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

2. Darwin
A new era of biology began on November 24, On this day Charles Darwin published On the Origin of Species by Means of Natural Selection
Darwin made two major points in his book:
He presented evidence that the many species of organisms presently inhabiting the Earth are descendants of ancestral species
He proposed a mechanism for the evolutionary process, natural selection

3. Evolutionary History Cuvier: paleontology Lyell: uniformitarianism
Darwin: evolution
Mendel: inheritance
Wallace: evolution
Linnaeus: taxonomy
Hutton: gradualism
Lamarck: evolution
Malthus: populations

4. Influences on Darwin Linneaus: classification
Cuvier: Fossils, the remains or traces of organisms from the past; along with classification, helped to lay the groundwork for Darwin’s theories.
Hutton and Lyell: Gradualism, the idea that profound change can take place through the cumulative effect of slow but continuous processes (opposite of catastrophism); had a strong influence of Darwin’s thinking
Lamarck: Evolution, idea that current species are modified descendents of previous/extinct species; even though mechanism was unsupported (through acquired traits), but concept was valid

5. Darwin’s Voyage Naturalist on HMS Beagle
5 year voyage around the globe from
Job: to catalogue all of the flora and fauna they encountered
Made significant discoveries to his theory in the Galapagos Islands

6. Darwin’s Publication
In 1844, Darwin wrote a long essay on the origin of species and natural selection, but he was reluctant to introduce his theory publicly, anticipating the uproar it would cause, most of scientific world did not believe in evolution or gradualism
In June 1858 Darwin received a manuscript from Alfred Russell Wallace. Wallace had developed a theory of natural selection similar to Darwin’s
Darwin quickly finished The Origin of Species and published it the next year, 1859

7. Darwin’s Theory
Descent with Modification - all organisms are related through descent from an ancestor that lived in the remote past
Modification through Natural Selection - populations of organisms can change over generations if individuals having certain heritable traits leave more offspring than others (differential reproductive success), organisms better “fit” to the environment will reproduce more successfully thereby passing on their better adapted genes.

8. Darwin’s Evidence
Darwin’s Finches – looked at beaks adapted to specific diets found on different islands

9. Darwin’s View
In the Darwinian view, the history of life is like a tree with multiple branches from a common trunk. The tips of the youngest twigs represent the diversity of living organisms as they exist today

10. Darwin’s Theory Explained
Evolutionary biologist Ernst Mayr – dissected the logic of Darwin’s theory into three inferences based on five observations
Observations:
Exponential fertility
Stable population size
Limited resources
Individuals vary
Heritable variation
Inferences:
Struggle for existence
Non-random survival
Natural selection (differential success in reproduction)

11. Darwin’s Theory Explained
Observation #1: For any species, population sizes would increase exponentially if all individuals that are born reproduced successfully
Observation #2: Nonetheless, populations tend to be stable in size except for seasonal fluctuations
Observation #3: Resources are limited
Inference #1: Production of more individuals than the environment can support leads to a struggle for existence among individuals of a population, with only a fraction of their offspring surviving

12. Darwin’s Theory Explained
Observation #4: Members of a population vary extensively in their characteristics; no two individuals are exactly alike
Observation #5: Much of this variation is heritable

13. Darwin’s Theory Explained
Inference #2: Survival depends in part on inherited traits. Individuals whose inherited traits give them a high probability of surviving and reproducing are likely to leave more offspring than other individuals
Inference #3: This unequal ability of individuals to survive and reproduce will lead to a gradual change in a population, with favorable characteristics accumulating over generations

14. Natural Selection
Differential success in reproduction that results from the interaction between individuals that vary in heritable traits and their environment
Can produce an increase over time in the adaptation of organisms to their environment
If an environment changes over time, natural selection may result in adaptation to these new conditions

15. Evidence for Natural Selection
Examined predation pressures
Transplanted guppies from pike-cichlid pools to killfish pools, measured ages and size over 11-year period
Average size and age at maturity of guppies of transplanted populations increased
Concluded that change in predator resulted in some population variations being favored over others

16. Evidence for Evolution
Homology
Embryology
Vestigial Structures
Similarities in Macromolecules
Biogeography
Fossils

17. Evolution Evidence: Homology
Anatomical resemblances that represent variations on a structural theme that was present in a common ancestor

18. Evolution Evidence: Embryology
Comparative embryology reveals additional anatomical homologies not visible in adult organisms
Shows common ancestry

19. Evolution Evidence: Vestigial
Remnants of structures that served important functions in the organism’s ancestors
Shows evolutionary history of a species and common ancestry

20. Evolution Evidence: Molecular
Biologists also observe homologies among organisms at the molecular level, including genes that are shared among organisms inherited from a common ancestor and the proteins encoded in those genes.

21. Evolution Evidence: Biogeography
The geographic distribution of species
Some mammals that have adapted to similar environments have evolved independently from different ancestors

22. Evolution Evidence: Fossil Record
The succession of forms found in the fossil record
Shows evolution of current species from ancestral species over time