chapter 13- how populations evolve
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Chapter 13- How Populations Evolve
• Artificial selection• Biogeography• Bottleneck effect• Comparative anatomy• Comparative embryology• Descent with modification• Directional selection• Diversifying selection• Evolution• Evolutionary adaptations• Fitness• Fossil record• Founder effect• Gene flow• Gene pool
• Genetic drift• Hardy-Weinberg equilibrium• Homologous structures• Microevolution• Modern synthesis• Molecular biology• Mutation• Natural selection• Neutral variation• Population• Population genetics• Sexual dimorphism• Species• Stabilizing selection
Evolution• Change over time
• Darwin called evolution- descent with modification until last paragraph of The Origin of Species
Evolutionary adaptations
• Inherited traits that increase an organism’s ability to survive and reproduce in a particular environment
Fossil record
• Ordered specimens in layers of sedimentary rock– Gives an idea of when certain adaptations appear– Younger fossils are on top of older ones
Comparative anatomy
• Comparing body structures– Homologous structures- features with similar structure
but a different function• Due to similar ancestry
Comparative embryology
• Study of structures that appear during development– Shows
common ancestry in vertebrates
Molecular biology
• Study of molecular basis of genes and gene expression– Universal genetic code= life is related– Similar DNA and proteins= close relationships
Darwin’s Theory points:
• Overproduction
• Competition
• Variety
• Adaptations
• Natural selection
• Speciation
Natural selection
• Unequal reproductive success– Those that are better
adapted to the env are more likely to survive, reproduce
– Mechanism for evolution
Natural selection in action:
– Galapagos finches- avg. beak size differs in wet vs. dry years
– Insecticide resistance- resistant insects survive and reproduce, next generation more are resistant
– Antibiotic resistance- similar to insecticide resistance
– Industrial melanism- moth coloring due to industrialization in England
Population
• Smallest unit that can evolve• Group of individuals of the same species living in
the same place at the same time• Population genetics- science of genetic change
in populations
Population vocab
– Modern synthesis- theory of evolution that includes genetics
– Species- group of populations whose individuals can reproduce and produce fertile offspring
Gene pool
• Collection of genes (alleles) in a population at any given time– Frequency of alleles can change
• Microevolution- change in gene pool over a few generations
• Gene pools will remain constant if the population isn’t evolving – IMPOSSIBLE!!
Hardy-Weinberg Equilibrium
– Frequency of each allele in the gene pool will remain constant unless acted on by other agents
– p2 + 2pq + q2 = 1– freq. WW + freq. Ww + freq. ww = 1– Used in human genetics to estimate how
many people carry alleles for certain inherited diseases
5 conditions for H-W equilibrium
• Large population• Pop. is isolated, no migration of individuals or gametes in
or out• Mutations don’t alter gene pool• Random mating• All individuals are equal in reproductive success; natural
selection does NOT occur
• IMPOSSIBLE
Causes of microevolution
• Natural selection• Genetic drift- change in gene pool due to change
– Bottleneck effect- random, drastic reduction of population size(ex: flood, earthquake)
– Founder effect- colonization of a new location by a small # of individuals (ex: birds fly to new island)
Causes of microevolution con’t
• Gene flow- genes move into or out of a population (by fertile individuals or gametes)
• Reduces genetic differences between pop.’s
• Mutation- change in DNA, causes new alleles– ** ultimate source of genetic variation– Raw material for evolution
Variation
• Prevalent in most populations• Especially sexually reproducing species• Many traits are the result of several genes• Polymorphic- characteristic with 2 or more forms
– ABO blood type• Can also happen in a cline- change in characteristic due
to geography– As latitude increases, mammal/bird size increases
• Is measured in gene or nucleotide diversity
Variation con’t
• Produced by sexual recombination and mutation– Most of the time it’s harmful, occasionally it creates a better
variation– Asexual org’s- mutation generates– Sexual org’s- sexual recombination generates
How natural selection affects variation
• Heterozygote advantage- have greater reproductive success– Ex: malaria/sickle-cell
• Balanced polymorphism- ability of natural selection to maintain 2 or more phenotypes
• Neutral variation- no selective advantage (human fingerprints)
• Endangered species have reduced variation– Why? Small gene pool
Fitness
• Relative contribution to gene pool of next generation– More fit- reproduce more have well-adapted young that
reproduce– Fittest individual- passes most genes to next generation
Outcomes of natural selection– Stabilizing- favors intermediates
• Occurs in stable env’s• Prevails most of the time in
populations
– Directional- favors an extreme phenotype
• Occurs during env. change or when there is migration to new habitat
– Diversifying- favors both extremes
• Can lead to balanced polymorphism• Occurs when habitat has 2 different
env conditions
Natural selection DOES NOT produce perfection, due to:
• Historical constraints– Need to use already existing structures
• Compromising for adaptations– Blue-footed booby- fast in water, clumsy on land
• Not all evolution is adaptive– Chance happens, ex: bottleneck (could remove good genes)
• Selection is only on existing variations
Sexual selection may produce sexual dimorphism
• Sexual dimorphism- distinction between secondary sex characteristics in males and females– Plumage, antlers, dances, manes– Intrasexual selection- fighting for females– Intersexual selection- mate choice, impressing other sex by
dance, feathers etc• Many times it’s not advantageous to stick out to predators but it’s necessary
for passing on genes
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