Download Evolution of Populations

Chapter 15 recap
Conditions for
natural
selection
 variation in population, limited resources.
 individuals are naturally selected
Conditions for
evolution
 variation in population, natural selection, LOTS of time
 populations evolve (NOT individuals)
Evolution of Populations
What Darwin
didn’t know
Biology
1930’s
Genetic
Variation
Sources of
genetic
variation
 how traits are passed betw/ generations
 how variation in pop’n appeared
 Mendel’s work was linked with Darwin’s theory
 genes control heritable traits (proteins!!)
 Watson & Crick
 showed molecular nature of DNA, mutations and
genetic variation
 many genes have at least 2 alleles (versions)
 individual can be heterozygous or homozygous
 genes code for proteins which determine traits
 mutation: change in DNA sequence
 may or may not cause change in trait
 random
 caused by chemicals or radiation in env’t
 may or may not affect fitness of individual
 sexual reproduction: gene shuffling in gametes
 independent assortment of chromosomes during meiosis
 crossing over during meiosis
 studied in a population
 group of individuals (same species) that can interbreed
 gene pool: all genes (all alleles) present in population
 relative frequency (%) of alleles in population
 number of times allele occurs in gene pool
 e.g. Person A is RR: 2 ‘R’ alleles in population
Evolution in
genetic terms
 any change in the frequency of alleles in a population
Natural
selection
 leads to changes in allele frequencies in a population
 change in allele frequency = evolution
Single gene
trait
 controlled by 1 gene with 2 alleles
 3 possible genotypes and 2 possible phenotypes
 e.g. brown lizard with red and black forms present
 red form more easily seen = less fit
 black form move faster = more fit
 change in allele frequency in population = evolution
 controlled by 2 or more genes each with 2 or more alleles
Polygenic trait
 many possible genotypes and phenotypes
 bell curve of phenotypes
 population phenotype distribution affected in 3 ways:
 directional selection:
 individuals at 1 end of curve have more fitness
 e.g. bird beaks: shortage of small seeds
 birds with larger beaks able to eat
large seeds and survive
 stabilizing selection:
 individuals near center of curve more fitness
 e.g. birth mass of human babies
 disruptive selection:
 individuals at ends of curve have more fitness
 e.g. bird beaks: medium size seed shortage
 birds with larger and smaller beaks have
an advantage.
 curve splits into 2 distinct phenotypes
 new species can form
Genetic Drift
 small populations: chance occurrences can change allele
frequency.
Founder
effect
 small groups of individuals leave and colonize new habitat
 diff’t allele frequencies than parent population
 “founders” alleles determine new population
frequencies.
 given enough time: new species form.
HardyWeinberg
principle
 how does evolutionary change operate?
 what are the conditions where there is NO evolution?
 genetic
equilibrium
 no change in allele frequencies = no evolution
 5 conditions must be met to keep equilibrium from
generation to generation.
1. mating must be random – all members of pop have equal
opportunity to produce offspring
(NO: males compete for females and females are picky)
2. populations must be large: >10,000 so no genetic drify
(NO: not all populations are that large)
3. no movement into or out of the population: no mixing of
gene pool with gene pool down the road
( NO: animals migrate)
4. no mutations: gene mutations = new alleles in the pop’n
(NO: mutations can happen at any time)
5. no natural selection: all genotypes must survive and
reproduce equally (no advantages)
(NO: variation exists in every population)
 in some populations some conditions are met some of the
time.
 if conditions not met = genetic equilibrium disrupted =change
in allele frequencies = evolution
Formation of Species
Speciation
Species
 formation of new species.
 group of organisms that:
 can breed and produce fertile offspring.
 shares a gene pool.
Reproductive
isolation
How do new species form?
 groups of organisms become different enough that they:
 can no longer breed.
 no longer share a gene pool.
Behavioral
isolation
 2 species don’t interbreed because they have different
courtship rituals
 e.g. bird songs
Geographic
isolation
 2 species don’t interbreed because they are physically
separated
 e.g. mountains, river, ocean, etc.
Temporal
isolation
 2 species don’t interbreed because they reproduce at
different times
 e.g. flowers pollinating at different times
Patterns of Evolution
Extinction
 extinction is a normal part of life on earth
 99% of all species that ever lived are extinct
 natural selection: competition for limited resources
 species that don’t adapt to env’t changes
don’t survive
 natural disasters
 collapse of entire food webs
Adaptive
radiation
 single species evolves into many different species
 ancient reptile evolves into dinosaur species
 ancient mammal evolves into many mammal species
Convergent
evolution
 unrelated organisms evolve with similar characteristics
 streamline body, flukes, tailfins
 shark (fish)
 penguin (bird)
 dolphin (mammal)
 seal (mammal)
Coevolution
 change over time in 1 species causes change in another
species
 flowers and pollinators
 plants and plant-eating insects
Punctuate
equilibrium
 long time periods in earth’s history where little change in
species occurs (equilibrium)
 brief time periods with lots of change in species
(evolution)