Date: ________________________________________

10.3 Gene Pools & Speciation

Gene Pool

· A gene pool consists of all the genes and their different alleles, present in an interbreeding population at any given time.

· The gene pool is constantly changing.

·

· Immigration also adds new alleles; changes the allele frequency

· Alleles can be removed by natural selection if disadvantageous

· Large Gene Pool: __________________________________ (many different alleles)

· Small Gene Pool: ______________________________in alleles present (common in inbreeding)

· Allele Frequency: how often a particular allele appears in a population

Evolution and Alleles

· Gene pools are generally relatively ______________________________________________

· New alleles can be introduced and old alleles can disappear.

· A result of evolution - after many generations 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 passed on to as many offspring.

·

What is a Species?

·

· share the same gene pool

· produce fertile offspring!

· Individuals of different species that cannot interbreed under natural conditions are said to be ____________________________ from each other and have separate gene pools.

· Example: A horse (Equus ferus caballus) and donkey (Equus africanus asinus) can mate to produce a mule

· However, mules are ___________________. Horses and donkeys are of different species.

Mule

· Members of the same species have similar ____________________________________________ characteristics that can be measured.

· They are genetically ___________________ from other species (own identical karyotype).

· They have a common ___________________ (a common ancestor)

Microevolution

· Species continuously evolve

· The changes in gene (allele) frequencies and phenotypic traits that occur within a species are referred to as _________________________.

· Example:

· After the ground finches populated the different islands, they evolved over time to form a new species of tree finches.

Speciation

· ______________________________________________________________________________.

· For speciation to occur, individuals from the original species must become _____________________________________________ from the rest of the individuals.

· This often occurs after a population of a species extends its range by migrating, creating a barrier between gene pools within a population and thus preventing the individuals from mating.

· This leads to genetic isolation.

· Genetic isolation explains the large number of endemic species on the Galapagos Islands.

· Endemic species:

· Example: Lava lizards of the Galapagos Islands

One species is present on all the main islands; six smaller islands have closely related but different species formed by migration to island and subsequent divergence.

·

Mechanisms of Reproductive Isolation

· A reproductive isolating mechanism is any behavioural, structural, or biochemical trait that prevents individuals of different species from reproducing successfully together; can lead to speciation.

· Reproductive isolating mechanisms include:

1. ______________________________________

2. ______________________________________

1. Prezygotic Mechanisms

Reproductive isolating mechanism that prevents interspecies mating and fertilization.

Prezygotic mechanisms include:

Mechanism

Description

Example

Behavioural Isolation

Many frogs have unique calls that only attract females of their species.

Temporal Isolation

Flowers that release pollen in the spring are reproductively isolated from flowers that release pollen in summer.

Ecological Isolation

Mountain bluebird lives at high elevations while eastern bluebird lives at lower ones.

Mechanical Isolation

Male and female genitalia of each species are uniquely shaped and are physically incompatible with other species.

Gametic Isolation

Many marine animals release their sperm and eggs into open water. The sperm recognizes eggs of their own species through chemical markers.

2. Postzygotic Mechanisms

· Reproductive isolating mechanism that prevents maturation and reproduction in offspring from interspecies reproduction.

· Mechanisms include:

Mechanism

Description

Example

Zygotic Mortality

Some species of sheep and goat can mate, but zygote doesn’t live.

Hybrid Inviability

When tigers and leopards are crossed – ends in a miscarriage or stillborn.

Hybrid Infertility

Mules are the sterile offspring of a horse and a donkey cross.

Exceptions to the Species Rule

· The term species only applies to sexually reproducing organisms.

· Hybrids are sometimes capable of mating with a parent species producing fertile offspring. However, the hybrid is still not considered a member of the parent species.

Polyploidy

· When a cell contains three or more sets of chromosomes

· Human body cells are diploid = _______

· Triploid = _____

· Tetraploid = _______

· Polyploidy is the result of the failure of the 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 and 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 because they cannot form homologous pairs during prophase I.

Autopolyploidy

·

Allopolyploidy

·

· While polyploidy may cause a new species to form, it is more likely to happen in plants than in animals.

· For many animals, having an abnormal number of chromosomes and extra genes causes developmental problems.

· Think back to the nondisjunction disorders discussed in Genetics.

· Most hybrids are infertile and have reduced lifespans.

· That being said, polyploidy can occur in less complex animals.

Polyploidy: The Red Viscacha

· The red viscacha (Tympanotomys barrerae) is a rodent native to Argentina.

· It has 102 chromosomes (2n=102) - the highest of any mammal, with cells being twice the normal size!

· Its closest living relative is the Andean viscacha-rat (Octomys mimax) which has 2n=56 chromosomes.

· It is believed that an Octomys ancestor produced tetraploid offspring (4n=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

· The genus Allium includes onion, leeks, garlic, and chives.

· Polyploidy has occurred frequently in many species of Allium.

· Allium canadense (wild onion) is 2n = 14.

· However, variants of 2n=28 exist (such) as Allium lavendulae).

· Allium angulosum and Allium oleraceum are two species that occur in Luthania; one is a diploid with 2n=16 and one is a tetraploid 4n=32.

Speciation

· Intraspecific:

· Interspecific Hybridization:

Modes of Speciation

· There are two modes of speciation:

1. ______________________________________

2. ______________________________________

Allopatric Speciation

· Allopatric speciation is the _________________________________________________________

____________________________________________________________________________.

· Once separated, populations can no longer exchange genetic information.

· Over generations, these populations become less similar because any mutation that occurs is not shared between populations; environmental conditions differ and eventually they could develop reproductive isolating mechanisms.

· The geographic splitting of a species into two populations can occur in a number of ways, such as:

· development of ____________________________

· ___________________________________________

· ______________________________________ activities that disrupt a habitat

Geographic Separation leads to Allopatric Speciation

Example: 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 two separate populations.

· Now the species on both sides are ________________ and cannot successfully ________________, even when placed together.

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

·

· Individuals within the population become genetically isolated from the larger population.

· Can occur gradually or suddenly.

· Can be a result of polyploidy or hybrid formation.

Example: Sympatric Speciation

The hawthorn fly, native to North America, 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 two populations.

Sudden Sympatric Speciation

· A single mutation can render an individual unable to reproduce with other members of the population.

· If two such individuals share the same mutation or if a single individual is able to reproduce asexually, a reproductively isolated population, a new species may result.

Polyploidy and Hybrid Speciation

· ____________________________ can result in sympatric speciation.

· Mutations causing polyploidy double the number of chromosomes in an individual.

· Polyploids produce fertile offspring when mated with each other but produce sterile offspring when mated with the original species.

Patterns of Evolution

As we have seen, natural selection leads to predictable outcomes:

· Closely related species share many homologous structures, even though they no longer serve the same function.

· Species have vestigial structures that once served a useful purpose in their ancestors.

When considered on a grander scaler, these and other predictable outcomes lead to recognizable patterns.

(A) Divergent Evolution

· _____________________________________________________________________________.

· A single parent species is put under (at least) two different selective pressures.

Specie A

Specie B

· With time, two or more related species will develop and become more and more dissimilar as they adapt to their environment.

Example: Divergent Evolution

· Ontario forests are home to many rodents which all evolved from a single common ancestor that existed millions of years ago.

· Over time, several different species have evolved unique features to make them successful in their environment.

· Competition between species is minimized as new species diverge to fill specialized niches.

·

Adaptive Radiation

· _____________________________________________________________________________.

· The relatively ________________________________ 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.

· 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

Example of Adaptive Radiation: Darwin’s Finches

· When Darwin first visited the Galapagos Islands in 1835, he noted that there existed a variety of 14 species of finches that all seemed to be derived from a common ancestor.

· Darwin observed that the sizes and shapes of the beaks of the finches varied, as did their diet.

· From his observations, he concluded that a common original species likely lived on the mainland of South America and was blown several hundred miles off course to these islands.

· This flock would then have subsequently populated and diversified into many different forms, adapting to the many different opportunities the islands offered for finding food.

· 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 and were able to exploit a wide variety of new resources with little or no competition from other species, their bill size would be naturally selected for.

· This process of modification over time to fill a variety of niches is adaptive radiation.

Grant’s Research

· There has since been intense research into Darwin’ finches.

· In particular, Peter and Rosemary Grant have shown that _________________________________ are closely correlated and when one changes, the other also changes.

· Peter and Rosemary spent part of each year since 1973 in a tent on a tiny barren volcanic island in the Galapagos. They caught, weighed, measured, and identified hundreds of small birds and recorded their diets every year.

· In their natural laboratory, the 100-acre island called Daphne Major, the Grants watched the struggle for survival among individuals in two species of birds:

· Medium ground finch, Geospiza fortis

· Small ground finch, Geospiza fuliginosa

Reason for Adaption

· The struggle is mainly about food – different types of seeds – and the availability of that food is dramatically influenced by year-to-year weather changes.

· Both species feed on small seeds, though G. fortis can also eat larger seeds.

· There was a severe drought in 1977. That year, the vegetation withered. Seeds of all kinds were scarce. The small soft seeds were quickly exhausted by the birds, leaving mainly large tough seeds.

· Under these drastically changing conditions, the struggle to survive favoured the G. fortis with the larger beaks for opening the hard seeds.

· Smaller finches with less-powerful beaks perished.

· The big-beaked G. fortis just happened to be the ones favoured by the particular set of conditions Nature imposed that year.

· The Grants observed that the offspring of the birds that survived the 1977 drought tended to be larger, with bigger beaks.

· So the adaptation to a changed environment led to a larger-beaked finch population in the following generation.

· Adaptation can go either way, of course! As the Grants later found, unusually rainy weather in 1984-1985 resulted in more small soft seeds and fewer of the large, tough ones.

· Sure enough, G. fuliginosa were the birds best adapted to eat those seeds because of their smaller beaks and thus were able to survive and produce the most offspring.

Progressive Change

· Changes to the beaks of finches in the Galapagos Islands is a chief example that natural selection increases the frequency of characteristics that make individuals better adapted and decreases the frequency of other characteristics leading to changes within the species.

· Since better-adapted individuals survive, they can reproduce and pass on characteristics to their offspring.

· Individuals that are less well adapted have lower survival rates and less reproductive success.

(B) Convergent Evolution

·

· As a result they develop similar traits even though they do not have a common ancestor with the trait.

· These are analogous structures.

Example: Cacti & Euphorbia

· These are different of plant that live in completely different regions in the world. However, they both live in very dry environments.

· Cacti evolved in deserts of South America and are native to the Americas.

· Euphorbia, which look similar to cacti, first evolved in South Africa and are found in Africa, Eurasia, and Australia.

· Independently, both species

· They both have sharp spines and thick green stems to perform photosynthesis and store water.

Other Examples of Convergent Evolution

· Insect wings and bird wings

· Swimming carnivores: shark vs. dolphins vs. penguins

· All have similar _______________________ including fins or flippers, and streamlined body shapes that allow them to move through water.

(C) Coevolution

· The process in which

· The evolution of one species is linked to the evolution of another.

· Develops as a result of mutualistic symbiotic relationship.

Examples of Coevolution

· The Brazil nut has a _______________________________. The Agouti is the only mammal with jaws and teeth ______________ enough to bite open the shell.

· The Madagascar long-spurred _______________ has a ___________________ called spurs which contain nectar. It is pollinated by a ____________________whose tongue is 30 cm long and reach the nectar but in the process it helps the orchids with pollination by moving pollen from plant to plant.

Gradualism vs. Punctuated Equilibrium

What is the pace of evolution? How quickly do new species and entirely new groups evolve?

There are two theories to explain the patterns of evolution that take place over very long periods of time.

Theory of 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.

·

Theory of Punctuated Equilibrium

· Sometimes the fossil record shows a new species appearing quite suddenly and then remaining little changed over time.

· The theory of Punctuated Equilibrium suggests that most evolutionary changes occur in relatively rapid spurts, and are followed by long periods of little or no change.

Three Main Assertions:

·

· Speciation occurs in small isolated populations – so few transitional fossils

·

Patterns of Natural Selection

· Selective pressures may result from any number abiotic or biotic factors: : diseases, climatic conditions, food availability, predators, choice of mate.

· These selective pressures can result in different patterns of natural selection:

1. ___________________________________

2. ___________________________________

3. ___________________________________

Scenario

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

1. Stabilizing Selection

·

· Example: What if there were only medium flowers for the hummingbirds?

· The birds with long bills require _________________________ and _____________.

· The birds with short bills ________________________________________________.

· The medium sized bills are the _____________ and will be ________________ for.

2. Directional Selection

·

· If a new habitat has plants with long flowers, birds with longer bills will be favored for by the environment.

· Longer-billed birds will obtain more food and will be more likely to live to reproduce and pass their genes on to the next generation.

· Eventually, the bill length of the population will increase and a new mean phenotype is selected for.

3. Disruptive Selection

·

· Example: 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.

Polymorphism

· The existence of two or more forms/phenotypes within a population.

· Example: Peppered moths

Transient Polymorphism

·

· Our example of the peppered moth during the Industrial Revolution 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 had a greater chance of surviving and so the black allele began replacing the previously common peppered allele.

Balanced Polymorphism

· When two different forms/varieties coexist in the same population in a stable environment (because of natural selection).

· Example: Sickle cell anemia in Africa

· Caused by a recessive allele

· The heterozygous condition causes patients to exhibit a condition known as sickle-cell trait

· Red blood cells appear normal

· Only half the hemoglobin is abnormal

· Produces a mild anemia

· Shorter life span for these 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 severe anemia

· The heterozygotes survive and reproduce

· Sickle-cell anemia is an example of balancing selection because the sickle cell allele frequency is maintained by the heterozygote advantage.

11