lecture 3 Flashcards
(24 cards)
evolution
is a change in the genetic composition of a population of a species over
time
Hardy-Weinberg Principle
the genetic composition (allele frequencies) within a
population does not change (~evolution does not occur) unless one of the following things occur:
1) no mutations
2) no migrations
3) infinitely large population size
4) random mating
5) no selection
mutations
can create an allele
non random mating vs random mating
Non-random mating among individuals in a population (certain individuals
more desirable e.g. blonde individuals only mate with other blondes) – Ifrandommating–allelefrequencieswillstaythesame
migration
can add or subtract from the gene pool
small populations
limited gene pool and mating opportunities
Natural selection
– Selectivepressures(orchangingabioticandbioticfactors)→varying
fitness among individuals → favourable traits are passed on to future generations at a higher frequency → genetic composition of the population changes over time
three general types of selection
stabilizing, disruptive, directional
frequency
a size range is removed, climatic event, predator removal
fitness
fitness of remaining size range increases
average size
average size of population changes over generations
Evidence of Natural Selection in Nature?
Eg. Peppered Moth - England
• Inhabit dense woods and rest on tree trunks during the day
• Bird species search tree trunks and feed on these moths
• Survival of the moth depends on being camouflaged against the tree trunk
• Two forms: light and dark
• Colour: determined by a single gene → colour reflects genetic variation among
individuals
• Early 1800s – occasional dark forms, but primarily light form
• Early 1900s – dark form became more common in heavily industrialized areas
– Why?
– In industrialized areas:
– Soot deposited on tree trunks
– Dark colour –more favourable
– Genetic composition of population changed over generations
Does exploitation cause evolution?
– Commercial Fishing is inherently selective – preferentially harvest larger fish (directional selection)
• Conover & Munch (2002) Science 297: 94-96
– Atlantic silverside (Menidia menidia) experiments in lab – selectively removed larger fish
– selectively removed smaller fish
– removed fish at random
– measured fish for 4 generations
– mean weight of harvested fish declined over the 4 generations – Smaller fish produce fewer eggs and fewer offspring
– The amount of fish available to harvest decreased over time…
Does exploitation cause evolution in the wild:
– Fishing pressure can significantly change the genetic composition of
populations in 20-50 yrs
– Size-selection is one of the primary reasons why overexploited fish
populations do not recover
Evolution can result in new species…
biological species concept
BiologicalSpeciesConcept–
distinguish species based on their
potential to interbreed and produce fertile offspring
– implies that reproductive isolation (or genetic isolation) defines a
•
species because reproduction is the means of transferring genetic information (DNA)
How does speciation occur?
1st : Exchange of genes among individuals of a population (gene flow)
stop – some individuals become reproductively isolated from other individuals
– –
– Allopatric (geographic) speciation – individuals are geographically isolated by a physical barrier (e.g. river, mountain, unsuitable habitat)
– Sympatric speciation – subpopulations are isolated without geographical isolation (e.g. timing or location of breeding)
– (disruptive selection favours divergent phenotypes)
• 2nd: isolated subpopulation experiences different selection pressures (→
different favourable traits → varying fitness)
• 3rd: genetic composition of subpopulation changes over generations (1o
via natural selection)
• 4th: After generations, if the isolated subpopulation can no longer
interbreed with the origin population → speciation (If subpopulations can interbreed – no speciation)
allopatric speciation
(geographic) speciation – individuals are geographically isolated by a physical barrier (e.g. river, mountain, unsuitable habitat)
sympatric speciation
subpopulations are isolated without geographical isolation (e.g. timing or location of breeding)
– (disruptive selection favours divergent phenotypes)
E.g. Cichlids - Nicaragua
Example of Sympatric speciation
• • •
•
Small, isolated, low productivity lakes Midas & Arrow cichlid species Disruptive Selection
– divergent food preferences (competition) → different food and feeding habitats • Two morphotypes – different feeding morphology, body shape
Reproductive isolation
– differences in courtship behaviour → non-random mating (premating)
What maintains speciation?
Speciation is maintained through reproductive isolation of the subpopulations…
Isolating mechanisms -
isolating mechanisms
mechanisms that restrict exchange of genes between subpopulations
- premating
- postmating
Premating
prevent mating
– Separation of mating events in space and time
– Behaviour, mechanical or structural incompatibility
postmating
reduced survival or reproductive success of offspring
