Transcript Life: The Science of Biology, 8e

David Sadava H. Craig Heller Gordon H. Orians
William K. Purves David M. Hillis
Biologia.blu
B – Le basi molecolari della vita e
dell’evoluzione
The Origin of Species
The Origin of Species
• What are species?
• How do new species arise?
• What happens when newly formed species
come together?
• Why do rates of speciation vary?
The Origin of Species - What are species?
Species literally means “kinds”.
We recognize most species by their
appearance.
Many species change little over large
geographic ranges.
The Origin of Species - What are species?
Linnaeus described species based on
their appearance—the morphological
species concept.
Members of species look alike because
they share many alleles.
He originated the binomial system of
nomenclature.
The Origin of Species - What are species?
Species can be thought of as branches
on the tree of life.
Speciation: The process by which one
species splIts into two or more daughter
species, often gradually.
The Origin of Species - What are species?
Speciation may be a gradual process
The Origin of Species - What are species?
The biological species concept,
proposed by Ernst Mayr:
“Species are groups of actually or
potentially interbreeding natural
populations which are reproductively
isolated from other such groups.”
This does not apply to asexually
reproducing organisms.
The Origin of Species - How do new species arise?
Allopatric speciation occurs when
populations are separated by a physical
barrier. Also called geographic
speciation.
Thought to be the dominant mode of
speciation.
The Origin of Species - How do new species arise?
Allopatric speciation
The Origin of Species - How do new species arise?
Barriers can form as continents drift, sea
level changes, glaciers advance and
retreat, climate changes.
The environments in which the isolated
populations live are different, and so the
populations evolve differently.
The Origin of Species - How do new species arise?
Allopatric speciation can also occur if
some individuals cross a barrier to form
a new, isolated population.
The 14 species of finches on the
Galàpagos (Darwin’s finches) arose
from a single species that colonized the
islands from South America.
The islands have different environments,
and are sufficiently far apart for
speciation to occur.
The Origin of Species - How do new species arise?
Allopatric speciation among Darwin’s finches (part 1)
The Origin of Species - How do new species arise?
Allopatric speciation among Darwin’s finches (part 2)
The Origin of Species - How do new species arise?
Sympatric speciation does not require
physical isolation.
Disruptive selection is required, such as
in black-bellied seed crackers.
The Origin of Species - How do new species arise?
Sympatric speciation most commonly
occurs by polyploidy—duplication of
the whole set of chromosomes.
Chromosome duplication in a single
species is autopolyploidy; combining
of chromosomes from two species is
allopolyploidy.
The Origin of Species - How do new species arise?
Tetraploids are soon reproductively isolated from diploids
The Origin of Species - How do new species arise?
The Hawaiian Islands have 800 species
of Drosophila, many restricted to one
island.
Many of these species have resulted
from founder events—species are
descendents of individuals that
dispersed among the islands.
The Origin of Species - How do new species arise?
Founder events lead to allopatric speciation
The Origin of Species - What happens when the newly formed species come together?
Prezygotic reproductive barriers
operate before fertilization occurs.
• Habitat isolation—e.g., Rhagoletis flies
in the Hudson River valley.
• Temporal isolation—mating periods do
not overlap.
• Mechanical isolation—differences in
size and shape of reproductive organs;
common in insects.
The Origin of Species - What happens when the newly formed species come together?
Gametic isolation - eggs of one species don’t
have appropriate chemical signals for sperm
of another species; or sperm is not able to
attach to and penetrate the egg.
Behavioral isolation - individuals reject or fail
to recognize potential mating partners. Floral
traits of plants can influence the behavior of
pollinators, and thus whether plants can
hybridize.
The Origin of Species - What happens when the newly formed species come together?
Hawkmoths favor flowers of one columbine species (part 1)
The Origin of Species - What happens when the newly formed species come together?
Hawkmoths favor flowers of one columbine species (part 2)
The Origin of Species - What happens when the newly formed species come together?
If populations are reunited before complete
reproductive isolation has developed,
interbreeding can occur.
If hybrid offspring are fit and interbreed with
both populations, gene pools are combined
and no speciation occurs.
If hybrid offspring are less fit, reinforcement
may result in more prezygotic barriers.
A hybrid zone can develop, and may persist
for a long time while reinforcement develops.
Hybrid zones make good natural laboratories
for the study of speciation.
The Origin of Species - What happens when the newly formed species come together?
Hybrid zones may be long and narrow
The Origin of Species – Why do rates of speciation vary?
Rates of speciation among groups vary greatly.
Many factors influence the likelihood of
speciation.
Species-rich groups are more likely to speciate
faster than species-poor groups.
Speciation rates are likely to be faster in
species with poor dispersal abilities, which
can be separated by even narrow barriers.
Populations with specialized diets are more
likely to speciate.
The Origin of Species – Why do rates of speciation vary?
Dietary shifts can promote speciation
The Origin of Species – Why do rates of speciation vary?
Sexual selection also appears to increase rates
of speciation.
Example: birds with promiscuous mating
systems.
A)Birds of paradise—33 species
B)Manucodes—5 species
The Origin of Species – Why do rates of speciation vary?
Sexual selection in birds can lead to higher speciation rates
The Origin of Species – Why do rates of speciation vary?
An evolutionary radiation is the proliferation
of many species from a single ancestor.
If the resulting species live in a wide array of
environments, it is an adaptive radiation.
Adaptive radiation is likely to occur in
environments with abundant resources.
A population may encounter underutilized
resources when colonizing a new area that
contains few species, such as islands.
Adaptive radiations have followed mass
extinctions.