Evolution by Natural Selection

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” Struggle for life 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 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

Resulting from sexual reproduction Genetic Diversity 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 Selecting agents – specific environmental factors that favor and influence the likelihood that certain characteristics will be passed on 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 Two identical fields of clover Cows act as the selecting agent by eating the taller plants first Seeds from both fields were collected and grown under identical conditions Tall plants rarely reproduced Only short plants produced seeds Cows selectively ate plants with tall gene Seeds from ungrazed field produced tall, short but mostly medium sized plants Seeds from grazed field produced mostly short plants

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) Concept allows for comparisons of genes within a population to determine if genetic changes are occurring of genes of two different populations to determine similarities in genetic makeup Concept is an argument for evolution because most of these conditions cannot be met

Processes That Drive Natural Selection All the genes of all individuals in a population Constant genetic makeup over several generations (unchanging gene pool) indicates that evolution is not taking place A changing gene pool indicates evolution is taking place

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 No gene flow occurring 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 Populations become separate genetically Process has occurred only if gene flow between isolated populations does not occur even after barriers are removed Separation of species is not enough to generate new species

Theory of Natural Selection as a Mechanism for Evolution Proposed 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