Download - Evolution
5b:Evolution
5b:Evolution
DarwinDarwin observed several things on his voyage on the Beagle:
Geological oddities: marine shells in the mountains, for example
Fossils: differed from modern examples
Animals: similar to some in England but different as well- the Patagonian hare, for example
DarwinDarwin’s observations:
Geographic change in species: Variation due to environmental differences
Galapagos finches:many species with different beaks, living on different islands, eating different things
Galapagos tortoises: One type of tortoise per island
Adaptation: a feature that allows the organism to survive better in its environment
Natural SelectionDarwin’s observations led him to propose a method by which adaptations might arise: Natural Selection
The 4 steps, revisited:
Heritable variations passed to offspring
More offspring produced than environment can support
Favorable traits in some offspring lead to those having a higher survival rate
Over time, more and more of the population possesses the favorable traits
Natural Selection
IMPORTANT:
Variation arises from RANDOM changes that happen to arise from genetic mutations-- there is no directedness or ability to predict future needs
Some mutations are detrimental, and some are neutral
Natural Selection
IMPORTANT:
Natural selection is ongoing continuously because the environment is constantly changing
Natural Selection
Because resources are always limited, some individuals will fail to survive and reproduce-- thus removing their genes from the population
Failing to reproduce is functionally the same as failing to survive
Natural SelectionFitness varies among individuals
It is the reproductive success of one individual compared to others in the population
More fit animal:
Uses more resources
Avoids death
Leaves more offspring
Natural vs. Artificial SelectionHumans practice artificial selection on domestic animals and plants
Individuals with desired traits are bred, while those with undesired traits are not
Eventually, offspring change to resemble the predetermined idea the humans had
Ex. Any domestic animal, vegetables
Natural vs. Artificial Selection
In artificial selection, the result is predesired, and the breeding is directed
In natural selection, the result is determined by the environment with no direction or desire involved
Natural Selection- TermsSelection pressure: What acts on an animal to either increase or decrease its chances to survive and/ or reproduce
Selected for: the trait conveys advantage
Selected against: the trait conveys disadvantage
Adaptations
Adaptations may take many generations to evolve
Explain why animals are suited to their environment and ‘lifestyle’
The more ‘adaptive’ a trait is, the greater the advantage that the individual has
Convergent Evolution
Convergent Evolution: when unrelated species share similar characteristics
Ex: Flippers on manatees, penguins and sea turtles
Ex: The horny toad of the US, and the thorny devil of Australia
Wallace
Important to realize that Darwin was not the only guy to come up with Natural Selection- Alfred Wallace came to the same conclusions studying different organisms, and they first published at the same time
Evidence for Evolution
Fossils:
Fossils frequently show the pattern of a succession of species from simple to more complex, though this is not universal
Transitional fossils: Archaeopteryx
Whales: see p. 226
Evidence for EvolutionBiogeography: the study of the distribution of organisms
Different mix of organisms when geography separates areas
Ex: Cacti and Spurges in deserts
Ex: Marsupials in Australia
Americas and Australia connected at one time, but marsupials able to evolve separately from placental mammals in Australia
Evidence for EvolutionAnatomy:
Vestigial structures: fully developed in one group, but reduced and nonfunctional in others
Ex: Appendix, tailbone in humans
Ex: Hipbones in snakes
Ex: Wings in flightless birds
These occur because organisms inherit structures from their ancestors
Evidence for EvolutionAnatomy:
Homologous structures: Anatomically similar structures, frequently used for different purposes, explained by a common ancestor
Ex: Horse legs vs. bird wings. vs. bat wings vs. whale flippers vs. human arms
Homologous StructuresExample from humans:
Pharyngeal pouches- seen in all vertebrate embryos
Develop into gills in fish and amphibians
Develop into tonsils, inner ear canal, thymus and glands in humans
It is easier (therefore more likely to arise randomly) to modify an existing structure than create an entirely new one
Evidence for Evolution
Molecular Evidence:
All living organisms use the same biological molecules-- DNA, RNA, ATP, etc.
DNA code is the same for all living organisms
Many of the same genes have been modified to result in the wide variety we see
Gene sequences are more different the further apart organisms are evolutionarily
Evolution on a Small Scale
Remember: individual organisms can not evolve, populations/ species evolve
Microevolution: small measurable changes in a population from generation to generation
Hardy-Weinberg EquilibriumHardy-Weinberg Equilibrium allows us to measure small changes in the frequency of alleles in a population
This is how we know microevolution is occurring
Population genetics is the study of the occurrence and flow of genes in populations
We will look at the example of peppered moths
Peppered MothsPeppered moths can be either light or dark, and this is controlled by a single set of alleles:
D= dark color
d= light color
We know the frequency of genotypes in the population:
4% DD
32% Dd
64% dd
Peppered MothsFrom genotypes, we can figure out the frequency of each allele in the population:
.04 + .16 =.2 D
.16 + .64 =.8 d
Frequency of each gamete type will be the same as the frequency of occurrence of each allele
We can use a Punnett square to figure out the gene frequencies in the next generation
Peppered Moths
.2D.2D .8d.8d
.2D.2D .04DD .16Dd
.8d.8d .16Dd .64dd
** This is a punnett square for freqencies, not individuals
Peppered MothsNotice: the allele frequencies in the next generation are EXACTLY THE SAME
This means that sexual reproduction alone can not change the frequency of gene/ allele frequencies in a population, provided some assumptions are met
Notice the dominant allele does not increase in frequency
This is Hardy-Weinberg equilibrium
The Assumptions
1. No mutations: allelic changes do not occur
2. No gene flow into or out of the population
3. Random mating
4. No genetic drift- large population, changes in frequency due to chance are insignificant
5. No selection: one genotype is not favored over another
HW Equilibrium
However, those assumptions are basically never met
Changes in allele frequency do occur, and we can see those by comparing RL to the numbers predicted by HW equilibrium
Also, we know that deviation from the assumptions is what causes evolution
Back to the moths
After the industrial revolution, peppered moths that were light did not blend into trees with lots of soot on their bark, and they were eaten by birds
The frequency of D then increased in the population to around 80%
Causes of microevolution
Genetic mutations
The ultimate source for allele differences
Mutations can be harmful in one environment and helpful in another
Causes of microevolutionGene flow:
Movement of alleles among populations by migrating animals
Can increase variation in a population by introducing new mutations
Can prevent speciation by making the gene pool the same across populations
Causes of microevolution
Nonrandom mating:
Assortive mating: individuals mate with those that are similar
Ex. tall people mating with each other
Causes two groups of homozygotes to become more common, and hets to become less common
Sexual selection: Favors characteristics that increase the chance of mating
Ex. Male birds with bright colors or crazy feathers
Animals that compete for mates
Causes of microevolutionGenetic drift: changes due to chance
Allele frequencies ‘drift’ over time
More common in small populations, like on islands or other isolated areas
Can result in the loss of rare alleles completely, and fixation of others as they are the only ones left
Causes of microevolution
Bottleneck Effect: Catastrophic loss of most of a population, only a few individuals survive by chance
Those few are all that is left to pass on to later generations
Ex. cheetas and poor sperm
Founder Effect: Rare alleles are more common in a population isolated from the main population
Only a few individuals founded the new population, so their alleles are the ones that are represented
Ex. Amish have higher rate of two limb mutations
Causes: Natural SelectionThere are several types of selection, so it gets its own category
Directional Selection: an extreme phenotype is more successful and population shifts in that direction
Ex. the peppered moths
Ex. drug resistance in bacteria
Ex. Anacondas
Natural SelectionStabilizing Selection: an intermediate phenotype is best, extremes are selected against
Ex. male anacondas
Disruptive Selection: Two or more extremes are favored over the intermediate
Favors polymorphism: the occurrence of different forms in the same species
Ex. Snail with two different shell colorings, in two different habitats
Perfection?NO!! Animals do NOT tend to be perfectly adapted
Evolution does not start from scratch, it is modifying what is already there
Compromises: there may be costs to the adaptive benefit
Sexual selection may not result in adaptive traits- giant feathers may be energetically expensive and make it difficult to fly, for example
Maintenance of Variation
Populations will always show variation
New mutations always arising
Gene flow may be occurring
Diploidy and heterozygotes:
Only alleles that are expressed in phenotype can feel selective pressures
Heterozygous animals can protect alleles that might otherwise be selected against
The homozygous recessive will still occur occasionally
Example of Sickle CellsIndividuals with sickle cell are homozygous for S and typically die very young due to change in shape of RBCs
Individuals that are heterozygous are OK, because cells are normal shape until in low O2 environment
Individuals that are homozygous A are usually the most fit (most of us fall into this group)
Sickle Cell However, individuals with African descent have higher frequency of S- more sickle cell disease, but also:
Heterozygote is immune to malaria
So, in parts of Africa w/ malaria, heterozygote is selected for, but homozygotes continue to exist to do statistical frequency of each genotype in the next generation