chapter 23~ microevolution- small changes in the genetics of populations
TRANSCRIPT
• Chapter 23~ Microevolution- small changes in the genetics of populations
Populations
• The Smallest Unit of Evolution• One common misconception about
evolution is that individual organisms evolve, in the Darwinian sense, during their lifetimes
• Natural selection acts on individuals, but populations evolve
• Genetic variations in populations– Contribute to evolution
Individuals are selected, but populations evolve• Population:
– a localized group of individuals belonging to the same species
• Species: – a group of populations whose
individuals have the potential to breed and produce fertile offspring
• Gene pool: – all of the genes in a population
at any one time
Population genetics
• Is the study of how populations change genetically over time
• Integrates Mendelian genetics with the Darwinian theory of evolution by natural selection
• Focuses on populations as units of evolution
Traits• Polymorphism -
different forms of traits– Anatomical/Structural
(form)– Physiological
(function) – Behavioral (response)
• Qualitative vs. Quantitative Traits
• Alleles- versions of genes that are inherited• Genotype is inherited not the phenotype• Mutation and sexual recombination produce
the variation that makes evolution possible• Two processes, mutation (creating alleles)
and sexual recombination (shuffling alleles)– Produce the variation in gene pools that
contributes to differences among individuals
Stable Gene Pools• Mendelian
inheritance– Preserves
genetic variation in a population
• In a given population where gametes contribute to the next generation randomly, allele frequencies will NOT change= genetic equilibrium
Generation1
CRCR
genotypeCWCW
genotypePlants mate
All CRCW
(all pink flowers)
50% CR
gametes50% CW
gametesCome together at random
Generation2
Generation3
Generation4
25% CRCR 50% CRCW 25% CWCW
50% CR
gametes50% CW
gametesCome together at random
25% CRCR 50% CRCW 25% CWCW
Alleles segregate, and subsequentgenerations also have three typesof flowers in the same proportions
• Mutations– Are changes in the nucleotide
sequence of DNA– Cause new genes and alleles to arise
Mutation types:
1. Lethal2. Neutral3. Beneficial
Variation Creators• Mutation rates
– Tend to be low in animals and plants– Average about one mutation in every
100,000 genes per generation– Are more rapid in microorganisms
• In sexually reproducing populations, sexual recombination– Is far more important than mutation
in producing the genetic differences that make adaptation possible
Hardy-Weinberg Equation
• p=frequency of one allele (A); • q=frequency of the other allele (a);
p+q=1 (p=1-q &
q=1-p)• p2=frequency of AA genotype; • 2pq=frequency of Aa plus aA genotype;• q2=frequency of aa genotype;
p2 + 2pq + q2 = 1
Hardy-Weinberg TheoremServes as a model
for a non-evolving population (equilibrium)
5 conditions:• 1- Very large
population size;• 2- No gene flow;• 3- No mutations;• 4- Random
mating;• 5- No natural
selection
Natural selection
• Natural selection is the primary mechanism of adaptive evolution
• Differential success in reproduction– Results in certain alleles being passed
to the next generation in greater proportions
• Fitness: – contribution an individual makes to
the gene pool of the next generation
• 3 types of Natural Selection:– A. Directional– B. Disruptive– C. Stabilizing
Natural Selection Types
• Directional selection– Favors individuals at one end of the
phenotypic range
• Disruptive selection– Favors individuals at both extremes
of the phenotypic range
• Stabilizing selection– Favors intermediate variants and
acts against extreme phenotypes
A Closer Look at Selection Types
• From the range of variations available in a population– Natural selection increases the
frequencies of certain genotypes, fitting organisms to their environment over generations
Sexual selection
• Sexual dimorphism: secondary sex characteristic distinction
• Sexual selection: selection towards secondary sex characteristics that leads to sexual dimorphism
Sexual Selection• Intrasexual selection
– Is a direct competition among individuals of one sex for mates of the opposite sex
• Intersexual selection– Occurs when individuals of one sex
(usually females) are choosy in selecting their mates from individuals of the other sex
– May depend on the showiness of the male’s appearance
Population variation
• Balanced Polymorphism: coexistence of 2 or more distinct forms of individuals (morphs) within the same population
• Geographical variation: differences in genetic structure between populations (cline)
Variation preservation• Prevention of natural
selection’s reduction of variation
• Balanced polymorphism Heterozygote advantage (hybrid vigor; i.e., malaria/sickle-cell anemia)
Microevolution
• Gene Flow: – genetic exchange
due to the migration of fertile individuals or gametes between populations (reduces differences between populations)
– Immigration vs. Emigration
Genetic drift• changes in the gene pool of a small
population due to chance (usually reduces genetic variability)
CRCR
CRCW
CRCR
CWCW CRCR
CRCW
CRCW
CRCWCRCR
CRCR
Only 5 of 10 plantsleaveoffspring
CWCW CRCR
CRCW
CRCR CWCW
CRCW
CWCW CRCR
CRCW CRCW
Only 2 of 10 plantsleaveoffspring
CRCR
CRCR CRCR
CRCRCRCR
CRCR
CRCR
CRCR
CRCRCRCR
Generation 2p = 0.5q = 0.5
Generation 3p = 1.0q = 0.0
Generation 1p (frequency of CR) = 0.7q (frequency of CW) = 0.3
Microevolution• The Bottleneck Effect:
– type of genetic drift resulting from a reduction in population
– Caused by natural disasters or human activity
– the surviving population is no longer genetically representative of the original population Original
population
Bottle-neckingevent
Survivingpopulation
• Understanding the bottleneck effect– Can increase understanding of how
human activity affects other speciesBottlenecking a population of organisms tends to reduce genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction.
The Evolutionary Enigma of Sexual Reproduction
• Sexual reproduction– Produces fewer reproductive offspring than
asexual reproduction, a so-called reproductive handicap
Asexual reproduction
Female
Sexual reproduction
Female
Male
Generation 1
Generation 2
Generation 3
Generation 4
Why Sex?
• If sexual reproduction is a handicap, why has it persisted?– It produces genetic variation that
may aid in disease resistance
Why Natural Selection Cannot Fashion Perfect Organisms
• Evolution is limited by historical constraints
• Adaptations are often compromises• Chance and natural selection
interact• Selection can only edit existing
variations