Download changing populations

Darwinian Evolution
How do organisms respond to a changing environment?
Darwinian Revolution
November 24, 1859
Charles Darwin (1809-1882) publishes “On the Origin of Species by Means of Natural Selection”
Darwin’s publication focused biologists’ attention on the great
diversity of organisms.
He presented evidence that species today were the descendants
of ancestral species
He proposed a mechanism for the evolutionary process; called
natural selection
Darwinian Revolution
The period into which Darwin was born was one of great progress in all areas of science. Natural history was
no exception and for some time exploration of the fossil record had already suggested that the Earth was
far more ancient than Christian texts claimed.
Fossils suggested exotic forms of life never seen nor described in biblical texts. This generated enormous
debate among the aristocratic classes. Not only had the cultural histories of Western civilization always held
species to be constant and unchanging but this view was common to both religion (Old Testament) and
philosophy (Aristotle).
Darwinian theory challenged these traditional views in a profound manner- it questioned not only creation
(and with it the very role of God) but even provided a rational scientific explanation for how creationism
should be replaced.
Linnaeus (classification)
Hutton (gradual geologic change)
Lamarck (species can change)
Malthus (population limits)
Cuvier (fossils, extinction)
Lyell (modern geology)
Darwin (evolution, nutural selection)
Mendel (inheritance)
American Revolution
1750
Wallace (evolution, natural selection)
French Revolution
U.S. Civil War
1800
1850
1900
1795 Hutton proposes his theory of gradualism.
1798 Malthus publishes “Essay on the Principle of Population.”
Before Origin of Species
Carolus Linnaeus (1707-1778)
Interpreted adaptations as evidence that the Creator
had designed each species for a specific purpose
Was a founder of taxonomy, classifying life’s diversity
“for the greater glory of God”
Paleontology, the study of
fossils, was pioneered by French
scientist Georges Cuvier (1769-1832).
Cuvier opposed gradualism.
Instead he advocated
catastrophism and speculated
that each boundary between
geological layers represented a
catastrophe
This attention to taxonomic diversity and study of fossils helped to lay the groundwork for Darwin’s ideas
Before Origin of Species
Gradualism is the idea that profound change can take place through the cumulative effect of slow but
continuous processes
Geologists James Hutton (1726-1797) and Charles Lyell (1797-1875) perceived that changes in Earth’s surface
can result from slow continuous actions still operating today.
These ideas exerted a strong influence on Darwin’s thinking
Before Origin of Species
Jean-Baptiste Lamarck (1744-1829) hypothesized that an organism
could pass on characteristics it acquired during its lifetime to its
offspring. He even suggested that the environment gives rise to these
changes in animal development. The mechanisms he proposed,
however, were unsupported by evidence
Today this is called Lamarckian Evolution.
Voyage of the Beagle
Even as a boy Charles Darwin had a consuming interest in nature
Soon after receiving his B.A. degree Darwin was accepted on board the HMS Beagle, which was about to
embark on a voyage around the world
Darwin’s interest in the geographic distribution of species was kindled by the Beagle’s stop at the Galápagos
Islands near the equator west of South America.
ure 22.5
England
NORTH
AMERICA
EUROPE
ATLANTIC
OCEAN
PACIFIC
OCEAN
Galápagos
Islands
HMS Beagle in port
SOUTH
AMERICA
AUSTRALIA
Andes
Darwin in 1840,
after his return
AFRICA
Cape of
Good Hope
Cape Horn
Tierra del Fuego
Tasmania
New
Zealand
Voyage of the Beagle
Darwin began to perceive adaptation to the environment and the origin of new species as related processes
(a) Cactus eater. The long,
sharp beak of the cactus
ground finch (Geospiza
scandens) helps it tear
and eat cactus flowers
and pulp.
Figure 22.6a–c
(c) Seed eater. The large ground
finch (Geospiza magnirostris)
has a large beak adapted for
cracking seeds that fall from
plants to the ground.
(b) Insect eater. The green warbler
finch (Certhidea olivacea) uses its
narrow, pointed beak to grasp insects.
On The Origin of Species
In 1844, Darwin wrote a long essay on the origin of species and natural
selection but, anticipating the uproar it would cause, delayed publication.
In June 1858 Darwin received a manuscript from Alfred Russell Wallace who
had developed a theory of natural selection similar to his own. Darwin quickly
finished The Origin of Species and published it the next year.
Sirenia
Hyracoidea (Manatees
(Hyraxes) and relatives)
Barytherium
Mammuthus
Stegodon
Platybelodon
Mammut
Deinotherium
Elephas Loxodonta Loxodonta
maximus africana
cyclotis
(Africa)
(Asia)
(Africa)
Moeritherium
Millions of years ago
Years ago
Darwin developed two main ideas
* Evolution explains life’s unity and diversity
* Natural selection is a cause of adaptive evolution
In the Darwinian view,
the history of life is
like a tree with
multiple branchings
from a common trunk
to the tips of the
youngest twigs that
represent the diversity
of living organisms
Natural Selection
From Wikipedia.....
Natural selection is a natural law by
which genetically heritable traits become
more or less common in a population over
successive generations.
Natural genetic variation within a
population of organisms means that some
individuals will survive and reproduce more
successfully than others in their current
environment
Natural selection acts on the phenotype
but the inheritable, genetic basis of any
phenotype which gives a reproductive
advantage will become more common in a
population (see allele frequency)*
* Allele frequency to follow later.
Don’t forget to google “Darwin Awards”
Personal Favourite
Natural Selection
(a) A flower mantid
Natural selection can produce an increase over time
Malaysia
in the adaptation of organismsin
to their
environment
(b) A stick mantid
in Africa
Top: Flower Mantid (Malaysia)
Bottom: Stick Mantid (Africa)
Natural Selection
If an environment changes over time natural selection may result in adaptation to these new conditions
Reznick and Endler transplanted guppies from pike-cichlid pools to killifish pools
EXPERIMENT
and measured the average age and size of guppies at maturity over an 11-year period (30 to
60 generations).
Pools with killifish,
but not guppies prior
to transplant
Predator: Killifish; preys
mainly on small guppies
Experimental
transplant of
guppies
Guppies:
Larger at
sexual maturity
than those in
“pike-cichlid pools”
Predator: Pike-cichlid; preys mainly on large guppies
Guppies: Smaller at sexual maturity than
those in “killifish pools”
Figure 22.12
Natural Selection
RESULTS
185.6
161.5
67.5 76.1
Males
Females
Age of guppies
at maturity (days)
Weight of guppies
at maturity (mg)
After 11 years, the average size and age at maturity of guppies in the transplanted
populations increased compared to those of guppies in control populations.
85.7 92.3
48.5
58.2
Males
Females
Control Population: Guppies
from pools with pike-cichlids
as predators
Experimental Population:
Guppies transplanted to
pools with killifish as
predators
CONCLUSION Reznick and Endler concluded that the change in predator resulted in different variations
in the population (larger size and faster maturation) being favored. Over a relatively short time, this altered
selection pressure resulted in an observable evolutionary change in the experimental population.
Natural Selection
In humans, the use of drugs artificially selects for pathogens that through chance mutations are resistant to
the drugs’ effects. Natural selection is a cause of adaptive evolution
Percent of HIV resistant to 3TC
Researchers have developed numerous drugs to combat HIV- but using these medications selects for viruses
resistant to the drugs
Patient
No. 1
Patient No. 2
Patient No. 3
Weeks
Natural Selection
One common misconception about evolution is that individual organisms evolve, in
the Darwinian sense, during their lifetimes
Natural selection acts on individuals, but populations evolve. It is variation that
allows populations to evolve.
Members of a population may show no variation
for a particular trait.
Members of a population may show two or more
variations for a particular trait.
Galahs are monomorphic in respect to plumage.
Lupins are polymorphic for colour.
Population Genetics
How do genes affect groups?
Populations & Gene Pools
MAP
AREA
CANADA
ALASKA
Population genetics is the study of how populations change genetically over time. It integrates Mendelian
genetics with the Darwinian theory of evolution by natural selection and focuses on populations as units of
evolution
Beaufort Sea
Porcupine
herd range
N
TE OR
RR TH
IT WE
O S
RI T
ES
Fortymile
herd range
ALASKA
YUKON
•
Fairbanks
•
Whitehorse
A population is a localized group of
individuals that are capable of interbreeding
and producing fertile offspring
The gene pool is the total aggregate of genes
in a population at any one time- this includes all
the alleles present in the population
Allele Frequencies
Allele frequency is a measure of the relative frequency of an allele in a population.
Most genes show only one allele in most of the population; this is called the wild-type allele.
For variation to occur, however, more than one allele of a gene must exist; this is called polymorphism.
Non-evolving populations
Allele frequencies stay constant if...
* There is no random mating
* Matings are fertile
* There are no mutations
* There is no migration
* The population is large
Evolving Populations
Allele frequencies are subject to various
“change agents”.You need to know the
types of change agents and the
consequences for the population.
Evolving Populations: Mutations
Mutations are the origin of genetic variation. While often harmful to the individual they can also be
neutral or beneficial.
Point Mutations
Block Mutations
Substitution
Insertion
Deletion
Insertion
Deletion
Duplication
Inversion
Translocation
Aneuploidy
Evolving Populations: Gene Flow
Gene flow involves the migration of individuals between populations. This may result in very rapid changes
in allele frequencies.
Immigration may
increase the variety of
alleles present in the
population
Emigration may
decrease the variety of
alleles present in the
population
Evolving Populations: Genetic Drift
Genetic Drift describes how allele frequencies can fluctuate unpredictably from one generation to the
next. This process ends to reduce genetic variation and has greater effects in small populations.
CW CW
CR C R
CR C R
Only 5 of
10 plants
leave
offspring
CR C W
CW CW
CR C R
CR C R
CR C W
CW CW
CR C R
CR C W
CR C W
CR C R
CW CW
CR C W
CR C R
CR C R
CR C W
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
Only 2 of
10 plants
leave
offspring
CR C R
CR C R
CR C R
CR C R
CR C R
CR C R
CR C R
CR C R
CR C W
CR C W
Generation 2
p = 0.5
q = 0.5
CR C R
CR C R
Generation 3
p = 1.0
q = 0.0
Evolving Populations: Bottleneck Effect
Blam!
In the bottleneck effect a sudden change in the
environment may drastically reduce the size of a population
The surviving gene pool may no longer be reflective of the
original population’s gene pool
(a) Shaking just a few marbles through the
narrow neck of a bottle is analogous to a
drastic reduction in the size of a population
after some environmental disaster. By chance,
blue marbles are over-represented in the new
population and gold marbles are absent.
Figure 23.8 A
Original
population
Bottlenecking
event
Surviving
population
Evolving Populations: Founder Effect
The founder effect occurs when a small, unrepresentative sample becomes isolated
from a larger population.
Captive breeding programs in zoos represent a founder effect. Care must be taken to avoid
inbreeding and maximise genetic diversity.
Speciation
How do new species arise?
The Biological Concept of Species
Organisms belong to the same species if they can interbreed and produce viable, fertile offspring.
* All human races belong to the same
species as we can reproduce.
* Domestic dogs also belong to the same
species as they can interbreed eg.
Spoodles...
* Horses and donkeys, however, are
separate species; even though they can
interbreed the offspring are infertile.
* Obviously a whale and a gold fish are
different species. We won’t have much
success trying to breed a gold whale by
normal standards.
* Also, the biological species concept
cannot be applied to asexual organisms.
The Biological Concept of 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.
Speciation
Speciation is a process whereby a parent species splits into daughter species. For this to happen the gene
pool of the original species must divided into separate gene pools and remain reproductively isolated for long
enough that the individuals are eventually unable to breed between the populations.
These isolating mechanisms can be classified in two ways.
Pre-reproductive Isolation
Impede mating between species or hinder fertilization
Geographic:
Temporal:
Behavioural:
Morphological:
individuals may be separated by seas, mountains, distances or habitat
individual may breed during different times of year or day
individual may have different courtship patterns
individuals may have different reproductive structures so that mating is physically impossible.
Post-reproductive Isolation
These do not prevent mating but do prevent fertile young from being produced.
Gamete mortality: gametes do not survive
Zygote mortality: zygote forms but does not survive
Hybrid sterility:
offspring are born infertile
Pre-Reproductive Isolation
Prezygotic barriers impede mating or hinder fertilization if mating does occur
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)
(a)
(f)
(c)
Post-Reproductive Isolation
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)
Allopatric Speciation
Allopatric speciation occurs where gene flow is interrupted or reduced because a population is divided
into two or more geographically isolated subpopulations
Once geographic separation has occurred one or both populations may undergo evolutionary change during
the period of separation
A. harrisi
A. leucurus
Allopatric Speciation
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
Mating experiments
after several generations
Some flies
raised on
maltose medium
Allopatric Speciation
RESULTS
22
9
8
20
Male
Male
Maltose
Starch
Female
Starch Maltose
Mating frequencies
in experimental group
CONCLUSION
Different
Same
populations population
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
18
15
12
15
Mating frequencies
in control group
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.
Extinction
A variety of causes can lead to extinction.
* Loss of habitat
* Competition for resources with a better adapted organism
* Catastrophic events can produce a mass extinction of species. For example:
oMeteorite strike 65 mya (Cretaceous period). This explosion caused major fires, tidal
waves and threw up enough dust to block enough sunlight to cool the planet.
o Quaternary Period– Earth’s orbit changed raising temp in Antarctic Ocean
* Human activity increases extinction, habitat destruction/salination, introduction of foreign
plants and animals, competing with or attacking native flora and fauna, chemical use