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TRANSCRIPT
Processes of Evolution
Chapter 18 Part 2
18.6 Maintaining Variation
Natural selection theory helps explain diverse
aspects of nature, including differences between
males and females, and the relationship
between sickle-cell anemia and malaria
Sexual Selection
With sexual selection, some version of a trait gives an individual an advantage over others in attracting mates
Distinct male and female phenotypes (sexual dimorphism) is one outcome of sexual selection
Sexual Selection
Balanced Polymorphism
Balanced polymorphism
• A state in which natural selection maintains two or more alleles at relatively high frequencies
• Occurs when environmental conditions favor heterozygotes
Example: Sickle cell anemia and malaria
• HbA/HbS heterozygotes survive malaria more often than people who make only normal hemoglobin
Sickle Cell Anemia and Malaria
Fig. 18-13a, p. 287
Fig. 18-13b, p. 287
Fig. 18-13c, p. 287
18.7 Genetic Drift—
The Chance Changes
Genetic drift
• A random change in allele frequencies over time
• Can lead to a loss of genetic diversity, especially in small populations
Fixation has occurred when all individuals in a population are homozygous for one allele
Genetic Drift and Population Size
Genetic Drift and Population Size
Bottlenecks
Bottleneck
• A drastic reduction in population size brought about by severe pressure
• After a bottleneck, genetic drift is pronounced when a few individuals rebuild a population
• Example: Northern elephant seals
The Founder Effect
Founder effect
• Genetic drift is pronounced when a few individuals start a new population
Inbreeding
• Breeding or mating between close relatives who share a large number of alleles
• Example: Old Order Amish in Lancaster County, Pennsylvania (Ellis-van Creveld syndrome)
18.3-18.6 Key Concepts
Patterns of Natural Selection
Natural selection is a microevolutionary process
Depending on the population and its
environment, natural selection can shift or
maintain the range of variation in heritable traits
18.8 Gene Flow
Gene flow
• Physical movement of alleles caused by individuals moving into and away from populations
• Tends to counter the evolutionary effects of mutation, natural selection, and genetic drift on a population
• Example: Movement of acorns by blue jays
Gene Flow Between Oak Populations
18.7-18.8 Key Concepts
Other Microevolutionary Processes
With genetic drift, change can occur in a line of
descent by chance alone
Gene flow counters the evolutionary effects of
mutation, natural selection, and genetic drift
18.9 Reproductive Isolation
Speciation
• Evolutionary process by which new species form
• Reproductive isolating mechanisms are always part of the process
Reproductive isolation
• The end of gene exchange between populations
• Beginning of speciation
Four Butterflies, Two Species
Reproductive Isolating Mechanisms
Reproductive isolating mechanisms prevent interbreeding among species
• Heritable aspects of body form, function, or behavior that arise as populations diverge
• Prezygotic isolating mechanisms prevent pollination or mating
• Postzygotic isolating mechanisms result in weak or infertile hybrids
Prezygotic Isolating Mechanisms
Temporal isolation
Mechanical isolation
Behavioral isolation
Ecological isolation
Gamete incompatibility
Mechanical Isolation
Behavioral Isolation
Postzygotic Isolation Mechanisms
Reduced hybrid viability (ligers, tigons)
• Extra or missing genes
Reduced hybrid fertility (mules)
• Robust but sterile offspring
Hybrid breakdown
• Lower fitness with successive generations
Reproductive Isolating Mechanisms
Fig. 18-17, p. 290
Different
species! Prezygotic isolating mechanisms
Temporal isolation: Individuals of
different species reproduce at different
times.
Mechanical isolation: Individuals cannot
mate or pollinate because of physical
incompatibilities.
Behavioral isolation: Individuals of
different species ignore or do not get
the required cues for sex.
Ecological isolation: Individuals of
different species live in different places
and never do meet up.They interbreed
anyway.
Gamete incompatibility: Reproductive
cells meet up, but no fertilization occurs.
Zygotes form,
but . . .
Postzygotic isolating mechanisms
Hybrid inviability: Hybrid embryos die
early, or new individuals die before they
can reproduce.
Hybrid sterility: Hybrid individuals or
their offspring do not make functional
gametes.
No offspring, sterile offspring, or weak offspring that die before reproducing
18.10 Allopatric Speciation
Allopatric speciation
• A physical barrier arises and ends gene flow between populations
• Genetic divergence results in speciation
• Example: llamas, vicunas, and camels
Allopatric Speciation
The Inviting Archipelagos
Winds or ocean currents carry a few individuals of mainland species to remote, isolated islands chains (archipelagos) such as Hawaii
Habitats and selection pressures that differ within and between the islands foster divergences that result in allopatric speciation
Allopatric Speciation on an Isolated Archipelago
Fig. 18-21a, p. 293
B Later, a few
individuals of a new
species colonize
nearby island 2.
Speciation follows
genetic divergence
in the new habitat.
C Genetically different
descendants of the
ancestral species may
colonize islands 3 and 4
or even invade island 1.
Genetic divergence and
speciation may follow.
A A few individuals of a
mainland species reach
isolated island 1. In the new
habitat, populations of their
descendants diverge, and
speciation occurs.
Fig. 18-21b, p. 293
Insects, spiders
from buds
twisted apart
by bill, some
nectar; high
mountain rain
forest
Akekee (L.
caeruleirostris)
Insects,
spiders, some
nectar; high
mountain rain
forest
Nihoa finch
(Telespiza
ultima)
Insects, buds,
seeds, flowers,
seabird eggs;
rocky or
shrubby slopes
Mamane
seeds ripped
from pods;
buds,
flowers,
some berries,
insects; high
mountain dry
forests
Maui parrotbill
(Pseudonestor
xanthophrys)
Rips dry branches
for insect larvae,
pupae, caterpillars;
mountain forest
with open canopy,
dense underbrush
Apapane
(Himatione
sanguinea)
Nectar, especially
of ohialehua flowers;
caterpillars and other
insects; spiders;
high mountain
forests
Akepa
(Loxops
coccineus)
Palila(Loxioides
bailleui)
Fig. 18-21c, p. 293
Tree snails,
insects in
understory;
last known
male died in
2004
Bark or leaf
insects, some
nectar; high
mountain rain
forest
Kauai Amakihi
(Hemignathus
kauaiensis)
Bark-picker;
insects, spiders,
nectar; high
mountain rain
forest
Akiapolaau
(Hemignathus
munroi)
Akohekohe
(Palmeria
dolei)
Iiwi (Vestiaria
coccinea)
Probes, digs
insects from
big trees; high
mountain rain
forest
Mostly nectar
from flowering
trees, some
insects, pollen;
high mountain
rain forest
Mostly nectar
(ohia flowers,
lobelias, mints),
some insects;
high mountain
rain forest
Maui Alauahio
(Paroreomyza
montana)
Poouli
(Melamprosops
phaeosoma)
18.11 Other Speciation Models
Populations sometimes speciate even without a
physical barrier that blocks gene flow
• Sympatric speciation
• Parapatric speciation
Sympatric Speciation
In sympatric speciation, new species form within a home range of an existing species, in the absence of a physical barrier
A change in chromosome number (polyploidy) can cause instant speciation
On Lord Howe Island, species of palms are reproductively isolated
Sympatric Speciation in Wheat
Fig. 18-22, p. 294
T. aestivum
(one of the
common
bread
wheats)
Triticum
monococcum
(einkorn)
Unknown
species of
Triticum
spontaneous
chromosome
doubling
T. tauschii
(a wild
relative)
T. turgidum
(wild emmer)
14AA X 14BB 14AB 28AABB X 14DD 42AABBDD
A By 11,000 years ago,
humans were cultivating
wild wheats. Einkorn has
a diploid chromosome
number of 14 (two sets of
7). It probably hybridized
with another wild wheat
species having the same
number of chromosomes.
B About 8,000 years ago,
the allopolyploid wild emmer
originated from an AB hybrid
wheat plant in which the
chromosome number
doubled. Wild emmer is
tetraploid, or AABB; it has
two sets of 14 chromosomes.
There is recently renewed
culinary interest in emmer,
also called farro.
C AABB emmer probably
hybridized with T.
tauschii, a wild relative
of wheat. Its diploid
chromosome number
s 14 (two sets of 7 DD).
Common bread wheats
have a chromosome
number of 42 (six sets
of 7 AABBDD).
Sympatric Speciation in Palms
Parapatric Speciation
In parapatric speciation, populations in contact along a common border evolve into distinct species
Hybrids in the contact zone are less fit than individuals on either side
Parapatric Speciation
Fig. 18-24c, p. 295
T. barretti
hybrid zone
T. anophthalmus
Different Speciation Models
18.9-18.11 Key Concepts
How Species Arise
Speciation varies in its details, but it typically
starts after gene flow ends
Microevolutionary events that occur
independently lead to genetic divergences,
which are reinforced as reproductive isolation
mechanisms evolve
18.12 Macroevolution
Macroevolution
• Large-scale patterns of evolutioary change
• Includes patterns of change such as one species giving rise to multiple species, the origin of major groups, and major extinction events
Coevolution
Two species in close ecological contact act as agents of selection on each other (coevolution)
• Predator and prey
• Host and parasite
• Pollinator and flower
Over time, the two species may come to depend on each other
Coevolution
Fig. 18-25, p. 296
proboscis
nectar tube
10 cm
Stasis and Exaptation
Stasis
• A lineage exists for millions of years with little or no change (e.g. coelacanth)
Exaptation (preadaptation)
• Some complex traits in modern species held different adaptive value in ancestral lineages (e.g.feathers in birds and dinosaurs)
Adaptive Radiation
Adaptive radiation
• A burst of speciation that occurs when a lineage encounters a new set of niches
Key innovation
• A structural or functional adaptation that allows individuals to exploit their habitat in a new way
Extinction
Extinction
• The irrevocable loss of a species from Earth
Mass extinctions
• Extinctions of many lineages, followed by adaptive radiations
• Five catastrophic events in which the majority of species on Earth disappeared
Adaptive Radiation of Mammals Following the K-T Extinction
Fig. 18-26a, p. 297
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Evolutionary Theory
Evolutionary biologists try to explain how all species are related by descent from common ancestors
Genetic change is the basis of evolution, but many biologists disagree about how it occurs
18.12 Key Concepts
Macroevolutionary Patterns
Patterns of genetic change that involve more
than one species are called macroevolution
Recurring patterns of macroevolution include
exaptation, adaptive radiation, and extinction
