Organic Evolution Chapter 6
Evolution - Defined • Evolution – a change in the genetic composition of a population over time. • A change in the frequency of certain alleles. • On a larger scale, evolution can be used to refer to the gradual appearance of all biological diversity.
Darwin’s Revolutionary Theory • The Origin of Species focused attention on the diversity of life, similarities as well as differences, and the adaptations organisms have for particular environments.
Darwin’s Revolutionary Theory • Charles Darwin presented evidence that many modern organisms are descended from ancestral species that were different.
Darwin’s Revolutionary Theory • Prevailing view of the world was that the Earth was only a few thousand years old and that all life had been created at the beginning and remained unchanged.
Pre-Darwinian Evolutionary Ideas • Several ancient Greek philosophers thought life changed through time. • Aristotle recognized fossils as forms of ancient life. • He developed the scala naturae (scale of nature). • Each form of life had a rung on the ladder. • Organisms were arranged in order of complexity. • The ancient Greeks didn’t propose an evolutionary mechanism.
Pre-Darwinian Evolutionary Ideas • Lamarck was the first to suggest an explanation for evolution. • Inheritance of acquired characteristics • Didn’t hold up to testing.
A Mechanism for Evolution • Darwin presented a mechanism for evolution – natural selection. • Organisms that are in some way more successful at reproduction will pass on more of their genes. • Over time the traits responsible for that success will become widespread in the population. • This theory holds up very well!!
Alfred Russell Wallace • Wallace independently developed a theory of natural selection. • He sent his manuscript to Darwin, spurring him to finally publish his ideas. • Both ideas were presented to the Linnean Society in 1858. • Darwin finished On the Origin of Species and published it in 1859.
Uniformitarianism • Charles Lyell’s principle of uniformitarianism: • Laws of physics & chemistry present throughout history of Earth. • Past geological events similar to today’s events. • Principles of Geology
Uniformitarianism • Natural forces could explain the formation of fossil-bearing rocks. • Lyell concluded the age of the earth must be millions of years. • He stressed the gradual nature of geological changes.
Uniformitarianism and the Age of Earth • Darwin studied the work of Lyell closely. He took the first volume of Lyell’s Principles of Geology on the Beagle. He received the second volume while on the voyage. • He concluded that Earth must be much older than 6000 years. • Perhaps these slow changes could work on living things as well…..
Evolution in Need of a Mechanism • Darwin was not the first to have the thought that organisms change through time. • His grandfather, Erasmus Darwin, and others suggested that life evolves as environments change. • But a mechanism for that change was needed.
Darwin (1809 – 1882) • Darwin had a lifelong love of nature. • His father wanted him to study medicine. • This was not what Darwin wanted and he didn’t finish.
Darwin • After leaving medical school he attended Cambridge University with the intention of entering the clergy. • His mentor and botany professor, John Henslow, recommended him for a position as ship’s naturalist aboard the Beagle.
The Voyage of the Beagle • Darwin started out on a five year trip around the world aboard the Beagle in 1831. He was 22. • As ship’s naturalist he spent his time on shore collecting thousands of plant and animal specimens and making important observations.
The Voyage of the Beagle • Darwin saw that the plants and animals that he found in temperate areas of South America were more similar to tropical South American species than they were to temperate European species.
The Voyage of the Beagle • The fossils he found in South America were more like modern South American species than European species.
The Voyage of the Beagle • During the voyage he read Lyell’s Principles of Geology. • He had Lyell’s ideas in mind as he traveled and observed the geology of South America.
The Voyage of the Beagle • He experienced an earthquake in Chile and observed that the coastline had risen several feet. • He also found marine fossils high in the Andes Mountains. • Darwin concluded that the mountains were formed by a series of such earthquakes.
The Voyage of the Beagle • Darwin became interested in the geographic distribution of organisms after visiting the Galapagos Islands.
After the Voyage • After returning, Darwin realized that adaptation to the environment and the origin of new species were closely linked processes. • Galapagos finch species have evolved by adapting to specific conditions on each island.
Natural Selection • After reading a paper by Thomas Malthus concerning the fact that human populations increase faster than limited food resources, Darwin noticed the connection between natural selection and this ability of populations to overreproduce.
Natural Selection • Only a small fraction of all offspring produced by any species actually reach maturity and reproduce. • Natural populations normally remain at a constant size.
Natural Selection • Those that survive may have heritable traits that increased their chances of survival. • They will pass those traits on. • The frequency of those traits will increase.
Artificial Selection • Artificial selection – people selectively breed organisms with desired traits. • Darwin noticed that considerable change can be achieved in a short period of time.
Natural Selection • Natural selection occurs when organisms with particular heritable traits have more offspring that survive & reproduce.
Natural Selection • Natural selection can increase the adaptation of an organism to its environment.
Natural Selection • When an environment changes, or when individuals move to a new environment, natural selection may result in adaptation to the new conditions. • Sometimes this results in a new species.
Natural Selection • Individuals do not evolve; populations evolve. • Evolution is measured as changes in relative proportions of heritable variations in a population over several generations.
Natural Selection • Natural selection can only work on heritable traits. • Acquired traits are not heritable and are not subject to natural selection.
Natural Selection • Environmental factors are variable. • A trait that is beneficial in one place or time may be detrimental in another place or time.
Darwinian Evolutionary Theory: Evidence • The main premise underlying evolutionary theory is that the living world is always changing. • Perpetual change in form & diversity of organisms over the last 700 million years can be clearly seen in the fossil record.
Fossils • Fossils are remnants of past life preserved in the earth. • Complete remains – insects in amber. • Petrified skeletal parts infiltrated with silica or other minerals. • Or traces of organisms such as molds, casts, impressions, trackways, or fossilized excrement.
The Fossil Record • Fossils provide support for the idea that life changes through time. • Fossil intermediates • Whales descended from land mammals. • Birds descended from one branch of dinosaurs. • The oldest fossils are of prokaryotes.
Dating Fossils • Geological time can be measured in sedimentary rock layers. • The Law of Stratigraphy • Dates oldest layers at the bottom and youngest at the top. • Time is divided into eons, eras, periods and epochs.
Dating Fossils • Radiometric dating methods are based on the decay of naturally occurring elements into other elements. • Different methods used for different time periods.
Dating Fossils - example • 40K has a half life of 1.3 billion years – meaning half of the 40K will have decayed to 40Ar and 40Ca. Half of what remains will decay in the next 1.3 billion years. • Measure ratio of remaining 40K to the amount of 40K originally there (remaining 40K plus 40Ar and 40Ca).
Fossil Record • The fossil record of macroscopic organisms begins in the Cambrian period: 505–570 MYA. • Fossil bacteria and algae, casts of jellyfishes, sponges spicules, soft corals, and flatworms are found in Precambrian rocks. • Mostly microscopic
Evolutionary Trends • The fossil record shows that species arise and go extinct repeatedly throughout geological history. • Trends appear in the fossil record – directional changes in features or patterns of diversity.
Evolutionary Trends • The evolution of horses from the Eocene epoch (57.8 MYA) to the present is a well studied trend. • Body size – increasing • Foot structure – fewer toes • Tooth structure – larger grinding surface
Common Descent • Darwin proposed that all organisms have descended from a single ancestral form. • Life history is shown as a branching tree called a phylogeny.
Homology • The phrase “descent with modification” summarizes Darwin’s view of how Evolution works. • All organisms descended from common ancestor. • Similar species have diverged more recently. • Homology – when similar structures result from shared ancestry.
Anatomical Homologies • Homologous structures – variations on a structural theme that was present in a common ancestor. • Example – vertebrate forelimbs have different functions, but share the same underlying structure.
Anatomical Homologies • Vertebrate embryos have a tail and pharyngeal pouches. • These structures develop into different but homologous structures in adults. • Gills in fishes • Part of ears & throat in humans.
Ontogeny & Phylogeny • There are many parallels between ontogeny (an individual’s development) and phylogeny (evolutionary descent). • Embryological similarities • Features of an ancestors ontogeny can be shifted earlier or later in a descendant's ontogeny.
Ontogeny & Phylogeny • Heterochrony – evolutionary change in timing of development. • Characteristics can be added late in development and features are then moved to an earlier stage. • Ontogeny can be shortened in evolution. • Terminal stages may be deleted causing adults of descendants to resemble youthful ancestors. • Paedomorphosis • Retention of ancestral juvenile characters by descendant adults.
Developmental Modularity and Evolvability • Heterotopy– a change in the physical location of a developmental process in an organism’s body. • Process must be compartmentalized into semi-autonomous modules to be expressed in new location • Ex: Location of toepad development in geckos.
Developmental Modularity and Evolvability • Evolvability– denotes the great evolutionary opportunities created by semi-autonomous developmental modules whose expression can be moved from one part of the body to another. • Allows for “experimentation” with the construction of many new structures.
Vestigial Organs • Vestigial organs – remnants of structures that served important functions in an ancestor. • Remnants of pelvis and leg bones in snakes • Appendix in humans