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1

evolution

is a change in the genetic composition of a population of a species over
time

2

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

3

mutations

can create an allele

4

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

5

migration

can add or subtract from the gene pool

6

small populations

limited gene pool and mating opportunities

7

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

8

three general types of selection

stabilizing, disruptive, directional

9

frequency

a size range is removed, climatic event, predator removal

10

fitness

fitness of remaining size range increases

11

average size

average size of population changes over generations

12

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

13

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...

14

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

15

Evolution can result in new species...

biological species concept

16

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)

17

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)

18

allopatric speciation

(geographic) speciation – individuals are geographically isolated by a physical barrier (e.g. river, mountain, unsuitable habitat)

19

sympatric speciation

subpopulations are isolated without geographical isolation (e.g. timing or location of breeding)
– (disruptive selection favours divergent phenotypes)

20

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)

21

What maintains speciation?

Speciation is maintained through reproductive isolation of the subpopulations...
Isolating mechanisms -

22

isolating mechanisms

mechanisms that restrict exchange of genes between subpopulations
1. premating
2. postmating

23

Premating

prevent mating
– Separation of mating events in space and time
– Behaviour, mechanical or structural incompatibility

24

postmating

reduced survival or reproductive success of offspring