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EvolutionGenetically-controlled changes in physiology, anatomy and behaviour over time
Microevolution: evolutionary changes within a speciesMacroevolution: evolutionary changes within larger
taxonomic units
Speciation: the development of two or more (cladogenesis) genetically differentiable species from
a single common ancestor
Not all evolutionary changes result inspeciation
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 sinceThe 1940’s (Francis Crick and James Watson)
Nucleic Acids
A nucleic acid is a complex, high-molecular-weight, biochemical macromolecule composed of nucleotide chains that convey genetic information.
Chromosomes Genes are arranged into paired, thread-like structures calledchromosomes within the cells of an organism
The locus is the pointat which a particulargene is found on thechromosome
Human beings have46 chromosomes,arranged into 23pairs
Cells with completesets of pairs are calleddiploid, while spermcells are haploid(1/2 of each pair)
THOUSANDS OF GENESON 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 notmatch perfectly, reproduction cannot occur (prevents interbreeding)
Offspring will resemble parents because genes must match ateach locus, but the offspring will differ from both parents in traits for which there was more than one allele
GenomeThe complete range of genes present in a speciesThere are over 25,000 different genes in the human genome.
Phenotypic variations
Differences in physiology, anatomy or behaviour of differentspecies or different individuals of the same species
Phenotypic differences may result from genetic differences(genotypic variation), but this is not necessarily true – relates tonature vs. nurture arguments
Phenotype may differ betweenorganisms of the same genotype because of environmental differencesor between organisms in the same environment due to genotype.
EpigeneticsEnvironment can affect gene expression
Natural SelectionTraits that provide an advantage for reproduction are selected for, whereas disadvantaged traits are selected against.
Allopatric speciationThe formation of new species by geographic isolation
Theory heavily favoured by Charles Darwin
Honeycreepers, Hawaii
Sympatric speciationThe 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 distinctconditions or mating duringday vs. night (insects)
Distinct courtship rituals
Genetic diversity of cichlids isvery high in African lakes, dueto sympatric speciation
Source: Gotelli (2007)
Population Bottleneck
If the population of aspecies decreases toa very small size andthen recovers, thenan extended period oflow genetic diversityensues
Do cheaters never prosper?
Pangaea250 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
Nothofagus pumilio and Nothofagus betuloides
Torres del Paine, Patagonia, Argentina
Nothofagus fusca
South Island, New Zealand
80 MYA: Both part of GondwanaBUT…
Recent DNA analyses
Nothofagus spp. of New Zealand and Australiaare 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 Zealandand South America are consistent with vicariance
Important considerations•DNA analysis holds promise for the investigating questions oflong-range dispersal vs. vicariance when used in combinationwith 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 phenotypically-similar life-forms of different taxa in similar climates
CactaceaeNorth American deserts
EuphorbiaceaeSouthern African deserts
I. PLANTS
ConvergentEvolution
(A form of homoplasy)
Similar niches can bepredictably occupiedby phylogenetically-distinct, but phenotypically similarorganisms when great distances
II. ANIMALS
• 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
Great American Interchange
The Influence of Geographic Barriers
Distinct marine biota
Similar marine biota
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