Evolution Origin of Life and Speciation
What is Natural Selection? • Who came up with the theory of natural selection? • Name some criteria necessary for natural selection to occur.
Name types of evidence for evolution. • Fossil record • Homologous structures • Vestigial structures • Embryonic structures • Molecular record
Origin of Life • We have said that all organisms have ancestors, but not all organisms have descendants. What do we mean by that? • What about the first organism? How do you think life first began on Earth?
Origin of Life • What do you think the first organism was like?
Early Earth • Early Earth was formed about 4.6 billion years ago and was very different than earth today. • How do you think it might have been different?
Early Earth • The atmosphere was very different than it is now, containing little or no oxygen. • Earth was too hot for liquid water. • Once the surface cooled enough for rocks to form, the surface was covered with volcanic activity.
Early Earth • About 3.8 billion years ago the Earth cooled enough for liquid water to remain. • Thunderstorms drenched the planet and oceans covered most of the surface.
Could organic molecules have evolved under these conditions? • In the 1950’s Stanley Miller and Harold Urey tried to simulate the conditions of early Earth. • They showed how several amino acids could be created under those conditions.
Miller and Urey’s Experiment • They passed sparks (representing lightening) through a mixture of hydrogen, methane, ammonia, and water (representing the atmosphere)
The Big Picture • Miller and Urey showed that the mixtures of organic compounds necessary for life could have arisen on primitive earth!
Hypothesis of the Origin of Life • The leap from a mixture of organic molecules to a living cell is large. • Tiny bubbles of organic molecules (called proteinoid spheres) have characteristics of living systems such as selectively permeable membranes and means of storing and releasing energy. They may have become more and more like living cells over time.
Hypothesis of the Origin of Life • Experiments have shown that under the conditions of early Earth, small RNA sequences could have formed and replicated on their own. This could have created a simple RNA-based form of life from which the DNA system could have evolved.
Hypothesis of Origin of Life • How certain do you think this hypothesis is? Do you think it will ever be changed? Do you think it will be changed during your lifetime?
Origin of Life • Evidence indicates that about 200-300 million years after the accumulation of liquid water on Earth, cells similar to modern bacteria were common.
Changing Earth • Photosynthetic bacteria became common and oxygen began to accumulate in the atmosphere and the ozone layer formed. • The rise in oxygen caused some life forms to go extinct, while others evolved ways to use oxygen for respiration.
Hypothesis of Origin of Eukaryotic Cells-Endosymbiotic Theory • What is a eukaryotic cell? • Prokaryotic cells began to evolve internal cell membranes- this was the ancestor to eukaryotic cells. • Smaller prokaryotes began living inside this ancestor and over time it became an interdependent relationship. What does this mean?
Lynn Margulis’ Endosymbiotic Theory • One group which entered the cell had the ability to use oxygen to generate ATP. These evolved into mitochondria. • Another group of prokaryotes which carried out photosynthesis evolved into chloroplasts.
Evidence for Endosymbiotic Theory • Mitochondria and chloroplasts have many characteristics of free living bacteria: 1- contain DNA similar to bacterial DNA 2- have ribosomes of similar size and structure to those of bacteria 3- reproduce by binary fission like bacteria
Speciation • Speciation: the formation of new species • What is a species? • As new species evolve, the populations become reproductively isolated from each other. (cannot interbreed and produce fertile offspring)
Isolating Mechanisms: • Behavioral Isolation: differences in courtship or reproductive strategies that prevent breeding • Geographic Isolation: populations separated by physical barriers • Temporal Isolation: reproduce at different times
Patterns of Evolution • Adaptive Radiation: when a species evolves into several different forms that live in different ways • Can you think of an example we have discussed, or any other example, of adaptive radiation?
Patterns of Evolution • Example of adaptive radiation: Darwin’s finches-more than a dozen species evolved from a single species
Patterns of Evolution • Convergent Evolution: unrelated organisms come to resemble one another due to similar selective pressures • Example? • What is divergent evolution?
Divergent Evolution • occurs when two or more biological characteristics have a common evolutionary origin but have diverged over evolutionary time. This is also known as adaptation or adaptive evolution. • example, the vertebrate limb is one example of divergent evolution. The limb in many different species has a common origin, but has diverged somewhat in overall structure and function.
Structures that are similar due to evolutionary origin, such as the forearm bones of humans, birds, porpoises, and elephants, are called homologous. Structures that evolve separately to perform a similar function are analogous. The wings of birds, bats, and insects, for example, have different embryological origins but are all designed for flight.
Patterns of Evolution • Coevolution: when two species evolve together, in response to changes in each other • Can you think of an example?
Coevolution • Example: flowers and pollinators, flowers and plant-eating insects
Gradual versus Punctuated Evolution Gradual: slow and steady change Punctuated: long, stabile periods interrupted by brief periods of rapid change
Can we see evolution occur? • Can you think of an example of an organism that evolves “quickly”? One that has evolved during your life time?
Population Genetics • The study of traits and changes in populations.
Gene Pool • All mechanisms of evolution involve changes in the gene pool. • A gene pool is the combined genetic material of all the members of a given population.
Microevolution • The change in a population’s alleles over a period of time. • These changes manifest themselves in the organism’s phenotype. • Since individuals do not evolve, a population must be watched to detect any change in genetic modification.
Allelic Frequencies • The number of each allele is a fraction of all the genes for a particular trait. • These fractions are known as allelic frequencies. • The constant state of allele frequencies is called genetic equilibrium.
Hardy-Weinberg Principle • Developed to determine if a population is evolving. • Authors of the theorem set up parameters, which do not exist in nature, to be followed when determining the allele frequencies of any population…
Hardy Weinberg conditions • The population must be very large in size. • It must be isolated from other populations (no gene flow) • No mutations • Random mating • No natural selection
Mathematical Wedding of Mendel and Darwin: The Hardy Weinberg Theorem • p+q = 1 • p2 + 2pq + q2 = 1 • p represents the frequency of the dominant allele • q represents the frequency of the recessive allele • p2 represents the frequency of the homozygous dominant phenotype • 2pq represents the frequency of the heterozygous phenotype • q2 represents the frequency of the homozygous recessive phenotype
Causes for Microevolution • Genetic Drift : The random change in gene pools due to random events. • Examples: migrations, natural disasters, isolation • Bottleneck effect: genetic drift occurring after a random population reducing event • Founder’s effect: the effect of establishing a new population by a small number of individuals, carrying only a small fraction of the original population's genetic variation. • As a result, the new population may be distinctively different, both genetically and phenotypically, from the parent population from which it is derived. • In extreme cases, the founder effect is thought to lead to the speciation and subsequent evolution of new species.
Genetic Drift and the Founder Effect • Polydactyly -- extra fingers or sometimes toes -- is one symptom of Ellis-van Creveld syndrome. • The syndrome is commonly found among the Old Order Amish of Pennsylvania, a population that experiences the "founder effect." • Genetically inherited diseases like Ellis-van Creveld are more concentrated among the Amish because they marry within their own community, which prevents new genetic variation from entering the population.
Causes for Microevolution • Gene Flow • The movement of alleles into and out of a population • Migration of an organism into different areas can cause allelic frequency changes • Immigration • Emigration