The origin of species chapter 24.
Evolutionary theory must explain how new
species originate.
Two basic patterns in which evolution of one
species into one or more other species
occurs.
The origin of species
Anagenesis: accumulation of changes over
time gradually transforms a species into a
different species.
Cladogenesis: Gene pool splits into two or
more pools which each give rise to new
species.
The origin of species
Biological Species Concept
The species is a basic biological unit and
humans seem to intuitively recognize
species.
The origin of species
Why do species exist?
Why don’t we see a smooth continuous
blending of one species into another?
The origin of species
Because intermediate forms are selected
against.
If they were not selected against, then the
two forms would merge into one as their
gene pools mixed.
Species definitions
John Ray (1627-1705) gave first general
definition of a species.
A species consists of all individuals that can
breed together and produce fertile offspring.
A female donkey mated to a male horse
produces what?
A mule (which is sterile)
Hence, donkeys and horses
are separate species.
Biological Species Concept
Ray’s idea updated into the biological
species concept.
“Species are groups of actually or
potentially interbreeding natural
populations, which are reproductively
isolated from other such groups.” Ernst
Mayr.
Reproductive Isolation
There are a large number of potential
barriers that prevent different species
producing viable, healthy adults.
These include both prezygotic and
postzygotic isolating mechanisms (i.e.,
barriers that come respectively before and
after mating).
Review pages 474-475
But what about organisms that do not mate
with another individual?
E.g. single-celled animals, bacteria, fungi
and many plants reproduce asexually.
In practice, many organisms are assigned to
species based on morphology or DNA.
Speciation
Classically, speciation has been viewed as a
three stage process:
1. Isolation of populations.
2. Divergence in traits of separated populations
(e.g. mating system or habitat use).
3. Reproductive isolation of populations that
maintains isolation when members of different
populations come into contact.
Speciation
Two ways in which speciation can occur.
Allopatric speciation occurs when a gene
pool is geographically divided into two
Sympatric speciation occurs without
geographic separation of the populations..
Allopatric speciation
Occurs when a population is divided by a barrier.
Can occur because a barrier develops or because
some members of population disperse to a new
area.
Once separated, the gene pools diverge as each
population adapts to its local environment. Over
time isolating mechanisms are likely to develop.
Allopatric speciation
If after many generations members of the
allopatric populations are brought back
together they may or may not be able to
produce fertile offspring.
Even if they can do so, those offspring may
have intermediate characteristics which suit
them to neither of the parental environments
and thus they will be selected against.
Allopatric speciation
If intermediates are selected against, we
would expect isolating mechanisms
(barriers to reproduction) to be strongly
favored by selection.
Ultimately the two populations would
become different enough to be unable to
interbreed successfully and so become new
species.
Allopatric speciation
Examples.
Two species of closely related antelope squirrels
live on opposite sides of the Grand Canyon. The
canyon is a barrier to their dispersal.
In contrast, birds and other species that disperse
well have not undergone speciation on opposite
sides of the canyon
Allopatric speciation
Different Galapagos Islands contain
different species of finches, which have
evolved in the approximately 2 million
years since the islands were first colonized
from the South American mainland.
Allopatric speciation
Diane Dodd investigated development of
reproductive barriers in fruit flies.
Raised populations for several generations
on either starch or maltose medium. Fly
populations diverged each becoming better
at digesting its food source.
Allopatric speciation
When flies from “starch populations” and
from “maltose populations” brought
together they were significantly more likely
to mate with flies of their own population.
Indicates that reproductive barriers between
species can begin to form quickly.
Sympatric Speciation
In sympatric speciation, speciation takes
place in geographically overlapping
populations.
Mechanisms of sympatric speciation
include polyploidy and nonrandom
mating that reduces gene flow.
Sympatric Speciation
Polyploidy is common in plants and many species
have resulted from accidents in cell division that
produce extra sets of chromosomes.
For example a diploid plant (2n chromosomes)
may become a tetraploid (4n). The tetraploid
cannot produce fertile young with diploid plants
because young will be triploid (3n chromosomes),
but can self-pollinate or mate with other
tetraploids.
Sympatric Speciation
Polyploidy can thus result in speciation in just one
generation.
Polyploidy can also occur when two different
species produce a hybrid. The offspring are often
sterile because chromosomes cannot pair up
during meiosis. However, the plant can often
reproduce asexually.
Sympatric Speciation
Subsequently, various mechanisms can
convert a sterile hybrid into a fertile
polyploid called an allopolyploid.
The allopolyploids are fertile with each
other, but not other species and so are a new
species.
Sympatric Speciation
Many important crops are polyploids. For
example, the wheat used for bread is an
allohexaploid (six sets of chromosomes,
with two sets from each of three different
species).
Sympatric Speciation
Non-random mating. Reproductive
isolation can occur when genetic factors
enable a subpopulation to exploit a resource
not used by the parental population.
Sympatric Speciation
Example: North American apple maggot
fly. Original breeding habitat was hawthorn
fruits on hawthorn trees, but about 200
years ago some populations colonized apple
trees.
Apples mature faster than haws (hawthorn
fruit) so apple-feeding flies have been
selected for rapid development.
Larvae
Damaged apple
http://www.virginiafruit.ento.vt.edu/
AppleMaggot.html
http://www.eppo.int/
QUARANTINE/insects/
Rhagoletis_pomonella/RHAG
PO_images.htm
Adult
http://bugguide.net/node/view/248877
Sympatric speciation in applemaggot flies
Natural selection favors divergence because
hawthorn fruits ripen 3-4 weeks after apples. As a
result, hawthorn fly larvae experience cool
temperatures before pupating whereas apple fly
larvae experience warmer temperatures.
Hawthorn flies and apple flies thus depend on
different temperature signals to time their pupation
and emergence the next spring and have different
developmental timetables.
Sympatric Speciation
Apple-feeding flies now temporally isolated
(isolated in time) from hawthorn-feeding
flies. Speciation appears well underway.
A protein electrophoresis study by Feder et
al. (1988,1990) showed that the two
populations are genetically distinct.
Sympatric Speciation
Lake Victoria cichlids. Lake Victoria about
12,000 years old but home to more than 500
species of cichlids (fish).
There has been rapid speciation and some of
it appears to have been caused by nonrandom mating in which females choose
males based on their appearance.
Sympatric Speciation
Researchers studied two closely related species
one which has a blue-tinged back and the other a
red-tinged back.
In an aquarium with natural light females mated
with males of their own species exclusively.
However, in an aquarium under monochromatic
orange light (where blue and red could not be
distinguished), females mated indiscriminately
and offspring were fertile.
Sympatric Speciation
Researchers concluded mate choice by
females based on coloration is main barrier
keeping gene pools separated.
Because fertile young are produced in
interspecific crosses the speciation probably
occurred recently.
Phylogeny and Systematics
Phylogeny is the evolutionary history of a
species or group of species.
Systematics is science of understanding the
diversity and relatedness of organisms.
Phylogeny and Systematics
Traditionally morphological similarities
used to infer evolutionary relationships.
More recently, comparisons of DNA, RNA
and other molecules used to infer
relationships: molecular systematics.
Phylogeny and Systematics
Phylogenetic trees are based on common
ancestry and data from various sources used
to construct them:
Fossil evidence
Molecular evidence
Morphological evidence
Phylogeny and Systematics
In constructing phylogenies important to
distinguish between homologous structures
(similar due to common descent) and analagous
structures (similar because of convergent
evolution).
Australian sugar glider a marsupial and North
American flying squirrel a eutherian mammal are
examples of convergent evolution. Both possess
gliding membrane, but otherwise only distantly
related.
Phylogeny and Systematics
Analagous structures that have evolved
independently are called homoplasies.
Deciding whether structures are
homologous or analagous requires various
types of evidence to be assessed.
Phylogeny and Systematics
Corroborating similarities in other
structures
Fossil evidence
Complexity of characters being compared.
The more points of resemblance there are
between two structures the less likely it is
they evolved independently.
Phylogeny and Systematics
Evaluating molecular homologies.
Comparison of DNA sequences usually
done using computer programs that match
up sequences taking into account effects of
insertions and deletions.
Phylogeny and Systematics
Science of systematics dates to Linnaeus in
the 18th century who devised basic systems
of binomial nomenclature and
hierarchical classification in use today.
All organisms have a unique binomial name
E.g. Humans are Homo sapiens
Phylogeny and Systematics
Organisms are classified into hierarchical
classifications that group closely related
organisms and progressively include more
and more organisms.
Phylogenetic trees
Branching diagrams called phylogenetic
trees summarize evolutionary relationships
and hierarchical classification is represented
in finer branching of phylogenetic trees.
Phylogenetic trees
In a phylogenetic tree the tips of the
branches specify particular species and the
branch points represent common ancestors.
Cladistics and construction of
phylogenetic trees
Cladograms are diagrams that display
patterns of shared characteristics.
If shared characteristics are due to common
ancestry the cladogram forms basis of a
phylogenetic tree.
Cladograms
Within a tree a clade is defined as a group
that includes an ancestral species and all of
its descendants.
Cladistics is analysis of how species may
be grouped into clades.
Shared derived characters
Cladograms are largely constructed using
shared derived characters.
These are characteristics that are
evolutionary novelties, new developments
that are unique to a particular clade.
Shared derived characters
Shared derived characters are unique to the
clade. For example, for mammals hair is a
shared derived character
Shared primitive characters
Shared primitive characters are characters that are
shared beyond the taxon we are interested in.
Among vertebrates the backbone is an example
because it evolved in ancestor of all vertebrates.
If you go back far enough in time shared primitive
characters will be shared derived characters. Thus,
the backbone is a shared derived character that
distinguishes vertebrates from all other animals.
Constructing a cladogram
Outgroup comparison is used to begin
building a cladogram.
An outgroup is a close relative of the
members of the ingroup (the various
species being studied) that provides a basis
for comparison with the others.
Constructing a cladogram
The outgroup in a cladogram is determined
using data different from that being used to
construct the cladistic tree.
The outgroup “roots” the tree. The
outgroup is based on the assumption that
homologies in the outgroup and ingroup are
primitive characters.
Constructing a cladogram
Having the outgroup for comparison
enables researchers to focus on those
characters derived after the separation from
the outgroup to figure out relationships
among species in the ingroup.
Constructing a cladogram
Cladogram of various vertebrates: leopard,
tuna, salamander, turtle and lamprey.
Use lancelet as outgroup (is a chordate, but
has no backbone).
Table summarizes data about character
traits and which organisms possess them.
Constructing a cladogram
In the cladogram new characters are marked on
the tree where they originate and these characters
are possessed by all subsequent groups.
The cladogram of vertebrates is a step towards
constructing a phylogenetic tree, but such a tree
would need to be based on much more data.
Unfortunately, additional data and additional
species make it hard to decide on a best tree.
Identifying the “best” trees
When constructing a phylogenetic tree that
involves many species there are billions of
possible ways to arrange a tree.
We try to build tress that are the most
likely. Generally these are trees that are the
most parsimonious (require the fewest
evolutionary changes) to construct.
Homology and analogy
The most parsimonious tree may not always
correct.
If analogy versus homology mistakes are
made the tree will be incorrect.
Which of the next two trees is the best tree?
Homology and analogy
If mammal and bird four-chambered hearts
are homologous, then tree A is most
parsimonious.
However, lots of data suggest birds and
reptiles more closely related, so tree B is
better tree. Four-chambered heart evolved
more than once.
Phylogenetic trees are hypotheses
Important to remember that phylogenetic trees are
hypotheses for the evolutionary pathways.
Trees that we will have most confidence in will be
supported by multiple lines of evidence (e.g.
molecular, morphological and fossil evidence).
Molecular clocks
Trees of relatedness can be dated by using
fossil evidence, but also by using molecular
clocks.
Based on observation that some genes
appear to evolve at fairly constant rates.
Molecular clocks
Assumption is that the number of changes
in genes is proportional to the amount of
time since two species branched from their
common ancestor.
Molecular clocks are calibrated against the
fossil record.
Molecular clocks
Molecular clocks are not perfect as genes
may evolve in fits and starts (because of
effects of selection) and not be very
clocklike.
Molecular clocks
Some good markers to use for molecular
clocks are silent mutations (changes in
genes that do not change the amino acid
coded for) because these will have no effect
on selection. However, these are most
useful over only relatively short time
periods.
Applying a molecular clock: HIV
HIV is descended from viruses found in
chimps and monkeys (SIV simian
immunodeficiency virus).
To date the time the virus jumped to
humans scientists have compared current
HIV-1 M samples to some from tissue
samples preserved in 1959.
Applying a molecular clock: HIV
Samples showed virus has evolved at steady
rate and by extrapolating back using the
molecular clock have estimated that HIV-1
M first infected humans in the 1930’s.