Descent with Modification: A Darwinian View of Life

1. Descent with Modification: A Darwinian View of Life Chapter 22

2. Overview: Darwin Introduces a Revolutionary Theory • A new era of biology began on November 24, 1859 – the day Charles Darwin published On the Origin of Species by Means of Natural Selection • This manuscript: • Focused biologists’ attention on the great diversity of organisms

3. Darwins Two Major Points from Origin of Species • Presented evidence that many species of organisms presently inhabiting the Earth are descendents of ancestral species (common descent) • Proposed a mechanism for the evolutionary process (natural selection) • a population’s allele frequency can change over generations if individuals that possess certain heritable traits leave more offspring than others • Results in evolutionary adaptation – accumulation of inherited characteristics that enhance organisms’ ability to survive and reproduce in specific environments • evolution– change over time in genetic composition of a population and could eventually lead to new species

4. Darwin’s Ideas Challenged Traditional Views • The Darwinian revolution challenged traditional views of a young Earth inhabited by unchanging species • In order to understand why Darwin’s ideas were revolutionary, it is important to examine his view in context of other Western ideas about Earth and its life • There was MUCH resistance to the idea of evolution

5. On the Origin of Species • Shook the deepest roots of Western culture • Challenged a worldview that had been prevalent for centuries

6. The Scale of Nature and Classification of Species • Aristotle (Greek Philosopher) • viewed species as fixed and unchanging • Scale of Nature – all living things are arranged on a scale of increasing complexity • Old Testament of the Bible • holds that species were individually designed by God and therefore are perfect • interpreted organismal adaptations as evidence that each species was designed to serve a specific purpose • Carolus Linnaeus (Swedish Botanist) • developed two-part (binomial) system of naming organisms – still used today • taxonomy – branch of biology concerned with naming and classifying organisms

7. Study of fossils (paleontology) helped lay groundwork for Darwin’s ideas Fossils - remains or traces of organisms from the past Usually found in sedimentary rock, which appears in the strata Fossils in each strata provide a glimpse of some of the organisms that populated Earth at the time that layer formed Fossils, Cuvier, and Catastrophism

8. Fossils, Cuvier, and Catastrophism • Georges Cuvier - noted that sedimentary strata contain unique groups of organisms • Deeper strata contain species that are often very different from those present today • Rather than follow evolution, Cuvier believed in Catastrophism - the idea that each strata boundary corresponds to some catastrophic event, such as flood

9. Theories of Gradualism • James Hutton introduced the theory of gradualism • Gradualism - holds that geological changes are the cumulative product of slow, continuous processes • Charles Lyell introduced the theory of uniformitarianism • Uniformitarianism - incorporates gradualism, and refers to the idea that geologic processes have not changed throughout earth’s history • Importance: the Earth must be very old. • Exerted a strong influence on Darwin’s thinking—applied the principle of gradualism to biological evolution

10. Lamarck’s Theory of Evolution • Jean Baptiste Lamarck placed fossils in an evolutionary context and was the first person to present a functional mechanism describing evolution • He used two common ideas of his time: • Use and disuse - organs that are used become stronger and more functional; those that are not used deteriorate • Inheritance of acquired characteristics - after acquiring a new trait, an organism is able to pass it on to offspring • Lamarck’s theory was based on observations; they were unsupported by evidence

11. Lamarck v/s Darwin

12. Darwin’s Proposals • In The Origin of Species, Darwin proposed that species change through natural selection • 19th century—generally believed species remained unchanged since their creation • Darwin’s Research : • As a boy and into adulthood, Charles Darwin had a consuming interest in nature • Soon after Darwin received his B.A. in theology, he was accepted on board the HMS Beagle which was about to embark on a voyage around the world

13. The Voyage of the Beagle • During his travels on the Beagle, Darwin: • Observed and collected many specimens of South American plants and animals • Observed various adaptations of plants and animals that inhabited many diverse environments • Geologic observations - found fossils of organisms high in the Andes Mountains; inferred that rocks containing fossils must have been raised there by a long series of earthquakes • Galapagos Islands • Learned that most species there occur nowhere else in the world, but many resemble species in South America • Species on the islands are related but, like finches, were adapted to different lifestyles

14. Galapagos Finches

15. Darwin’s Focus on Adaptation • Darwin reassessed all observations made during the voyage • He began to perceive adaptation to the environment and the origin of new species as closely related processes • Species adapt to succeed in changing/different environments • Confirmed years later—Galapagos finches’ beaks and behaviors are adapted to the specific foods available on their home islands • In 1844, Darwin wrote a long essay on the origin of species and natural selection but reluctant to publish, anticipating the uproar it would cause • In 1858, Darwin received a manuscript from Alfred Russell Wallace who had developed a theory of natural selection similar to Darwin’s • Darwin quickly finished The Origin of Species and published it next year

16. The Origin of Species • Evolution explains life’s unity and diversity • Natural selection is a cause of adaptive evolution • Descent with Modification • Think of descent in terms of a branching tree • Over time, organisms enter/experience new environments which lead to changes in the organisms • These changes cause divergence, the branches of the tree • Closely related species are closest together in the tree • The trunk represents a common ancestor • The Linnaean classification scheme fits the concept of an evolutionary tree; genus and species may differ, but still belong in the same family

17. Natural Selection and Adaptation • Darwin’s Observations: • All species have the potential to overproduce offspring. If they continued to do so, they would use up all of the world’s resources • Populations are stable, except for seasonal fluctuations, or unless changed by man altering the environment • Environmental resources are limiting • Individuals within a population have varying characteristics • Much of the variation is heritable (genetic)

18. Summary of Darwin’s Ideas • Natural selection is differential success in reproduction (unequal ability of individuals to survive and reproduce). • Natural selection occurs through an interaction between the environment and the variability inherent among the individual organisms making up populations. • The product of natural selection is the adaptation of populations of organisms to their environment.

19. Natural Selection & Adaptation

20. Thomas Malthus • His ideas had a profound influence on Darwin’s thinking: • “man will overpopulate the planet unless there is disease, war, or famine” - eventually things will start dying • Populations in nature DO NOT GROW EXPONENTIALLY – there are always limits to population growth!

21. Artificial Selection • Humans have modified other species over many generations by selecting and breeding individuals that possess desired traits • Results in plants and animals bred as livestock/pets that often bear little resemblance to their wild ancestors (ex. wild mustard)

22. Summary of Natural Selection • Overpopulation - more organisms are born than can survive • Variation within a population - there will be many variation for different traits among individuals • Competition within the population - individuals will compete for survival: food, mates, shelter, etc. • Survival of the fittest - those with traits best suited to the environment will be more likely to survive • Reproduction - individuals that survive will pass their traits on to the next generation

23. Key Points to Remember • Population - group of interbreeding individuals belonging to a particular species and sharing a common geographic area • A population is the smallest unit that can evolve • Natural selection occurs because of interaction between organisms and their environments • Natural selection works to increase or decrease the occurrence of heritable traits • Individuals are selected, but populations evolve

24. Evolution • Theory - an accepted hypothesis that has been tested over and over again without yet being disproved • Definition - Evolution is the change in the overall genetic makeup (allele frequency) of a population over time • Three Basic Componentsa.  Individuals cannot evolve.  Populations evolve.b.  Natural selection is the mechanism of evolution.c.  Evolution occurs by chance (NOT GOAL ORIENTED).

25. Darwin’s Theory Explains a Wide Range of Observations • Natural Selection in Action: • The evolution of insecticide resistance in insect populations • Note: an environment does not create resistance, it selects for resistant individuals that were already present in the population.

26. Evolution of Insecticide Resistance • By spraying crops with poisons to kill insects, humans have unwittingly favored the reproductive success of insects with inherent resistance to poisons. • Resistant individuals survive and reproduce, passing the gene for resistance to offspring. • Additional applications of the same insecticide will be less effective, and the frequency of resistant insects in the population will grow.

27. Darwin’s Theory Explains a Wide Range of Observations • The example of the insect resistance highlights 2 important points about natural selection: • It is an editing mechanism, not a creative force; it only acts on existing variation in population; cannot create favorable traits • It favors traits that increase fitness in current, local environment; what is adaptive in one situation may not be adaptive in another

28. Evidence of Evolution • Evidence that the diversity of life is a product of evolution pervades every research field of biology. • Comparative Anatomy • Anatomical Homologies • Embryological Homologies • Molecular Homologies • Biogeography • Continental Drift • Geographic Distribution of Species • Fossil Record Evidence • Succession of Fossil Forms

29. Comparative Anatomy • The study of different structures contributes to scientists’ understanding of the evolution of anatomical structures and of evolutionary relationships. • Homologous structures • Analogous structures • Vestigial Structures

30. Homologies Homologous structuresare similar structures occurring in different species that are believed to be derived from a common ancestor. Analogous structuresare similar structures occurring in different species that are believed to be the result of convergent evolution (similar environmental pressures).

31. Homology • Species with common ancestry should display underlying similarities, even in features that no longer match in function. • Homology refers to similarities in characteristics resulting from common ancestry • 3 types: anatomical, embryological & molecular

32. Anatomical Homologies • The forelimbs of human, cats, whales, and bats share the same skeletal elements, even though the appendages have very different functions • The most likely explanation is that the basic similarity of these forelimbs is the consequence of the descent of mammals from a common ancestor – rather than all being uniquely engineered and unrelated.

33. Homologous Structures – Figure 22.14

34. Anatomical Homologies • Many homologies that are not obvious in adult organisms become evident when we look at embryonic development: • All vertebrate embryos have tails posterior to the anus, as well as structures called pharyngeal (throat) pouches • Develop into gills in fishes and parts of ears/throat in humans

35. Embryological Homologies Closely related organisms go through similar stages in their embryonic development. For example, all vertebrate embryos go through a stage in which they have gill pouches on the sides of their throats.

36. Early Embryonic Development

37. Vestigial Homologies • Vestigial organs - structures that have marginal, if any, importance to living organism, but had important functions in organism’s ancestors

38. Vestigial Structures The skeletons of some snakes retain vestiges of the pelvis and leg bones of walking ancestors. We would not expect to see these structures if snakes had an origin separate from other vertebrate animals.

39. Molecular Homologies • All life forms use the same genetic machinery of DNA/RNA and the genetic code is essentially universal • Due to this, all species may have descended from a common ancestor • Humans and bacteria share genes inherited from a distant ancestor • Homologies and the Tree of Life • Molecular homologies basically follow the tree of life - more closely related organisms have more similar homologies.

40. Biogeography • Species tend to be more closely related to other species from the same area than to other species with the same way of life but living in different areas. • Austraila is the home of marsupials (develop in pouch) – distinct from eutherians (live elsewhere and develop in uterus) • Some similar mammals that have adapted to similar environments have evolved independently from different ancestors (ex. Squirrels)

41. Biogeography • Darwin’s experience in the Galapagos were pivotal to his thinking: • Islands generally have species found no where else in the world, yet it was observed by Darwin that most island species are closely related to species from the nearest mainland or neighboring island. • This explains why 2 islands with similar environments in different parts of the world are populated not by closely related species, but rather by species that resemble those of the nearest mainland.

42. NORTH AMERICA Sugar glider AUSTRALIA Flying squirrel Biogeography • Some similar mammals that have adapted to similar environments • Have evolved independently from different ancestors • This is convergent evolution

43. Fossil Record • We can see the development of present day species from ancestral species • The succession of forms observed in the fossil record is consistent with other inferences about the major branches of descent in the tree of life • Prokaryote fossils precede all eukaryotic fossils • Darwinian view predicts evolutionary transitions leave signs in fossil record • Paleontologists have discovered fossils of many transitional forms

44. Summary of Darwin’s Theory Individual organisms in nature differ from one another and some of this variation is inherited Organisms in nature produce more offspring than can survive – and many that survive do not reproduce Members of each species must compete for resources Individuals best suited to their environment survive and reproduce most successfully – they pass their traits onto their offspring Species change over time – this is caused by natural selection – new species arise and other species disappear Species alive today have descended with modifications from species that lived in the past All organisms on Earth are united into a single tree of life by common descent