Organic Evolution Chapter 6 Part 1 Evolution Defined

Organic Evolution Chapter 6 (Part 1)

Evolution - Defined l Evolution – a change in the genetic composition of a population over time. l A change in the frequency of certain alleles.

A Revolutionary Theory l 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 l First Ideas: l l Aristotle recognized fossils as forms of ancient life. l l Several ancient Greek Philosophers thought life changed through time. 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 l Lamarck was the first to suggest an explanation for evolution. l l Inheritance of acquired characteristics Didn’t hold up to testing.

Lamarck Inheritance of Acquired Characteristics: organisms by striving to meet the demands of the environment acquire adaptations and pass them on by heredity to offspring l Transformational: Claims that individual organisms transform their characteristics through the use or disuse of parts l

Darwin’s Revolutionary Theory l Charles Darwin suggested Common Descent: that many modern organisms are descended from ancestral species that were different.

Darwin’s Revolutionary Theory l 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 l 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. l Over time the traits responsible for that success will become widespread in the population. l This theory holds up very well!! l

Darwin’s Revolutionary Theory l Variational: based on the genetic variation and distribution in populations

Darwin (1809 – 1882) Darwin had a lifelong love of nature. l His father wanted him to study medicine. l l This was not what Darwin wanted and he didn’t finish medical school.

Darwin l After leaving medical school he attended Cambridge University with the intention of entering the clergy. l 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. l As ship’s naturalist he spent his time on shore collecting thousands of plant and animal specimens and making important observations. l

The Voyage of the Beagle l 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 l The fossils he found in South America were more like modern South American species than European species.

The Voyage of the Beagle l Darwin became interested in the geographic distribution of organisms after visiting the Galapagos Islands.

Voyage Darwin realized that adaptation to the environment and the origin of new species were closely linked processes. l Galapagos finch species have evolved by adapting to specific conditions on each island. l

Contributor: Charles Lyell l During the voyage he read Charles Lyell’s Principles of Geology. l He had Lyell’s ideas in mind as he traveled and observed the geology of South America.

Uniformitarianism l Charles Lyell’s principle of uniformitarianism: l l Laws of physics & chemistry present throughout history of Earth. Past geological events similar to today’s events.

Uniformitarianism Natural forces could explain the formation of fossil-bearing rocks. l Lyell concluded the age of the earth must be millions of years. l He stressed the gradual nature of geological changes. l

Uniformitarianism l Darwin studied the work of Lyell closely. He concluded that Earth must be OLD. l Perhaps these slow changes could work on living things as well…. .

Contributor: Thomas Malthus l 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.

Contributor: Alfred Russell Wallace independently developed a theory of natural selection. l He sent his manuscript to Darwin, spurring him to finally publish his ideas. l Both ideas were presented to the Linnean Society in 1858. l Darwin finished On the Origin of Species and published it in 1859. l

Natural Selection l Only a small fraction of all offspring produced by any species actually reach maturity and reproduce. l Natural populations normally remain at a constant size. l Those that survive may have heritable traits that increased their chances of survival. They will pass those traits on. l The frequency of those traits will increase l

Natural Selection

Artificial Selection l Artificial selection – people selectively breed organisms with desired traits. l Darwin noticed that considerable change can be achieved in a short period of time.

Natural Selection l When an environment changes, or when individuals move to a new environment, natural selection may result in adaptation to the new conditions. l Sometimes this results in a new species.

Natural Selection l Individuals do not evolve; populations evolve. l Evolution is measured as changes in relative proportions of heritable variations in a population over several generations. l Natural selection can only work on heritable traits. l Acquired traits are not heritable and are not subject to natural selection.

Part 2: Evidence to Support Evolution l The main premise underlying evolutionary theory is that the living world is always changing. l Perpetual change in form & diversity of organisms over the last 700 million years can be clearly seen in the Fossil Record.

Fossils l Fossils are remnants of past life preserved in the earth. l l l 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 l Fossils provide support for the idea that life changes through time. l Fossil intermediates l l l Whales descended from land mammals. Birds descended from one branch of dinosaurs. The oldest fossils are of prokaryotes.

Dating Fossils l Geological time can be measured in sedimentary rock layers. l The Law of Stratigraphy Dates oldest layers at the bottom and youngest at the top. l Time is divided into eons, eras, periods and epochs. l

Dating Fossils l Radiometric dating methods are based on the decay of naturally occurring elements into other elements. l Different methods used for different time periods.

Dating Fossils - example has a half life of 1. 3 billion years – meaning half of the 40 K will have decayed to 40 Ar and 40 Ca. Half of what remains will decay in the next 1. 3 billion years. l Measure ratio of remaining 40 K to the amount of 40 K originally there (remaining 40 K plus 40 Ar and 40 Ca). l 40 K

Fossil Record The fossil record of macroscopic organisms begins in the Cambrian period: 505– 570 MYA. l Fossil bacteria and algae, casts of jellyfishes, sponges spicules, soft corals, and flatworms are found in Precambrian rocks. l l Mostly microscopic

Evolutionary Trends l The fossil record shows that species arise and go extinct repeatedly throughout geological history. l Trends appear in the fossil record – directional changes in features or patterns of diversity.

Evolutionary Trends l The evolution of horses from the Eocene epoch (57. 8 MYA) to the present is a well studied trend. l l l Body size – increasing Foot structure – fewer toes Tooth structure – larger grinding surface

Part 3: More Supporting Evidence

Common Descent-Mini Theory #2 Darwin proposed that all organisms have descended from a single ancestral form. l Life history is shown as a branching tree called a phylogeny. l

Homology- Evidence For Common Descent l The phrase “descent with modification” summarizes Darwin’s view of how Evolution works. All organisms descended from common ancestor. l Similar species have diverged more recently. (Closer branches=more similarities) l l Homology – when similar structures result from shared ancestry.

Anatomical Homologies #1: Homologous structures – variations on a structural theme that was present in a common ancestor. l Example – vertebrate forelimbs have different functions, but share the same underlying structure. l

Anatomical Homologies #2: Vertebrate embryos have a tail and pharyngeal pouches. (Embryo Similarities) l These structures develop into different but homologous structures in adults. l l l Gills in fishes Part of ears & throat in humans.

Ontogeny & Phylogeny l There are many parallels between ontogeny (an individual’s development) and phylogeny (evolutionary descent). Features of an ancestors ontogeny can be shifted earlier or later in a descendant's ontogeny. l Development= Fertilized Egg To Death l l l Example: Paedomorphosis Retention of ancestral juvenile characters by descendant adults.

Vestigial Organs l Vestigial organs – remnants of structures that served important functions in an ancestor. Remnants of pelvis and leg bones in snakes l Appendix in humans l

Molecular Homologies l Similarities can be found at the molecular level, too. The genetic code is universal - it is likely that all organisms descended from a common ancestor. l Different organisms share genes that have been inherited from a common ancestor. l l Often, these genes have different functions, like the mammalian forelimbs.

Part 4: Multiplication of Species-Mini Theory #3 Speciation refers to the formation of new species. l Defining a species is difficult… l l l Descent from common ancestral population. Ability to interbreed. Maintenance of genotypic & phenotypic cohesion. Reproductive barriers prevent species from interbreeding. l Where do they come from?

Examples of Reproductive Barriers l Premating barriers prevent mating from occurring in the first place. l l Ex. Not recognizing mating rituals, structural differences Postmating barriers impair growth, survival, or reproduction of the offspring. Ex. Infertile hybrids, not viable fertilized egg l Hybrids are offspring of members of two closely related species. l

Allopatric Speciation Allopatric (another land) populations occupy separate geographic areas. l Over time, reproductive barriers may evolve so that they could not interbreed. l The geographical separation can arise in two ways: l Vicariant speciation is initiated when climatic or geological changes fragment a species’ habitat, forming impenetrable barriers. l Founder events occur when a small number of individuals disperse to a distant place where no other members of their species exist. l

Sympatric Speciation Sympatric (same land) speciation occurs when speciation occurs in one geographic area – a lake for example. l Individuals within the species become specialized on a food type, shelter, part of the lake etc. l l Eventually reproductive barriers evolve (no interbreeding!).

Parapatric Speciation l Parapatric Speciation – geographically intermediate between allopatric and sympatric speciation. l Two species are parapatric if their geographic ranges are primarily allopatric but make contact along a borderline that neither species successfully crosses.

Adaptive Radiation l Adaptive radiation – the production of ecologically diverse species from a common ancestral stock. l Common in lakes & islands – sources of new evolutionary opportunities.

Adaptive Radiation l Archipelagoes increase opportunities for both founder events and ecological diversification. l l l Entire archipelago isolated from the continent. Each island is geographically isolated from the others. Ex: Galápagos Islands

Part 5: Gradualism (Theory #4) l Darwin’s theory of gradualism proposes that small differences accumulate over time producing the larger changes we see over geologic time. l Certainly, this process is always at work, but probably does not account for all changes.

Phyletic Gradualism l Relative rates of evolution during and between speciation events? l FOSSIL RECORD? Doesn't show smooth evolutionary transitions. Darwin's explanation was that the record was incomplete; evolution really was gradual, but most of the record had been lost. Claimed that where a complete fossil record could be found, it would show a gradual morphological change through time in lineages, and where lineages became split, gradual divergence of species

Punctuated Equilibrium l Punctuated equilibrium states that phenotypic evolution is concentrated in relatively brief events of branching speciation followed by periods of stasis.

Lake Turkana, Africa Site (Lake Turkana, Northern Kenya) = very complete sedimentary record. Williamson (1981) documented the changes in snail shell patterns. l Measured between 5 and 24 characters in 3300 specimens from 13 species‘ lineages. Concluded that snails in all 13 lineages showed no change l for prolonged periods, with occasional periods of punctuational change.

Theory #5: Natural Selection Variation exists within any population. l When natural selection acts to favor one trait over another that trait will increase in the population. l The population has evolved, not any one individual. l

Populations l Population – a localized, interbreeding group of individuals of a particular species.

Microevolution l Microevolution – evolutionary changes in the frequency of alleles in a population. Polymorphism – occurrence of different allelic forms of a gene in a population. l If there is only one allele for a gene in the population – every individual is homozygous for the trait – it is fixed in the population. l

Where Does Variation Come From? l Natural selection acts on the variation that is already present in the population. l But, where did that variation come from? l Two processes provide the variation in gene pools. Mutation l Sexual recombination l

Mutation New genes or alleles only result by mutations. l Mutations are changes in the nucleotide sequence of DNA. l

Sexual Recombination l Sexual recombination is a much more common way of producing variation in populations. Reshuffling of allele combinations already present in the population is how variation is maintained in populations. l Sexual reproduction rearranges alleles into fresh combinations every generation. l

Genetic Drift l The smaller the sample, the greater the chance of deviation from expected results. These random deviations from expected frequencies are called genetic drift. l Allele frequencies are more likely to deviate from the expected in small populations. l

The Bottleneck Effect l Sometimes a catastrophic event can severely reduce the size of a population. l l The random assortment of survivors may have drastically different allele frequencies. Bottleneck effect

The Bottleneck Effect l The actions of people sometimes cause bottlenecks in other species. l l l N. California elephant seal population reduced to 20100 individuals in the 1890 s. Current population > 30, 000. Variation drastically reduced – 24 genes with 1 allele. http: //www. sealexperience. com/index. html

The Founder Effect l Founder effect – When a small group of individuals becomes separated from the population and form a new population, the allele frequencies in their gene pool may be different than the original population.

Modes of Selection Stabilizing – removes the extremes. l Directional – variants at one of the extremes are favored. l Disruptive – variants at both extremes are favored. l

Macroevolution l Macroevolution refers to grand scale events in evolution. Evolution of new structures l Major trends in the fossil record l

Speciation and Extinction Through Geological Time l. A species has two possible fates: Become extinct or l Give rise to new species. l l Speciation and extinction rates vary among species. l Lineages with high speciation and low extinction produce the greatest diversity.

Mass Extinctions l Mass extinctions are episodic events where many species go extinct at the same time. l l Permian extinction – 225 MYA – half the families of shallow-water marine invertebrates and 90% of the marine invertebrate species went extinct over a few million years. Cretaceous extinction – 65 MYA – marks the end of the dinosaurs as well as many other species.