Chapter 13- How Populations Evolve

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

Biogeography

• Geographic distribution of species

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

Artificial selection

• Selective breeding of plants and animals

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

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