D.2: Species and Speciation
Gene Pool: ________________________________________________________________________________________
__________________________________________________________________________________________________
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 nondisjunction disorders) or if they do survive they are infertile because meiosis doesn’t occur since the
individual lacks homologous chromosomes.)
Prezygotic
Mechanism
Behavioural Isolation
Temporal Isolation
Description
Different species used different
mating rituals to find a mate
Different species breed at different
times of the year
Ecological Isolation
Species occupy different habitats
within a region
Mechanical Isolation
Differences in morphological
features make two species
incompatible
Gametes are unable to recognize
each other to allow fusion of nuclei.
Gametic Isolation
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.
Postzygotic
Mechanisms
Zygotic Mortality
Hybrid Inviability
Hybrid Infertility
Description
Mating and fertilization are possible, but
zygote doesn’t develop properly
A hybrid develops but either dies before
birth, or cannot survive to maturity
Hybrid is born, and 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
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: ______________________________________________________________________________
___________________________________________________________________________________________
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 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