Evolution I & II Flashcards
How species changes into new species
There exists variation in a population for natural selection to act on, where mutations lead to genetic variation and thus phenotypic variation.
Microevolution
As the environment changes, there are new selection pressures which give organisms having favourable traits (beak) a selective advantage in the local conditions since they are better able to (find food). This allows them to be selected for and have higher chance of survival and reproduction, passing on their alleles to their offspring, increasing frequency of favourable alleles in the population.
Macroevolution
As this population is geographically isolated from other populations of the same species under different selection pressures/niches, disruption of gene flow occurs between the sub-populations. Each sub-population evolves independently of each other and their allele frequencies will change as they accumulate different genetic mutations, and are subjected to genetic drift and natural selection.
Over many generations, each population will become reproductively isolated and no longer able to interbreed to form viable, fertile offspring. Hence new species is formed by allopatric speciation.
Microevolution vs Macroevolution
Both occur because of change in allele frequencies in a population over successive generations.
- Micro does not form new species while Macro forms new species
- Micro occurs over a relatively shorter period of time while Macro occurs over longer periods of time
Definitions of species
Biological: Group of organisms capable of interbreeding to form viable, fertile offspring
species are reproductively isolated from one another, share a common gene pool and have same chromosome number, and usually have similar morphological, physiological and behavioural form
Organisms can be interbred to see if they produce fertile viable offspring
But cannot be applied to asexually-reproducing organisms and extinct species whose breeding behaviour cannot be observed
Genetic: Group of genetically compatible interbreeding organisms in a natural population genetically isolated from other such groups
Different genetic species are genetically distinct since evolved independently, undergo changes that lead to behavioural changes (do not interbreed in nature, but sometimes GI =/= RI)
Genetic data from mitochondrial and nuclear DNA to identify species can be unambiguous in deducing evolutionary relationships
But tech req to study DNA sequences is expensive
Ecological: Group sharing same ecological niche
Niche: place where org lives and interacts with env
Every organism has a niche
Sometimes unrelated species occupy similar niche
Morphological: Group sharing similar body shape, size, structural features
Can be applied to all organisms
Degree of difference req to indicate separate species is highly subjective. Large morphological diff can exist within a species and some superficially similar but diff evo origins
Phylogenetic: smallest group of organisms that share most recent common ancestor
Obtained by comparing homologous morphological structures/molecular sequences
Avoids mistakenly classifying orgs based on superficial morphological similarities as chars based on common ancestry/homology
Accuracy of phylogenetic tree dep on availability, diversity and accuracy of source data
Natural selection
Process in which individuals that have certain inherited traits tend to survive and reproduce at higher rates than other individuals because of those traits
- To maintain constancy of numbers
Keeps size of population relatively constant as many offspring die before they reach reproductive age, esp since there is overproduction of offspring in most species which would lead to exponential increase if all survived - Competition
Natural selection occurs when selection pressures such as limited resources, predators, disease and extreme weather conditions are imposed on a population. - Variation in a population
Is a pre-requisite for evolution by natural selection. - Causes survival of the fittest
Organisms carrying favourable traits are selected for. - Increases favourable allele frequencies
- Forms new species
Homologous structures
Molecular homology: similarity in DNA, RNA and amino acid sequences e.g. cytochrome C gene and p53 protein
Forelimbs of the bat, lizard, human and whale have the same bone arrangement even though they have very different functions : flying, lifting, swimming, climbing.
Natural selection had resulted in the different forms of the pentadactyl limb to suit specialised functions
It was modification of the five-digit forelimb of a common ancestor from which they all descended.
How natural selection brought about evolution
Random mutation resulting in new alleles and sexual reproduction led to variation in size, shape and length of the forelimb bone in the population.
Selection pressure, possibly the availability of food or the need to escape from ground-predators, resulted in the favouring of bats with forelimbs that enabled flight.
higher chance of survival and reproduction and passing on their alleles to their offspring, conferring an ability to fly. Over time, this resulted in frequency of allele responsible for ability to fly to increase.
Heterozygote advantage
Homozygote recessives are selected against
Homozygote dominants are selected against
Heterozygotes are selected for as they are resistant to XXX while they do not suffer from YYY
This is called heterozygote advantage, a form of balancing selection where both alleles are selected for under specific circumstances
Advantages of molecular methods
- All known life is based on nucleic acids so studies of any types of taxa can use DNA sequence data
- It is objective
Molecular character states are unambiguous (AGCT) while morphological characteristics can be difficult to distinguish as there could be many overlaps in different character states - It is quantifiable
Molecular data is easily converted to numerical form for mathematical and statistical analysis
The degree of relatedness can be inferred and quantified by calculating the number of nucleotide differences between species - mtDNA does not undergo recombination hence any differences in DNA is purely due to accumulations in mutations and thus we can estimate time of speciation
- DNA from both dead and living organisms can be used and no need for entire specimen
- Molecular methods provide more characters than morphological characters as molecular differences are not always phenotypically visible (esp if introns, intergenic spaces compared)
- They can be used to compare between species that are morphologically indistinguishable and to assess relationships between species that are so phylogenetically distant that they share very few morphological similarities
Examples of microevolution and macroevolution
Micro: antibiotic resistance of bacteria, pigmentation of peppered moth (lighter form and melanic form which was favoured when allowed camouflage on dark bark post-industrialisation against birds)
Macro: Galapagos finches
