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Processes of Evolution
Chapter 18
2Processes of Processes of EvolutionEvolution
Microevolution
Microevolution pertains to the evolutionary changes within a population.
Populations are all the members of a single species occupying a particular area.
Population genetics - study of genetic changes within a population
- The various alleles at all the gene loci in all individuals make up the gene pool of the population.
- It is customary to describe the gene pool of a population in terms of gene frequencies.
3Processes of Processes of EvolutionEvolution
Gene Frequencies
Suppose in a Drosophila population there are:36% flies homozygous dominant for long wings
48% heterozygous for long wings 16% homozygous recessive for short wings
In population of 100 flies there would be:36 LL, 48 Ll and 16 ll
Number of L alleles would be (2 X 36) + 48 = 120Number of l alleles would be (2 X 16) + 48 = 80
There are 120 L alleles and 80 l alleles
4Processes of Processes of EvolutionEvolution
Gene Frequencies
To determine frequency of each allele:Calculate its percentage from total # of alleles in population.
For dominant allele L = 120/200 = 0.6For recessive allele l = 80/200 = 0.4
The sperm & eggs produced by this population should have the same frequencies. You can calculate the expected ratios of genotypes in the next generation by using a Punnett square.
5Processes of Processes of EvolutionEvolution
Gene Frequencies
Punnett Squareeggs
0.6 L 0.4 lsperm 0.6 L 0.36LL 0.24 Ll
0.4 l 0.24 Ll 0.16 ll
Genotypes frequencies = 0.36 LL + 0.48 Ll + 0.16 ll = 1
6Processes of Processes of EvolutionEvolution
Gene Frequencies
Note that the frequency of each allele in the next generation is the same as it was in the previous generation.
Sexual reproduction alone cannot bring about a change in allele frequencies.
Also, the dominant allele does not increase from one generation to the next. It does NOT become more common.
7Processes of Processes of EvolutionEvolution
Hardy-Weinberg
The Hardy-Weinberg principle - mathematics:
p + q = 1 p2 + 2pq + q2 = 1
p = frequency of dominant allele
q = frequency of recessive allele
p2 = frequency of homozygous dominant individuals
q2 = frequency of homozygous recessive individuals
2pq = frequency of heterozygous individuals
8Calculating Gene Pool FrequenciesUsing the Hardy-Weinberg Equation
9Calculating Gene Pool FrequenciesUsing the Hardy-Weinberg Equation
Go to Worksheet Problems Now
Do Practice Problems 18.1 (p. 303) now
10Processes of Processes of EvolutionEvolution
Hardy-Weinberg
The Hardy-Weinberg principle:
Allele frequencies in a population will remain constant assuming five conditions are met:
No Mutations - alleles do not change
No Gene Flow - no migration in or out of population
Random Mating - individuals pair by chance
No Genetic Drift - populations are large such that gene frequencies don’t change by chance alone
No Selection - particular genotypes not selected
11Processes of Processes of EvolutionEvolution
Hardy-Weinberg
In real life, the Hardy-Weinberg conditions are rarely, if ever, met.
Thus, allele frequencies in a population DO change from one generation to the next.
The significance of the Hardy-Weinberg principle is that it tells us what factors cause evolution:
Those that violated the conditions listed.
- Evolution can be detected by noting any deviation from a Hardy-Weinberg equilibrium.
12Processes of Processes of EvolutionEvolution
Microevolution
The accumulation of small changes in the gene pool over a relatively short period of time is called microevolution.
Example:
Industrial Melanism:• Pepper moths in Great Britain - Before Industrial Revolution, light-colored moths more
common than dark-colored moths (< 10% dark) - After Industrial Revolution, dark-colored moth more common than light-colored moths. (By 1950s > 80% dark & 94% dark in 1960) - After Clean Air Act of mid-1950s: By 1994 only 19% dark
13Industrial Melanism and Microevolution
When vegetation is light-colored, dark moths are seen & eaten by birds
When vegetation is dark due to pollution, light
moths are seen & eaten by birds
14Processes of Processes of EvolutionEvolution
Causes of Microevolution
1. Genetic MutationsThe raw material for evolutionary changeProvides new combinations of allelesSome might be more adaptive than othersMany traits are polymorphic - - Two or more distinct phenotypes are present in
a population. - Examples: Human freckles
ABO blood types
15Processes of Processes of EvolutionEvolution
Causes of Microevolution
2. Gene Flow (Gene Migration)Movement of alleles between populations when:
Gametes or seeds (in plants) are carried into another population
Breeding individuals migrate into or out of population
Gene flow can increase variation in a population by introducing novel alleles
Continual gene flow makes gene pools similar & reduces differences among populations. This can prevent speciation from happening.
16Gene Flow
There is interbreeding between populations; thus gene flow occurs
among the populations. So they are sub-species
of species Elaphe obsoleta
17Processes of Processes of EvolutionEvolution
Causes of Microevolution
3. Nonrandom MatingWhen individuals do not choose mates randomly - Inbreeding: •Mating with relatives. Increases frequency of recessive abnormalities.
Assortative mating: Individuals select mates with their phenotype Individuals reject mates with differing phenotypeCauses population to subdivide into two phenotypic classes
Homozygotes increases; heterozygotes decrease
18Processes of Processes of EvolutionEvolution
Causes of Microevolution
3. Nonrandom Mating (cont’d) Sexual selection:
Males compete for the right to reproduceFemales choose to mate with males possessing a particular phenotype
Example:
Elaborate tail of peacocks may be due to female peahens choosing males with grander tails.
All of these mechanisms can cause an increase in homozygotes
19Processes of Processes of EvolutionEvolution
Causes of Microevolution
4. Genetic DriftRefers to changes in allele frequencies of a gene pool due to chance.
More likely to have a large effect on smaller populations where the sampling error is a larger part of the population.
Can cause the gene pools of two isolated populations to become dissimilar
Some alleles are lost and others become fixed (unopposed)
20Genetic Drift
21Processes of Processes of EvolutionEvolution
Genetic Drift
Bottleneck EffectSometimes a natural disaster, or humans, might cause a near extinction event.
This prevents a majority of individuals, and their genotypes, from entering the next generation
Example: - Cheetahs: Extreme genetic similarity is believed to be due to a bottleneck
They suffer from infertility because of intense inbreeding after that time
22Processes of Processes of EvolutionEvolution
Genetic Drift
Founder EffectWhen rare alleles occur at higher frequency in a population isolated from general population
Happens when a new population is started from just a few individuals
The alleles carried by population founders are dictated by chance
Examples: - Amish of Lancaster County, PA have high
incidence of dwarfism & polydactylism - Lake Maracaibo, Venezuela has high incidence
of Huntington disease
23Founder Effect
Rare recessive form of dwarfism linked to
polydactylism is very common in Amish of
Pennsylvania
1/14 individuals carries recessive allele
24Processes of Processes of EvolutionEvolution
Natural Selection
Process that results in adaptation of a population to the biotic and abiotic environments
Requires:
Variation - The members of a population differ from one another
Inheritance - Many differences are heritable genetic differences
Differential Adaptiveness - Some differences affect survivability
Differential Reproduction – Some differences affect likelihood of successful reproduction
25Processes of Processes of EvolutionEvolution
Natural Selection
Results in:A change in allele frequencies of the gene pool Improved fitness of the population
Natural selection is the major cause of microevolution
26Processes of Processes of EvolutionEvolution
Types of Selection
Most traits are:• polygenic - controlled by more than one pair of
alleles located at different loci• variations in such traits result in a bell-shaped
curves
Three types of selection occur:1. Directional Selection2. Stabilizing Selection3. Disruptive Selection
27Processes of Processes of EvolutionEvolution
Types of Selection
1. Directional SelectionOccurs when one extreme phenotype is favored The curve shifts in one direction Examples: •When bacteria become resistant to antibiotics •Human struggle against malaria Plasmodium & mosquito evolution of resistance to treatments
•Gradual increase in size of horse
28Directional Selection
29Processes of Processes of EvolutionEvolution
Types of Selection
2. Stabilizing Selection
Occurs when an intermediate phenotype is favored
Can improve adaptation of population to a relatively constant environment
The peak of the curve increases and tails decrease
Examples:
•When human babies with low or high birth weight are less likely to survive
•Swiss starlings clutch size
30Stabilizing Selection
Swiss starlings optimal clutch size is 4-5 eggs
31Processes of Processes of EvolutionEvolution
Types of Selection
3. Disruptive Two or more extreme phenotypes are favored over any intermediate phenotypes
The curve has two peaks
Examples:
•When Cepaea snails vary because a wide geographic range causes selection to vary
32Disruptive Selection
Dark shells more prevalent in forested
areas
Light shells near low-lying vegetation
33Processes of Processes of EvolutionEvolution
Maintenance of VariationsGenetic variability
Populations with limited variation may not be able to adapt to new conditions & become extinct
Maintenance of variability is advantageous to population
Only exposed alleles are subject to natural selection:
● Thus, heterozygotes can be a protector of recessive alleles that might otherwise be weeded out.Allows even lethal alleles to remain in population at low frequencies virtually forever
34Processes of Processes of EvolutionEvolution
Maintenance of Variations
Heterozygote Advantage:Lethal recessive alleles may confer advantage to heterozygotes Sickle cell disease is detrimental in homozygote However, heterozygotes more likely to survive malaria than homozygous dominants because malaria parasite is unable to live in their red blood cells while it destroys the RBCs of homozygotes.
Sickle cell allele occurs at higher than expected frequency in malaria prone areas
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35Sickle-cell Disease
36Processes of Processes of EvolutionEvolution
Macroevolution
Macroevolution
- Any evolutionary change at or above the level of the species
● Speciation
- Splitting of one species into two or more species
- Transformation of one species into a new species over time.
37Processes of Processes of EvolutionEvolution
Definition of a Species
Morphological
Can be distinguished anatomically
Physical traits differ
Specialist decides what criteria probably represent reproductively isolated populations
Most species described this way
38Processes of Processes of EvolutionEvolution
Species Definitions
Biological Species Concept
A group of populations that can breed among themselves to produce fertile offspring
Are reproductively isolated from other such populations; unable to reproduce with members of other groups
The organisms share a gene pool
Very few actually tested for reproductive isolation
Cannot be applied to asexual organisms or those only known from the fossil record
39Biological Species Definition
These three species of flycatchers are
reproductively isolated since they do not
reproduce with each other
40Processes of Processes of EvolutionEvolution
Reproductive Isolating Mechanisms
Reproductive isolating mechanisms are any structural, functional or behavioral characteristics that prevent successful reproduction from occurring between different groups of organisms.
Two general types: Pre-zygotic Mechanisms - Discourage attempts to mate ● Post-zygotic Mechanisms - Prevent hybrid offspring from developing or breeding
41Processes of Processes of EvolutionEvolution
Prezygotic Mechanisms
1. Habitat Isolation
- Occupy different habitats & are less likely to meet & reproduce
2. Temporal Isolation
- Species live in same habitat but reproduce at different times
3. Behavioral Isolation
- Species have their own courtship rituals
- Firefly flashes; cricket chirping; chemical signals
42Temporal Isolation
43Processes of Processes of EvolutionEvolution
Prezygotic Mechanisms
4. Mechanical Isolation
- Reproductive parts are incompatible
5. Gamete Isolation
- If gametes meet, they do not fuse to become a zygote.
44Processes of Processes of EvolutionEvolution
Postzygotic Mechanisms
1. Zygote Mortality - Hybrid zygote might be created but dies2. Hybrid Sterility - Hybrid zygotes develop into sterile adults - Example: Mule is a cross between a horse & a donkey.
They are usually sterile
3. Reduced F2 Fitness
- If hybrids can reproduce, their offspring cannot.
45Processes of Processes of EvolutionEvolution
Two Modes of Speciation
1. Allopatric Speciation
Two geographically isolated populations of one species
Become different species over time
Can be due to differing selection pressures in differing environments
Examples:
California salamanders separated by Central Valley
Green iguanas in Galapagos Islands are thought to be ancestors of marine & rhinoceros iguanas
46Allopatric Speciation
47Processes of Processes of EvolutionEvolution
Two Modes of Speciation
2. Sympatric Speciation
One population develops into two or more reproductively isolated groups
No prior geographic isolation
Examples:
Tetraploid hybridization in wheat (polyploidy)Results in self fertile species
Reproductively isolated from either parental species
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48Processes of Processes of EvolutionEvolution
Adaptive Radiation
When many new species evolve from a single ancestral species.
Occurs when members of species become adapted to the different environments
This is an example of allopatric speciation
Examples:1. Hawaiian honeycreepers 2. Galapagos finches
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