Allopatric and Sympatric Speciation Sky *These are based on how the gene flow is interrupted* Allopatric speciation, also known as other country speciation, is when populations are separated by a geographical barrier. Geographic barriers can be any type of naturally occurring formation that causes isolation between various areas. Examples of geographic barriers are mountains, islands, glaciers, lakes, oceans, rivers and canyons. New species begin to form when isolation occurs because they have to adapt to different environmental conditions based on where they are located. This is the most common form of speciation. Sympatric speciation, or same country speciation, is when there is reproductive isolation between a population. Some barriers that prevent reproduction would be differences in mating (mating time, rituals, etc.), differences in the shape of genitalia and the production of sterile offspring (ex: the Liger, a species that cannot reproduce). Allopatric speciation may sometimes start off the process of sympatric speciation by first creating the geographic barriers which then leads to the reproductive isolation.
Anagenesis and Cladogenesis Libby Anagenesis and cladogenesis are both similar and different in many different ways. To start, anagenesis is the collective changes that transform one species into a different species with different characteristics. Anagenesis is also called phyletic evolution. Cladogenesis, is the separation of a gene pool into two or more gene pools. Said pools then eventually produce one or more new species. Branching evolution is another name for cladogenesis. The main difference between the two is that anagenesis is evolution within a lineage and cladogenesis is evolution that results in the splitting of a lineage. Anagenesis is just a single lineage while cladogenesis involves evolution in a branching pattern with many new species evolving from a single parent species.
Sexual selection: Patricia a special type of natural selection in which the sexes acquire distinct forms either because the members of one sex choose mates with particular features or because in the competition for mates among the members of one sex only those with certain traits succeed. The English version: one of the indiduals in the mating pair acquire some unique traits because of competition for mates or the environment. Nonrandom mating: Nonrandom mating occurs when the probability that two individuals in a population will mate is not the same for all possible pairs of individuals. The mates are not randomly mating, they are mating because of desirable traits that would help the offspring survive, the mating calls, and the appearance of the mate. Assortative Mating: and a form of sexual selection in which individuals with similar phenotypes. SIMILARITIES: These three types have to do with reproduction.
Prezygotic Isolating Mechanisms: Jana How do different species that are capable of producing fertile offspring keep from hybridization leading to speciation in their natural environments? Behavioral Isolation: species differ in their mating rituals Differences in visual signals Different mating calls (ex: frogs/birds/insects) Different chemical pheromones released to attract mates (ex: moths) Electroreception- electrical discharges released and electroreceptors detect (ex: African electric fish) Ex: blue footed booby dancing rituals Temporal Isolation: species breed at different times or have different reproductive seasons Different species of plants that grow in the same area have different flowering seasons (ex: lettuce; spring vs. summer) Different peak breeding seasons (ex: frogs) Habitat Isolation: species are in the have geographical area but do not encounter each other Ex: lions in open grassland and tigers in forests Ex: scrub oak in non-fertile soil vs. valley oak in fertile soil & open grassland
Founder Effect/Bottleneck effect Brian The Founder Effect is a cause of genetic drift attributable to colonization by a limited number of individuals from a parent population. This effect involves the dispersal of organisms to form an isolated population. In this process, many alleles may be lost in the new population. Just by chance, some rare alleles may be at a high frequency, while others may be missing. Gene pools are highly limited under the Founder Effect. The Bottleneck Effect is a type of genetic drift resulting from a reduction in population (natural disaster) such that the surviving population is no longer genetically representative of the original population. This effect also reduces genetic variation in a population. The next generation may experience different alleles from the original parent population. There may be less of an ability to adapt to new environmental pressures. Genetic variability only increases over time when random mutations occur within the population.
Geographic Isolation/ Reproductive Isolation Taylor Species are geographically isolated when they occur in different areas (often separated by a physical barrier such as a river or mountain range). In contrast, species that do not mate due to habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, gametic isolation, reduced hybrid viability, or reduced hybrid fertility are said to be reproductively isolated (there is no physical barrier between the species). Both isolations cause species to be created. Geographical Isolation-
#17 Convergent Evolution/Divergent Evolution Andrew Convergent evolution, by definition, is the process whereby organisms not monophyletic independently evolve to have similar traits due to the organisms similar environmental surrounds. The organisms who undergo convergent evolution are classified as different species because of their differing genetic makeup, but their physical characteristics became increasingly alike. This is also an example of homologous structures because the increasingly similar phenotypes of the organisms are creating similar bone structures with organisms of a distinct species. These bone structures may resemble a close comparison in appearance but their ancestral origins have no similarity because of the different genotypes. Examples of convergent evolution include the marsupials of Australia closely resembling the phenotypes of placental mammals across the world. Marsupials and placental mammals are completely different species of animal and have no similar ancestral background; however, through geographical isolation, the marsupials have become increasingly similar in appearance to worldwide placental animals. On the other hand, Divergent evolution, is the accumulation of differences within a single species that can eventually lead to the formation of new species. The most iconic example of divergent evolution is Charles Darwin s research of the finches in the Galapagos islands. Darwin thought up his theory of evolution partly based on what he discovered from extensively observing the species of finches in the Galapagos. Darwin uncovered that the different species of finches actually used to descend from the same species at one point. Originally, the different species of birds that cover the Galápagos Islands were once a single species of bird. It was through geographical isolation which entails different environmental landscapes that eventually lead to the development of other species. The other species of finches in the Galápagos changed in ways that benefitted their survival in their specific island in the Galápagos. Islands that had larger seeds favored the survival of finches with larger beaks, therefore the island became predominantly inhabited by large beaked finches which eventually became a new species of finch. This same principle applies to small beaked finches in the islands in the Galápagos that had mainly smaller seeds which favored the survival of smaller beaked finches
Adaptive radiation/polymorphism/dimorphism Jae Adaptive radiation is when organisms abruptly change from their ancestors get accustomed to their surroundings, causing new species to form. The phenomenon starts off with an ancestor which undergoes speciation through the changes in the environment such as natural disasters, different resources available and new environmental niches. An epitome of cladogenesis, adaptive radiation links with polymorphism and dimorphism due to the fact that adaptive radiation catalyzes alterations in phenotypes. The two types of changes in phenotypes being polymorphism and dimorphism. Polymorphism is where two distinct types of phenotypes form due to numerous reasons such as genetics, environmental factors and random occurrences. In contrast, dimorphism is when the male and female of a specific show extremely different aspects and characteristics.
Disruptive/stabilizing/diversifying selection Sarah Disruptive Selection: Organisms with the two extreme variations of a trait are adaptive, while other organisms with traits in between are selected against. The midrange traits become much less common, while the two extreme traits remain. Stabilizing Selection: Organisms with moderate traits are adaptive while organisms with extreme variations of traits are selected against. Midrange phenotypes remain common. Directional Selection: Organisms at one extreme end of the trait variation spectrum are adaptive, while organisms with traits at the other end of the spectrum are selected against. Allele frequencies move in a consistent direction, so that one extreme trait becomes the most common phenotype over time.
Zach Darwin s Theory/Lamarck s Theory Darwin and Lamarck had differing theories on natural selection regarding traits. Darwin s theory was that animals have different traits and those who are the fittest are able to reproduce while those who don t have the beneficial traits slowly die off. Those who are able to reproduce pass on their traits to their offspring and over time, those traits will become more and more apparent in a population. This example can be illustrated through giraffes, as giraffes born with longer necks were able to reach higher food and thus survive and reproduce. Those who did not have longer necks eventually died off and that trait became less apparent as time went on. The long neck trait is then inherited to the offspring and spread out to the population. Lamarck s is slightly different. Lamarck believed that organisms inherited traits in their lifetime through constant use and passed on those traits to their offspring. Lamarck also believed that traits that were not used, or in disuse, would eventually disappear as those that were used would get passed on. This example could also be illustrated with giraffes but in a different context. Based on Lamarck s theory, Giraffes had to stretch their necks to acquire their food and overtime they passed on this trait they acquired to their offspring. Lamarck also theorized that the more an organ or trait was used, the bigger it would get, just as a blacksmith s arm muscles get bigger through constant use. This is also why scientists believe that humans lost tails from past ancestors and why scientists believe that we may lose our appendix through its disuse. We believe Darwin s theory over Lamarck s through evidence and the disproving of facts. Lamarck s theory has been disproved as scientists found that traits organisms acquire are not passed on to offspring, just as a professional runner s fitness may not be passed on to his/her future child. Darwin s theory has been proven true through research as the only way traits are passed on are through genes and genes can only be passed on by those who survive.
Gradualism and punctuated equilibrium are two ways in which evolution of a species can occur. A species can evolve by only one of these, or by both Morgan Gradualism : the hypothesis that evolution proceeds chiefly by the accumulation of gradual changes Gradual divergence over long spans of time Assure that big changes occur as the accumulation of many small ones Punctuated Equilibrium: the hypothesis that evolutionary development is marked by isolated episodes of rapid speciation between long periods of little or no change. Rate of speciation is not constant Rapid burst of change Long periods of little or no change Species undergo rapid change when they first bud from the parent population
Gene Pool/ Gene Flow/ Genetic Drift Rebecca Gene pools, genetic drift, and gene flow are all part of microevolution. Gene pool is the total aggregate of genes in a population at any one time and a collection of genes in a population. A change in the gene pool of a population over a succession of generation. Mutations in a gene pool create genetic variation. A change in an organism's DNA may cause an original source of genetic variation, which is raw material for natural selection. If mating randomly occurs, and there are no advantages or disadvantages to a certain gene, then the gene pool will most likely stay the same over time. Each generation, genes can be shuffled around but it depends on each allele s frequency. Genetic drift, along with natural selection, mutation, and migration is a basic mechanism of evolution. In some generations, some populations may leave more offspring that have a better chance of surviving than the last generation. Genetic drift changes in the gene pool of a small population due to chance. It usually reduces genetic variability. The bottleneck effect is a type of genetic drift. The bottleneck effect results from a reduction in population such that the surviving population is no longer genetically representative in the original population. When genes are exchanged due to the mixing of populations, the result is gene flow. Gene flow is popular among the migrating crowd. The species that move from place to place at a specific time of the year tend to interact with multiple over populations. Once they get to the place they want migrate to, the species have plenty of opportunities to mate with other populations which allows them to gain some genetic diversity. This reduces differences between populations.
Natural and Artificial Selection Alex Natural Selection: Populations of organisms can change over the generations if individuals having certain heritable traits leave more offspring than others Artificial Selection: The intentional reproduction of individuals in a population that have desirable traits. Comparing Natural and Artificial Selection: Natural selection is when different factors in nature mainly fitness, ability to produce healthy offspring, determine how long a species will survive in the wild. This is shown with the finches on the Galapagos Islands and how the different finches evolved over time to change their beaks so they were able to eat the most abundant food sources available on each island. On the other hand Artificial Selection is when people specifically breed or breed another species because they are looking for the most desirable traits. This is seen in farmers and the vegetables they grow because they want the biggest and best tasting vegetables, but yet are also resistant to different species of bugs so that they have more crop yield. Darwin s Finches (Natural Selection): Artificial Selection:
Prezygotic and Postzygotic Barriers Kyra Prezygotic barriers: Official Definition: Impede mating between species or hinder the fertilization of ova if members of different species attempt to mate A more simple breakdown: Species cannot mate because the sperm and egg just don t match, or species have barriers which prevent any possible mating Some Factors that can affect this: Habitat, (example, if one species lives in the water and another lives on land, they will not encounter each other and will not mate), Behavioral (example, the blue footed boobies have a special mating dance, so if a different species tries to mate, they do not know the dance, so they can t mate), Temporal (Example, skunks of 2 different species near each other, one mates in the winter, one mates in the summer. They never run into each other, so they will not mate), Mechanical (example, flowers; pollination anatomy) Gametic (example frogs, their egg coat receptors will only accept the sperm of their own species, so even if another species near the eggs try to fertilize them, they cannot be fertilized, so they cannot make a new species) Postzygotic barriers Official Definition: Fertilization occurs, but the hybrid zygote does not develop into a viable, fertile adult More simple explanation: Mating is successful but the offspring either does not survive or do not survive long after that, or cannot produce offspring of their own Examples: Reduced hybrid viability (frogs; zygotes fail to develop or reach sexual maturity), Reduced hybrid fertility (mule; horse x donkey; cannot backbreed), Hybrid breakdown (cotton; 2nd generation hybrids are sterile)
Divergent/Phyletic speciation Jasmine A speciation is defined as the origin of a new species. There are two different patterns of speciation: phyletic and divergent which are both common. A gradual change in a single population is known as phyletic speciation. The organisms which undergo the changes differ a lot from the original organism that they are considered new species. This could be simply shown by a line of species. For example, let say Species A undergoes a change. Naturally, Species B would follow behind Species A and then C and so on. However, this happens gradually over time. The evolution of a horse and the evolution of a human have been classified as phyletic speciation. Another type of speciation is divergent speciation. Divergent speciation is a change branching off from the past species. If Species A undergoes a change, Species B and C will branch off of species A. It results from reproductive isolation (the impede mating between species and ). This type of speciation has been proven from fossil evidence and the mechanisms of biological evolution. The differences between phyletic and divergent speciation is that with phyletic speciation, the changes form a line (from A to B, B to C). For divergent, a branching tree forms (B and C branch off A). But one thing they have in common is that they both may become extinct in the process.
Homologous/analogous structures Katie Analogous structures are structures in species that have similar functions and look similar in the external form, but came from separate evolutionary paths. Homologous structures descended from a common ancestor and may appear similar in internal structure, but serve different functions. Analogous and homologous structures both can relate to the environment in which the species lives. If different populations of a species migrate to different habitats, then homologous structures may develop, with the functions of structures evolving to adapt to the functions required in a certain habitat. Analogous structures may develop in species that live in similar environments, and therefore their structures may appear similar in appearance and have similar functions, yet have descended from different evolutionary paths.
#16: Hardy Weinberg Equilibrium Katie The Hardy-Weinberg Theorem serves as a model genetic structure for non-evolving populations. 5 Conditions of a non-evolving population must be met for this equilibrium to exist: Large Population No immigration or emigration There is random mating No natural selection No mutation The Hardy-Weinberg Equation: p² + 2pq + q² =1, (or 100%) where: p is the frequency of the dominant allele (A) q is the frequency of the recessive allele (a) p² is the frequency of the homozygous dominant genotype (AA) 2pq is the frequency of the heterozygous genotype (Aa) q² is the frequency of the homozygous recessive genotype (aa) Important: p+q=1 and p²+2pq+q²=1
Micro and Macroevolution Payton I. Microevolution a. Definition i. Evolutionary change within a species or small group of organisms over a short period of time (AKA- Evolution on a small scale) b. Example i. Galapagos Finches c. How it occurs so quickly i. Gene Flow- Organisms carrying the optimal allele migrate to another population ii. Mutation- A mutation in the DNA of an organism changes it to the optimal allele iii. Genetic Drift- Just by luck, there are more organisms produced that have the optimum allele than initially expected iv. Natural Selection- The organisms with the optimum allele present have a better chance of outlasting and reproducing than their mediocre counterparts d. Picture b. Example i. Humans evolving from apes c. Picture II. Macroevolution a. Definition i. The origin of new taxonomic groups *The major difference evolution within a between the two is the fact that one is SINGLE SPECIES and the other is evolution that creates a whole NEW SPECIES*
Mechanical & Gametic Isolation Sarah Mechanical isolation and gametic isolation are both forms of prezygotic reproductive isolation. Reproductive isolation hinders mating between species and makes fertilization of ova impossible for members of a different species. Both mechanical and gametic isolation are prezygotic reproductive barriers, meaning the offspring between species are unable to be conceived. Mechanical isolation is when the reproductive organs of closely related species do not fit together, preventing sperm transfer, while gametic isolation is when the sperm of one species cannot fertilize the eggs of another closely related species. There are two types of gametic isolation: biochemical barriers and chemical incompatibility. Biochemical barriers prevent the sperm from penetrating the egg. Chemical incompatibility prevents sperm from surviving in the female reproductive tract. An example of gametic isolation is the way in which sea urchins reproduce. Sea urchins reproduce by releasing sperm and eggs into surrounding waters where they hopefully fuse and form zygotes. Because of gametic isolation, different species are unable to fuse with these gametes.
Survival of the Fittest Darwin's theory of survival of the fittest is one of the mechanisms of natural selection. Survival will depend on successful traits, or adaptations. While the biological concept of fitness is defined as reproductive success. Fitness is a reproductive success judged by the number of surviving offspring left in the next generation. It is often a combination of survival, mating success, and the number of offspring per mating. Large female frogs and fish lay more eggs than do smaller females, and thus may leave more offspring in the next generation. Therefore only the individuals that are best equipped to survive and reproduce will pass on the highest frequency of genes to the next populations. Their success may not only be seen by the difference in color but by the success they have in attracting mates. In some species larger males tend to mate more frequently than smaller males. Ex. When the environment of the Rock Pocket mice turned from white to black, only the black mice where the fittest or most adaptable to the environment, and able to hide from predators, and therefore pass on their alleles while reproducing.
Sexual Selection/ Nonrandom Mating/ Assortative Mating Sexual Selection- Mode of natural selection in which some individuals out-reproduce others because they are better at securing mates. Nonrandom Mating- occurs when the probability that two individuals in a population will mate is not the same for all possible pairs of individuals. Assortative Mating- is a mating pattern and a form of sexual selection in which individuals with similar phenotypes mate with one another more frequently than would be expected under a random mating pattern. Assortative mating and non random mating are mating patterns that form the concept of sexual selection, a form of natural selection, because the preference for particular mates causes some organisms to be unfavored and therefore not be chosen by females to mate with, preventing the passing on of their genes, and filtering them out of the popular because they are not desirable. Assortative mating is a form of nonrandom, which mating pairs are based off phenotype. There are two types of assortative mating, positive, where organisms mate with those similar to their own phenotypes, such as a tall person mating with another tall person. This is extremely common when it comes to mating. Negative is when an organism avoids mating with those similar to themselves.
Lamarck s Theories Darwin vs Charles Darwin and Jean-Baptiste Lamarck both had ideas about how life on earth got to be. They had similar and many different ideas. They both thought life gradually changed to be better suited for their environment, and that all organisms are related in some way. Another similarity was their belief that life evolved from simpler organisms to many more complex organisms. Lamarck individually believed that change was made by what an organism wants or needs. For example, he said giraffes that stretch their necks transmit acquired longer necks to their offspring. He also believed that disuse of a body part leads to the loss of that part, and the constant use of a different one causes that part to increase. Darwin however, stated that organisms, even of the same species, have variations and those who are better suited to survive in their environment will pass that trait onto their offspring. Those who were not well suited eventually died off. This is how he believed evolution occurs. For example, elephants with longer trunks lived longer because they could reach food and water, while the shorter trunked elephants died off. Those with longer trunks then passed on the long trunks to their offspring. There s also the thought of natural selection with Darwin. Those who blend in with the surroundings or are more fit will be more camouflaged from prey, therefore passing on those traits.
Microevolution Change in allele frequencies within a single species or population Can be observed over a short period of time o Occurs over short time scale, but builds up over time to from macroevolution Examples of microevolution: o Genetic drift: changes in the gene pool of a small population due to chance (usually reduces genetic viability) The Bottleneck Effect Founder Effect Macroevolution The origin of new taxonomic groups Evolutionary patterns on a larger scale (above the species level) Examples of macroevolution: Speciation: origin of new species Anagenesis (phyletic evolution): accumulation of heritable changes Evolution within a lineage Cladogenesis (branching evolution): budding of new species from a parent species that continues to exist (biological diversity) Evolution that results in the splitting of a lineage Microevolution vs. Macroevolution Microevolution occurs on a much smaller scale than macroevolution as well as a shorter time period than macroevolution. Small changes caused by microevolution build up over time, thus contributing to macroevolution. Both micro- and macroevolution rely on the same sources of genetic change, like natural selection and mutation.
Alanna Doyle Evolution Cooperative Review Founder Effect/ Bottleneck Effect The Founder Effect and the Bottleneck Effect are different types of genetic drift. They ultimately result in a loss of genetic variation in a population. The Bottleneck Effect: This occurs when there is a decrease in population for at least one generation and the original species is not an accurate and full representation the population anymore. The new and surviving generation is not genetically the same as the original population. This can occur from an environmental disaster, the hunting of a species to the point of extinction, or destruction of habitat resulting in the deaths of organisms. (http://evolution.berkeley.edu) In Generation 1, there are high numbers of all three colors, but Generation 4 does not have white balls, therefore, represents a decrease in genetic variation. Founder Effect: This is the loss of genetic variation that occurs when individuals from a large population creates a new population. This new population is established from a small number of colonizing ancestors and they remain isolated from other colonies. Therefore, there is a small mating population and not much genetic variation.
Phyletic Speciation / Divergent Speciation Phyletic Speciation: A process of gradual change in a single population. Divergent Speciation: A species evolves into many different species. Both of these types of speciation involve the evolution of one species into different species. The difference between them is that phyletic speciation if the change a species into only one new species. Divergent speciation is when one species evolves into multiple different species with different characteristics to fit their environment and selective pressures. Examples: This species of butterfly evolves from a blue color to a red color This species of butterfly starts out as a darker blue color but evolves into a green species and and light blue species.
#17 Convergent/ Divergent Evolution Convergent Evolution occurs when animals occupying similar environments come to resemble each other even though they might be hardly related. Selective pressures in the similar environment results in similar adaptations. Whales and other sea mammals are a good example of this phenomena. Whales and dolphins resemble sharks and other fish in shape and other external features, however whales originated from land animals and contain lungs rather than gills. Another example is plants in deserts. These plants adapt to grow bulbs and other similar external features, however their flowers reveal different evolutionary origins. Divergent Evolution occurs when a population becomes separated from the rest of the species and follows its on evolutionary path due to different needs. Divergent evolution can lead to not only different local ecotypes but new species all together. An example of divergent evolution is the speciation of bears. During a massive split off of a glacier a long time ago, a brown bear population was geographically isolated from the rest of the species. Due to harsh conditions, this brown bear population evolved into the polar bear. These selective pressures in this new environment forced the development of a new species due to divergent evolution. In convergent evolution, populations that have little evolutionary relationship come to resemble each other due to similar selective pressures. In divergent evolution however, populations start evolutionary similar and evolve into new species. So convergent is convergent and divergent is diverging.
Disruptive/Directional/Stabilizing Selection 1. Stabilizing Selection a. Preserves midrange phenotypes/alleles in a population, decreases alleles/phenotypes on the extremes b. genetic diversity decreases as the extremes decrease, the mean for a particular trait value stabilizes because of this c. Example: Baby birth weight - babies that are too light are typically not developed enough and have lower survival chances; babies that are too heavy might have increased birthing complications, this leads to adaptation over time for babies to have the middle (intermediate) weight. Image shows stabilizing selection as the butterfly population becomes more concentrated at intermediate value, with no longer any butterflies at the extremes. 2. Directional Selection a. Alleles at one extreme or another become favored, causing allele frequency to slowly shift overtime to that phenotype, this is typically due to environmental change. b. Example: The phenotype of the rock pocket mouse in the darker environment shifted from light mouse coloring to dark mouse coloring in order to camouflage with its environment. Image shows the shift of the butterfly phenotype from lighter coloring to darker coloring. 3. DIsruptive Selection a. Alleles/phenotypes at each extreme are favored and the intermediate alleles aren t b. Example: If there was an environment with light and dark rocks and snails that live in that environment have a range of coloring from white to beige to black, the white and black snails (the extreme phenotypes) will thrive while the beige (intermediate phenotype) will die out since they don t match the environment. Image shows the shifting of butterfly phenotype away from the intermediate coloring and towards the extremes of lighter and darker coloring.
4. Pre/Post-zygotic Barriers Prezygotic barriers are walls that would dissuade an organism from breeding or cause an issue between the fertilization in a species. Some examples of prezygotic barriers are habitat, temporal, behavioral, mechanical and gametic isolations. Habitat isolation is when a species if found in the same general area, but is not in the same vicinity, meaning they do not interact. For example, certain birds keep their nests up high in the canopy, but others may settle in the middle of the trees and not come in contact with the birds higher up. Temporal isolation is when species breed at varying times either by season, time of day or year, meaning they cannot breed since they will not meet during this time. For instance, if an animal mates during the day, it will not run into the same breed that is trying to mate at night. Behavioral isolation is when organisms have different breeding tactics like behavioral patterns or customs to attract a mate such as a dance or mating call. Mechanical isolation is when organisms have certain differences in body type or shape that prevent successful breeding. An example is the genitals of certain insects being unable to fit properly. Gametic isolation is when one species is unable to properly fertilize another species. This causes the sperm to die inside the female reproductive tubes. For example, when sea urchins eject their sperm into the waters, but red urchin sperm will not infuse with purple urchins. Postzygotic barriers are barriers that occur during fertilization, such as the the zygote being unable to develop into a fertile adult organism. Some of these include reduced hybrid viability, reduced hybrid fertility and hybrid breakdown. Reduced hybrid viability means the hybrid has two different species as parents and it ends up impairing the hybrid s growth like the Ensatina salamander, that cannot fully develop and stays frail and susceptible to attack. Reduced hybrid fertility means that a hybrid may be born healthy, but may not be fertile or able to reproduce. This happens when the number of chromosomes in the parents differ and the offspring has chromosomes that will not be able to pair up. For example, horses have 64 chromosomes and donkeys have 62, so mules end up having 63. Hybrid breakdown is when the hybrid may be perfectly normal (strong and able to breed), in the first generation, but lose the ability to thrive or reproduce in later generations like certain plants. For example, certain rice hybrids are strong for the first pick, but their seeds cannot reproduce themselves.
Griffin White Period 6, AP Biology Analogous and Homologous Structures Analogous structures are structures on different species that have the same or similar functions but do not have the same evolutionary origin. These structures could be on any different species as long as the function is the same. One example would be a wing on an insect and the wings of a bird. Both allow the species to fly but the two are not related. On the other hand, homologous structures do not need to have the same function but exist because of a common evolutionary path. This can be seen in some species that do not have a need for an arm but still have the skeletal system that shows where the arm used to be. An example would be the bones in the wing of a bat and a human arm. The bat s wing has an arm bone in its wing as both the human and bat share an evolutionary origin. Analogous structures and homologous structures differ in the characteristics that define them. Analogous structures Homologous structures