Biology Defined: A Look at Chemical Evolution (PDF)

Biology defined: The study of life. Not just living things, but how life began and how it is changing, and has changed, relative to the environment.  Why is Biology significant? Other than the physical world around us, physics, chemistry, astronomy, geology, which all contributed to the fo...

Biology defined: The study of life. Not just living things, but how life began and how it is changing, and has changed, relative to the environment.  Why is Biology significant? Other than the physical world around us, physics, chemistry, astronomy, geology, which all contributed to the formation of first life, nothing can be considered significant without LIVING organisms to encounter, interpret, and adapt to their surroundings. Examples of why Accounting, Arts, Computer Science, etc. are insignificant without life.  Life as we know it - Earth is approximately 4.6 billion years old. Life first evolved as a simple, self- replicating molecule about 3.85 billion years ago. As more complex organisms evolved, they either adapted to their surroundings, or they perished. Evolution is FACT! Chemicals required for life as we know it: Hydrogen (H) – part of water molecule Carbon (C) – complex carbon molecules Oxygen (O) – other part of water Nitrogen (N) – amino acid formation The Atoms and Molecules of Ancient Earth The chemical evolution hypothesis explains how complex carbon-containing compounds (and eventually life) likely formed from simpler molecules. When Did Chemical Evolution Take Place? Radiometric dating can be used to estimate the age of the Earth and when life first appeared.  Each element has a characteristic number of protons. The number of neutrons can vary; forms of an element with different numbers of neutrons are called isotopes (12C or 13 C). Radioactive isotopes have unstable nuclei that emit particles of radiation (energy) to form new daughter isotopes. This is known as radioactive decay.  How Old Is the Earth? Meteorites formed 4.58 bya, and the Moon formed 4.51 bya. Earth must be about the same age, but no direct radiometric dating is possible because Earth was initially molten.  When Did Life Begin? The oldest fossils (from 3.85 bya) consist of carbon grains that have high levels of 12C relative to other heavier carbon isotopes. Living organisms preferentially take in this lighter 12C from their surroundings.  The Building Blocks of Chemical Evolution Most cells are 96 percent hydrogen (H), carbon (C), nitrogen (N), and oxygen (O).  Chemical Evolution The chemical evolution theory proposes that the simple molecules present on ancient Earth reacted with one another to create larger, more complex molecules. Formation of formaldehyde (H2CO) and hydrogen cyanide (HCN) is the first step in chemical evolution and requires energy input. The Roles of Temperature and Concentration in Chemical Reactions High temperatures and high concentrations cause more reactant collisions and faster reaction rates. Computer models show that significant amounts of H2CO and HCN form under temperature and concentration conditions that were present on ancient Earth about 4 bya. How Did Chemical Energy Change during Chemical Evolution? The formation of H2CO and HCN is a critical step in chemical evolution because energy from sunlight has been converted to chemical energy. The Composition of the Early Atmosphere Volcanic gases (mostly CO2, N2, and H2O) probably dominated Earth’s early atmosphere, but H2, NH3, and CH4 were also present in sufficient amounts to form H2CO and HCN.  Linking Carbon Atoms Together The potential energy present in carbon compounds (e.g., CH2O) allowed the formation of complex organic compounds, some of which are found in organisms today.  Early Origin-of-Life Experiments To test whether the first stages of chemical evolution would have occurred on ancient Earth, in 1953 Stanley Miller combined methane (CH4), ammonia (NH3), and hydrogen (H2) in a closed system with water and applied heat and electricity as an energy source.  The products that resulted from Miller's experiment included HCN and H2CO, important precursors for more complex organic molecules, as well as amino acids— compounds that are fundamental to life.  The Early Oceans and the Properties of Water Life originated in and is based on water because water is a great solvent (substances dissolve easily in it). How Does Water’s Structure Correlate with Its Properties? The structure of water is unusual; its small size, bent shape, highly polar covalent bonds, and overall polarity are unique. Water also has several striking physical properties: it expands as it changes from a liquid to a solid, and it has an extraordinarily large capacity for absorbing heat. Water is Denser as a Liquid than as a Solid Hydrogen bonds in ice connect water molecules in an open crystal pattern. In liquid water, there are fewer hydrogen bonds and the water molecules can pack more tightly, making water more dense than ice. Early Earth Environment Water’s temperature-buffering capacity would have protected dissolved HCN and H2CO from energy sources in Earth's early environment (such as asteroid bombardment, volcanism, and sunlight) that could have broken them apart. What Was Water’s Role in Chemical Evolution? Chemical evolution proceeded in the ocean, and the simple organic molecules that formed from reduced-carbon compounds were preserved from destructive energy sources by water’s high specific heat.  Protein Structure and Function The four steps of chemical evolution (each requiring energy-input) are: (1) production of small molecules containing reduced carbon… (2) creation of a prebiotic soup of amino acids, sugars, and nitrogenous bases… (3) linkage of these organic subunits to make the large organic molecules important in cells today… and (4) evolution of a self-replicating molecule.  Amino Acids and Polymerization More recent experiments, similar to Miller’s, have shown that amino acids and other organic molecules form easily under conditions that accurately simulate those on ancient Earth. All organisms have proteins built from a collection of only 20 different amino acids.  The Cell Theory In the late 1660s, Robert Hooke and Anton van Leeuwenhoek were the first to observe cells. Where Do Cells Come From? The cell theory states that all organisms are made of cells and all cells come from preexisting cells. Around 1860 Louis Pasteur proved that cells arise from cells and not by spontaneous generation.  Because all cells come from preexisting cells, single-celled organisms in a population are related to a single common ancestor, and all cells in a multicellular organism also descend from a single ancestral cell. Organelles Divide a Large Cell into Compartments Eukaryotes solve the problem of size by dividing their cell volume into compartments.  Organelle Examples Mitochondria – power generators by catabolizing glucose Chloroplasts – site of photosynthesis Golgi Apparatus – packaging of products Ribosomes – protein synthesis Nucleus – houses DNA Endoplasmic reticulum – synthesis and metabolism Vacuoles – water storage in plants  The Endosymbiosis Theory The first eukaryotes were probably single-celled organisms with a cytoskeleton and a nucleus but no cell wall. Symbiosis occurs when individuals of two different species live in physical contact; endosymbiosis occurs when an organism of one species lives inside an organism of another species.  The endosymbiosis theory proposes that mitochondria originated when a bacterial cell took up residence inside a eukaryote about 2 billion years ago. The endosymbiosis theory also contends that chloroplasts originated in an analogous way.  Do the Data Support the Endosymbiosis Theory? Observations consistent with the endosymbiosis theory include the following: 1. Mitochondria and chloroplasts are about the size of an average bacterium. 2. Both organelles replicate by fission, as do bacteria, and have their own ribosomes to manufacture their own proteins. 3. Both organelles have double membranes, consistent with the engulfing mechanism. 4. Mitochondria and chloroplasts have genes that code for the enzymes needed to replicate and transcribe their own genomes.  Mitochondrial gene sequences are much more closely related to the sequences from bacteria than to sequences from the nuclear DNA of eukaryotes—indicating that the mitochondrial genome came from a bacterium rather than a eukaryote. Similarly, chloroplast genes resemble bacterial genes more closely than they resemble plant genes.  The Tree of Life The cell theory and the theory of evolution by natural selection imply that all species are descended from a single common ancestor at the root of a family tree of all organisms—the tree of life. Parsimony Hypothesis The simplest answer makes the most sense when dealing with complex issues.  Linnaean Taxonomy Before we can understand the relatedness of organisms, we need to classify them. In Linnaeus’ taxonomic system for classifying organisms, each organism is given a unique two- part scientific name consisting of the genus and the species.  Linnaean Taxonomy Established in 1735 by Carolus Linnaeus (real name Karl von Linne). Created a naming scheme known as binomial nomenclature.  Taxonomic Levels Linnaeus’ system is hierarchical with nested taxa. The taxonomic levels from least to most specific are kingdom, phylum, class, order, family, genus, & species. The process of narrowing species into smaller groups is known as circumscription.  A New Level Recently Domain was added to the list. It goes in front of Kingdom and includes: Bacteria, Archaea, and Eukarya. These three Domains actually include several Kingdoms with different names than the classic taxonomy, but their specifics are still be worked-out. How Many Kingdoms Are There? Linnaeus proposed just two kingdoms, plants and animals. An alternative five-kingdom system based on phylogeny was proposed in the 1960s. This required an understanding of prokaryotic vs. eukaryotic traits, as well as other characters of commonality. Characters of Commonality Prokaryotic – lack nuclear wall Eukaryotic – true nucleus Unicellular – made of 1 cell Multicellular – made of >1 cell Autotrophic – make your own food Heterotrophic – get food elsewhere Saprophytic – eat decaying stuff  Using Molecules to Understand the Tree of Life Carl Woese and colleagues studied small subunit rRNA, a molecule found in all organisms, as a means for understanding the evolutionary relationships among groups of organisms. Number of nucleotides in the human genome: 3.1 billion in the nucleus of every cell in your body.  Closely related species should have rRNA sequences that are more similar than those from distantly related species because they share a more recent common ancestor. Thus, small subunit rRNA sequences can be used to produce a phylogenetic tree showing probable evolutionary relationships.  The rRNA Tree The tree of life indicates three major groups of organisms: the eukaryotes (Eukarya) and two groups of prokaryotes —the Bacteria and the Archaea. Woese proposed a new taxonomic level called the domain. Each of the three domains (Bacteria, Archaea, and Eukarya) includes several related kingdoms. The Tree of Life Is a Work in Progress Analysis of other molecules has confirmed many of the major findings from rRNA analysis.  Other Evidence for Relatedness Among Groups Vertebrates (53,000 of the 2,000,000 named species). –Development – Undergo the same order and type of cellular division during development. –Embryology – All have fish-like larval stage with gills and a post-anal tail. –Biochemistry – All share certain same DNA, enzymes, cellular processes. –Morphology – Homology between groups, such as forelimbs in all mammals. The Tree of Life – On a 1-Year Time Scale  Phylogenies and the History of Life Life has existed on Earth for some 3.85 billion years. What patterns can be discerned in the tree of life as species appeared and disappeared over the course of history? Why do those patterns exist?  The history of life is studied primarily by using phylogenetic trees and the fossil record. The evolutionary history of a group of organisms is called a phylogeny. A phylogenetic tree shows ancestor- descendant relationships among evolutionary groups (usually species or populations). Fossils are physical evidence left by organisms from the past. The fossil record includes all fossils that have been found and recorded.  Using Phylogenies How do biologists read a finished phylogenetic tree, and how are trees put together in the first place?  Populations are represented by branches, and nodes show where ancestral groups split into descendant groups. Adjacent branches are sister taxa (a taxon is any named group of organisms), and a polytomy is a node where more than two descendant groups branch off. Tips are branch endpoints and represent living (extant) groups or a group’s end in extinction.  All phylogenies shown in this text are rooted, meaning that the most ancient node of the tree is shown at the bottom. The location of this node is determined using an outgroup, a taxonomic group that diverged before the rest of the taxa being studied. An ancestor and all its descendants form a monophyletic group (also called a clade or lineage).  How Do Researchers Build Phylogenies? Morphological, developmental, biochemical, and genetic characteristics are used to estimate phylogenetic relationships among species.  The phenetic approach computes a statistic that summarizes the overall similarity among populations, based on the data. A computer program then compares the similarities among populations and builds a tree that clusters the most similar populations and places more divergent populations on more distant branches.  Researchers using the cladistic approach focus on synapomorphies, the shared derived characters of the species under study. When many such traits have been measured, a computer program is used to identify which traits are unique to each monophyletic group and then places the groups on a tree in the correct relationship to one another.  The cladistic approach can run into difficulties in cases of convergent evolution. Biologists then use parsimony to try to identify the phylogenetic tree that minimizes the overall number of convergent evolution events. This approach assumes that convergent evolution should be much rarer than similarity due to shared descent.  Parsimony is a principle of logic stating that the most likely explanation or pattern is the one that implies the least amount of change or the least complexity. Whale Evolution: A Case History Traditionally, phylogenetic trees based on morphological data place whales outside of the artiodactyls—mammals such as cows, deer, and hippos that have hooves, an even number of toes, and an unusual pulley-shaped ankle bone (astralagus).  DNA sequence data, however, suggest a close relationship between whales and hippos. A phylogenetic tree showing closely related whales and hippos is less parsimonious than the tree based on morphological data because it requires the evolution and then loss of the astralagus in whales. How similar is DNA between taxa? Bacteria – 5% Yeast – 20% Insect – 40% Tunicate – 60% Fish – 75% Amphibian – 85% Reptile – 87% Bird – 87% Other mammals – average 93% Chimpanzee – 99%  Has neither direction nor purpose – organisms are not designed in advance; It is entirely contingent; It is continuous and continuing – all biologists are studying moving targets changing with time; Extant organisms are evolutionary experiments with functional properties that have resulted from past conditions.  Using the Fossil Record The fossil record is the only source of direct evidence about what prehistoric organisms looked like, where they lived, and when they existed.  How Do Fossils Form? Most fossils form when an organism is buried in sediment before decomposition occurs.  Limitations of the Fossil Record There are several features and limitations of the fossil record that must be recognized: habitat bias, taxonomic bias, temporal bias, and abundance bias.  Paleontologists—scientists who study fossils—recognize that they are limited to asking questions about tiny and scattered segments on the tree of life. Yet analyzing fossils is the only way scientists have of examining the physical appearance of extinct forms and inferring how they lived.  Life’s Timeline Major events in the history of life are marked on the timeline shown which has been broken into four segments (the Precambrian, the Paleozoic, the Mesozoic, and the Cenozoic).  Each of these three fossil beds is extraordinary because each includes more than one habitat and the fossils of soft-bodied as well as hard-shelled animals.  These fossils give researchers a compelling picture of life in the oceans 525–515 Ma. Few, if any, species in the Ediacaran faunas are also found in the Burgess Shale–type assemblages 20–40 million years later. New species of sponges, jellyfish, and comb jellies are abundant, as well as entirely new groups, principal among them the arthropods and mollusks. The Doushantuo Microfossils Researchers identified sponges, cyanobacteria, and multicellular algae in samples dated 570–580 Ma. They also found what they concluded were animal embryos in early stages.  This conclusion was based on the fact that the samples contained one-celled, two-celled, four-celled, and eight-celled fossils, along with individuals containing larger cell numbers whose overall size was the same. This is exactly the pattern that occurs during cleavage in today’s animals.  The Ediacaran Faunas Sponges, jellyfish, comb jellies, and traces of other animals dated 544–565 Ma are found in these Australian deposits. Fossils from this 20-million-year interval indicate that shallow-water marine habitats contained a diversity of animal species. The Burgess Shale Faunas Virtually every major animal group is represented in the Burgess Shale fossils.  Speciation occurs when populations of the same species become genetically isolated by lack of gene flow and then diverge from each other due to selection, genetic drift, or mutation.  Populations can become genetically isolated from each other if they occupy different geographic areas, if they use different habitats within the same area, or if one population cannot breed with the other.  Populations can be recognized as distinct species if they are reproductively isolated from each other, if they have distinct morphological characteristics, or if they form independent branches on a phylogenetic tree.  When populations that have diverged come back into contact, several outcomes are possible.  Speciation Populations that experience reduced gene flow may diverge genetically as a result of mutation, genetic drift, and/or natural selection. This genetic divergence may eventually lead to speciation, the formation of new species.  Defining and Identifying Species A species is a distinct type of organism, formally defined as an evolutionarily independent population or group of populations.  Gene flow reduces genetic differences among populations, so evolutionary independence starts with lack of gene flow. Once gene flow stops, mutation, selection, and drift can act on populations independently and genetic divergence can occur. Genetic divergence, in turn, may lead to speciation.  The Biological Species Concept The biological species concept considers populations to be evolutionarily independent if they are reproductively isolated from each other (no gene flow).  Prezygotic isolation occurs when individuals from different populations are unable to mate. Postzygotic isolation occurs when individuals from different populations can breed, but the offspring produced have low fitness.  The concept of reproductive isolation can be applied only to populations that overlap geographically. If the species do not overlap, they are considered geographically isolated, not reproductively isolated. The Morphospecies Concept Evolutionary independence can also be estimated by looking at population morphology, because different appearances often evolve when populations experience different natural selection, mutations, and genetic drift. These different forms will persist only if gene flow is restricted.  This morphospecies concept can be applied easily but is rather subjective.  The Phylogenetic Species Concept On phylogenetic trees, an ancestral population plus all its descendants is called a monophyletic group. A phylogenetic species is the smallest monophyletic group on a tree; by definition, it is isolated from gene flow with other groups.  The phylogenetic species concept is precise and is applicable to all populations. Because current phylogenetic trees are based mainly on molecular data, they tend to receive the most support for defining species.  Populations that have their own identifying traits but are not distinct enough to be considered a separate species are called subspecies.  Seaside sparrow subspecies were believed to be genetically isolated because the populations are geographically isolated and young birds breed near their hatching ground.  Biologists compared gene sequences from different seaside sparrow populations that were subspecies under the morphospecies concept and determined the phylogeny shown next. They found that seaside sparrows belong to two monophyletic groups, one living on the Atlantic Coast and one living on the Gulf Coast.  One subspecies thought to be nearing extinction was shown to be genetically indistinguishable from the Atlantic Coast sparrows and thus did not need to be individually preserved.  Isolation and Divergence in Allopatry Physical separation of populations occurs when a group colonizes a new habitat (dispersal) or when a new physical barrier divides a population (vicariance). Speciation that begins with physical isolation via either dispersal or vicariance is known as allopatric speciation. Populations that live in different areas are said to be in allopatry.  Biogeography—the study of how species and populations are distributed geographically—can tell us how colonization and range-splitting events occur. Dispersal and Colonization Isolate Populations Colonization events often cause speciation because the physical separation reduces gene flow and colonists are likely to experience genetic drift. If the new habitat differs from the original habitat, natural selection may also cause divergence of the new population.  Genetic drift and natural selection during and following colonization events appear to have caused repeated speciation in many groups of island organisms.  Vicariance Isolates Populations When an existing population is split by a new physical barrier, vicariance has occurred. The last ice age caused vicariance as a result of glaciation, which created physical barriers. In addition, continental drift—the movement of continental plates explained by the theory of plate tectonics— separated species physically.  Ostrich Moa Emu & Cassowary Rhea Kiwi  A series of vicariance events can lead to a series of speciation events resulting from genetic isolation followed by genetic divergence due to mutation, selection, and genetic drift. Isolation and Divergence in Sympatry Populations or species that live in the same geographic region (close enough to mate) live in sympatry.  Researchers used to think that speciation could not occur among sympatric populations because if gene flow is possible, it would tend to eliminate evolutionary differences.  In some situations, natural selection can overcome limited gene flow to cause sympatric speciation. Often this happens when populations become isolated by habitat preference.  Many insects and parasites are associated with specific hosts. When some individuals in a population switch hosts, they experience different selection pressures and reduced gene flow with the original population. Host switching is an important cause of speciation in sympatric populations.  If two populations have diverged and if divergence has affected when, where, or how individuals in the populations mate, it is unlikely that interbreeding will take place. In cases such as this, prezygotic isolation exists. When this happens, mating between the populations is rare, gene flow is minimal, and the populations continue to diverge.  When prezygotic isolation does not occur, populations may successfully interbreed, causing gene flow that might eventually eliminate differences that evolved during their separation. Other possible outcomes are reinforcement, hybrid zones, and speciation by hybridization.  Reinforcement Sometimes individuals from populations that had been separated can mate with each other, but the hybrid offspring are aborted, die young, or survive but are sterile. This is known as postzygotic isolation.  Postzygotic isolation causes natural selection against interbreeding. Selection for traits that isolate populations reproductively is called reinforcement.  Reinforcement occurs only when species live in the same geographic area. If two species never actually interbreed in the wild, natural selection will not act to reduce interbreeding.  Hybrid Zones Sometimes hybrid offspring, which possess traits that are intermediate between the two parental populations, are healthy and capable of breeding. A geographic area where interbreeding between the two populations is common and there are lots of hybrid offspring is called a hybrid zone.  What is Microevolution? The change in an organism on a time scale we can appreciate Selective pressures are placed on an organism and they either acclimate or perish Oftentimes, this change is due to plasticity within a species able to encounter environmental stressors and survive them  Microevolution Examples Microbial resistance to drugs Insect resistance to pesticides Domesticated organisms Examples in humans that should be acting, but are not because we have compassion: – Cancer when caused by avoidable environmental conditions – Allergies – Stupidity Evolution by Natural Selection The theory of evolution by natural selection, jointly proposed by Charles Darwin and Alfred Russel Wallace in 1858, is THE underlying tenet of life.  Darwin described evolution as descent with modification, meaning that change over time produced modern species from ancestral species.  Evidence for Change through Time Fossils are traces of organisms that lived in the past; those that have been found and described in the scientific literature make up the fossil record.  Researchers now use radioactive isotopes to assign absolute ages to the geologic time scale. Geologic data suggest that Earth is about 4.6 billion years old, and fossil data suggest the earliest signs of life in rocks is about 3.85 billion years old.  Extinction Many fossils provide evidence for extinct species unlike any known living organisms. Darwin interpreted extinction as evidence that species are dynamic and can change.  Transitional Forms The law of succession means that extinct fossil species are typically succeeded, in the same region, by similar species. Darwin interpreted this pattern as evidence that extinct forms are the ancestors of modern forms and that species change over time.  Many transitional forms have been discovered with traits that are intermediate between older and younger species.  What is the Reason for Change? Earth's topography and environment have changed drastically over time. Our planet and its species are dynamic. Populations must adapt to all existing environmental stressors, such as competition, predation, and climate, or else perish.  Vestigial traits are functionless structures that are similar to functioning structures in related species. Vestigial traits are evidence that trait structure and function change over time. Vestigial or not?  Evidence That Species Are Related Darwin collected mockingbirds from the Galápagos islands on his five-year voyage on HMS Beagle. The mockingbirds were superficially similar, but different islands had distinct species. Darwin proposed that the different island species were similar because they had descended with modification from a common ancestor.  This pattern of similar yet distinct species is commonly found among island groups worldwide. Related species share a phylogeny, or family tree, and their relationships can be depicted graphically in a phylogenetic tree.  Structural Homologies Structural homology refers to similarity of morphological traits, such as the same general limb structure in vertebrates.  Darwin interpreted structural homologies as a product of descent with modification. Developmental Homologies Developmental homology refers to similarity in embryo morphology and/or pattern of tissue differentiation. For example, all vertebrates have gill pouches and tails early in embryonic development.  Developmental homologies are routinely observed at two levels: in the overall morphology (that is, form) of embryos, and in the fate of particular embryonic tissues. Evidence supports the fact that this pattern exists because the common ancestor of all vertebrates was a fishlike animal with gill pouches and a tail.  Genetic Homologies Structural and developmental homologies (phenotype) result from genetic homology — a similarity in the DNA sequences of genes from different species (genotype).  Evidence suggests that all organisms on Earth are descended from a single common ancestor. Consequently, almost all organisms use the same 64 mRNA codons to specify the same amino acids.  Homology, in biology, refers solely to similarities due to common descent, or divergent evolution. However, sometimes organisms have similar traits that are not inherited from a common ancestor. These are called homoplasy and are analogous traits or convergent traits. How Do Biologists Distinguish Homology from Analogy? Sometimes species that inhabit similar environments have similar traits due to convergent evolution. Convergent evolution occurs when natural selection leads to similar solutions to the challenges posed by a particular habitat, predator, or climate.  The key to differentiating homology from analogy is determination of whether or not the similar structures or genes derive from a common ancestor. Darwinism and the Pattern Component of Evolution Darwin and others have provided convincing evidence to support the theory of evolution BY natural selection, which states that species are related and change through time.  A population experiences natural selection whenever the following four conditions apply: (1) individuals vary in their traits; (2) some of these variations are heritable; (3) some individuals survive and reproduce better than other individuals; and (4) differential survival and reproduction (Darwinian fitness) is influenced by the heritable traits of individuals.  Some heritable traits help an individual to survive or reproduce better than other individuals. These are the traits that will, over time, increase in frequency in the population, causing evolution — the genetically based change in a population's traits over time.  The Nature of Natural Selection and Adaptation Although natural selection appears to be a simple process, research has shown that it is often misunderstood.  1) Selection Acts on Individuals, but Evolutionary Change Occurs in Populations.  Individuals do not change during natural selection. Those that are selected simply produce more surviving offspring than other individuals do, causing a change in the genetic makeup of the population.  Acclimation occurs when an individual changes in response to changes in the environment, but adaptation occurs only when a population changes in response to natural selection.  2) Evolution Is Not Progressive According to modern evolutionary theory, there is no such thing as "higher" or "lower" organisms. There are only more ancestral and more derived groups with different adaptations that allow the groups to thrive in different environments. 3) Not All Traits Are Adaptive Adaptation is not a perfect process. Not all traits are adaptive, and even structures that currently function in organisms may be subject to genetic or historical constraints.  Evolutionary Processes A population is a group of individuals from the same species that live and breed together. The four mechanisms that cause evolution are natural selection, genetic drift, gene flow, and mutation. Natural Selection and Sexual Selection Natural selection occurs when individuals with certain heritable phenotypes survive and reproduce better than others. The alleles responsible for the increased reproduction then increase in frequency.  Heterozygote advantage is a pattern of natural selection in which heterozygous (two different alleles) individuals have higher fitness than homozygous (two identical alleles) individuals. Natural selection maintains genetic variation in a population. Different patterns of natural selection exist, each with its own causes and consequences.  Directional Selection Directional selection occurs when natural selection increases the frequency of one allele. This type of selection reduces population genetic diversity over time.  Stabilizing Selection Stabilizing selection occurs when individuals with intermediate traits reproduce more than others, thereby maintaining intermediate phenotypes in a population.  Disruptive Selection Disruptive selection is the opposite of stabilizing selection; it occurs when intermediate phenotypes are selected against, and extreme phenotypes are favored. Disruptive selection maintains genetic variation but does not change the mean value of a trait.  Disruptive selection can cause speciation (the formation of new species) if individuals with one extreme of a trait start mating preferentially with individual that have the same trait.  Sexual Selection Mate choice often plays an important role in speciation. Sexual selection is selection for enhanced ability to attract mates and is a form of natural selection.  The fundamental asymmetry of sex is that females usually invest more in their offspring than males do. Females typically produce relatively few offspring, and their fitness is limited primarily by the ability to gain resources necessary to produce and rear young.  Males can father many offspring, and male fitness is limited primarily by the ability to acquire mates. Sexually selected traits should therefore be found primarily in males. Sexual Dimorphism  Genetic Drift Genetic drift is a change in allele frequencies due to chance. It causes allele frequencies to drift up and down randomly over time. Genetic drift is random with respect to fitness. Allele frequency changes are not adaptive.  Genetic drift is most pronounced in a small population. Over time, genetic drift can lead to the random loss or fixation of alleles. When random loss or fixation occurs, genetic variation in the population declines.  Genetic Drift in Natural Populations Genetic drift is of great concern to conservation biologists because populations found on nature reserves or in zoos are especially susceptible to it. Over long periods of time, genetic drift can have an effect even in moderately large populations.  How Do Founder Effects Cause Genetic Drift? A founder event occurs when a group leaves a population, emigrates to a new area, and starts a new population. If the founding group is small, its allele frequencies probably differ from those of the source population. This sampling effect on the new population's allele frequencies is called a founder effect.  Founder events and founder effects are especially common in the colonization of isolated habitats such as islands, mountains, caves, and ponds. How Do Population Bottlenecks Cause Genetic Drift? A sudden decrease in population size, called a population bottleneck, can lead to a genetic bottleneck—a sudden reduction in the number of alleles in a population. Genetic bottlenecks are commonly caused by disease outbreaks and natural catastrophes, and genetic drift often occurs in the resulting small population.  Gene Flow Gene flow, the movement of alleles from one population to another, occurs whenever individuals leave one population, join another, and reproduce. Gene flow reduces genetic differences between the source and recipient populations.  Gene flow can increase the average fitness of individuals in a population where genetic diversity was lost due to a nonadaptive process such as genetic drift (heterozygous advantage); Gene flow can decrease the average fitness of individuals in a population where natural selection produced adaptation to a specific habitat (introduced species).  Mutation Although most evolutionary mechanisms reduce genetic diversity, mutation restores genetic diversity and creates new alleles. Because errors are inevitable, mutation is always adding new alleles into populations at all gene loci. Mutation generally results in deleterious alleles but does occasionally produce advantageous alleles.  Mutation as an Evolutionary Mechanism For a given allele, mutation rates are often too low to affect allele frequencies significantly in and of themselves. For significant evolutionary change to occur, another evolutionary mechanism must act on alleles created by mutation.