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BIOLOGY
Chapter 18 EVOLUTION AND THE ORIGIN OF SPECIES
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CHAPTER OUTLINE
18.1 - Understanding Evolution
18.2 – Formation of New Species
18.3 – reconnection and Rates of Speciation
All organisms are products of evolution adapted to their environment.
(a)Saguaro (Carnegiea gigantea) can soak up 750 liters of water in a single rain storm, enabling these cacti to
survive the dry conditions of the Sonora desert in Mexico and the Southwestern United States.
(b)The Andean semiaquatic lizard (Potamites montanicola) unlike most lizards, is nocturnal and swims.
Scientists still do no know how these cold-blood animals are able to move in the cold (10 to 15°C)
temperatures of the Andean night.
INTRODUCTION
• ALL species of living organisms evolved at
some point from a different species.
• Evolution is an ongoing process.
• The theory of evolution is a unifying theory
of biology
• The framework that biologist use when
asking questions about the living world.
• Provides direction about living things
that result from experiment after
experiment.
•
18.1 UNDERSTANDING EVOLUTION
Evolution by natural selection – how species change over
time
• Plato – species were static and unchanging
• Comte de Buffon (18th century) – observed various
geographic regions with different plant and animal
populations (even with similar environments)
• Hutton (same time) – geological change occurred
gradually by accumulation of small changes from
processes over long periods of time
• Lyell (19th century) – Earth older that popular belief =
more time for change in species – friend of Darwin
• Lamarck – inheritance of acquired characteristics
through modification of individuals caused by
environmental factors could be inherited by offspring
(use or disuse of a structure within lifetime).
CHARLES DARWIN AND NATURAL SELECTION
• Mid-19th century – TWO naturalists
described the actual mechanism for
evolution independently
• Darwin – 1831-1836 travelled to S.
America, Australia, & southern tip of
Africa on H.M.S Beagle
• Wallace – 1848-1852 travelled to Brazil,
collected insects in rainforest & 18541862 travelled to Malay Archipelago
FIGURE 18.2
• Galapagos - Darwin observed that beak shape varies
among finch species on the different islands.
• He postulated that the beak of an ancestral species had
adapted over time to equip the finches to acquire different
food sources.
FIGURE 18.3
•
Both (a) Charles Darwin and (b) Alfred Wallace observed similar patterns. Darwin
called the mechanism natural selection (“survival of the fittest”).
•
Natural selection = More reproduction of individual with favorable traits that survive
environmental changes because of those traits
•
Both wrote scientific papers on natural selection that were presented together before
the Linnean Society in 1858.
CHARLES DARWIN AND NATURAL SELECTION
• Outcome of 3 principles that operate in
nature.
1. Most characteristics are inherited
2. More offspring are produced than are
able to survive, so resources are limited
(competition).
3. Offspring vary in regard to their
characteristics and those variations are
inherited
PROCESSES AND PATTERNS OF EVOLUTION
• Evolution only takes place if there is
variation in genes within a population –
MUST be genetically based!
• Mutations – source of new alleles, can
reduce fitness, increase it, or not effect
it at all
• Sexual reproduction – unique
combinations of alleles
• Heritable traits that help survival &
reproduction in a specific environment =
adaptation – “fits” the environment
PROCESSES AND PATTERNS OF EVOLUTION
• Divergent evolution – from common
ancestor, new species with different traits
because of different environments
• Convergent evolution – similar
environments, evolve similar phenotypes,
but have no recent common ancestor
• Evolution – physical changes in a
population OVER ENORMOUS SPANS OF
TIME
FIGURE 18.5
Flowering plants evolved from a common ancestor. Notice that the (a) dense blazing
star (Liatrus spicata) and the (b) purple coneflower (Echinacea purpurea) vary in
appearance, yet both share a similar basic morphology. (credit a: modification of work
by Drew Avery; credit b: modification of work by Cory Zanker)
EVIDENCE OF EVOLUTION - FIGURE 18.6
1. Fossils – organisms of past are not same as today’s, also shows progression
over time
In this (a) display, fossil hominids are arranged from oldest (bottom) to newest
(top). As hominids evolved, the shape of the skull changed.
An artist’s rendition of (b) extinct species of the genus Equus reveals that these
ancient species resembled the modern horse (Equus ferus) but varied in size.
EVIDENCE OF EVOLUTION - FIGURE 18.7
2. Anatomy and Embryology
a) Homologous structures - similar construction of appendages indicates
that these organisms share a common ancestor.
b) Vestigial structures – exist in organisms, but have no apparent function
(wings on flightless birds.
EVIDENCE OF EVOLUTION - FIGURE 18.8
c) Convergence of form – similar environments/selection pressures
The white winter coat of the (a) arctic fox and the (b) ptarmigan’s plumage are
adaptations to their environments. (credit a: modification of work by Keith Morehouse)
d) Embryology – structures absent in some groups often appear in embryonic forms &
disappear by the time the adult or juvenile form is reached
All vertebrate embryos, including humans, have gill slits and tails in early development.
EVIDENCE OF EVOLUTION
3) Biogeography – patterns in geographic distribution
explained in conjunction with movement of tectonic plates.
•
Broad groups evolved before Pangea break up
•
Unique diversification of areas since Pangea break up
4) Molecular Biology – structures of molecules of life
reflect descent with modification
•
DNA used as genetic code for all of life
•
Universality of genetic code, DNA replication &
expression
•
Components of ribosomes and structures of
membranes
MISCONCEPTIONS
1. Evolution is just a theory – Scientifically, a theory is
understood to be a body of thoroughly tested and
verified explanations for a set of observations of the
natural world
2. Individuals evolve – change in genetic composition of a
POPULATION over time (many generations)
3. Evolution explains the origin of life – NO, it only
explains HOW populations change and diversify over
time since the origin of life
4. Organisms evolve on purpose – it’s not intentional,
variation is already in the population, it is not goal
directed
• A trait fit in one environment at one time could be
fatal at some point in the future.
18.2 FORMATION OF NEW SPECIES
•
Species – group of individual organisms that
interbreed and produce fertile, viable offspring
•
•
Share external and internal characteristics,
develop from DNA
NOT based on appearance –
•
•
domestic dogs look different, but can
interbreed
Bald eagles and African fish eagles look
similar, but offspring would be infertile
FIGURE 18.9
The (a) poodle and (b) cocker spaniel can reproduce to produce a breed known as (c)
the cockapoo. (credit a: modification of work by Sally Eller, Tom Reese; credit b:
modification of work by Jeremy McWilliams; credit c: modification of work by Kathleen
Conklin)
FIGURE 18.10
The (a) African fish eagle is similar in appearance to the (b) bald eagle, but the two
birds are members of different species. (credit a: modification of work by Nigel Wedge;
credit b: modification of work by U.S. Fish and Wildlife Service)
SPECIATION
•
The formation of two species from one species
•
MUST evolve in such a way that it becomes impossible
for individuals from the two new populations to
interbreed.
•
Allopatric speciation – allo=other & patric=homeland
•
•
Geographic separation of population from a
parent species
Sympatric speciation - sym=same & patric=homeland
•
Occurs within a parent species remaining in one
location
FIGURE 18.11
The only illustration in Darwin's On the Origin of Species is (a) a diagram showing speciation
events leading to biological diversity. The diagram shows similarities to phylogenetic charts
that are drawn today to illustrate the relationships of species. (b) Modern elephants evolved
from the Palaeomastodon, a species that lived in Egypt 35–50 million years ago.
ALLOPATRIC SPECIATION
•
Populations become geographically discontinuous so the
free-flow of alleles is prevented.
•
If the separation lasts of a long period of time, the two
populations will evolve along different trajectories
•
•
Typically caused by environmental conditions such as
climate, resources, predators, and competitors
• Nature of the geographic separation necessary to
isolate populations depend entirely on the biology of
the organism and its potential for dispersal.
Two categories
1. Dispersal – move to new location
2. Vicariance – natural situation physically divides
population
FIGURE 18.12
The northern spotted owl and the
Mexican spotted owl inhabit
geographically separate
locations with different climates
and ecosystems. The owl is an
example of allopatric speciation.
FIGURE 18.13
Adaptive radiation – dispersal throughout area, finding specific niche
or isolated habitat
The honeycreeper birds illustrate adaptive radiation. From one
original species of bird, multiple others evolved, each with its own
distinctive characteristics.
SYMPATRIC SPECIATION - FIGURE 18.14
Aneuploidy results when the gametes have too many or
too few chromosomes due to nondisjunction during
meiosis. In the example shown here, the resulting
offspring will have 2n+1 or 2n-1 chromosomes
SYMPATRIC SPECIATION - FIGURE 18.15
Autopolyploidy results when mitosis is not
followed by cytokinesis = two or more complete
sets of chromosomes.
SYMPATRIC SPECIATION - FIGURE 18.16
Alloploidy results when two species mate to produce
viable offspring. In the example shown, a normal gamete
from one species fuses with a polyploidy gamete from
another. Two matings are necessary to produce viable
offspring.
REPRODUCTIVE ISOLATION
•
The ability to interbreed between two populations
•
Prezygotic barriers – block reproduction from taking place,
including preventing fertilization when attempting
reproduction
•
•
•
•
Temporal isolation – differences in breeding schedule
Habitat isolation – populations no longer overlap
Behavioral isolation – presence or absence of a
specific behavior preventing reproduction
• Gametic barrier – differences in gamete cells or
reproductive organs
Postzygotic barriers – after zygote formation, don’t survive
embryonic stage
•
Hybrid inviability - born sterile
REPRODUCTIVE ISOLATION - FIGURE 18.17
These two related frog species exhibit temporal reproductive isolation. (a) Rana aurora
breeds earlier in the year than (b) Rana boylii. (credit a: modification of work by Mark R.
Jennings, USFWS; credit b: modification of work by Alessandro Catenazzi)
FIGURE 18.18
Speciation can occur when two populations occupy different habitats. The habitats
need not be far apart. The cricket (a) Gryllus pennsylvanicus prefers sandy soil, and the
cricket (b) Gryllus firmus prefers loamy soil. The two species can live in close proximity,
but because of their different soil preferences, they became genetically isolated.
FIGURE 18.19
The shape of the male reproductive organ varies among male damselfly species, and is
only compatible with the female of that species. Reproductive organ incompatibility
keeps the species reproductively isolated.
FIGURE 18.20
Some flowers have evolved to attract certain pollinators. The (a) wide foxglove flower is
adapted for pollination by bees, while the (b) long, tube-shaped trumpet creeper flower
is adapted for pollination by humming birds.
HABITAT INFLUENCE - FIGURE 18.21
New food source at lower levels, so populations don’t interact as much.
Cichlid fish from Lake Apoyeque, Nicaragua, show evidence of sympatric
speciation. Lake Apoyeque, a crater lake, is 1800 years old, but genetic
evidence indicates that the lake was populated only 100 years ago by a
single population of cichlid fish. Nevertheless, two populations with distinct
morphologies and diets now exist in the lake, and scientists believe these
populations may be in an early stage of speciation.
18.3 RECONNECTION - FIGURE 18.22
After speciation has occurred, the two separate but closely
related species may continue to produce offspring in an area
called the hybrid zone. Reinforcement, fusion, or stability may
result, depending on reproductive barriers and the relative
fitness of the hybrids.
VARYING RATES OF SPECIATION
FIGURE 18.23
In (a) gradual speciation, species diverge at a slow, steady
pace as traits change incrementally.
In (b) punctuated equilibrium, species diverge quickly and then
remain unchanged for long periods of time.
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