D.2: Species and Speciation

Gene Pool: ________________________________________________________________________________________

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The gene pool is constantly changing- Mutations add ne w 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: ___________________________________________________________________________________

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? ____________________________________________________________________________________________

__________________________________________________________________________________________

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.

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: _________________________________________________________________________

Postzygotic Isolation: _________________________________________________________________________

(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.)

Prezygotic

Mechanism Description ExampleBehavioural Isolation Different species used different

mating rituals to find a mateMany frogs have unique calls that only attract females of their species

Temporal Isolation Different species breed at different times of the year

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

Ecological Isolation Species occupy different habitats within a region

The mountain bluebird lives at high elevations while the eastern bluebird lives a lower ones

Mechanical Isolation Differences in morphological features make two species incompatible

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

Gametic Isolation Gametes are unable to recognize each other to allow fusion of nuclei.

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

Postzygotic

Mechanisms Description ExampleZygotic Mortality Mating and fertilization are possible, but

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

Hybrid Inviability A hybrid develops but either dies before birth, or cannot survive to maturity

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

Hybrid Infertility Hybrid is born, and lives but is sterile Mules are the sterile offspring of a horse and a donkey cross

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 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: 2n Triploid= 3n; Tetraploid = 4n

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.

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

Autopolyploidy: ___________________________________________________________________________

Allopolyploidy: ______________________________________________________________________________

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SPECIATION

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

Allopatric vs. Sympatric Speciation

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. 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….

Ex: 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.

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. Can be a result of behavioural changes

Example: Hawthorne Fly 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 vs. Convergent Evolution

DIVERGENT EVOLUTION

___________________________________________________________________________________________ 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

________________________________________________________________________________________ (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

Ex: Darwin’s Finches 13 species of finches that live on the

Galapagos Islands evolved from a single species.

The original species likely lived on the mainland of South America and had a medium-sized bill used to feed on medium-sized seeds.

CONVERGENT EVOLUTION

When ______________________________________________________________________________________ As a result they develop similar traits even though they do not have a common ancestor with the trait. These are analogous structures

Structures that are _____________________________________________________________________ The trait is not found in the most recent common ancestor Ex: wings of birds, insects, and bats

Ex. 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.

EX: 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)

Evolutionary Patterns:

Gradualism vs. 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.

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: _________________________________________________________________________________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 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

Disruptive Selection: ________________________________________________________________________________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.

Polymorphism The existence of 2 or more forms/phenotypes within a population Ex: Peppered Moth

Transient Polymorphism When one allele is in the process of displacing another. 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 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 ½ the hemoglobin is abnormal Produces a mild anemia Shorter life span for these red blood cells

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

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