Transcript Population Genetics 16
Gene Pool
- all alleles of all individuals in a population species: interbreed & produce fertile offspring a shared gene pool
The Gene Pool
Different species do NOT exchange genes by interbreeding Different species that interbreed often produce sterile or less viable offspring e.g. Mule
Populations
- A group of the same species living in an area No two individuals are exactly alike
(variations)
More
Fit
individuals survive & pass on their traits
Adaptation
Process when a population becomes better suited to its environment finches had to evolve in order to FIT into environment Populations living in different places become increasingly different as each becomes adapted to its own environment
Speciation
Process of species formation One species may split into 2 or more species A species may evolve into a new species Requires very long periods of time
At Shambala Preserve, not far from Hollywood, Patrick the liger looks ready for his close-up. The preserve, run by
The Birds
star Tippi Hedren, bills itself as "a haven for endangered exotic big cats." Though few ligers exist, they may not exactly be endangered, since they are not thought to exist in the wild. It's a matter of geography and genetics: Lions generally live in Africa, tigers in Asia. Plus, interspecies breeding tends to result in "diminished fitness of the offspring," according to Ronald Tilson, director of conservation at the Minnesota Zoo in Apple Valley.
• Read "Ligers Make
Dynamite
Leap Into Limelight" •
—Photograph © by Bill Dow/Shambala Preserve
Divergent evolution
- Related populations become less similar as they respond to different environments Accumulate differences Coevolution – process when 2+ species change in response to each other E.g. plants & birds
Homologous structures – feature that share a common ancestor different function, similar structure E.g. whale flipper & human arm Analogous structure – feature that has same function with similar structure But very different internal anatomy & embryological development E.g. bird wing and moth wing
Vestigial structures – structure that no longer serves an important function E.g. tail bone, wisdom teeth, appendix
Convergent evolution
occurs when the environment selects similar phenotypes but ancestral type is very different E.g. sharks & porpoises (fish & mammal)
Causes of variation
Influenced by environment & heredity Variations in genotype by: Mutation Recombination Random of genes during meiosis fusion of gametes
Changes in equilibrium due to…
Immigration – movement INTO a population Emigration – movement OUT of a population
Genetic drift
change in gene pool of small population due to lack of contributing organism e.g. Failure to produce offspring or too many offspring significant effect of random events or chance
4 types of natural selection
Stabilizing selection – individuals with average traits – fittest E.g. lizard size Directional selection – environment ‘directs’ individuals with extreme trait to be fittest E.g. anteater tongue Disruptive selection – individuals with either extreme are fittest – it ‘disrupts’ the norm E.g. limpet shell color Sexual selection – female attraction to male E.g. peacock
Isolating mechanisms
Geographic isolation – physical separation of members of population MOST COMMON way for new species to form E.g. canyon, climate Reproductive isolation – barriers to successful breeding between groups in population E.g. sterile offspring, death before birth, mating times
Rate of speciation
Gradual formation? gradualism punctuated equilibrium – sudden shift in form Takes a few thousand years instead of a few million
Modern Synthesis Theory
Combines Darwinian selection and Mendelian inheritance Population genetics study of genetic variation within a population Emphasis on quantitative characters
Modern Evolutionary Thought
Modern Synthesis Theory
(1940s) – comprehensive theory of evolution Fisher & Wright Previously many did not accept that Darwin’s theory of natural selection could drive evolution
A. Fisher S. Wright
Modern Synthesis Theory on evolution
GENES are responsible for the inheritance of characteristics POPULATIONS, not individuals, evolve due to natural selection & genetic drift SPECIATION due to the gradual accumulation of small genetic changes
Microevolution
Changes occur in gene pools mutation, natural selection, genetic drift, etc.
due to Gene pool changes cause more VARIATION in individuals in the population E.g.: Bacteria becoming unaffected by antibiotics (resistant)
Gene Pools
If all members of a population are homozygous for a particular allele, then the allele is fixed in the gene pool.
The Hardy-Weinberg Theorem
Used to describe a non-evolving population.
Shuffling of alleles by meiosis and random fertilization have no effect on the overall gene pool.
Natural populations are NOT expected to actually be in Hardy-Weinberg equilibrium.
Assumptions of the H-W Theorem
1.
2.
3.
Large population size - small populations can have chance fluctuations in allele frequencies ( storm).
e.g.
, fire, No migration - immigrants can change the frequency of an allele by bringing in new alleles to a population.
No net mutations - if alleles change from one to another, this will change the frequency of those alleles
Assumptions of the H-W Theorem
3.
4.
Random mating - if certain traits are more desirable, then individuals with those traits will be selected and this will not allow for random mixing of alleles.
No natural selection - if some individuals survive and reproduce at a higher rate than others, then their offspring will carry those genes and the frequency will change for the next generation.
Hardy-Weinberg Equilibrium
The gene pool of a non-evolving population remains constant over multiple generations; i.e.
, the allele frequency does not change over generations of time.
The Hardy-Weinberg Equation:
1.0 = P 2 + 2P q + q 2
where P 2 = frequency of PP genotype 2P q q 2 = frequency of Pq plus qP genotype = frequency of qq genotype
P q
1.0 = P
2
+ 2P
q
+
q 2
P PP Pq q Pq qq
1 : 2 : 1
Directional selection 29
Diversifying selection favors extreme over intermediate phenotypes.
- Occurs when environmental change favors an extreme phenotype.
Stabilizing selection favors intermediate over extreme phenotypes.
- Reduces variation and maintains the current average.
- Example = human birth weights.
30
31
Natural selection maintains sexual reproduction -Sex generates genetic variation during meiosis and fertilization. -Generation-to-generation variation may be of greatest importance to the continuation of sexual reproduction.
-Disadvantages to using sexual reproduction: Asexual reproduction produces many more offspring. -The variation produced during meiosis greatly outweighs this disadvantage, so sexual reproduction is here to stay.
32
Sexual selection leads to differences between sexes a. Sexual dimorphism is the difference in appearance between males and females of a species.
-Intrasexual selection is the direct competition between members of the same sex for mates of the opposite sex.
-This gives rise to males most often having secondary sexual equipment such as antlers that are used in competing for females.
-In intersexual selection (mate choice), one sex is choosy when selecting a mate of the opposite sex.
-This gives rise to often amazingly sophisticated secondary sexual characteristics; e.g.
, peacock feathers.
33
34
Five Agents of Evolutionary Change
Mutation Mutation rates are generally so low they have little effect on Hardy-Weinberg proportions of common alleles.
ultimate source of genetic variation Gene flow movement of alleles from one population to another tend to homogenize allele frequencies 35
Five Agents of Evolutionary Change
Nonrandom mating assortative mating individuals mate - phenotypically similar Causes frequencies of particular genotypes to differ from those predicted by Hardy-Weinberg.
36
Five Agents of Evolutionary Change
Frequencies of particular alleles may change by chance alone.
important in small populations founder effect allelic pool) - few individuals found new population (small bottleneck effect - drastic reduction in population, and gene pool size 37
Forms of Selection
Disruptive selection Selection eliminates intermediate types.
Directional selection Selection eliminates one extreme from a phenotypic array.
Stabilizing selection Selection acts to eliminate both extremes from an array of phenotypes.
38
Kinds of Selection
39
Selection on Color in Guppies
Guppies are found in small northeastern streams in South America and in nearby mountainous streams in Trinidad.
Due to dispersal barriers, guppies can be found in pools below waterfalls with high predation risk, or pools above waterfalls with low predation risk.
40
Evolution of Coloration in Guppies
41
Selection on Color in Guppies
High predation environment - Males exhibit drab coloration and tend to be relatively small and reproduce at a younger age.
Low predation environment - Males display bright coloration, a larger number of spots, and tend to be more successful at defending territories.
In the absence of predators, larger, more colorful fish may produce more offspring.
42
Why is genetic variation important?
variation no variation global warming
survival EXTINCTION!!
43
Forces that affect allele freq.
1. Mutation 2. Migration 3. Selection 4. Random (genetic) drift Selection and migration most important for livestock breeders.
44