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Speciation: How Species
Form
Lesson 6
SPECIATION
• Microevolution: changes in allele frequencies and
phenotypic traits within populations and species; can
result in new species.
• Speciation: The evolutionary formation of new
species.
• Species: members of groups or populations that
interbreed or have the ability to interbreed with each
other under natural conditions.
– reproductively isolated from other groups
– evolve independently
Reproductive Isolation
• Whether or not different species interbreed successfully in
the wild can be difficult to examine.
• Differentiating species is mainly done by physical
appearance/morphology, however, subspecies (or races)
are morphologically distinct subpopulations that can
interbreed.
• Where morphology is not possible, behaviour or
reproductive isolating mechanisms are used.
• A reproductive isolating mechanism is any behavioural,
structural or biochemical trait that prevents individuals
of different species from reproducing successfully
together.
Means of Reproductive Isolation:
PREZYGOTIC (prevention of interspecies mating or
fertilization)
• Prevention of mating:
– Ecological isolation: different habitat, niches,
– Temporal isolation: different reproductive cycles
(timing of day, seasons)
– Behavioural isolation: different signals
Which type is this?
Means of Reproductive Isolation:
• Prevention of fertilization:
– Mechanical isolation: structural differences
in reproductive organs (arthropod, flowers)
– Gametic isolation: molecular recognition of
sperm and egg (water animals)
Different penis structures of damselflies
Means of Reproductive Isolation:
• POSTZYGOTIC (prevent
maturation and reproduction in
offspring from interspecies
reproduction)
– Zygotic Mortality: different
species can mate but no
embryos develop to maturity,
– Hybrid Inviability: baby
hybrids aren't viable, don't live
long
– Hybrid Infertility: baby hybrid
viable but not fertile.
Reproductive isolation and
Speciation
• Reproductive isolation may lead to
speciation.
• The gene pool is isolated, any mutation and
selection that occurs is no longer shared;
• any significant evolutionary changes that
occur in either population (new or old) will
result in the formation of separate
species.
In class work
• Pg 363, Q 13-18
Modes of Speciation
• Sympatric Speciation: evolution of populations
within the same geographic area into separate
species. (ex. grey tree frogs)
• Chromosomal changes (plants) and nonrandom mating (animals) alter gene flow
• More common in plants
• Polyploidy (3 or more sets of chromosomes) can
lead to speciation
Modes of Speciation
• Allopatric Speciation: evolution of populations
into separate species as a result of geographic
isolation. Ex. water, canyon, mountain range,
human construction (dams, highways, canals)
• Darwin’s finches pg 336
In class work
• Pg 365, Q 19-24
Adaptive Radiation
• Adaptive radiation is an increase in the
morphological (what they look like) and
ecological (where they live) diversity of a
species eventually resulting in the formation
of new species.
Adaptive Radiation
• It usually occurs very rapidly when a species colonizes a
new environment (Darwin’s Finches) OR by survivors
after a massive extinction event (Mammals after the
extinction of the Dinosaurs).
Adaptive Radiation
• Darwin’s Finches: An ancestral finch population
got blown off the mainland of South America onto
the Galapagos Islands. Over time that finch
species evolved to fulfill all the niches on the
islands and thereby give rise to the variety of
finches seen on the islands.
• Mammals after the extinction of the Dinosaurs:
With Dinosaurs out of the way, mammals were
able to grow bigger and fill all the niches vacated
by the larger reptiles, which explains the wide
diversity of forms we see in mammals today.
Adaptive Radiation
• It supports evolution by showing that
groups of organisms (i.e. mammals) are all
related to each other and came from a
common ancestor that inhabited new
environments and evolved to adapt to
these environments.
• All mammalian forelimbs contain the same
bones which shows that they all evolved
from a common ancestor.
Divergent and Convergent Evolution
• Divergent
– Species that were once similar diverge or become
increasingly distinct
– Eg/ Darwin’s finches
• Convergent
– A pattern of evolution in which similar traits arise
because different species have independently
adapted to similar environmental conditions
– Eg/ bats and birds
CONVERGENT EVOLUTION
• Species from different evolutionary branches may
come to resemble one another if they live in very
similar environments.
• Example:
1. Sidewinder (Mojave Desert) and
Horned Viper (Middle East Desert)
CONVERGENT EVOLUTION EXAMPLE
• 1. Ostrich (Africa) and Emu
(Australia).
Macroevolution
• Large scale evolutionary change significant enough to
warrant the classification of groups into genera or
even higher-taxa level.
• For example
– the separation of eubacteria and archaebacteria.
– Cambrian explosion – rapid speciation and
diversification in the animal kingdom for about 40
million years starting 565 mya. Early arthropods,
echinoderms, molluscs, primitive chordates
• Burgess Shale in B.C.
• 2 major theories for macroevolution / the rate of
evolution:
Theory of Gradualism
• The accumulation of many small and ongoing
changes and processes.
– When new species first evolve, they appear very
similar to the originator species and only become
more distinctive, as natural selection and genetic
drift act on both species.
– One would expect then to find many transitional
species in the fossil record.
– This is explained by an incomplete fossil record,
and the possibility that intermediate forms were not
preserved.
Theory of Punctuated Equilibrium
• Eldredge and Gould (1972) rejected the explanation of
the incomplete fossil record and proposed the Theory of
Punctuated Equilibrium
• Relatively rapid spurts of change followed by long
periods of little or no change.
– Species evolve very rapidly in evolutionary time
– Speciation usually occurs in small isolated
populations and thus intermediate fossils
(transitional species) are very rare.
– After the initial burst of evolution, species do not
change significantly over long periods of time.
Which one do we use today?
• Today both theories are needed to understand the
fossil record.
• It is widely accepted that both gradual and rapid
evolutionary processes are at work.
• Example:
– Before a major extinction event, an environment may
be host to many well adapted species and
evolutionary change would be slow.
– An environmental crisis resulting in extinction of many
species would leave many niches empty.
– Surviving species have many new opportunities and
experience disruptive selection, evolving into many
species which fill the niches.
– Once they become well adapted, stabilizing selection
kicks in again and more gradual change occurs.
Consequences of Human
Activities
• Human-made barriers may prevent gene flow
between the split populations
• Isolated populations may undergo adaptive
radiation
• Severely fragmented populations may
eventually die out if there is insufficient
genetic diversity
• E.g. giant panda in China
Speciation and Mass
Extinctions
• Five major mass extinctions have been
identified
In class work
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•
•
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Pg 370, Q 25-30
Final Evolution Quiz Wed May 16
Evolution unit test Wed May 23
Assignment, pg 375, due Wednesday May
23