Evolution Natural Selection Evolution of Populations Microevolution vs. Macroevolution

Evolution By definition, a change in a species over time A well supported and widely accepted scientific theory A major theme in biology Helps to explain biodiversity

Evolution of Populations It is the population as a whole that evolves…NOT the INDIVIDUAL  An organism cannot change its genetics or inherited traits ONLY POPULATIONS CAN EVOLVE!!!

Microevolution Change in allele frequencies of a population over generations Small-scale evolution Allele frequencies may be altered by three major factors:  Natural selection, genetic drift, and gene flow

Natural Selection Process in which individuals with certain heritable characteristics survive and reproduce at a higher rate than other individuals Explains how adaptations arise  Adaptations - inherited traits that increase fitness and survival  Fitness is a measure of reproductive success Not always the strongest, fastest, etc

Conditions for Natural Selection Variation – differences in a population due to mutation and recombination Overproduction of offspring Struggle for existence or competition Relationship between fitness and survival

Variation Natural selection only plays on those variations that are genetic Some variation may be discrete characters (single gene traits; few phenotypes)  Different forms are called morphs  Population may be referred to as polymorphic Most heritable variation consists of quantitative characters – vary along a continuum (polygenic)

Natural Selection Alleles may be passed to the next generation in different proportions Those with variations that are better suited tend to produce more offspring Acts more directly on phenotype and can alter frequency distribution in three ways (for polygenic traits):

Directional Selection – individuals with one phenotypic extreme are favored Disruptive Selection – individuals with both phenotypic extremes are favored Stabilizing Selection – individuals with the intermediate phenotype are favored

Genetic Drift Unpredictable fluctuation in allelic frequencies due to random changes  More likely in small populations Examples:  Founder effect – small group of individuals become isolated from the original population  Bottleneck effect – small group of survivors New gene pool may not reflect the original population

Gene Flow Occurs when a population gains or loses alleles by additions to and/or subtractions from the population  Immigration or emigration Tends to reduce differences and make populations more similar

Hardy-Weinberg Theorem Describes a population that is NOT evolving  States that frequencies of alleles and genes will remain constant unless they are acted on by forces other than segregation and recombination  Called genetic equilibrium Unrealistic and not very likely due to conditions…

Five conditions for Hardy-Weinberg Equilibrium 1. No mutations 2. Random mating 3. No natural selection 4. Extremely large population size (with no genetic drift) 5. No gene flow

Hardy-Weinberg Equation Used to determine if equilibrium is maintained or if evolution is occurring p + q = 1 p 2 + 2pq + q 2 = 1 p = frequency of dominant allele q = frequency of recessive allele P 2 is homo dom, 2pq is hetero, and q 2 is homo rec.

Macroevolution Evolutionary change above the species level  Includes large scale changes Includes speciation – the process by which new species arise Biological species concept:  Defines a species as a population or group of populations whose members have potential to interbreed and produce viable, fertile offspring

Reproductive Isolation Existence of biological barriers that keep members of two species from producing viable, fertile offspring (hybrids)  Prezygotic barriers – prevent mating or hinder fertilization  Postzygotic barriers – prevent a fertilized egg from developing into a fertile adult

Reproductive Isolation  Speciation

Prezygotic Barriers Habitat Isolation – species live in different habitats or geographic areas Behavioral isolation – members of one species do not respond to mating signals or behaviors of other species Temporal isolation – breed at different times, seasons, years, etc Mechanical isolation – anatomically incompatible Gametic isolation – gametes cannot fuse to form a zygote

Postzygotic Barriers Reduced hybrid viability – genetic incompatibility causes development of zygote to cease Reduced hybrid fertility – offspring is sterile and cannot reproduce Hybrid breakdown – two species mate, producing viable, fertile hybrids; hybrids however, produce weak or sterile offspring

Speciation Allopatric – new species forms due to geographic isolation  Confirmed when individuals from new population cannot successfully mate with parent population Sympatric – small part of the population becomes a new species without being geographically separated  May result from a part of the population switching to a new habitat, food, or other resource

Adaptive Radiation When many new species arise from a single common ancestor  Occurs when a few organisms move to new distant areas or after numerous extinctions  Example include Darwin’s finches Dinosaurs Mammals

Coevolution When two organisms evolve in response to each other  Common in close relationships like predator-prey, parasite-host, or mutualistic relationships

Convergent Evolution Process by which unrelated organisms come to resemble each other due to similar functions or environmental pressures  Leads to analogous structures

Rate of Speciation Gradualism – proposes species descended from a common ancestor gradually, acquiring unique adaptations and morphological changes very slowly Punctuated Equilibrium – describes periods of stasis punctuated by sudden changes in populations