Evolution
- The Role of Natural Selection in Evolution
- Evolution-descent with modification
- Most potent at individual level
- Individuals do not evolve, only populations
- Natural selection-processes where beneficial genes are passed on and harmful or less valuable genes are not as likely to be passed on
- Theory of Natural Selection
- Populations become genetically adapted to their surroundings over time
- Four postulates of Natural Selection
- There is variation among individuals in a population
- Some of this variation is heritable
- In each generation some individuals survive and reproduce better than others
- Survival and reproduction are not random, but are tied
to individual variations. Individuals with the most favorable variations are
selected for.
- Misconceptions
- Survival of the Fittest
- Survival alone is not good enough
- Survival and reproduction is the key
- Struggle for life
- Not necessarily open conflict
- But finding the necessary resources for survival and reproduction
- Genetic Variation
- Spontaneous mutations
- Genetic recombination
- Genetic Diversity influences the process of natural selection -- e.g., resulting from mutation
- Spontaneous mutation changes in DNA that cannot be tied to any particular causative agent
- Cosmic radiation, mutagenic chemicals
- Naturally occurring genetic mutation rates are low (~1 in 100,000)
- Possible outcomes when a gene is altered
- Mutation so minor that it has no effect
- Mutation harmful
- Mutation is beneficial
- In order for mutations to be passed to the next generation, they
must happen in cells that will become sex cells (leading to genetic recombination)
- Genetic Diversity influences the process of natural selection -- e.g., resulting from sexual reproduction
- Does not generate new genetic information
- Genes recombine into new mixtures
- Genetic recombination: each individual has unique set of genes, half
donated by mother, half by father
- Acquired characteristics do not influence natural selection
- not genetically determined gained during life of
organism cannot be passed on to other generations
- Processes that drive Natural Selection
- Differential Survival
- Some individuals possess characteristics enabling them to preferentially survive and pass on their genes
- Differential Reproductive Rates
- Some organism may be able to better utilize resources to produce more offspring
- Differential Mate Selection
- Some individuals possess characteristics enabling them to be more frequently chosen as mates
- Size, aggressiveness, attractiveness to opposite sex
- Differential Survival
- Certain traits favor survival
- Can be for finding shelter, food, disease resistance, etc.
- Ex. Pesticide resistance
- Differential Reproductive Rates
- Differential Mate Selection
- Also called Sexual Selection-differential reproductive success due to variation in obtaining mates
- Hardy-Weinberg Concept
- Conditions necessary for genetic makeup to remain constant
- Mating must be completely random
- Mutations must not occur
- Migration of individual organisms into and out of the population must not occur
- The population must be very large
- All genes must have an equal chance of being passed onto the next generation (no natural selection)
- Processes That Drive Natural Selection
- Evidence for the Theory of Evolution
- Species and populations are changing genetically through mutations, adaptations, environmental changes, selective breeding, extinctions
- Evolution occurs by small steps
- All species use same DNA code and amino acid building blocks
- Difficult to eliminate a structure that becomes part of a process controlled by genes (example: appendix)
- Closely related species have similar DNA and embryological development
- Fossil records show changes in kinds of organisms.
- Species appear and subsequently become extinct
- New techniques and discoveries support evolution
- Misconceptions about the Theory of Evolution
- Evolution happened only in the past
- Lots of evidence exists for current genetic changes
- Evolution has a specific goal
- Natural selection favors organisms that best fit the current environment
- Random events can have major influence on natural selection
- Changes in environment cause mutations that are needed for an organism to survive
- Mutation are random events (not necessarily adaptive)
- Individual organisms evolve
- Individuals may adapt by changing behavior or physiology but they cannot change their genes
- An existing species may have arisen from another present-day species
- The species in question may have had a common ancestor
- Species
- Population of organisms whose members have the potential to interbreed naturally to produce fertile offspring but do not interbreed with other groups
- Populations that demonstrate gene flow between them
- Gene flow - movement of genes from one generation to the next or from one region to another
- Smallest irreversible step in the evolutionary process
- Two key ideas in these definitions
- Species is a population of organisms (not individuals)
- Offspring must be fertile
- Identifying species
- Biological species concept
- Species can be identified by their inability to interbreed
- Not a practical way to distinguish species
- Morphological species concept
- Method of using physical characteristics to identify species
- Useful but not foolproof
- Speciation
- Range geographical area over which a species is found
- Geographic isolation portion of a species becomes reproductively isolated from the rest of the gene pool by geographic change
- Geographical barriers - geological features that keep portions of the species from exchanging genes
- Speciation - process of generating new species
- Over a long period of time, accumulated genetic differences may result in subspecies
- How New Species Originate
- Three step process:
- Geographic isolation occurs
- Selective agents favor specific valuable genetic combinations
- Genetic differences become so great that reproduction between two groups is impossible
Result: Populations become separate genetically
- Process has occurred only if gene flow between isolated populations does not occur even after barriers are removed
- Separation of members of a species is not enough to generate new species
- Theory of Natural Selection as a Mechanism for Evolution
- Proposed (independently!) by Darwin and Wallace (1858)
- Based on the following assumptions
- All organism produce more offspring than can survive
- No two organisms are exactly alike
- Organisms are in a constant struggle for survival
- Individuals that possess favorable characteristics for their environment have higher rate of survival and produce more offspring
- Favorable characteristics become more common in the species and unfavorable characteristics are lost
Website maintained by Andy Long.
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