Population Genetics

Variation of Traits Within a Population Causes of Variation Traits vary and can be mapped along a bell curve, which shows that most individuals have average traits, whereas a few individuals have extreme traits. Variations in genotype arise by: 1.) Mutations 2.) Recombination (Crossing-Over) 3.) Random pairing of gametes

The Gene Pool Members of a species can interbreed & produce fertile offspring Species have a shared gene pool Gene pool – all of the alleles of all individuals in a population

The Gene Pool Different species do NOT exchange genes by interbreeding Different species that interbreed often produce sterile or less viable offspring (i.e., mule)

Populations A group of the same species living in an area No two individuals are exactly alike (variations) Individuals with great fitness can survive & reproduce to pass on their traits

Hardy-Weinberg Genetic Equilibrium Says: Allele frequencies in the gene pool do not change unless acted upon by certain forces. Hardy-Weinberg genetic equilibrium is a theoretical model of a population in which NO evolution occurs and the gene pool of the population is stable.

5 Assumptions of H-W Equilibrium 1.) “All traits allow for equal fitness” 2.) “No new alleles spontaneously added” 3.) “No gene flow can occur” 5.) “No sexual selection (being picky about a mate) can occur” 4.) “No genetic drift can occur”

Hardy-Weinberg Genetic Equilibrium Used to describe a non-evolving population.  Natural populations are NOT expected to actually be in Hardy-Weinberg equilibrium. Deviation from Hardy-Weinberg equilibrium usually results in evolution

Microevolution Changes occur in gene pools due to mutations, natural selection, & genetic drift Gene pool changes cause more VARIATION in individuals in the population This process is called MICROEVOLUTION Example: Bacteria becoming unaffected by antibiotics (resistant)

Causes of Microevolution Genetic Drift - The change in the gene pool of a small population due to chance Natural Selection Success in reproduction based on heritable traits results in selected alleles being passed to relatively more offspring Gene Flow -Genetic exchange due to the migration of fertile individuals or gametes between populations

Causes of Microevolution Mutation A change in an organism’s DNA Mutations can be transmitted in gametes to offspring Non-Random Mating (Sexual Selection) - Mates are chosen on the basis of the best traits

Factors that Cause Genetic Drift Bottleneck Effect A drastic reduction in population (volcanoes, earthquakes, landslides …) Reduced genetic variation Smaller population may not be able to adapt to new selection pressures

Modes of Natural Selection

Modes of Natural Selection Directional Selection Favors individuals at one end of the phenotypic range (at one extreme) Disruptive Selection Favors individuals at both ends of the phenotypic range (at both extremes) over intermediate phenotypes

Modes of Natural Selection Stabilizing Selection Favors intermediate over extreme phenotypes Reduces variation and maintains the current average The typical bell curve shape

Forming New Species “Speciation”

Speciation Formation of new species One species may split into 2 or more species A species may evolve into a new species Requires very long periods of time

What is a “Species”? Mechanisms Contributing to Speciation: According to the biological species concept, a species is a group of organisms that can successfully interbreed but cannot breed with other groups. Mechanisms Contributing to Speciation: 1.) Allopatric Isolation (different geographical areas) 2.) Sympatric Isolation (same geographical area) 3.) Reproductive Isolation (difference in reproductive times, seasons, mating calls, or anatomy)

Allopatric Isolation What are now different species may have at one time been one species, but became separated due to geographical isolation Can be caused by the formation of a geographic barrier, such as a canyon or river Allopatric Isolation: Speciation that occurs when biological populations of the same species become isolated from each other, such that they now inhabit different geographic regions Prevents gene flow As time passes, they become different species

Allopatric Isolation

Sympatric Isolation Sympatric Isolation: Speciation that occurs with organisms living in the same geographic region Often due to reproductive isolation Reproductive isolation results from the separation of population subgroups by barriers to successful breeding. Can be due to different: Mating seasons Mating calls Anatomy

Sympatric Isolation

Reproductive Isolation “Prezygotic Barriers”

Reproductive Isolation “Postzygotic Barriers” “Reduced hybrid viability” = miscarriage during embryonic development “Hybrid breakdown” = F1 generation is fertile; later generations are not

Rates of Speciation In the gradual model of speciation (gradualism), species undergo small changes at a constant rate. Under punctuated equilibrium, new species arise abruptly, differ greatly from their ancestors, and then change little over long periods.

Rates of Speciation