10 3 Gene Pools and Speciation Gene Pool

10. 3 Gene Pools and Speciation

Gene Pool �All the alleles present in the reproducing members of an interbreeding population at a given time. �The gene pool is constantly changing �Mutations add new alleles to the population �Immigration also adds new alleles; changes the allele frequency �Alleles can be removed by natural selection if disadvantageous

�Large gene pool �–a lot of variety in traits (many different alleles) �Small gene pool �Little variation in alleles present (common in inbreeding) �Allele Frequency: how often a particular allele appears in a population (will discuss further in D. 4)

Evolution and Alleles �Gene pools are generally relatively stable over time, but not always �New alleles can be introduced and old alleles can disappear �A result of evolution: after many generation of natural selection, alleles proven to be more advantageous tend to be more frequent. �Alleles that are disadvantageous to an organism's survival are not passes on to as many offspring. �Without changes in allele frequency, evolution cannot occur

What is a species? �All the members of a population that can interbreed under natural conditions; share the same gene pool. �Produce fertile offspring! �Individuals of different species cannot interbreed under natural conditions – they are reproductively isolated from each other and they have different gene pools.

Species �A horse (Equus ferus caballus) and donkey (Equus africanus asinus) can mate to produce a mule �However, mules are infertile. Horses and donkeys are of different species. Horse + donkey mule

What is a Species? �Members of the same species have similar physiological/morphological characteristics that can be measured �They are genetically distinct from other species (own identical karyotype) �They have a common phylogeny (a common ancestor)

Speciation �The formation of an entirely new species �For speciation to occur, individuals from the original species must become reproductively isolated from the rest of the individuals �A barrier is created between gene pools within a population – thus preventing the individuals from mating. This leads to genetic isolation

Genetic Isolation �Prezygotic Isolation: prevent fertilization and a zygote from forming �Postzygotic Isolation: a zygote is formed but does not mature into a viable, reproducing adult �(The 2 species are similar enough to allow their gametes to combine, but they have different diploid numbers. The hybrid offspring may not have enough genetic information to survive-(similar to non-disjunction disorders) or if they do survive they are infertile because meiosis doesn’t occur since the individual lacks homologous chromosomes. )

Mechanism Description Behavioural Different species used Isolation different mating rituals to find a mate Temporal Different species breed at Isolation different times of the year Ecological Isolation Species occupy different habitats within a region Mechanical Differences in Isolation morphological features make two species incompatible Gametic Gametes are unable to Isolation recognize each other to allow fusion of nuclei. Example Many frogs have unique calls that only attract females of their species Flowers that release pollen in the spring are reproductively isolated from flowers that release pollen in the summer. The mountain bluebird lives at high elevations while the eastern bluebird lives a lower ones Male and female genetalia of each species are uniquely shaped and are physically incompatible with other species Many marine animals release their sperm and eggs into open water. The sperm recognize eggs of their own species through chemical markers.

Mechanisms Description Zygotic Mating and Mortality fertilization are possible, but zygote doesn’t develop properly Hybrid A hybrid develops but Inviability either dies before birth, or cannot survive to maturity Hybrid is born, and Infertility lives but is sterile Example Some species of sheep and goat can mate, but zygote doesn’t live. When tigers and leopards are crossed – ends in a miscarriage or stillborn offspring Mules are the sterile offspring of a horse and a donkey cross

Just for fun…. Some hybrids �Zebra (male) + equine (female) zebroid/zorse (don’t occur in nature) �Lion + tiger liger (male) (female) (territories don’t overlap, so only seen in captivity, however, may have occurred naturally in the past) Photoshopped?

�Liger named Hercules

Just for Fun… Some hybrids �Sheep + goat toast of botswana • Camel + llama cama

Exceptions to the species rule �Term species only applies to sexually reproducing organisms. �Hybrids are sometimes capable of mating with a parent species and producing fertile offspring. However, the hybrid is still not considered a member of the parent species. �Ring species – members of adjacent populations interbreed successfully but members of widely separated populations do not.

Polyploidy �When a cell contains three of more sets of chromosomes �Human body cells are diploid: 2 n �Triploid= 3 n �Tetraploid = 4 n �Polyploidy is the result of the failure to nuclei to separate during meiosis

�In plants, polyploidy is much more common than in animals. �The extra set of chromosomes leads to more vigorous plants which produce bigger fruits or storage units, or are more resistant to disease �Having extra sets of chromosomes has the consequence of making errors in replication more common.

Polyploidy can lead to speciation �If one plant is triploid another is tetraploid, they may not be able to form a zygote �This can lead to speciation - the evolving population changes significantly enough so that the production of offspring becomes impossible �(Note: organisms with an odd number of sets of chromosomes (i. e. triploid) are usually sterile b/c cannot form homologous pairs during prophase I)

�Autopolyploidy: increase in the number of chromosomes within the same species � Allopolyploidy: when the chromosomes number in a sterile hybrid becomes doubled and produces fertile hybrids

Polyploidy speciation �While polyploidy may cause a new species to form, it is more likely to happen in plants rather than in animals. �For many animals, have an abnormal number of chromosomes and extra genes causes developmental problems. �Think back to the nondisjunction disorders we discussed in grade 11. �Most hybrids a infertile and have reduced lifespans �That being said, polyploidy can occur in less complex animals

Polyploidy: the Red Viscacha

Polyploidy: the Red Viscacha �The red viscacha (Tympanotomys barrerae) is a rodent native to Argentina. �It has 102 chromosomes (2 n=102) - the highest of any mammal! �Its closest living relative is the Andean viscacha-rat (Octomys mimax) which has 2 n=56 chromosomes.

Polyploidy – Red Viscacha �It is believed that an Octomys ancestor produced tetraploid offspring (4 n=112) that became reproductively isolated from the parent species. �With time, some of the additional chromosomes were lost (naturally selected against) �Research shows that the Red Viscacha has only 2 of every autosome pair but there are several genes that exist in 4 copies.

Polyploidy - Allium �Allium - includes onion, leeks, garlic, and chives �Polyploidy has occurred frequently in many species of Allium

Polyploidy - Allium �Allium canadense (white onion) is 2 n = 14. �However, variants of 2 n=28 exist (such) as Allium lavendulae) Allium canadense Allium lavendulae

Polyploidy - Allium angulosum Allium oleracium � 2 n =16 � 4 n= 32

Speciation �INTRASPECIFIC: Process in which one or more species arise from a previously existing one �INTERSPECIFIC HYBRIDIZATION: Process in which 2 different species give rise to a new species

Allopatric Speciation �When a single species is separated into 2 geographically isolated populations. �Once the populations are physically separated they can no longer exchange genetic info. �Over many generations, the populations will gradually become less and less alike. �Any mutation that occurs in one population, will not be shared with the other.

Allopatric Speciation…. �Differences in the environments of the 2 populations can lead to different forms of natural selection. �With time, it is likely that the 2 populations will have evolved some sort of reproductive isolating mechanism. �The geographical isolation can be caused by development of mountain ranges, continental drift, human construction activities that disrupt a habitat….

Allopatric Speciation

Ex: Allopatric Speciation � 2 mya, a thin strip of land called the Isthmus of Panama formed to separate the Caribbean Sea from the Pacific Ocean and permanently divide species such as the wrasse into 2 separate populations. �Now the species distinct and cannot successfully interbreed when placed with each other.

Speciation on the Galapagos Islands �The speciation that occurred on the Galapagos islands is the result of Allopatric speciation. �Animals migrated to the islands, and became geographically and reproductively isolated from parent species on the main land.

Sympatric Speciation �When a new species evolves from within a large population. �Individuals within the population become genetically isolated from the larger population. �Can occur gradually or suddenly.

Ex: Sympatric Speciation �The hawthorn fly lays its eggs on the fruits of hawthorn trees. When apple trees were introduced into its environment, some of these flies began laying their eggs on apples. Today the species consists of 2 populations.

Divergent Evolution �The large –scale evolution of a group into many different forms �A single parent species is put under (at least) 2 different selective pressures. �With time, 2 or more related species will develop and become more and more dissimilar as they adapt to their environment

�Species that have evolved from divergent evolution share a common ancestor and often share homologous traits (i. e. Pentadactyl limb)

Adaptive Radiation �A type of divergent evolution �the relatively rapid evolution of a single species into many new species (that are similar but distinct from each other) �Happens because variation in the population allow certain members to exploit a slightly different niche in a more successful way

Adaptive Radiation… �each new species fills a different ecological niche �occurs when a variety of new resources, that are not being used by other species become available �More common in periods of environmental change

Adaptive Radiation Ex: Darwin’s Finches � 14 species of finches that live on the Galapagos Islands evolved from a single species. �Darwin observed that the size and shapes of the finch beak varied with their diet. �The original parent species likely lived on the mainland of South America and had a medium-sized bill used to feed on medium-sized seeds. �On the mainland, if a finch developed a small bill to eat small seeds or a large bill to eat large seeds, they likely would have been in competition with other bird species and would not be naturally selected for.

�However, if these small and large billed finches lived on the Galapagos Islands, where there wasn’t other bird species (yet), there would be no competition for their ideal food source and they would occupy a new ecological niche. �As a result, their bill size would be naturally selected for.

Convergent Evolution �When 2 different species evolve to occupy similar ecological niches. �As a result they develop, similar traits even though they do not have a common ancestor with the trait. �These are analogous structures

Analogous Structures �Structures that are similar function but evolved independently in each species �The trait is not found in the most recent common ancestor �Ex: wings of birds, insects, and bats

Examples of Convergent Evolution �Cacti and euphorbia �These are different species of plant that live in completely different regions in the world. �However, they both live in very dry environments.

�Cacti evolved in the deserts of South America and are native to the Americas. �Euphorbia evolved in South Africa and are found in Africa, Eurasia, and Australia �As a result, both have evolved adaptations independently that allow them to survive in their environments. �Since their environments are so similar, their adaptations are similar too. �They both have sharp spines and thick green stems to perform photosynthesis and store water.

Convergent Evolution – Swimming Carnivore �Shark vs. dolphin vs. penguin vs. ichthyosaur (extinct) �They all have similar body shapes including fins or flippers and streamlined body shapes that allow them to move through the water (occupy a common niche) penguin bird

Evolutionary Patterns Divergent Evolution Convergent Evolution Parallel Evolution

Gradualism and Punctuated Equilibrium �What is the pace of evolution? �How quickly do new species and entirely new groups evolve? � 2 Theories to explain the patterns of evolution that take place over very long periods of time: gradualism and punctuated equilibrium

Gradualism �As new species evolve, they appear very similar to the original species and only gradually become more distinctive. �Over long periods of time, the small changes accumulate, resulting in dramatically different organisms. �The fossil record would show many transitional/ intermediate forms.

Punctuated Equilibrium �Sometimes the fossil record shows a new species appearing quite suddenly and then remaining little changed over time. � 3 main assertions of punctuated equilibrium �New species evolve rapidly �Speciation occurs in small isolated populations – so few transitional fossils �After initial change, additional changes are slow

Gradualism Punctuated Equilibrium

Patterns of Natural Selection �Sometimes abiotic or biotic factors can result in different patterns of natural selection. �Can result in directional, stabilizing, or disruptive selection

Scenario �Hummingbirds use their bills to feed on nectar from flowers. �Hummingbird populations can have varying lengths of bills (short, medium, long)

Stabilizing Selection �When the average phenotype within a population is favoured by the environment. � Selection pressures remove the extreme varieties �Ex: What if there were only medium flowers for the hummingbirds? �The birds with long bills require more nutrients and energy �The birds with short bills may not get enough food. �The medium sized bills are the best and will be selected for.

Directional Selection �This is when one extreme of a variation is naturally selected for. �Ex: If a new habitat has plants with long flowers, birds with longer bills will be favoured for by the environment. �These birds will have more food and will be more likely to live to reproduce and pass their genes on to the next generation �Thus, a new mean phenotype is selected for

Disruptive Selection �Favours individuals with variations at opposite extremes of a trait over the intermediate variations. �Ex: If there were both short flowers and long flowers, this would best suit short bills and long bills for hummingbirds. The medium sized billed hummingbirds wouldn’t be selected.

Examples: �Stabilizing Selection: �Average birth weight in human babies is favoured over low birth weight or high birth weight.

Examples: �Disruptive Selection: �Red crossbills (Loxia curvirosta) �Beaks cross on either the left or the right, allowing the bird to access seeds from conifer cones. �The intermediate form is a straight bill and it is naturally selected against.

Examples: �Directional Selection: �In giraffes, long necks have be directionally selected for because they allow the animals to eat leaves from high branches.

Polymorphism �The existence of 2 or more forms/phenotypes within a population �Remember the peppered moths? ? ? (5. 1)

Peppered Moth

Transient Polymorphism �When one allele is in the process of displacing another. �Our example of the peppered moth during the Industrial Evolution is an example of Transient Polymorphism �Before the industrial revolution, the peppered allele was in higher frequency �When the trees turned black, the black moths has a greater chance of surviving and so the black allele began replacing the previously common peppered allele

Balanced Polymorphism �When 2 different forms/varieties coexist in the same population in a stable environment (because of natural selection) �Ex: Sickle cell anemia in Africa

Sickle Cell Anemia �A recessive allele.

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Sickle Cell Anemia �The heterozygous condition causes patients to exhibit a condition known as sickle-cell trait �Red blood cells appear normal �Only ½ the hemoglobin is abnormal �Produces a mild anemia �Shorter life span for these red blood cells

The Heterozygote Advantage �However, it is beneficial because the parasite that causes malaria cannot complete its life cycle inside the red blood cells �A heterozygote advantage �Some of the homozygous dominants will die because of malaria �Some of the homozygous recessive will die because of sever anemias �The heterozygotes survive and reproduce

Balancing Selection �A form of natural selection �Maintains genetic polymorphism within a population �Sickle-cell anemia is an example of balancing selection because the sickle-cell allele frequency is maintained by the heterozygote advantage.

Co-evolution �The process in which one species evolves in response to the evolution of another species. �(The evolution of one species is linked to the evolution of another) �Develops as a result of mutualistic symbiotic relationships

Ex: Agouti and Brazil Nut �The brazil nut has a hard protective shell. �The Agouti is the only mammal with jaws and teeth strong enough to bite open the shell

Ex: Madagascar long-spurred orchid and the hawk moth �The Madagascar long – spurred orchid has a long tube called spurs which contain nectar. �It is pollinated by a hawk moth whose tongue is 30 cm long and can reach the nectar �In the process it helps the orchids with pollination by moving pollen from plant to plant.

Ex: Dung Beetle and Orchidantha inouei �The dung beetle feeds on dung/feces. �This particular orchid produces a foul odour (similar to poop) to attract the dung beetle. �The dung beetle will unknowingly pick up pollen from the flower and deposit it onto another.

Ex: Orchid bee and bees �The “orchid bee” Ophrys apifera is an orchid that mimics a female bee. �Male bees will attempt to mate with the flower. �Instead they pick up pollen and deposit it on the next orchid (aiding in pollination) as they attempt to copulate with it