Download Macroevolution/Speciation

Evolution
Macroevolution/Speciation
Macroevolution
• Macroevolution is the occurrence of
large-scale changes in the characteristics
(traits) of life -- in effect, the evolution of
species and higher taxa.
• It is distinct from microevolution, which is
the sequence of changes that occur within
a single population.
• According to Charles Darwin and also the
modern synthesis, this distinction is
and purely a matter of scale.
– The modern evolutionary synthesis (often
referred to simply as the modern synthesis),
brings together Charles Darwin's theory of the
evolution of species by natural selection with
the theory of genetics as the basis for
inheritance.
Proposed Mechanisms for
Macroevolution
• Extrapolation of microevolutionary
processes......Gradualism.
• The second way in which macroevolution is
believed to occur is through sudden and rapid
changes......Punctuated Equilibrium.
Tiny microevolutions, over
sufficient time, add up and
accumulate in isolated
populations and eventually
in new species.
Based on the fact that there are
genes in all living organisms, and a
small change in them could cause
drastic changes in the organism,
resulting in a new species quite
Too long of a time scale!
•
Whilst speciation events have been demonstrated in the laboratory
and observed in the field, really dramatic differences between
species do not usually occur in directly observable timescales (it
occurs too quickly for the process to be shown in the fossil record.)
•
It is argued that, since macroevolution can not be confirmed by a
controlled experiment, it cannot be considered to be part of a
scientific theory.
•
However, evolutionists counter that astronomy, geology,
archaeology and the other historical sciences, like macroevolution,
have to check hypotheses through natural experiments.
•
They confirm hypotheses by finding out if they conform or fit with the
physical or observational evidence and can make valid predictions.
In this way, macroevolution is testable and falsifiable.
How is it studied then?
• Classical evidence of macroevolutionary
processes
– fossils (extinction and radiation)
– biogeography (global taxa distribution)
– comparative biology
• Phenetics and cladistics: tree-building
• Molecular evolution and molecular
phylogenetics
The tree of life: stems and leaves
extant spp
biogeographic/
geological events
extinct spp
• We need to infer the shape of the tree
(evolutionary relationships) from leaves (extant
taxa) and branches (extinct or fossil taxa), and
from knowledge of geological events ...
1. What was present, where? (Fossils)
2. What happened, when? (Geology, palaeoclimatology)
3. What is present, where? (Biogeography)
Determining evolutionary
relationships
•
Understanding structure similarities and
differences
– Homologous structures from a common
ancestor based on descent
– Analagous structures with similar functionality
but not dervied from common ancestor
– Convergent and divergent evolution
•
Derived characters vs. Ancestral
How is it studied then?
• Classical evidence of macroevolutionary
processes
– fossils (extinction and radiation)
– biogeography (global taxa distribution)
– comparative biology
• Phenetics and cladistics: tree-building
• Molecular evolution and molecular
phylogenetics
Comparative Biochemistry
• Many genes are found in all living things,
because we all use similar metabolism.
• Genes can also be described as homologous
and analogous.
• Considering homologous genes, the genes of
closely related species are or similar than genes
from more distantly related species. Increasing
time since the divergence of two species gives
increasing numbers of random mutations.
•
Cytochrome C is part of the electron transport system in the mitochondria. It
is found in all eukaryotes, and some aerobic prokaryotes as well. The
number of amino acid differences between the cytochrome c found in
different species is proportional to the time since they diverged.
Speciation
• Speciation refers to the appearance of a new species of
life on earth.
• Species occur when a parent species (also referred to as a
common ancestor) splits into two (or more)
reproductively-isolated populations
– each of which then accumulates changes from sexual
reproduction and/or random mutation (in addition to any other
various contributors to genetic change)..........
• until the populations are no longer capable of
interbreeding.
Macroevolution: Speciation
• Process whereby new species arise
• Generally one of two conditions
required:
– Geographic barriers
– Reproductive isolation
• Cumulative process of drift and
selection acting over many generations
Reproductive Isolation Mechanisms
(Pre-zygotic (before mating) vs. post-zygotic (after mating)
• Pre-zygotic
– different breeding seasons (either
yearly or time of day)
– different signals: sight, sound, smell,
behavior
– mechanical problems—genitalia don’t
fit together
– gametic isolation: sperm don’t detect
the egg or can’t fertilize it
• Post-zygotic
– hybrid inviability: hybrids don’t survive
to birth or adulthood
– hybrid sterility: as with the mule, a
sterile hybrid.
• Even having hybrids that are
significantly less fit (able to survive and
reproduce) than purebreds is an
isolating mechanism, a way to prevent
the two populations from fusing into
one population.
horse
(fert.)
donke
(fert.)
mule
(sterile)
Allopatric Speciation: The Great Divide
• The simplest and most common form of speciation
– 2 groups of one species are isolated geographically (mountains,
oceans, rivers).....
– and diverge into separate species.
• Or, the barrier develops slowly as conditions change: the gradual
formation of the Grand Canyon split a population into 2 isolated
groups, that have diverged into separate species, the Kaibab and
Albert squirrels.
Sympatric Speciation
•
Sympatric speciation means
speciation that occurs within
the same geographical
location.
•
An example: cichlid fish in
Lake Barmobi Mbo in
Cameroon, Africa—an isolated
volcanic lake.
– Nine species, all more closely
related to each other (by DNA
evidence) than to similar fish
in other lakes.
– Lake has no distinct
geographical zones, and the
fish can easily swim anywhere
in it.
– They feed in different
locations, but all breed in the
same location, close to the
bottom.
– The mechanism is not clear.
Rates and results of speciation
• Isolation, low dispersal, strong sexual selection
and reductions in population size all favor
speciation
• Speciation gives rise to adaptive diversity
• According to the theory of punctuated equilibria
speciation has been the major source of
adaptive change as opposed to phyletic
gradualism