Download Evolution and Speciation

Evolution and
Speciation
CHAPTER 23/24 KIND OF COMBINED
Populations Evolve
Individuals do not evolve, populations do
◦ A population is a localized group of individuals that are capable of interbreeding & producing
fertile offspring
Microevolution – a change in the genetic makeup of a population from
generation to generation
Population genetics – the study of how populations genetically change over
longer periods of time
Populations Evolve
Populations evolve over time by natural selection
◦ Variation
◦ Selection
◦ Time
Species
Biological species = a population or
group of populations whose
members have the potential to
interbreed in nature & produce
viable, fertile offspring
Why are the following not
considered the same species?
+
What makes a species?
Male Tiger + Lioness = Tigon
Male Lion + female Tiger = Liger
Lions and Tigers are not the same species
because their offspring are infertile.
What makes a species?
Zebra + Horse = Zorse
Zebras and Horses are not the same species because zorses are sterile.
Other Species Concepts
• Morphological
– A group of organisms that have the same phenotypic traits.
• Ecological
– A group of organisms that occupy the same niche.
• Phylogenetic
– A group of organisms that have the same, unique genetic
history.
Speciation
Speciation: The formation of a new species. Reproductive barriers result in new species over time.
Allopatric vs. Sympatric Speciation
◦ Allopatric = a population forms a new species
because it is geographically isolated from the
parent population.
◦ Sympatric = a small part of a population becomes
a new population due to the emergence of
reproductive barriers other than geographic
isolation.
Prezygotic vs. Postzygotic
Prezygotic = A reproductive barrier occurs
before fertilization.
Postzygotic = A reproductive barrier occurs
after fertilization.
Why Hardy-Weinberg Theorem doesn’t
work in real life
The proportion of genotypes (AA, Aa, aa) in a population will remain constant from generation to
generation if…
◦
◦
◦
◦
◦
The population is very large
There is no gene flow (immigration or emigration)
There are no mutations
Random mating is occurring
There is no natural selection
change
…because under these conditions, the frequency of the alleles (A & a) does not
Populations exhibiting these conditions are not evolving & are said to be in Hardy-Weinberg
equilibrium.
Evolution occurs when any of the above 5 occurs
4 Modes of Evolution:
Factors that disrupt HW-EQ
1. Natural selection
2. Genetic drift
3. Gene flow
4. Mutations
Modes of Evolution:
Factors that disrupt HW-EQ
1. Natural selection
◦ Changes in allelic frequencies due to a selective force
◦ Organisms with traits that are better suited for their environment will survive and
reproduce.
◦ Results in alleles being passed to the next generation in different proportions
Natural Selection
Based on 3 conditions:
1.
2.
3.
Variation: there are differences among the individuals in a population
Heritability: parents are able to pass on their traits
Differential reproductive success: how many offspring are produced that survive, in
comparison to the rest of the population.
1. Variation
1. Variation
Genetic Variation at the molecular level
Mutations- point or other
Recombination- during sexual reproduction
2. Heritability –
Organisms inherit traits from their parents
What are the three ways the sexual reproduction allows the reshuffling of alleles?
◦ Crossing Over
◦ Independent assortment
◦ Fertilization
2. Heritability
Preserving genetic variation- Diploidy
Diploidy maintains genetic variation
in the form of hidden recessive
alleles in heterozygotes.
◦ 2 Chromosomes (dad and mom)
3. Differential reproductive success:
Biological Fitness
Fitness: is the contribution an individual makes to the
gene pool of the next generation, relative to the
contributions of other individuals
◦ Being able to survive and reproduce
Relative fitness: is the contribution of a genotype to
the next generation, compared with contributions of
alternative genotypes for the same locus
Modes of Evolution:
Factors that disrupt HW-EQ
1. Natural selection
◦ Changes in allelic frequencies due to a selective force
◦ Organisms with traits that are better suited for their environment will survive and reproduce.
◦ Results in alleles being passed to the next generation in different proportions
2. Genetic drift
◦ Changes in allele frequencies due to chance
◦ Bottleneck effect – occurs when a population is drastically reduced in size due to an environmental disaster (ie:
fire, flood) & the gene pool of the survivors no longer represents that of the original population
◦ Founder effect – occurs when a small group of individuals is isolated from the larger population & the gene pool of
this splinter population does not reflect the source population
GENETIC DRIFT
OFFSRPRING
POPULATION A
POPULATION B
Bottleneck effect
Original
population
Bottlenecking
event
Surviving
population
Elephant seals
Founder effect

When a small group of individuals is isolated from the larger
population & the gene pool of this splinter population does not
reflect the source population.
Modes of Evolution:
Factors that disrupt HW-EQ
3. Gene flow
◦ Changes in allelic frequency due to the loss/gain of alleles
◦ Caused by emigration/immigration
4. Mutations
◦ Changes in allelic frequencies due to point mutations, chromosome rearrangements,
duplication, etc.
◦ Rate is low in animals & plants
◦ Rate is higher in viruses & bacteria b/c they have short life spans
Single Gene vs. Polygenic
Single gene:
◦ Only two phenotypes
◦ Often: either you have it or you
don’t
◦ Bar graph
Single Gene vs. Polygenic
Polygenic
◦ Many phenotypes
◦ Usually a range
◦ Bell shaped curve (normal
distribution)
3 Modes of Natural Selection
Directional selection – favors variants at one extreme
Disruptive selection – favors variants at both extremes
Stabilizing selection – favors intermediate variants