Chap 16: Evolution of Populations Darwin never knew how heredity actually worked Biologists connected Mendel’s work to Darwin’s in the 1930’s Advances in genetics since then has redefined many of Darwin’s ideas in genetic terms Neo-Darwinism

Chap 16: Evolution of Populations Single-gene & Polygenic Traits # of phenotypes produced for given trait depends on how many genes control trait Single-gene traits controlled by a single gene with two alleles Ex.- widow’s peak

Frequency of Phenotype (%) Widow’s peakNo widow’s peak Phenotype

Chap 16: Evolution of Populations Many traits controlled by two or more genes Called polygenic traits Each gene often has two or more alleles Result: one polygenic trait can have many possible genotypes & phenotypes Ex.- height Distribution of phenotypes usually takes on bell shape on graph (normal distribution)

Chap 16: Evolution of Populations How Common is Genetic Variation? All genes have at least two forms (alleles) Plants & animals often have many alleles for a single trait Some variation is “invisible” since it involves biochemical processes Organisms may also be heterozygous for many genes

Chap 16: Evolution of Populations Sources of Genetic Variation Two main sources of genetic variation: - mutations - genetic shuffling resulting from sexual reproduction

Chap 16: Evolution of Populations Mutations Any change in a sequence of DNA Can occur due to mistakes in replication or due to environmental factors (radiation) Do not always affect organism’s phenotype Those that do, may affect an organism’s fitness Other mutations have no effect on fitness

Chap 16: Evolution of Populations Gene Shuffling Even though parents provided you your genes, you don’t look exactly like them Most heritable differences due to gene shuffling that occurs during meiosis Each member of homologous pair moves independently: 23 chromosomes can produce 8.4 million combinations

Chap 16: Evolution of Populations Crossing-over also increases the # of different genotypes that can appear in offspring Sexual reproduction major source of variation within populations Does not change relative allele frequency on its own

Chap 16: Evolution of Populations Variation and Gene Pools Genetic variation studied in populations Population- group of individuals of the same species that interbreed Since they interbreed, share a common group of genes called a gene pool Gene pool- consists of all genes, including all different alleles, present in a population

Chap 16: Evolution of Populations Relative frequency of an allele is the # of times that allele occurs in the gene pool, compared with # of times other alleles for same gene occur Often expressed as a percentage In genetic terms, evolution is any change in the relative frequency of alleles in a population

Sample Population 48% heterozygous black 36% homozygous brown 16% homozygous black Frequency of Alleles allele for brown fur allele for black fur

Chap 16: Evolution of Populations Evolution versus Genetic Equilibrium Are there any conditions under which no evolution will occur? Is there any way to recognize when this is occurring? Answer is provided by the Hardy-Weinberg principle

Chap 16: Evolution of Populations Hardy-Weinberg principle: allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change Situation called genetic equilibrium

Chap 16: Evolution of Populations Five conditions required to maintain genetic equilibrium from generation to generation: - Must be random mating - Population must be very large - No movement into/out of population - No mutations - No natural selection

Chap 16: Evolution of Populations Some populations, these criteria met for long time periods If conditions are not met, genetic equilibrium disrupted, & population evolves

Chap 16: Evolution of Populations Random Mating All members of population have equal chance of producing offspring In natural populations, mating is rarely completely random Nonrandom mating means genes for certain traits are under strong selection pressure (sexual selection)

Chap 16: Evolution of Populations Large Population Genetic drift has less effect on large populations No Movement Into/Out of Population New individuals could bring new alleles into population Gene pool of population must be kept separate from others

Chap 16: Evolution of Populations No Mutations If genes mutate, then new alleles might be introduced into gene pool Gene frequencies will change

Chap 16: Evolution of Populations No Natural Selection All genotypes in the population must have equal chance of survival & reproduction No phenotype can have selective advantage over another

Chap 16: Evolution of Populations Evolution as Genetic Change Each time an organism reproduces, passes along a copy of its genes to offspring Can then view evolutionary fitness as an organism’s success in passing along genes Can view evolutionary adaptation as any genetically controlled functional, structural, or behavioral trait that affects ability to pass along genes to next generation

Natural selection never acts directly on genes Entire organism (not single gene) that either reproduces or does not (pass along genes) Can only affect which organisms survive & reproduce or do not Individual does not contribute its alleles to gene pool unless it survives & reproduces

Chap 16: Evolution of Populations If organism produces many offspring, its alleles stay in gene pool & may increase in frequency Evolution is change over time in relative frequencies of alleles in a population So, populations can evolve, not individual organisms, over time

Chap 16: Evolution of Populations Natural Selection on Single-gene Traits Natural selection on single-gene traits can lead to changes in allele frequency Ex.- population of lizards Normally brown, experiences mutation causing red and black phenotypes If red more visible to predators, those individuals less likely to survive & reproduce

Chap 16: Evolution of Populations Black lizards might warm up faster on cold days If high body temp. allows faster movement, then black moves faster Avoids predators & feeds easier: might produce more offspring than brown form If color change has no effect on fitness, then no selection pressure would occur

Chap 16: Evolution of Populations Natural Selection on Polygenic Traits Fitness of individuals close to one another on bell curve will not be very different Fitness can vary great deal from one end of a curve to the other Where fitness varies, natural selection can act

Chap 16: Evolution of Populations Genetic Drift N.S. not only source of evolutionary change In small populations, an allele can become more or less common by chance Probability can be used to determine results of crosses in large populations Smaller the population, farther the results might be from predicted values

Chap 16: Evolution of Populations This random change in allele frequency is known as genetic drift In small populations, individuals that carry a particular allele may leave more descendants than others, just by chance Over time, a series of chance events of this type can cause an allele to become common in a population

Chap 16: Evolution of Populations Genetic drift may occur when a small group of individuals colonizes a new habitat These individuals may carry alleles in a different frequency than the parent population Population founded will be genetically different from parent population Cause is chance (that particular alleles were in founding individuals), not N.S.

Chap 16: Evolution of Populations Natural selection can affect the distributions of phenotypes in any of three ways: - Directional selection - Stabilizing selection - Disruptive selection

Chap 16: Evolution of Populations Directional Selection Directional selection- when individuals at one end of curve have higher fitness than individuals in middle or at other end Range of phenotypes shifts towards one end due to some individuals failing to S&R

Chap 16: Evolution of Populations Ex.- Darwin’s finches (each with diff. beak size) Food shortage causes supply of small & medium-sized seeds to run low Birds with beaks that enable them to open large seeds will have advantage Big-beaked birds will have higher fitness Average beak size in population would probably increase

Directional Selection Food becomes scarce. Key Low mortality, high fitness High mortality, low fitness

Chap 16: Evolution of Populations Stabilizing Selection Stabilizing selection- when individuals near center of curve have higher fitness than individuals at either end Keeps center of curve at current position, but narrows the overall graph

Chap 16: Evolution of Populations Ex.- human birth weight Babies born much smaller than average less likely to be healthy, less likely to survive Babies much larger than average are likely to have difficulty being born Fitness of larger or smaller individuals is lower than that of average-sized individuals

Key Percentage of Population Birth Weight Selection against both extremes keep curve narrow and in same place. Low mortality, high fitness High mortality, low fitness Stabilizing Selection

Chap 16: Evolution of Populations Disruptive Selection Disruptive selection- when individuals at upper & lower ends of curve have higher fitness than individuals near middle Selection acts against intermediate phenotype If selection pressure is strong and lasts long enough, two distinct phenotypes produced

Chap 16: Evolution of Populations Ex.- If population of birds lives in area where medium-sized seeds become less common Large and small-sized seeds become more common Birds with unusually small or large beaks would have higher fitness Population might split into large seed eaters and small seed eaters

Disruptive Selection Largest and smallest seeds become more common. Number of Birds in Population Beak Size Population splits into two subgroups specializing in different seeds. Beak Size Number of Birds in Population Key Low mortality, high fitness High mortality, low fitness

Chap 16: Evolution of Populations Process of Speciation How do changes caused by natural selection and genetic drift lead to formation of new species? Gene pools of two populations must become separated for them to become new species Biological Species Concept

Chap 16: Evolution of Populations Isolating Mechanisms As new species evolve, populations become reproductively isolated from each other When members of two populations can no longer interbreed and produce fertile offspring- reproductive isolation Populations have separate gene pools Respond to N.S. or genetic drift differently

Chap 16: Evolution of Populations Reproductive isolation can develop in variety of ways, including: - Behavioral isolation - Geographic isolation - Temporal isolation

Chap 16: Evolution of Populations Behavioral isolation Occurs when two populations are capable of interbreeding, but have differences in courtship rituals or other reproductive behaviors Ex.- bird species using different songs to attract mates

Geographic isolation Two populations are separated by geographic barriers such as rivers, mountains, or bodies of water Ex.- Two species of squirrel (Albert & Kaibab) formed after Colorado river cut the Grand Canyon

Chap 16: Evolution of Populations Does not guarantee formation of new species: Barrier might be breached/broken down shortly If two populations still can interbreed, no new species has formed Also, geographic barriers can exist for some organisms, but not for others (birds can fly over a large river or mountain range)

Chap 16: Evolution of Populations Temporal isolation Two or more species reproduce at different times Ex.- plants Three plants all live in same forest, only release pollen on one day Each releases pollen on different day Impossible to pollinate one another

Chap 17-4: Patterns of Evolution Punctuated Equilibrium Darwin felt that biological change needed to be slow and steady – idea known as gradualism Fossil record confirms that some populations of organisms change gradually over time Also evidence that this pattern does not always hold true

Chap 17-4: Patterns of Evolution Some species have changed little since they appeared in the fossil record Much of the time these species are in a state of equilibrium Every now and then, something upsets the equilibrium Changes in populations then occur relatively rapidly

Chap 17-4: Patterns of Evolution Rapid evolution after long periods of stability can occur for several reasons: - Small pop. isolated from larger pop. - Small group migrates to new environment - Mass extinctions open up niches Term punctuated equilibrium used to describe pattern of long, stable periods interrupted by brief periods of rapid change