The origin of species, Lecture 7 Flashcards
Evolution requires …
changes in allele and genotype frequency (microevolution)
Four evolutionary forces which cause changes in allele and genotype frequency?
- mutation
- gene flow
- genetic drift
- natural selection
Mutation
(evolutionary forces which cause changes in allele and genotype frequency)
4 points
- changes in genetic code
- more important if occurs in cells that produce gametes
- current organism likely to be well adapted to environment, therefore more likely to be detrimental than beneficial
- generally low rates, small effects on large populations
Gene flow (evolutionary forces which cause changes in allele and genotype frequency) 2 points
- gene pool
- movement of genes between populations, eg: genetic exchange due to migration of fertile individuals or gametes between populations
Gene pool
All the alleles in a population are known as the gene pool
Genetic drift (evolutionary forces which cause changes in allele and genotype frequency) definition
chance alterations in population’s allele frequencies
Causes of genetic drift
(evolutionary forces which cause changes in allele and genotype frequency)
2
- bottlenecks
2. founder effects
Causes of genetic drift
(evolutionary forces which cause changes in allele and genotype frequency)
bottlenecks
- size of population is severely reduced
- reduces variation
Causes of genetic drift
(evolutionary forces which cause changes in allele and genotype frequency)
Example of bottlenecks
Northern elephant seal,
hunted by humans in the 19th century (1800s)
population reduced to 20-30 individuals
now 100,000 individuals
Causes of genetic drift
(evolutionary forces which cause changes in allele and genotype frequency)
Founder effects
one/few individuals of a species become founders of a new isolated population
rare alleles in old population, may become significant in new population:
- could be detrimental
- reduced genetic variation
- non-random distribution of genes
Causes of genetic drift
(evolutionary forces which cause changes in allele and genotype frequency)
Example of founder effects
Galapagos islands
Natural selection (evolutionary forces which cause changes in allele and genotype frequency)
Differential success in reproduction, i.e. some variants leaving more offspring
Three factors which drive natural selection
- climate
- avoidance of predators
- resistance to pesticides
Three types of natural selection
- stabilising
- directional
- disruptive/diversifying
Stabilising selection
both extremes are at a disadvantage
Directional selection
one extreme is at a disadvantage
Disruptive/diversifying selection
average individuals are at a selective disadvantage
Carrying capacity
high density population
Population growth
general trend
- initial rapid growth of a low density population
2. growth rate slows, maximum population size eventually reached
K-selection
(density dependent)
At high density populations, traits (adaptations) are favoured that enable reproduction with fewer resource.
R-selection
(density independent)
At low density populations, selection favours traits that promote rapid reproduction.
Speciation
The origination of a new species. driven by population divergence.
Population
A group of individuals of the same species living in a particular geographical location
Two types of speciation
allopatric
and sympatric
Allopatric speciation
Population forms a new species whilst geographically separated from its parent population
Where is allopatric speciation more likely?
smaller, isolated populations
Sympatric speciation
Occurs in geographically overlapping populations,
new species evolve from a single ancestral species whilst in the same geographic region
Process of allopatric speciation
3
- physical separation
- separated populations diverge
- become productively separated (unable to interbreed/exchange genes)
Example of allopatric speciation
ring species,
photo in notes
Example of sympatric speciation
autopolyploidy
Reproductive barriers
These ensure new species keep their separate identities
Two types of reproductive barriers
prezygotic and postzygotic
Prezygotic
reproductive barriers
impedes mating between species or fertilisation of ova is mating is attempted
postzygotic
reproductive barrier
prevents any ovum fertilised by another species from developing into a viable fertile adult
Reasons for prezygotic reproductive barriers
5
habitat isolation behavioural isolation temporal isolation - eg: orchids living in the same place flower on different days mechanical isolation gametic isolation
Reasons for postzygotic reproductive barriers
2
reduced hybrid fertility eg: mule
hybrid breakdown, offspring of hybrids are feeble or sterile
TWO RATES OF EVOLUTION
Gradualism
Punctuated equilibrium
Gradualism
rates of evolution
gradual evolutionary change, slow but constant rate of change
Punctuated equilibrium
rates of evolution
Appearance of new characteristics give rise to new species relatively rapidly and then persist unchanged for a long time (equilibrium)
new innovations result in a new spurt of evolution which would result in a ‘punctuation’ of the fossil record
Are both rates of evolution correct?
yes, differs between species
