Two main types of evolution Evolution simply refers to change over time. Biologists typically concentrate on studying one of two levels of evolution. Macroevolution: this refers to long-term changes in species. Ex: the evolution of multi-celled organisms from single-celled organisms Ex: the evolution of amphibian species from fish species And the second level… Microevolution: this refers to short-term changes in populations due to changes in the environment. Ex: the average beak depth of a finch species changes after a major drought. Ex: the percent of dark-colored moths in a population increases as pollution increased during the industrial revolution. So who was this Darwin guy? Charles Darwin is given much of the credit (and blame) for the theory of evolution. In reality, Darwin published his theory at the same time as another scientist, Alfred Wallace. Even before Darwin, scientists like Jean Baptiste de Lamarck, were already proposing that species change over time, and are not the same today as they were at their “creation”. Darwin remains so famous because his ideas on the mechanisms of evolution, such as natural selection, have continued to be a cornerstone of modern evolutionary theory. Evolution by Natural Selection Natural Selection: the process by which a population adapts to its environment due to individuals with favorable traits successfully reproducing more offspring than individuals with less favorable traits. It is called natural selection because the environment is “selecting” for individuals who are most fit in the current conditions. (The less fit individuals die and/or do not reproduce.) Requirements for Natural Selection Variation: individuals of a population are not identical. Even though the individuals of a population are of a single species, they exhibit much phenotypic variation. Limits on Population Growth: Populations produce more offspring than can survive. Therefore, there is a struggle for existence in that resources are limited. Due to limited resources not all offspring survive. And the third requirement… Differential Reproductive Success: The individuals most adapted (i.e.: they have the most favorable combination of traits) to the environment are more likely to survive and reproduce than individuals with a less favorable combination of traits. Therefore, not all traits are passed to future individuals in equal proportions. The Synthetic Theory of Evolution brings Mendel and Darwin together When Darwin proposed that populations evolve by natural selection, he knew that phenotypic variation was a crucial component of his hypothesis. In order for natural selection to work, favorable traits must be heritable. This means traits are passed from parents to offspring. Darwin had no way to explain how traits are inherited, so his hypothesis could not be fully supported. An Example of Evolution by Natural Selection In the mid-1900’s, antibiotics were looked at as miracle drugs that could combat and cure any bacterial disease. Today, many bacterial species are resistant to multiple antibiotics. This is of great concern to the medical community as well as the general public. How does a bacteria species become resistant to an antibiotic? Common Misconceptions about evolution and natural selection Misconception: Individual organisms adapt to their environment. “Survival of the fittest” means the strongest individual survives. Traits an organism acquires during its life can be passed on to offspring. Reality: Evolution can only occur in populations, and does not happen within the lifetime of an individual. Fitness is measured by an organism’s ability to produce viable offspring. An individual who is most fit produces the most healthy offspring. A weightlifter with big muscles does not pass this trait to his children. Natural Selection is not the only mechanism of evolution Hardy-Weinberg Equilibrium predicts when populations are not evolving (i.e.: when allele frequencies are stable, and not changing). If a population is changing genetically, then a biologist can investigate if it may be due to one of the following: 1. Nonrandom mating 2. Mutation 3. genetic drift 4. gene flow, and 5. Natural selection 1. Nonrandom Mating Failure to choose mates at random from the population. Causes Inbreeding within the same “neighborhood”. Assortative mating (like with like). Result Increases the number of homozygous loci. Does not in itself alter the overall gene frequencies in the population. 2. Mutations Inherited changes in a gene. Result May change gene frequencies (small population). Source of new alleles for selection. Often lost by genetic drift. 3. Genetic Drift Changes in the gene pool of a small population by chance. Types: 1. Bottleneck Effect 2. Founder's Effect Bottleneck Effect Loss of most of the population by disasters. Surviving population may have a different gene pool than the original population. Result Some alleles lost. Other alleles are over-represented. Genetic variation usually lost. Importance Reduction of population size may reduce gene pool for evolution to work with. Ex: Cheetahs Founder's Effect Genetic drift in a new colony that separates from a parent population. Ex: Old-Order Amish Result Genetic variation reduced. Some alleles increase in frequency while others are lost (as compared to the parent population). Importance Very common in islands and other groups that don't interbreed. 4. Gene Flow Movement of genes in/out of a population. Ex: Immigration Emigration Result Changes in gene frequencies. 5. Natural Selection Differential success in survival and reproduction. Result - Shifts in gene frequencies. Comment As the Environment changes, so does Natural Selection and Gene Frequencies. Result If the environment is "patchy", the population may have many different local populations. Can changes within a population lead to a new species? So far we have analyzed evolution within a population, which consists of organisms of a single species. One of today’s most compelling questions in biology is how speciation occurs. Speciation refers to the process by which a new species evolves from a pre-existing species. It has been difficult for biologists from all areas of specialty (i.e.: botany and zoology) to agree on a single definition for a species. This has complicated experimental studies of speciation. Macroevolution Biologists have long been questioning how the major taxonomic groups evolved to produce the incredible diversity of species we see today. For example, when all living organisms were single-celled there was much less diversity than there is today. How is it that from these single-celled ancestors, fish, amphibians, birds, mammals, etc. evolved? Species Definitions Morphological species concept: defines a species as a group of structurally similar organisms that differ from other described species. Biological species concept: defines a species as a group of organisms able to interbreed to produce viable offspring in nature. Isolating Mechanisms are needed for new species to arise. Geographic isolation: If a population splits into 2 due to environmental changes, the two populations may experience different evolutionary changes due to different natural selection pressures. Reproductive Isolating Mechanisms This type of isolation prevents interbreeding even when two populations live in the same geographic region. Prezygotic barriers: these barriers prevent fertilization between gametes of two different species. (ex: males of one population use a different mating call to attract females than a related species.) Postzygotic barriers: these barriers prevent hybrid offspring from either surviving or producing offspring. (ex: an embryo fails to develop and dies) The fossil record provides a look at the progression of life on Earth As you examine the types of organisms found in rock layers of different ages, some patterns emerge. The earliest fossils (i.e.: the oldest fossils) are of unicellular organisms only. Prokaryotic cells appear in the fossil record earlier than eukaryotic cells. Photosynthetic organisms appear in rock layers that are younger than the time at which oxygen began to accumulate in the atmosphere. The first fish fossils appear before amphibian fossils, amphibian fossils appear before reptile fossils, etc. Fossil Record Earliest - 3.5 billion years old. Earth - 4.5 billion years old. Example of transitional forms Remember, the fossil record does not show simply a progression of species leading to a modern species. The fossil record shows us the evolution of life is more like a branching tree. Horse’s leg bones 2. Homologies support Macroevolution Morphological homologies: the theory of evolution predicts that if a group of organisms share a common ancestor that they would have similar structural features even if they have different functions. Patterns of Evolution Divergent Evolution, defined: the process of two or more related species becoming more and more dissimilar These species will share homologous structures Convergent Evolution, defined: the process by which unrelated species become more similar as they adapt to the same kind of environment Their similar structures are considered analogous. Divergent Evolution Adaptive Radiation Example of Convergent Evolution Adaptive Radiation: a special case of divergent evolution Adaptive radiation, defined: evolutionary pattern in which many related species evolve from a single ancestral species. This occurs when many new niches open up in an environment. Examples include: evolution of many mammal species after extinction of the dinosaurs; evolution of many island species following colonization of recently formed islands. 3. Another example of morphological homology Vestigial Organs: these structures have little or no importance to the organisms that have them. 4. Embryology is the study of how an organism develops from a zygote Embryological development: By examining the growth and development of embryos of different species, similarities abound. For example, all vertebrate embryos have pharyngeal pouches. These structures become very different in the adults. For example, in fish they become gills, but in humans they form tubes that connect the middle ear with the throat. Also, all chordates have a notochord during development but this is not present in adults. 5. Molecular biology supports the theory of macroevolution It is difficult to find anatomical or embryological similarities that connect such distantly related organisms as plants, animals, and bacteria. However, there are abundant molecular similarities that tie these organisms together in the evolution of life. For example, all organisms use DNA, RNA, and ribosomes to make proteins. The genetic code is universal except for a few minor exceptions in bacteria. These diverse organisms also share many similarities in biochemical pathways such as respiration. Also, by examining proteins such as enzymes, scientists have found evolutionary relationships that were unexpected based on morphology.