Transcript 18.6-19 Evolution PowerPoint

Processes of Evolution
Chapter 18.6 and 19
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
Balanced Polymorphism

Balanced polymorphism

Example: Sickle cell anemia
and malaria
◦ A state in which natural
selection maintains two or
more alleles at relatively high
frequencies
◦ Occurs when environmental
conditions favor heterozygotes
◦ HbA/HbS heterozygotes survive
malaria more often than people
who make only normal
hemoglobin
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
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 – once a
population of 20 – now 17,000
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)
18.8 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
19.2 Comparing
Body Form and Function
Comparisons of body form and structures
yields clues about evolutionary
relationships
 Morphological divergence

◦ A body part from a common ancestor becomes
modified differently in different lines of descent

Homologous structures
◦ Similar body parts that reflect shared ancestry
◦ The same genes direct their development
Morphological Divergence
Morphological Convergence

Morphological convergence
◦ Evolution of similar body parts in different
lineages, not in a common ancestor

Analogous structures
◦ Body parts that evolved independently in separate
lineages in response to the same environmental
pressure
19.3 Comparing
Patterns of Development
Similar patterns of embryonic
development are the result of master
genes that have been conserved over
evolutionary time
 Evidence of evolutionary relationships

 PBS embryology clip
 Which Embryo?
Evidence for Evolution

Vestigial structures are inherited from ancestors,
but have lost much or all of their original function
due to different selection pressures acting on the
descendant.
◦ Ex. hipbones of bottlenose dolphins
 In ancestors, hipbones played a role in terrestrial locomotion.
As the dolphin lineage adapted to life at sea, this function was
lost
◦ Why would an organism possess structures with little or no
function?
 The presence of a vestigial structure does not affect an
organism’s fitness. Natural selection would not eliminate it.
Forever Young
Some differences between closely related
species resulted from changes in rate of
development
 Example: Chimpanzee and human skulls

◦ Early stages are similar
◦ Human skulls undergo less differential growth;
juvenile traits persist
Evidence for Evolution
Genetics & Molecular Biology

At the molecular level, the
universal genetic code and
homologous molecules
provide evidence of common
descent.
◦ DNA and RNA carry information
from generation to generation
and to direct protein synthesis
◦ Similar genes and proteins are
found in many organisms
 Hox genes – determine the headto-tail axis in embryonic
development
19.4 Comparing DNA and Proteins
◦ Cytochrome b – conserved between species –
mitochondrial protein in electron transfer chain
Molecular Clocks

The accumulation of neutral mutations in
the DNA of a lineage are like ticks of a
molecular clock that increase over time

DNA sequences of closely related species
are more similar than those of distantly
related ones
DNA Sequence Comparison
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
Reproductive Isolating Mechanisms
Postzygotic Isolation Mechanisms

Reduced hybrid viability

Reduced hybrid fertility
◦ ligers – male lion, female tiger
◦ tigons
◦ Extra or missing genes
◦ Mules(63 chromosomes)
 -male donkey (62 chromosomes) female horse (64
chromosomes)
◦ Robust but sterile offspring

Hybrid breakdown
◦ Lower fitness with successive generations
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
on an Isolated Archipelago
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
◦ Plams – difference in soil pH, etc.
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
Different Speciation Models
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
19.1 Taxonomy and Cladistics

Taxonomy
◦ The science of naming and classifying species

We group species based on what we know
about their evolutionary relationships
Linnaean Classification

Carolus Linneaus ranked organisms into ever
more inclusive categories (taxa)
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
Species
Genus
Family
Order
Class
Phylum
Kingdom
Domain
In the Linnaean system, each species is given a
unique, two-part scientific name
◦ Example: the dog rose, Rosa canina
◦ The first part is the genus name
 The second part is the species name
Linnaean Classification
Ranking Versus Grouping

Rankings do not necessarily reflect evolutionary
relationships (phylogeny)

Cladistics determines evolutionary relationships
by grouping species on the basis of shared,
quantifiable features (characters)

A clade is a group of species that share a set of
characters
Cladograms
If 98 out of 200 individuals in a
population express the recessive
phenotype, what percent of the
population would you predict would be
heterozygotes?
Q2 = 98/200 = .49
q = square root .49 = .7
p = .3 (p + q = 1 so p + .7 = 1)
2pq = 2 x .7 x .3 = .42 = 42%