Download Mechanisms of Evolution Macroevolution

Mechanisms of
Evolution
Macroevolution
Speciation
• MICROEVOLUTION - A change in the
frequency of alleles. Review population
genetics – Ch. 23.
• MACROEVOLUTION - Speciation (or
emergence of higher taxonomic levels)
• Speciation, or the origin of new species, is the
central process of macroevolution because
any higher taxon originates with a new species
novel enough to be the first member of that
taxon.
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• Darwin explored the Galápagos Islands
– And discovered plants and animals found
nowhere else on Earth
Figure 24.1
• The origin of new species, or speciation
– Is at the focal point of evolutionary theory,
because the appearance of new species is the
source of biological diversity
• Evolutionary theory
– Must explain how new species originate in
addition to how populations evolve
• Macroevolution
– Refers to evolutionary change above the
species level
• There are two patterns of speciation in the fossil
record: anagenesis and cladogenesis
• Anagenesis (phyletic evolution) - The transformation of
an unbranched lineage of organisms, sometimes to a state
different enough from the ancestral population to justify
renaming it as a new species.
• Cladogenesis (branching evolution) - The budding of
one or more new species from a parent species that
continues to exist; is more important than anagenesis. It is
more common and can promote biological diversity.
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• Two basic patterns of evolutionary change
– Anagenesis
– Cladogenesis
Figure 24.2 (a) Anagenesis
(b) Cladogenesis
Defining a Species
• Species - Latin term meaning “kind” or “appearance
• Linnaeus (founder of modern taxonomy) - described
species in terms of their physical form (morphology).
Morphology is still the most common method used for
describing species.
• Modern taxonomists also take into account genetic
makeup and functional and behavioral features when
describing species.
The Biological Species Concept
• The biological species concept relies on reproductive
isolation (proposed by Ernst Mayr, 1942)
• Biological species - A population or group of populations
whose members have the potential to interbreed with one
another in nature and to produce viable, fertile offspring,
but cannot produce viable, fertile offspring with members of
other species.
• 1. Largest unit of population in which gene flow is possible
• 2. Defined by reproductive isolation from other species in
natural environments (hybrids may be possible between
two species in the laboratory or in zoos)
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Species
(a) Similarity between different species. The eastern
meadowlark (Sturnella magna, left) and the western
meadowlark (Sturnella neglecta, right) have similar
body shapes and colorations. Nevertheless, they are
distinct biological species because their songs and other
behaviors are different enough to prevent interbreeding
should they meet in the wild.
(b) Diversity within a species. As diverse as we may be
in appearance, all humans belong to a single biological
species (Homo sapiens), defined by our capacity
to interbreed.
Figure 24.3 A, B
Reproductive Isolation
• Reproductive isolation
– Is the existence of biological factors that
impede members of two species from
producing viable, fertile hybrids
– Is a combination of various reproductive
barriers
Prezygotic and postzygotic
barriers to reproduction
• The gene pools of different species are
isolated from those of others by more
than one type of reproductive barrier.
The barriers that isolate gene pools are
either prezygotic or postzygotic,
depending on whether they occur before
or after fertilization.
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• Prezygotic barriers
– Impede mating between species or hinder
the fertilization of ova if members of
different species attempt to mate
• Postzygotic barriers
– Often prevent the hybrid zygote from
developing into a viable, fertile adult
Prezygotic Barriers
•
1. Habitat isolation
Two species living in different habitats in the same area encounter each other
rarely, even though they are not technically geographically isolated.
• Example: two species of garter snakes occur in the same areas but one species lives
mainly in water and the other is mainly terrestrial and they seldom come into contact.
•
2. Behavioral isolation
Species-specific signals and behaviors that attract mates are barriers among
closely related species.
• Example: Male fireflies of different species signal to females of the same species by
blinking their lights in a characteristic pattern; females discriminate among the different
signals and respond only to flashes of their own species.
• Includes behavioral responses to chemical attractants, courtship rituals, bird
and insect song, etc.
•
3. Temporal isolation
Two species that breed at different times of day, seasons, or years don’t mix their
gametes.
• Example: brown trout and rainbow trout cohabit the same streams, but brown trout
breed in the fall and rainbow trout breed in the spring.
ATTEMPT TO MATE
•
4. Mechanical isolation
Morphological differences prevent mating.
• Example: Male dragonflies use a pair of special appendages to clasp females during
copulation. The male’s clasping appendages do not fit the form of the females of other
species.
Prezygotic barriers impede mating or hinder fertilization if mating does occur
• Prezygotic and postzygotic barriers
Habitat
isolation
Behavioral
isolation
Temporal
isolation
Individuals
of different
species
Mechanical
isolation
Mating
attempt
HABITAT ISOLATION
TEMPORAL ISOLATION
BEHAVIORAL ISOLATION
(b)
MECHANICAL ISOLATION
(g)
(d)
(e)
(f)
(a)
(c)
Figure 24.4
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Prezygotic Barriers
ATTEMPT TO MATE
• 5. Gametic isolation
Gametes of different species that do meet rarely
complete fertilization (do not form a zygote).
– For animals that use internal fertilization the sperm of one
species may not be able to survive the internal environment of the
female reproductive tract of a different species.
– Cross-specific fertilization is also uncommon for animals that
utilize external fertilization due to a lack of gamete recognition.
Gamete recognition is based on specific molecules on the coats of
the egg that adhere only to complementary molecules on sperm of
the same species.
Gametic
isolation
Reduce
hybrid
fertility
Reduce
hybrid
viability
Hybrid
breakdown
Viable
fertile
offspring
Fertilization
REDUCED HYBRID
VIABILITY
GAMETIC ISOLATION
REDUCED HYBRID FERTILITY HYBRID BREAKDOWN
(k)
(j)
(m)
(l)
(h)
(i)
Postzygotic Barriers
When prezygotic barriers fail and a hybrid zygote forms, postzygotic
barriers prevent development of a viable, fertile hybrid.
•
1. Reduced hybrid viability (hybrid inviability)
Genetic incompatibility may cause the abortion of the hybrid at an
embryonic stage.
• Hybrids generally do not complete development, and those that do are frail
and soon die.
•
2. Reduced hybrid fertility (hybrid sterility)
If two species mate and produce viable hybrid offspring, reproductive
isolation is maintained if the hybrids are sterile. This prevents gene flow
between the parent species.
• One cause of this barrier is that if chromosomes of the two parent species
differ in number or structure, meiosis cannot produce normal gametes in the
hybrid. Mules.
•
3. Hybrid breakdown
In some crosses, the first generation hybrids are viable and fertile, but
when these hybrids mate with each other, or with either parent species,
the next generation is feeble or sterile.
• Example: Different cotton species can produce fertile hybrids, breakdown
occurs in the next generation when progeny of the hybrids die in their seeds or
grow into weak defective plants.
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Gametic
isolation
Reduce
hybrid
fertility
Reduce
hybrid
viability
Hybrid
breakdown
Viable
fertile
offspring
Fertilization
REDUCED HYBRID
VIABILITY
GAMETIC ISOLATION
REDUCED HYBRID FERTILITY HYBRID BREAKDOWN
(k)
(j)
(m)
(l)
(h)
(i)
Limitations of the Biol. Species Concept
• The biological species concept cannot be applied to:
• 1. Organisms that are completely asexual. Some protists
and fungi, some commercial plants (bananas), and many
bacteria are exclusively asexual.
• 2. Extinct organisms represented by fossils - must be
classified by morphology.
• 3. Sexual organisms about which little is known.
The species problem may never be completely
resolved. It is unlikely that a single definition will ever
apply in all cases.
Other Definitions of Species
• The morphological species concept
– Characterizes a species in terms of its body shape, size,
and other structural features; useful in the field; sometimes
difficult to apply.
• The paleontological species concept
– Focuses on morphologically discrete species known only
from the fossil record
• The ecological species concept
– Views a species in terms of its ecological niche
• The phylogenetic species concept
– Defines a species as a set of organisms with a unique
genetic history
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Modes of Speciation
• The evolution of reproductive barriers that keep
species separate is the key biological event in the
origin of new species.
– • An essential episode in the origin of a species occurs when
the gene pool of a population is separated from other
populations of the parent species.
– • This genetically isolated splinter group can then follow its
own evolutionary course as changes in allele frequencies
caused by selection, genetic drift, and mutations occur
undiluted by gene flow from other populations.
• There are two general modes of speciation: allopatric
speciation and sympatric speciation.
• Speciation can take place with or without
geographic separation
– Allopatric speciation
– Sympatric speciation
Figure 24.5 A, B
(a) Allopatric speciation. A (b) Sympatric speciation. A small
population becomes a new species
population forms a new
species while geographically without geographic separation.
isolated from its parent
population.
Allopatric (“Other Country”)
Speciation
• In allopatric speciation
– Gene flow is interrupted or reduced when a
population is divided into two or more
geographically isolated subpopulations
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Geographic Barriers
• 1. Geological processes can fragment a population into
two or more allopatric populations (having separate
ranges).
• • This can include emergence of mountain ranges,
movement of glaciers, formation of land bridges,
subsidence of large lakes.
• 2. Small populations may become geographically isolated
when individuals from the parent population travel to a new
location. (splinter populations)
• 3. The extent of geographical isolation necessary to
separate two populations depends on the ability of the
organisms to disperse due to the mobility of animals or the
dispersibility of spores, pollen and seeds of plants.
– • Example: the Grand Canyon is an impassable barrier to small
rodents, but is easily crossed by birds. As a result, the same bird
species populate both rims of the canyon, but each rim has several
unique species of rodents.
• Once geographic separation has occurred
– One or both populations may undergo
evolutionary change during the period of
separation
A. harrisi
A. leucurus
Figure 24.6
• In order to determine if allopatric
speciation has occurred
– Reproductive isolation must have been
established
EXPERIMENT Diane Dodd, of Yale University, divided a fruit-fly population, raising some
populations on a starch medium and others on a maltose medium. After many generations,
natural selection resulted in divergent evolution: Populations raised on starch digested starch
more efficiently, while those raised on maltose digested maltose more efficiently.
Dodd then put flies from the same or different populations in mating cages and measured
mating frequencies.
Initial population
of fruit flies
(Drosphila
Pseudoobscura)
Some flies
raised on
starch medium
Figure 24.7
Mating experiments
after several generations
Some flies
raised on
maltose medium
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Male
Maltose
Starch
Female
Starch Maltose
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9
8
20
Mating frequencies
in experimental group
Male
Different
Same
populations population
RESULTS
When flies from “starch populations” were mixed with flies from “maltose populations,”
the flies tended to mate with like partners. In the control group, flies taken from different
populations that were adapted to the same medium were about as likely to mate with each
other as with flies from their own populations.
Female
Different
Same
population populations
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15
12
15
Mating frequencies
in control group
CONCLUSION
The strong preference of “starch flies” and “maltose flies” to mate with
like-adapted flies, even if they were from different populations, indicates that a reproductive
barrier is forming between the divergent populations of flies. The barrier is not absolute
(some mating between starch flies and maltose flies did occur) but appears to be under way
after several generations of divergence resulting from the separation of these allopatric
populations into different environments.
Conditions Favoring Allopatric Speciation
• When populations are separated, speciation can occur as isolated
gene pools accumulate differences by microevolution. These
differences may cause a divergence in phenotype between the
isolated populations.
• 1. A small isolated population is more likely to change
substantially enough to become a new species than is a large
isolated population. There is more effect from genetic drift.
• 2. The geographic isolation of a small population usually occurs
at the fringe of the parent population's range (peripheral isolate).
As long as the gene pool is isolated from the parent population, a
peripheral isolate is a good candidate for speciation for three
reasons:
– a. The gene pool of the peripheral isolate probably differs from that
of the parent population from the outset. Fringe inhabiters usually
represent the extremes of any genotypic and phenotypic clines in an
original sympatric population. With a small peripheral isolate, there
will be a founder effect with chance resulting in a gene pool that is
not representative of the gene pool of the parental population.
– b. Genetic drift will continue to cause chance changes in the gene pool
of the small peripheral isolate until a large population is formed. New
mutations or combinations of alleles that are neutral in adaptive value
may become fixed in the population by chance alone, causing
phenotypic divergence from the parent population.
– c. Evolution caused by selection is likely to be different in the peripheral
isolate than in the parent population. The peripheral isolate inhabits a
frontier with a somewhat different environment, and it will probably be
exposed to different selection pressures than the parent population.
• Because of the severity of a fringe environment (It’s already at the
edge of the species range.), most peripheral isolates do not survive
long enough to undergo speciation. Though most peripheral
isolates go extinct, a small population can accumulate enough
genetic change to become a new species in only hundreds to
thousands of generations.
• NOTE: Geographic barriers by themselves are NOT biological
mechanisms of reproductive isolation and do not define species.
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