Download Chapter 23: Evolution of Populations / Lecture

Chapter 23: Evolution of Populations / Lecture notes
I.Population Genetics
A. Modern Evolutionary Synthesis
1.Mendel’s laws originally thought to contradict Gradualistic evolution
Darwin needed many minute gradual variations
Mendel’s work showed “either – or” variation
2.Population Genetics emerged in the 1930s
Focused on polygenic inheritance – provided the genetic explanation for gradual minute
variations in species.
3.The Modern Synthesis is born
Integrated genetics, evolution, paleontology, and taxonomy
Key architects: Theodosius Dobzhansky
Ernst Mayr
Emphasizes Natural Selection acting on whole populations
Gradualism
B. Populations and Gene Pools
1. Population – localized group of individuals of the same species\
Species – An interbreeding group of individuals
2. Differing populations of the same species may be geographically isolated
Boundaries may be sharp and distinct, or blurred with some exchange between
3. Gene Pool – Total genes in a population
Usually two or more alleles of each gene in the gene pool
These exist in relative frequencies (proportions)
If all the alleles of a gene are the same (all individuals are homozygous)
Then the gene pool is FIXED for that gene.
C. Hardy-Weinberg Theorem (1908)– Describes a non-evolving population
1.Definition – The relative frequencies of alleles in a gene pool remain constant over time
unless acted on by outside forces. Random fertilization and the events of meiosis have no
overall effect on allele relative frequencies (they balance out)
Copy diagram Pg. 448 Campbell.
2. This maintains Hardy-Weinberg Equilibrium
3. Hardy-Weinberg Equation – p2 + 2pq
+
q2 = 1
Frequency of RR + Frequency of Rr and rR + Frequency of rr = 1
With this, can calculate frequency of each allele if we know the genotype
Frequency.
4. Use in Health Sciences – 449 Campbell
5. Hardy-Weinberg equation fits well with Mendel’s discoveries.
6. Hardy-Weinberg Theorem Assumptions:
Very large population size (needed to cancel out chance genetic drift)
No migration (gene flow between populations may change frequencies
No net mutations (mutations may alter gene frequencies)
Random mating (sexual selection will favor certain alleles over others, changing
the relative frequencies)
No Natural Selection (If forces favor the survival of some alleles over others,
Gene frequencies will change).
Very rarely are all these assumptions true of a natural population
Therefore ------ we can expect changes in gene frequencies …… ei. Evolution.
II.Causes of MicroEvolution
Deviations from the assumptions of Hardy-Weinberg lead to changes, evolution, of gene
frequencies from generation to generation - MicroEvolution
Caused by Genetic Drift, Natural Selection, Gene Flow, and Mutation
Two main Causes: 1. Genetic Drift
2. Natural Selection
A. Genetic Drift – Change in allelic frequencies due to chance (Fig. 23.4)
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1. Consider flipping a coin 10 times, will it always yield 5 heads, 5 tails?
Simple chance will cause variation from the probability.
2. Smaller populations more susceptible to genetic drift ….. why?
3. The Bottleneck Effect – A disaster reduces a population significantly (Fig. 23.5)
That population rarely carries the exact allelic frequency of the original population
This small surviving population with frequencies varying from the original, then gives
rise to a new large population. This new pop. has changed from the original.
This new pop. also has less genetic variability – more vulnerable to extinctioin
Example – Cheetahs
4. The Founder Effect – A new colony is established by a small offshoot of the original
population.
The new founding group will not perfectly represent the gene frequencies of the
original population.
Their descendent population will then show differences from the original
B. Natural Selection – Differential reproductive success
C. Gene Flow – Flow of genes in and out of a population by migration of individuals
D. Mutation – Change in a gene code which may lead to a phenotypic change.
III. The Importance of Genetic Variation
A. Variation within and between populations
1. Variation within populations
Polymorphism – when a species population has two or more distinct forms of
a character. Each type is called a morph.
This applies only to discrete variation, not continuous variation
Measuring Variation
Gene diversity – Average percent of loci that are heterozygous
Nucleotide diversity – Percent of sites differing in nucleotide letters
Average difference between two individuals
2. Variation between populations
Geographic variation – Differences in gene pools between populations or subgroups
Cline – A graded change in a trait along a geographic axis (continuous variation)
Between isolated populations, discrete differences can be seen
B. Mutation and Sexual recombination
1. Brand new alleles only arise by mutation
These appear to be by chance
Only those affecting gametes will be passed on.
Most mutations are harmful
2. Favorable mutations
Very rare
A mutation is more likely to be favorable in a changing environment
3. Shorter life spans and generation time yields faster accumulation of mutations
4. Sexual Recombination – Shuffles the combinations of various traits
C. Diploidy and balanced polymorphism preserve variation
1. Being diploid, many recessive variations can hide for generations
This latent variation is protected from selection in heterozygotes
Maintaining a hidden pool of variation
2. Balanced Polymorphism – the ability of natural selection to maintain stable frequencies
of two or more forms.
Methods:
Heterozygote advantage (Ex. – Sickle cell anemia)
Frequency Dependent Selection – Survival and reproduction of one morph
declines if its frequency becomes too common.
3. Neutral Variation – Variation that offers no selective advantage or disadvantage.
IV. A Closer Look at Natural Selection
A. Evolutionary Fitness
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1. Darwinian Fitness – The contribution an organism makes to the next generation’s gene
pool relative to others in the population.
Can be quantified for specific genes – Relative Fitness
2. Survival alone doesn’t equal “Reproductive Fitness” Reproductive fitness is the key, and
a commonly used term today.
B. Types of Selection
Directional Selection – Causes a directional shift or change in a characteristic
Most common during times of environmental change
Diversifying selecton – Both extremes favored more than the middle range.
Occurs when environment is highly varied.
Stabilizing Selection – Acts against the extremes. Reduces variation maintaining current
Phenotypes.
C. Natural selection and sexual reproduction
Asexual reproduction can produce more offspring
Why then has sexual reproduction increased among organisms
Increases genetic variation
D. Sexual Selection
1. Leads to significant differences between the sexes. More differences than are required
by the simple biology of reproduction.
These differences called – Sexual Dimorphism
2. Caused by Sexual Selection
Intrasexual Selection – Within the same sex
Competition among individuals for mates
Intersexual Selection – Between the two sexes. More common form.
Called Mate choice.
Could sexually advantageous traits be disadvantageous in other contexts?
This leads to “trade-offs”
E. Natural Selection cannot lead to perfectly adapted organisms.
Beware “Adaptationist” Assumptions –
Reasons
Evolution is limited by historical constraints
Adaptations are often compromises
Not all evolution is adaptive
Selection can only favor existing variations
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