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Evidence for Evolution
Chapter 2
Evidence of change through time:
Living species
• Soapberry bugs
• Vestigial structures
• “Vestigial” genes (pseudogenes)
Evolution of beak length in soapberry bugs –
microevolution
• Native to southern US
• In Florida, native host plant is balloon vine
– which occurs primarily on southern tip of Florida, on
the Florida Keys, and less commonly, in central Florida
• Bugs feed by piercing the fruit with beak and
inserting it into the seeds
• Flat-podded golden rain tree introduced to central
Florida in 1926
• Rain tree fruit are much thinner than balloon vine
fruit
Evolution of beak length in soapberry bugs –
microevolution
• Soapberry bugs in c. Florida started using rain
trees as a host plant
• Populations of bugs living on thin-fruited rain
trees evolved shorter beaks
Fig. 2.1(a) Evolutionary change in soapberry bugs
Fig. 2.1(b) Evolutionary change in soapberry bugs
More on soapberry bugs – 1
• Beak length is a variable quantitative phenotype (also
heritable)
• Phenotypic distributions on the two host plant species
overlap
• But some bugs found on rain trees have much shorter
beaks than any bugs found on balloon vines (and vice
versa)
• Evolution of mean phenotype and the range of the
phenotype
• “Standing” heritable variation enables populations to make
evolutionary responses to changing environments (don’t
have to wait for the “right” mutations)
More on soapberry bugs – 2
• How do we know that this is really an evolutionary
change, rather than simply a plastic growth response?
• Why does natural selection favor shorter beaks in
populations that use rain trees as hosts?
– a long-beaked bug can exploit both thick and thin fruit
• See “Exploring the Literature” #11 at the end of this
chapter
• This appears to be an example of relatively modest
morphological change (microevolution), but could this also
be an example of speciation in progress?
Vestigial structures are evidence for
evolution
• Reduced wings in flightless birds (e.g., kiwi)
• Reduced or missing eyes in cave organisms (e.g.,
Mexican tetras)
– Eye development can be stimulated by transplantation
of lens tissue from an eyed relative
• Vestigial digits during embyronic development in
chickens
• Pelvises without hind limbs in whales
Fig. 2.8(a) Vestigial structural traits
Molecular vestiges – pseudogenes
• Pseudogenes – non-functional DNA
sequences that share similarity with
functional genes
• Arise through gene duplication followed by
accumulation of mutations that cause one
copy to become non-functional
• Example: a-globin gene and ya-globin
– members of the globin gene family
Evidence of change through time:
The fossil record
• Extinction
• Appearance of new forms in the fossil
record – “Law of Succession”
– glyptodonts and armadillos in S. America
– Diprotodon and wombats in Australia
• Transitional forms
– Archaeopteryx
Archaeopteryx fossil – 19th Cent. Engravings
of the London specimen
http://www.geo.ucalgary.ca/~macrae/t_origins/archaeopteryx/
http://www.talkorigins.org/faqs/archaeopteryx/info.html
Archaeopteryx
vs. modern
birds
www.geologyrocks.co.uk/ tut.php?id=13
Evidence of common ancestry – Homology
• Originally, and literally, the study of
likeness
• Similarities among species that are not the
result of functional necessity
• Example: the tetrapod forelimb
Fig. 2.11 Structural homologies – tetrapod forelimb
Homology is explained by common
ancestry
• Darwin argued that homologous similarities
were the result of inheritance from common
ancestors — humans, moles, horses,
dolphins and bats share similar limb
skeletons because they all inherited their
limbs from a common ancestor with a
similar number and arrangement of bones
“Tree Thinking”
• If:
– New species come from already existing species
– Some species go extinct without leaving descendants
– The total number of species does not decline over time
• Then:
– Some species must leave two or more descendants
– And, the history of life is a branching process that
results in an evolutionary tree that describes
relationships among species
Fig. 2.9 Darwin’s hypothetical evolutionary tree
More Tree Thinking
• Evolutionary trees define related groups of
species in a nested hierarchy (more closely
or more distantly related)
• Darwin realized that our ability to erect a
hierarchical classification system (e.g.,
genus, family, order, class) was a direct
consequence of branching evolution and
common ancestry
Fig. 2.10 An evolutionary tree for eight species of cats
A phylogeny (evolutionary tree) is an
hypothesis about relationships
• In the cat example, no one was around to
see it happen
• Other investigators reach different
conclusions about the details (e.g., lions and
tigers are each other’s closest relative =
sister taxa)
Molecular homology – processed
pseudogenes
• Processed pseudogenes are pseudogenes that lack the
introns and promoter regions found in their functional
equivalents
• Thought to result from reverse transcription of processed
mRNA
• The age of pseudogenes can be estimated by comparing the
their nucleotide sequences to those of their functional
partners (more differences = more time)
• In any phylogeny, pseudogenes that arose more recently
(i.e., younger) will be shared by fewer taxa than older
pseudogenes
Fig. 2.17(a) Processed pseudogenes used to test
Darwin’s hypothesis of common ancestry
Fig. 2.17(b) Processed pseudogenes used to test
Darwin’s hypothesis of common ancestry
Fig. 2.17(c) Processed pseudogenes used to test
Darwin’s hypothesis of common ancestry
Fig. 2.18 The geological time scale
Radiometric dating – 1
• Radioactive isotopes decay at known rates (= halflife)
• Relative amounts of parent and daughter isotopes
in a rock sample indicate the age of the rock
• Important assumptions:
– Amount of daughter isotope at time rock was formed is
known
– Parent and daughter isotopes have not entered or left
rock since its formation
Fig. 2.19 Radioactive decay
Radiometric dating – 2
•
•
http://www.earthsci.unimelb.edu.au/Thomas/lteng/engeimg/enge0507a.GIF
See Table 2.1 of Freeman and Herron (p. 61)