Transcript Chapter 14 – Origin of Species

The Origin of Species
• If microevolution (the genetic changes within
a population over generations) were all that
ever happened, the Earth would only be
inhabited by a highly adapted version of the
first form of life
• Fortunately for us, however, the world is full
of biological diversity – many different
varieties of life forms, consisting of many
millions of distinct biological species
Got biodiversity?
• Speciation (the
emergence of a new
species) is the bridge
between
microevolution and
macroevolution (the
broader pattern of
evolutionary change
over long periods of
time)
http://www.botany.uwc.ac.za/pssa/newsletter/images/sea-plants%20poster-A4.jpg
The Origin of Species
• A population is defined as a group of
interbreeding individuals belonging to a
particular species sharing a common
geographic area (ex. all the hermit crabs on
Fire Island National Seashore; all the squirrels
on SCCC campus or Long Island – depends on
how you define the area)
• A population is the smallest unit that can
evolve (individuals do NOT evolve)
What defines a species?
• Biological species concept:
– A species consists of a population whose
members have the potential to interbreed
with one another in nature to produce
viable, fertile offspring, but who cannot
interbreed with other such groups
– Species are based on their ability to
interbreed; not on physical similarities!
Origin of New Species
• Example: Eastern and Western Meadowlarks;
2 different species with similar shape and
coloration, but differences in song help
prevent interbreeding
• Barriers to breeding can be behavioral
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Origin of New Species
• Humans have
considerable
diversity, but we all
belong to the same
species because of
our ability to
interbreed
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Speciation happens
• It takes a reproductive barrier to keep
individuals of closely-related species from
interbreeding
• Reproductive barriers may be behavioral,
geographical, anatomical, or temporal
Prezygotic vs. Postzygotic Barriers
• Prezygotic – mechanisms preventing
formation of a zygote; these can be ecological,
behavioral, temporal, or mechanical
• Postzygotic – mechanisms preventing
organisms (zygotes and beyond) from
developing into a reproducing adult; these can
be
Prezygotic examples
• Geographical: Antelope squirrels occupying
different sides of the Grand Canyon
• Ecological: Lions and Tigers inhabit different
habitats and would rarely encounter each other
• Behavioral: Blue-footed boobies conduct
courtship rituals that other species would not
recognize
• Temporal: Different species of wild lettuce have
different blooming periods
Postzygotic examples
• Reduced hybrid viability - most hybrid
offspring of different species do not survive;
example – many species of salamanders are
able to mate, but the offspring do not survive
to reproduce themselves
• Reduced hybrid fertility – hybrid offspring
reach maturity, but are sterile
Postzygotic example
• Mules are the reproductive product of a horse
and a donkey mating
• Mules are viable (one got me nicely around
Bryce Canyon, Utah
without plunging me to
my death…), but they are
sterile
• Therefore, a horse and a
donkey must be considered
distinct species
Allopatric Speciation
• How do reproductive barriers arise?
• One of the clearest forms of species is
allopatric speciation caused by a geographic
barrier
• When a geographic barrier occurs, the
isolated populations each become adapted to
their own environment, such that over time,
they may no longer interbreed
(
speciation)
Allopatric Speciation
• Examples of allopatric
isolation: Antelope squirrels
on south and north rim of
Grand Canyon (a); migration
(b); volcanic activity (c)
(a)
(b)
(c)
Speciation
Sympatric Speciation
• Sympatric speciation – the process by which
new species arise within the range of another
species
• More controversial
• In this case, a new species does not arise from
geographic isolation
• Instead, a new species may arise by accident
when errors during cell division resulted in
organisms with extra sets of chromosomes
(very common in plants)
Sympatric Speciation
• New species formed by having extra sets of
chromosomes are considered to be polyploid
• Polyploid organisms have more than 2
complete sets of chromosomes
• For example, a polyploid containing 4 sets of
chromosomes will produce diploid (2n)
gametes! This species would be unable to
mate with normal diploid species (which
produce haploid gametes)
Error in
cell
division
Polyploid
cells undergo
meiosis
Selffertilization
3
2
1
Parent species
Diploid
Polyploid
(“tetraploid”)
Viable, fertile
tetraploid
species
Diploid
gametes
Chromosomes not
homologous
(cannot pair)
1
Species A
2n = 4
2
3
Gamete
n=2
Sterile hybrid
n=5
Species B
2n = 6
Gamete
n=3
Viable, fertile
hybrid species
2n = 10
3. However, ‘sterile’ hybrid
can reproduce asexually (as
many plants do), and if subsequent
errors in cell division occur,
chromosome duplications can result
in a fertile polyploid species!
Polyploid speciation
• Remember, polyploidy is a type of sympatric
speciation
• As many as 80% of all living plants today are
believed to have arisen by polyploidy!
• A polyploid contains twice (or sometimes
more) the genetic diversity as its diploid
predecessors, which provides an adaptive
advantage!
Polyploid speciation
• Many of the plants grown for food are
polyploids
– Oats and Barley
– Potatoes
– Bananas
– Peanuts
– Plums and Apples
– Wheat
– Coffee!
Allopatric vs. Sympatric Speciation
Adaptive Evolution
• The evolution of many diverse species from a
common ancestor is called adaptive radiation
• The adaptations of these species allow them
to fill new habitats or roles in their
communities (“niches”)
• Adaptive radiation typically occurs when a few
organisms colonize new, unexploited habitats,
or when environmental changes open up new
opportunities for the survivors
Adaptive Evolution
• The Galapagos Islands is one of the world’s
greatest showcases of adaptive radiation
• Each island arose ‘naked’ from underwater
volcanoes and were gradually clothed by
plants, animals and micro-organisms which
strayed from the South American mainland
• Darwin’s finches are a prime example of
adaptive evolution
Cactus-seed-eater
(cactus finch)
Tool-using insect-eater
(woodpecker finch)
Seed-eater
(medium ground finch)
Macroevolution
Rise of the mammals
• The extinction of the dinosaurs provided a
tremendous evolutionary opportunity to
mammals, who once lived in their shadows
Mass extinctions
Suggested Readings
•
•
•
•
The Selfish Gene, Richard Dawkins
The Blind Watchmaker, Richard Dawkins
Unweaving the Rainbow, Richard Dawkins
The Panda’s Thumb (and any other collection
of stories by), Stephen J. Gould
…some light reading for a winter night