Descent with modification

Descent with Modification - Darwin (22.1-22.3)

  1. Be Able to Describe How We Explained Patterns of Diversity Pre-Darwin

How Do We Explain Biodiversity?

Early Views of Biodiversity:

  • Attempts at understanding of biological diversity began with Aristotle (384-322 BC)

  • Developed notion that species have “fixed properties”

  • Each species had been created individually by God in the same form it has today (special creation) 

  • Because order is superior to disorder, God’s creation must follow a plan - graduation from inanimate objects up through “higher” forms of life 

  • Scale Naturae


Naturalists used structural similarities 

  • Carolus Linnaeus (1707-1778) established the framework of modern taxonomy in Systema Naturae 

  • Founder of modern taxonomy 


Paleontology - evidence that life has changed 

  • Georges Cuvier (1769-1832)

  • Fossils (such as mammoths and mastodons) are not like modern species and are not found on Earth because they have gone extinct 

  • Mary Anning (1799-1847) discovered several species of extinct marine reptiles 


Scientific Revolution (~15th - 17th Century)

  • Astronomy/Physics - Newton, Copernicus, Galileo

  • Theory of gravity using scientific approach 

  • Earth not the center of universe 

  • Geologists

  • Principle of uniformitarianism - the same processes operated in the past as in the present - the Earth is very old!

  • Natural History/Anatomy - Leonardo da Vinci, Vesalius 

  • Anatomy and Embryology; underlying similarity with animals 


Evolution - simple to complex 

  • Jean Baptiste Lamarck (1744-1829)

  • Hypothesized that different organisms originated separately by spontaneous generation from non-living matter - starting from the bottom of Scala Naturae 

  • Recognized the important role of the environment 

  • First to propose evolutionary change 

  • Species differ because they have different needs 

  • Proposed mechanism of evolution = inheritance of acquired characteristics


Thomas Malthus (1766-1834)

  • Economist who philosophized about populations 

  • Rate of human population growth is greater than the rate of increase in the food supply, so unchecked growth must lead to famine 

  • Competition for resources 

  • Inspiration for Darwin’s concept of “struggle for existence.”


  1. Be Able to Describe the Rules of Evolution and Describe Observations which Darwin Collected to Support the Theory 

Theory of Evolution

  • Charles Darwin (1809-1882) developed the theory of evolution based on 2 major hypotheses:

  1. Organisms have descended, with modification, from common ancestors-descent with modification 

  2. The chief cause of modification is natural selection 


Evolution by Natural Selection

  • The theory of evolution by Natural Selection was proposed by Darwin and Wallace (1858)

  • Charles Darwin (1809-1882)

  • Alfred Russell Wallace (1823-1913)


Voyage of the Beagle (1831-1836)

  • After graduating from Cambridge University, Darwin took a position as a naturalist on a 5-year voyage on the HMS Beagle

  • Collected specimens of South American plants and animals 

  • Noted that fossils resembled living species from the area in which they were found, and living species resembled other nearby species 


Adaptations

  • On the Galapagos Islands, Darwin collected many similar, but different specimens of finches, some unique to individual islands, others found on multiple islands 

  • Darwin observed many examples of adaptations within these birds 

  • Adaptations are inherited characteristics that enhance an organism’s survival and reproduction in specific environments


Adaptive Radiation

  • Animals unique to the islands resembled species on the nearby mainland of South America 

  • Darwin hypothesized that species from the mainland colonized and then diversified on the islands (adaptive radiation) 

  • Darwin perceived that new species could arise from ancestral forms through gradual accumulation of adaptations


Natural Selection

  • Darwin proposed natural selection as an explanation for adaption

  • Natural selection is a process in which individuals with certain inherited traits tend to survive and reproduce at higher rates because of their inherited traits 


Descent with Modification: 

  • Descent with modification by natural selection explains three broad observations: 

  1. The unity of life 

  2. The diversity of life 

  3. The ways organisms are suited to life in their environments 

  • All organisms are related by descent from a common ancestor that lived in the past 

  • Related organisms living in different habitats gradually accumulated diverse modifications to fit them to specific ways of life 


Diversity of Form: 

  • Shared characteristics among different species should the unity of life 

  • Ex. Darwin’s finch species all have short, stout beaks and similar body plans 

  • Differing species also have differing characteristics 

  • Finch species differ in beak shape and size, which are adapted to different food sources 

  • Darwin’s finches illustrate 3 key observations about life: 

  1. Organisms are adapted for life in their biotic and abiotic environments 

  2. The many shared characteristics (unity) of life 

  3. Display difference in morphology (diversity of life) 



Tree of Life: 

  • After On the Origin of Species (1859) was published differences and similarities among organisms came to be seen as products of their evolutionary history 


Darwin’s Theory of Evolution: 

  1. Evolution - charactertics of organisms change over time 

  2. Common descent - species diverged from common ancestors 

  3. Gradualism - differences evolve by small steps through intermediate forms 

  4. Natural Selection - Adaptations (features that enhance survival) are preferably passed on to the next generation 

  5. Population Change - evolution occurs by changes in the frequencies of different kinds of individuals in a population 


Lamarck Vs. Darwin: 

see Ipad notes


“Rules” of Evolution: 

  • Individuals do not evolve → populations evolve over time 

  • Natural selection can only increase or decrease inheritable traits that are a variable in a population 

  • The environment varies from place to place and over time → favorable traits vary with the environment 


3. Be familiar with lines of evidence that exist that support the theory of evolution 


Homology: 

  • Homology: similarity resulting from common ancestry, is another type of evidence for evolution 

  • Related species can have characteristics that have an underlying similarity yet function differently 

  • Homologous structures are anatomical resemblances that represent variations on a structural theme present in a common ancestor. 

  • Comparative embryology reveals anatomical homologies not visible in adult organisms 

  • Example: all vertebrate embryos have a post-anal tail and pharyngeal arches (vestigial in adult humans) 

  • The arches develop into structures with very different functions in adults different vertebrate groups


Homology & Evolutionary Trees:

  • Evolutionary Trees are diagrams that reflect hypithese about relationships among groups 

  • Homologies form nested patterns on the tree 

  • Relatedness is determined by the recent common ancestor, not by the proximity of the groups on the tree 

  • Evolutionary trees show relative timing of events, not actual dates 


Fossil Record: 

  • Darwin drew from the study of fossils, remains or traces of organisms from the past 

  • Many fossils were found in sedimentary rock, which appears in layers called strata 

  • Palentology is the study of the fossil record 


Gradual Change Over Time: 

  • Through studies of the fossil record, Darwin reasoned that the Earth must be older than the widely accepted age of a few thousand years 

  • If true, gradual processes could also account for substantial biological change 

  • The 1861 discovery of Archaeopteryx just after Darwin’s On the Orgin of Species, provided striking evidence for evolution by showing transitional form between dinosaurs and birds – sparking controversy at the time but now recognized as one of the most 

important fossil finds in history 


The Fossil Record: 

  • The fossil record provides evidence of the extinction of species, the origin of new groups, and changes within groups over time 

  • Ex. The fossil record supported the DNA-based hyporhtesis that cetaceans are close relatives of even-toes ungulates 

Continental Drift: 

  • The locations of continents and oceans have dramatically changed over the last 251 million years because of continental drift → movement of landmasses across Earth’s surface 

  • Pangaea existed on Earth about 250 million years ago 

  • Laurasis separated from Pangaea about 150 mya and split into North America, Europe and Asia 

  • Gondwana split into South America, Africa, Antarctica, and India 


Biogeography: 

  • Biogeography is the scientific study of the geographic distribution of species 

  • Understanding continental drift and modern species distribution helps predict when and where different groups evolved 

  • Ex. Freshwater fish in the family Galaxiidae live in South America and Australia 

  • Species share an ancestor dating back to Pangea 


Biographical Patterns: 

  • Phylogenetic and geographic relationships among extant maruspials 

  • Floristic realism of the world - dendrogram depicts the phylogenetic relationships 


Summary: 

Pre-Darwin Views: 

  • Aristotle: species as fixed, “Scala Naturae”

  • Linnaeus: taxonomy framework 

  • Cuvier and Anning: fossils show extinction 

  • Lamarck: first to propose evolutionary change 


Darwin and Natural Selection:

  • Descent with modification from common ancestors 

  • Natural selection as the mechanis of adaptation 

  • Observations from HMS Beagle: variation, adaptation (e.g. Galapagos finches), adaptive radiation 


Key Principles (“Rules” of Evolution)

  • Populations evolve, not individuals

  • Natural selection acts on heritable variation 

  • Environment shapes which traits are favorable 


Lines of Evidence for Evolution 

  • Homology (anatomy, embryology, molecular)

  • Fossil record (extinction, transitional forms) 

  • Biogeography (continental drift, species distributions) 



Big Picture: 

  • Evolution explains the unity of life, diversity of life, and adaptations to environments 

  • “Nothing in biology makes sense except in light of evolution”