Evolution: the change in allele frequency in a population over time
- Traits vary among individuals
- Individuals with certain traits reproduce and have more offspring
NOT forward thinking
(organisms are not adapted to future conditions)
Evolving Resistance
Mutation
Gene Transfer
Creates new genetic diversity by creating new alleles
Some new alleles are beneficial, others aren't
Everytime DNA replicates (binary fission), there could be a mutation, creating new alleles
New alleles are "transferred" by bacteria
Results in new genetic variation
Results in new genetic variation
4 criteria for natural selection:
(a) They have traits that make it more likely that they will survive
ONLY evolutionary process that leads to adaptation
Increases, decreases, or maintains genetic diversity
Natural Selection
Favors traits that enables an individual to survive
Interplay between an organism's traits & its environment determines what traits will predominate in any population
Increases genetic diversity
Decreases genetic diversity
Increases genetic diversity
Patterns in nature:
Patterns are explained by multiple evolutionary processes
Patterns:
What you measure
"the what"
Processes:
The process explains the pattern
"the why"
2 cases of natural selection:
Case 1. White Sands Lizards
Case 2. Darwin's Finches
Individuals evolved cryptic coloration in different habitats
Mutation:
mutation in mc l r gene
Selection:
Fitness (survival & reproduction) is higher for those who match the sand (background matching)
Occurs in populations, not individuals
Individuals don't evolve
Consistent differences in environment, habitats, and food resources among islands led to differences in beak morphology
Fitness:
Survival & reproduction, resulting in offpsring
"how well an individual does"
Factors related to fitness:
Natural selection acts on the phenotype & results in the evolutionary change in the genotype
Organisms can be fit in 1 environment and not in another
When individuals with a certain PHENOTYPE have more offspring than an individual with a different phenotype
4 modes of natural selection
Each of these modes:
Directional Selection
Disruptive Selection
Stabilizing Selection
Balancing Selection
Works against intermediate size (favors extreme)
Favors intermediates
Favors one extreme
Shifts the mean
Genetic variation decreases
Genetic variation increases
Maintains average
Genetic variation decreases
No single phenotype is favored in all populations of a species at all times
Genetic variation is maintained
Favors traits that enable an individual to mate
Nonrandom mating
If there is heritable variation in a trait, then successful variants will become more common over time
Sexual Selection
Fundamental Asymmetry of Sex
Male:
Female:
click to edit
Male fitness depends on the # of mates
Female fitness depends on ability to gain resources to produce healthy offspring
Intrasexual
Intersexual
Sexual Dimorphism:
Any trait that differs between males and females
Primary Sexual Traits
Sex organs (directly related to reproduction)
Secondary Sexual Traits
Traits that differ between the sexes, but are NOT related directly to fertilization of zygote (antlers, behavior, big horns)
In an evolutionary sense, failing to mate is the same as dying young
Predictions of Sexual Selection
- Males should be willing to mate with any female
- Increases attractiveness (to females)
- Increases success in male-male competition
What males do to "chosen":
Mutation
mutation to DNA
Introduces new alleles in a population
Change in the DNA sequence (chromosome changes)
Random with respect to fitness
(can be deleterious, beneficial, or benign)
New mutations are likely to have negative effect on fitness & should be eliminated by selection
When mutations are beneficial, it will increase in frequency if the individuals that have that mutation produce MORE offspring
Mutation tends to have bigger effects on oranisms with short generation times (bacteria)
Without mutation, there would be no genetic variation & evolution would STOP
Gene Flow
(Migration & Mating)
The movement of alleles from one population to another
Can be random with respect to fitness
Migrating individuals don't know whether it's adapted to this environment (could be more or less fit)
Causes allele frequencies in the 2 populations to be more alike/less different
(homogenization)
Advantageous traits become more common in a population over time
Changes allele frequencies of other alleles
Genetic Drift
Occurs when an individual leaves one population, joins another & breeds
In natural populations, chance events change allele frequencies
Fertilization
Random fluctuations in the number of alleles in a population. An allele may increase/decrease by chance over time
More pronounced in small populations than in large populations
Random with respect to fitness
Founder Effect
Bottleneck Event
Immigrants establish new population
New population is likely to have different allele frequencies than the source population, by chance
A sudden reduction in the number of (different) alleles in a population
Loss of alleles & change in allele frequencies due to drift
Larger populations are unlikely to change this quickly
1 of 2 alleles are randomly passed from parent to offspring during fertilization
NOT random with respect to fitness
Sperm is "cheap" & eggs are "expensive"
Non-Random Mating
DOES NOT lead to change in allele frequencies
No evolution
Leads to change in allele frequencies
In nature, mating IS NOT RANDOM with respect to sexual traits
Females/males choose a mate based on that trait
Sexual Selection
Genotype frequencies change
Inbreeding
The mating between relatives
Consequences of Inbreeding:
Selfing:
Relatives share a common ancestor
Inbreeding increases the rate at which natural selection eliminates deleterious alleles from a population
Hardy and Weinberg
- Frequency of AA genotype is pp or p^2
- Frequency of Aa or aA genotype is 2pq
- Frequency of aa genotype is qq or *q^2
Allele & genotype frequencies equal 1 (or 100%)
p + q = 1
p^2 + 2pq + q^2 = 1
*or*
AA + 2Aa + aa = 1
This is NOT realistic
If q allele = x, what is frequency?
(Chart on Slide 27)
x = 0.1
frequency = 0.1*0.1 = 0.01 = 1%
x = 0.2
frequency = 0.2*0.2 = 0.04 = 4%
x = 0.3
frequency = 0.3*0.3 = 0.09 = 9%
x = 0.4
frequency = 0.4*0.4 = 0.16 = 16%
x = 0.5
frequency = 0.5*0.5 = 0.25 = 25%
x = 0.6
frequency = 0.6*0.6 = 0.36 = 36%
x = 0.7
frequency = 0.7*0.7 = 0.49 = 49%
x = 0.8
frequency = 0.8*0.8 = 0.64 = 64%
x = 0.9
frequency = 0.9*0.9 = 0.81 = 81%
A null hypothesis: "allele frequencies in a population do not change over time"
as long as there is:
No selection: all individuals survive & reproduce
No genetic drift: infinitely large population size (no getting lucky)
No mutation: no new alleles
No gene flow: no allele gained or lost due to migration and mating
No sexual selection: random mating
Homology:
Similarity due to common descent
Behavior
Genetic
Structural
Development
Can study mutation in bacteria to understand mutation (caused by chemicals) in humans
Test drugs on mice before testing on humans
Use model organisms to identify genes of interest
Similarity in adult morphology
Similarities exist because we share a common fish ancestor
These structures exists because we share a common fish ancestor
Humans have lost the tail & gill pouch once born
Ex: cats bury their poop
Acclimate vs. Adapt
No change in alleles
Does not result in evolution
Transient change in an individual
Acclimate
Change in alleles in population
Results in evolution
Adaptation
Speciation:
The process of forming new species
Biological Species Concept: Focuses on the ability to exchange genes
Phylogenetic Species Concept: Based on reconstructing the evolutionary history of populations
Barriers to successful reproduction are called reproductive isolating mechanisms:
Prezygotic isolating mechanisms
Mechanisms that prevent formation of a zygote
Postzygotic isolating mechanisms
Mechanisms that prevent development into an adult
Individuals are prevented from mating
Individuals mate, but the "hybrid" offspring are often sterile, have low fitness or don't survive
PRE-reproduction barriers:
Obstacles to mating or to fertilization if mating occurs
Temporal Isolation
Behavioral Isolation
Ecological Isolation
Mechanical Isolation
Geographic Isolation
Gametic Isolation
Hybrid Sterility
Natural/sexual selection acts against hybrids
Hybrid Inviability
Considers populations to be evolutionarily independent if they are reproductively isolated from each other
Criticisms of biological species concept:
Form a distinct phylogenetic lineage (monophyletic group)
Disadvantages of the phylogenetic concept:
These requirements are VIOLATED if there IS:
For speciation to occur, gene flow must stop
Speciation is most likely to occur in geographic isolation
Examples of Geographic Isolation
2-Part Process
(a) They begin to become different through selection, drift, and mutation
Ocean Barrier
Fragmentation
River Barrier
STEP 1: Genetic Isolation
STEP 2: Genetic Divergence/Reproductive Isolation
STEP 3: Speciation
NO GENE FLOW
Allopatric
Different homeland
The isolation of populations begins when populations are NOT in physical contact
Sympatric
Same homeland
One species splits into 2 at a single locality despite never being geographically separated
Dispersal:
Individuals move into a new habitat, colonize & establish a new population
Vicariance:
Physical splitting of habitat
Disruptive selection
2 phenotypes are favored through disruptive selection
Those 2 phenotypes are so different that they are not recognized as mates & reproductive isolation evolves
Reinforcement
Initially incomplete isolating mechanisms are reinforced by natural selection until they are complete
2 populations may only be partially reproductively isolated
Selection acts to reinforce the distinction between 2 species & to prevent gene flow
Species-recognition cues:
How an individual can recognize a mate from the same species (song, color, behavior, smell)
These cues become more obvious when 2 distinct populations co-occur (are sympatric)
Adaptive Radiation
The rapid diversification of a lineage that results in many closely related species with a wide range of adaptations
3 Features of Adaptive Radiation:
Ecological Opportunity:
New resource is colonized by a species, and the descendants survive in a variety of habitats. Through selection, drift, and mutation, new species arise
Key Innovations:
Trait that allows descendants to live in new areas, exploit new resources & move in new ways
Examples:
