What was Darwin trying to explain? Observations Hypotheses

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1 What was Darwin trying to explain? Observations Hypotheses

2 Charles Darwin Englishman; bound for medicine then the clergy sidetracked by biology Beagle 1839 Journals 1859 The Origin of Species! Outlined the process of natural selection

3 Observations that suggested change over time Layered Fossils Extinctions Transitional forms Environmental/Geological change Apparent relatedness of species

4 Fossil record Fossils of complex organisms occur closer to the surface, in rocks of younger age, than those of simpler organisms. This layering was globally consistent suggesting they were different ages. Darwin had huge collections to examine, even in early 1800 s

5 Extinctions Some fossils clearly belonged to creatures no longer walking the Earth Irish elk More famous examples like T. rex

6 Transitional Forms & Environmental Change fossil sandwiches occurred in these layers; middle fossils intermediate in form between fossils above and below. Documented changes in traits through time

7 Presence of tiny, useless traits on contemporary species Anal spurs; coccyx Vestigial traits

8 Legless lepidosaurs Jon Sullivan; wildherps.com

9 Transitional Forms Darwin s predecessors Shared distribution of fossil species & similar looking contemporaries Law of Succession

10 Apparent Relatedness Darwin s contribution came after and almost certainly as a result of his voyage through the Galapagos island archipelago Charles Darwin (22) joins crew of HMS Beagle as gentleman s companion (ship s naturalist) & collects & catalogues everything. ( ) Plants, insects, fossils & Galapagos mockingbirds After his return he and his naturalist friends noted some things about his collections:

11 Different islands have distinct (but similar) species

12 Weight of evidence lead to his heretical proposal: H: Species on neighboring islands look similar because they are all descended from a common ancestor; The small differences between them are due to changes over time. A bold assertion & dangerous for the time. Species are not static!! So, he waited ~20 years after developing the idea to publish it

13 On the Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life -Charles Darwin, 1859 Darwin s blackbirds, grossbeaks, wrens and finches

14 Origin; Figure 1 What does this figure explain? Diversity Unity

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18 Darwin s hypothesis Accounted for phenotypic differences between species Differences are due to changes that accrue over time What about those phenotypic similarities between species? Similarities are what suggests common ancestry. We use similarities to determine which species are most closely related.

19 We recognize 3 types of similarities (homologies) Structural/Morphological Outward physical appearance Developmental Sequence & timing of developmental events; shared germinal tissues Genetic DNA sequence Darwin had these first two at his disposal.

20 Structural Why would a wing, a shovel, a grasping hand, a paddle, a rudder & a hinged pendulum be built from same bones, in same orientation & relative positions, unless the limb components of each organism evolved from a common ancestor?

21 Developmental Why should human embryos ever have gills and a tail; why should chick embryos ever have gills; and why should those structures appear at similar developmental stages & in same relative positions as those of fish? Fish & human jaws look very different, but develop from same population of embryonic cells. Why, unless?

22 Genetic Why should the same 64 codons specify the same building blocks in ALL organisms? Why should these strings of code, or genes, be the same across dissimilar organisms?

23 What was Darwin s contribution? Others had proposed evolution (of some variety) as a pattern that required explanation. Darwin provided the process, natural selection, that explained the pattern of descent with modification.

24 Darwin s postulates (hypotheses) 1. Traits in a population vary among individuals 2. Some traits are passed on to offspring (i.e. traits have a genetic basis and are heritable) 3. Due to battles for resources, some individuals produce more offspring than others 4. The subset of all offspring that survive are those that possess beneficial traits; these traits (and the individual carrying them) are naturally selected

25 Recall the mockingbirds Imagine: ancestral mockingbird population colonizes 4 Galapagos islands If all the postulates are satisfied, then Darwin predicted that mockingbird populations on each island would become different from their ancestral population, and increasingly well adapted to each of their respective environments, as better adapted individuals reproduced at disproportionately high rates.

26 Darwin introduced fitness to explain natural selection Fitness: the ability of an individual to produce fertile offspring, relative to that ability in other individuals. This is a measurable quantity, and allows us to define adaptation Adaptation: a heritable trait that increases the fitness of an individual relative to individuals lacking that trait. These are time and location (environmentally) dependent

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