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Evolution
Genetically-controlled changes in physiology, anatomy
and behaviour over time
Microevolution:
Macroevolution:
evolutionary changes within a species
evolutionary changes within larger
taxonomic units
Speciation:
(cladogenesis)
the development of two or more
genetically differentiable species from
a single common ancestor
Not all evolutionary changes result in
speciation
Physiology, anatomy and behaviour of species are
controlled by genes
Genes consist of molecules of deoxyribonucleic acid (DNA)
Components of DNA:
Sugars and phosphates, joined
by nitrogenous compounds
Physical structure known since
The 1940’s (Francis Crick and
James Watson)
Nucleic Acids
A nucleic acid is a complex, high-molecularweight, biochemical macromolecule
composed of nucleotide chains that convey
genetic information.
Chromosomes
Genes are arranged into paired, thread-like structures called
chromosomes within the cells of an organism
The locus is the point
at which a particular
gene is found on the
chromosome
Human beings have
46 chromosomes,
arranged into 23
pairs
Cells with complete
sets of pairs are called
diploid, while sperm
cells are haploid
(1/2 of each pair)
THOUSANDS OF GENES
ON EACH CHROMOSOME
Polymorphism
The specific chemical form of a gene at an individual locus
cause variation in the appearance of an organism
Different gene forms that exist at a given locus are called
alleles
Heterozygosity:
Different forms of
an allele at a locus
(a measure of
genetic diversity)
Implications for sexual reproduction
The genes themselves do not change or blend during
reproduction
If chromosomes and loci of the male and female do not
match perfectly, reproduction cannot occur
(prevents interbreeding)
Offspring will resemble parents because genes must match at
each locus, but the offspring will differ from both parents in
traits for which there was more than one allele
Genome
The complete range of genes present in a species
There are over 25,000 different genes in the human genome.
Phenotypic variations
Differences in physiology, anatomy or behaviour of different
species or different individuals of the same species
Phenotypic differences may result from genetic differences
(genotypic variation), but this is not necessarily true – relates to
nature vs. nurture arguments
Phenotype may differ between
organisms of the same genotype
because of environmental differences
or between organisms in the same
environment due to genotype.
Epigenetics
Environment can affect gene expression
Natural Selection
Traits that provide an advantage for reproduction are
selected for, whereas disadvantaged traits are selected
against.
Allopatric speciation
The formation of new species by
geographic isolation
Theory heavily favoured by
Charles Darwin
Honeycreepers, Hawaii
Sympatric speciation
The development of new
species within the same area
No physical barriers,
so how does it happen?
Distinct reproductive timing:
Ex: Different flowering times
of plants adapted to distinct
conditions or mating during
day vs. night (insects)
Distinct courtship rituals
Genetic diversity of cichlids is
very high in African lakes, due
to sympatric speciation
Population Bottleneck
If the population of a
species decreases to
a very small size and
then recovers, then
an extended period of
low genetic diversity
ensues
Do cheaters never prosper?
Source: Gotelli (2007)
Pangaea
Pangaea
250 million years BP
Supercontinent had split ~ 200 MY BP
Laurasia
North America, Asia
and Europe
Gondwana
Antarctica, Australia,
South America, Africa
and India
Vicariance or Dispersal ?
Vicariance theory suggests that ancestors of existing lineages
drifting with the repositioned land masses: slow speciation
Torres del Paine,
Patagonia, Argentina
South Island,
New Zealand
Nothofagus pumilio and
Nothofagus betuloides
Nothofagus fusca
80 MYA: Both part of Gondwana
BUT…
Recent DNA analyses
Nothofagus spp. of New Zealand and Australia
are more closely-related to each other than
to those of South America
This proves that dispersal was involved in Australia and
New Zealand, but the differences between those of New Zealand
and South America are consistent with vicariance
Important considerations
•DNA analysis holds promise for the investigating questions of
long-range dispersal vs. vicariance when used in combination
with modern observation and the fossil record
•Fossil record does not necessarily capture the first appearance
of a species
•Extinctions and reinvasions can occur, resulting in the
appearance of a continous presence in the fossil record
Analogous evolution of phenotypicallysimilar life-forms of different taxa in similar
climates
I. PLANTS
Cactaceae
North American deserts
Euphorbiaceae
Southern African deserts
Convergent
Evolution
(A form of homoplasy)
II. ANIMALS
Similar niches can be
predictably occupied
by phylogeneticallydistinct, but
phenotypically similar
organisms when
great distances
Great
American
Interchange
Great
American
Interchange
• Palaeozoogeographic event in which North
and South America became bridged after
150 million years of isolation
• Land and freshwater fauna
crossed the volcanic Isthmus
of Panama
• North-south climatic asymmetry
delayed and limited the success
of some of these interactions
The Influence of Geographic Barriers
Similar marine biota
Distinct marine biota
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