientists to question their beliefs: Maybe species had not been created in a perfect state, maybe they had changed over time.

12. New theories • Georges Cuvier: observed sudden changes in the fossil record and that many fossils have no living representatives. • Proposed a new idea that many species that had once existed were extinct • Concluded that earth’s surface had changed over time from observations of marine fossils high on mountaintops • Proposed that geologic forces unlike those we see today would have been necessary to raise sea floors to mountaintops in the short time span that Earth had existed (The belief at the time was that Earth was thousands-not millions- of years old.) NOT CORRECT • His idea became known as catastrophism-that catastrophic geologic events had caused the extinction of animals and then those surviving the events repopulated the Earth

13. New theories • Jean-Baptiste Lamarck: first to begin thinking about processes that drive evolution (change in a line of descent) • believed that species gradually improve over generations as they strive toward perfection, up the chain of being. • An unknown “fluida” in the body was responsible for body parts needing to change in the strive for perfection • Hypothesized that environmental pressures and internal needs cause changes in an individual’s body and then offspring inherit these changes. • Theories: Use and Disuse (“If you don’t use it, you lose it”) Inheritance of Acquired Characteristics (“I broke my leg so all of my children will be born with a broken leg.”)

14. New theories • Charles Darwin: theologist who was a naturalist aboard a ship called the Beagle on a survey expedition to South America • On his 5 year voyage, Darwin read Charles Lyell’s book Principles of Geology. • Lyell believed in the theory of uniformity (the idea that gradual, repetitive change had shaped Earth) • This theory said that catastrophe was not necessary to explain Earth’s surface and the fossils it contains in certain locations. • It said over great spans of time, everyday geologic processes such as erosion could have sculpted Earth’s current landscape. • This theory challenged the belief that the Earth was only 6,000 years old. • Lyell calculated that it would have taken millions of years for Earth’s surface to have been sculpted by these everyday geologic processes.

15. darwin

16. New theories • Darwin collected thousands of specimens on his journey. • These included fossils of an extinct armored mammal called glyptodons. • Glyptodons, Darwin believed, are ancestors of modern day armadillos. • Armadillos live only in places glyptodons once lived. • Both armadillos and glyptodons have/had helmets and protective shells consisting of bony plates.

17. New theories • Back in England, Darwin studied the notes and specimens he had from his voyage. • He also read an essay by Thomas Malthus. • Malthus correlated increases in the human population size with famine, disease, and war. • Malthus said that humans run out of food, living space, and other resources when they reproduce beyond the carrying capacity of the environment. • Humans must then compete with each for resources or develop new technology to increase their resources. • Darwin suggested that this applies to all populations, not just humans!!

18. New theories • Darwin also considered all of the species he had observed during his voyage. • He noticed that all members of the same species were not identical. • He noticed that, though the members of the same species had many traits in common, they might vary in size, color, or other traits. • He believed that having particular variations of a trait might give an individual a survival advantage over other members of the same species when competing for resources. • Certain variations of a trait might give an individual a survival and reproductive advantage over other members of the same species, making it better suited to its environment.

19. New theories • Darwin called this ability to survive and reproduce in a particular environment an organism’s fitness. • We define fitness as the degree of adaptation to a specific environment, and measure it as relative contribution to future generations. • A trait that improves an organism’s fitness is called an evolutionary adaptation, or adaptive trait.

20. New theories • Over generations, individuals with the most adaptive traits tend to survive longer and produce more offspring, thus contributing more to future generations. • This process, which Darwin called natural selection, could be a driving force of evolution. • More adaptive traits=more fit=longer survival=more offspring • If the adaptive traits of the organism are inheritable, more offspring with that trait will be produced by the better adapted organism that is living longer and producing more offspring, leading to the frequency of this trait increasing in the population of that organism over successive generations.

21. Principles of natural selection • Observations about populations: • Natural populations have an inherent reproductive capacity to increase in size over time. • As a population expands, resources that are used by its individuals (such as food and living space) eventually become limited. • When resources are limited, the individuals of a population compete for them. • Observations about genetics: • Individuals of a species share certain traits. • Individuals of a natural population vary in the details of those shared traits. • Shared traits have a heritable basis, in genes. Alleles (slightly different forms of a gene) arise by mutation. • Inferences: • A certain form of a shared trait may make its bearer better able to survive. • The individuals of a population that are better able to survive tend to leave more offspring. • Thus, an allele associated with an adaptive trait tends to become more common in a population over time.

22. New theories • Darwin wrote out his ideas about natural selection but waited over ten years to publish them. • Meanwhile, Alfred Wallace who was studying wildlife in the Amazon wrote an essay and sent it to Darwin. • Wallace’s essay contained the same thoughts as Darwin’s theory of natural selection! • A few weeks later, Wallace’s and Darwin’s theories were presented together at a scientific meeting, though neither man was at the meeting. • The next year, Darwin published On the Origin of Species, a book that laid out detailed evidence to support his theory. • His theory of evolution, or descent with modification, was more readily accepted than was his theory on how it occurs (by natural selection). • Experimental evidence from the field of genetics has now led to its widespread acceptance in the scientific community.

23. New theories • Part 2 of this chapter will consider the different forms of evidence we have that support Darwin’s theory. • Homework: • Pick one scientist before Darwin and explain what part of that person’s theory was correct (if any) and what part was incorrect (if any).