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Home , Allele frequency, Cline (biology), Evolution, Genetic variation, Gene flow, Gene pool

Population

MICROEVOLUTION On of … WHAT IS A SPECIES? • in one or more • Potential to interbreed • Produce fertile offspring

WHAT IS A ? EVOLUTION OF POPULATIONS • Group of interacting individuals The Synthetic of Evolution belonging to one species and living in – Applying population the same geographic area to the Darwinian model • A population • Genetics & Variability is the smallest • Non-Adaptive Evolution unit that can • & evolve. •

Figure 23.6

Genes & : • code for , how do new forms arise? , or behavior. • Point : change code, change . • for or leg length. • Alleles are alternative forms genes can have: red hair, brown hair, etc. DNA

RNA

protein chain of amino acids

Heyer 1 Population Evolution

Genetic variation: Events Are Random how do new forms arise? • Chromosomal mutations of base sequences. • Mutations are destructive alterations to previously existing complex systems. • Most mutations are neutral or detrimental — very few are beneficial. • The need for a mutation to arise does not increase its .

Genetic variation: Variation how do new forms arise? • Crossing over between maternal & paternal • Individuals vary in bell-curve of makes new . – = + environment. • Due to different environments or genotypes – But only gene differences are passed on.

# of ind. in popn. leg length amount of saliva produced rate of blinking

Non-genetic Phenotypic Variation Alleles to Gene Pools • Environmentally induced phenotypic variations within the same genotype

Alleles in an Alleles in all the individuals individual of the population (a) Map that (b) Map butterflies that emerge in spring: emerge in late summer: and brown black and

Figure 23.9

Heyer 2 Population Evolution

The Hardy-Weinberg & OF 1

CRCR CWCW genotype genotype Theorem ALLELES IN THE POPULATION Plants mate • If contribute to the next • Gene pool is the total Generation 2 generation randomly, Mendelian collection of genes All CRCW (all pink ) and their variations segregation and recombination of alleles 50% CR 50% CW (alleles) in a gametes gametes preserves genetic variation in a population Come together at random population

• Reservoir of Generation the frequencies of alleles and 3 \ variations from 25% CRCR 50% CRCW 25% CWCW genotypes in a population’s gene pool which the next 50% CR 50% CW remain constant from generation to generation derives its gametes gametes Come together at random generation genes

Generation • Polymorphic 4 Allelic frequency= 25% CRCR 50% CRCW 25% CWCW Alleles segregate, and subsequent 49% AA + 42% Aa + 9% aa = 70% A + 30% a also have three types of flowers in the same proportions Figure 23.4

A population in A population in Hardy-Weinberg equilibrium Hardy-Weinberg equilibrium

• p = frequency of occurrence of the CR in the population • p = frequency of occurrence of the CR allele in the population • q = frequency of occurrence of the Cw allele in the population • q = frequency of occurrence of the Cw allele in the population Then: Gametes for each generation are drawn at random from 64% CRCR, 32% CRCW, and 4% CWCW the gene pool of the previous generation: 80% CR 20% CW 80% CR 20% CW (p = 0.8) (q = 0.2) Gametes of the next generation: (p = 0.8) (q = 0.2) R R 64% C from + 16% C from R R R W C C homozygotes C C homozygotes R W Sperm C C CR CW R (80%) (20%) (80%) (20%) = 80% C = 0.8 = p W W 2 16% 16% 4% C from 16% C from p R W pq p2 pq + C C CRCW CWCW homozygotes CRCW heterozygotes R R W C

C = 20% C = 0.2 = q (80%) (80%) 16% 16% R W C C CRCW With random mating, these gametes will result in Eggs the same mix of plants in the next generation:

64% q2 4% 64% 2 4% W qp q R R W W W qp C C C C C R R W W C C C C C (20%) (20%) If the gametes come together at random, the genotype 64% CRCR, 32% CRCW, and 4% CWCW 64% CRCR, 32% CRCW and 4% CWCW plants frequencies of this generation are in Hardy-Weinberg equilibrium: again! R R R W W W 64% C C , 32% C C , and 4% C C Figure 23.5 Figure 23.5

A population in BIOLOGICAL EVOLUTION Hardy-Weinberg equilibrium • Change in a population’s gene pool over as a result of a • If p and q represent the relative frequencies of change in frequency of an allele the only two possible alleles in a population at a particular , then 2 2 q p + 2pq + q = 1 • p2 = frequency of the genotype homozygous for the first allele • q2 = frequency of the genotype homozygous for the first allele • 2pq = frequency of the heterozygous genotype 70% A + 30% a 50% A + 50% a

Heyer 3 Population Evolution

BIOLOGICAL EVOLUTION BIOLOGICAL EVOLUTION • Change in a population’s gene pool over time as a result of a change • Change in a population’s gene pool over time as a result of a in frequency of an allele change in frequency of an allele • But according to Hardy-Weinburg Equilibrium: if mating is random, • But according to Hardy-Weinburg Equilibrium: if mating is the frequency of alleles in a population remains constant over time. random, the frequency of alleles in a population remains constant over time.

• Therefore, population evolution is a product of non-random mating.

70% A + 30% a 70% A + 30% a

A population in Hardy-Weinberg equilibrium BIOLOGICAL EVOLUTION

• The five conditions for Hardy-Weinberg equilibrium: ¸ Large Remember! — ¸ No significant • Natural selection works on phenotype ¸ is trivial compared to recombination ¸ Random mating – But only genotype is inherited ¸ No significant natural selection • If any/several of these conditions are not met, changes • Natural selection works on individuals in may occur – But only populations evolve ÿ non-equilibrium = evolution

Non-Adaptive Evolution: EVOLUTION OF POPULATIONS Most Likely in Small Populations • Genetics & Variability • • Non-Adaptive Evolution • Genetic Bottleneck • Fitness & Natural Selection • Founder Effect • Sexual Selection • Gene Flow •

Heyer 4 Population Evolution

Genetic Drift • Random changes in gene frequencies. Non-Adaptive Evolution – it can lead to homozygosity. • Genetic Drift • Genetic Bottleneck • Founder Effect • Gene Flow • Assortative Mating

VARIATION IN Genetic Bottleneck POPULATIONS

• Few survive a population crash – – seals (~20)

Founder Effect Non-Adaptive Evolution

• Genetic Drift • Genetic Bottleneck • Founder Effect • Gene Flow • Assortative Mating

• A few individuals disperse to new place – ABO frequencies of Dunkers in PA vs. Germany.

Heyer 5 Population Evolution

Non-Adaptive Evolution Gene Flow

• Genetic Drift • Breeding w/ outsiders • Genetic Bottleneck – Spaniards in Central America • Founder Effect • Gene Flow • Assortative Mating

Non-Adaptive Evolution: Non-adaptive Evolution: Significant only in small populations. Nonrandom mating * Genetic Drift: random changes in gene freq. • Assortative Mating: with those who look * Genetic Bottleneck: few survive pop. crash most like you. ÿ cheetahs & elephant seals • Sexual Selection: coming soon! * Founder Effect: few disperse to new place ÿ ABO frequencies of Dunkers in PA vs. Germany. * Gene Flow: breeding w/ outsiders ÿ Spaniards in Central America * Æ↑ homozygosity. * ÆØ homozygosity.

Terms used in Natural Selection EVOLUTION OF POPULATIONS • Fitness: measure of how many genes you pass on to future generations. • Genetics & Variability • Differential representation of genes in future • Non-Adaptive Evolution generations due to differential survival to • Fitness & Natural Selection reproductive maturity. – Modes of Selection – Requires heritable (genetic) variation among • Sexual Selection individuals. • If differential survival is based upon expressed genotypic differences – it may lead to changes in population gene frequency.

Heyer 6 Population Evolution

Fitness and Selection “” • Darwinian fitness is a relative measure – how many offspring does one individual leave relative to others in the population. • Inheritance acts upon genotype “ • Selection acts upon phenotype – morphology, physiology, or behavior of the fittest” • Agents of selection – physical environment – biological environment • con- or hetero-specifics.

Modes of Selection Modes of Selection

• Fitness and Selection • • • Diversifying Selection

Heterozygote Advantage and the Stabilizing Selection: Sickle Gene Birth-weights

• HbS is a recessive allele for the gene • 1/11 African Americans are carriers • 1/500 are homozygous recessive (sickle cell )

Heyer 7 Population Evolution

MALARIA AND : and Sickle Cell Anemia

Directional Selection: Modes of Selection The Pepper Moth Biston betularia • Fitness and Selection • Stabilizing Selection • Directional Selection • Diversifying Selection

Industrial in early 1900’s

EVOLUTION OF PEST Directional Selection: RESISTANCE The Pepper Moth Biston betularia • Red gene confers resistance to • Insecticide application • Only individuals carrying red gene survive • Red gene increases in population

Heyer 8 Population Evolution

MALARIA TODAY MALARIAL RESISTANCE

• Mostly under control in 1947 • Common today in tropical countries • Kills 2.7 million per year • Mostly children

Diversifying Selection: Modes of Selection African Seedcrackers • Fitness and Selection • Stabilizing Selection • Directional Selection • Diversifying Selection – a.k.a.

These feed on of two sedge species.

Diversifying Selection: Diversifying Selection: African Seedcrackers African Seedcrackers

“Balanced ”

Heyer 9 Populaon Evoluon

Diversifying Selecon: Diversifying Selecon • Frequency‐dependent selecon • Different phenotypes favored in different parts of the population’s range – Predators learn to focus on most common phenotype – Minor alternate phenotypes escape noce • E.g., a : graded change in a trait along a geographic axis Figure 23.14 Heights of yarrow plants grown in common garden EXPERIMENT Researchers observed that the Parental population sample On pecking a moth image average size of yarrow plants (Achillea) growing on the blue jay receives a the slopes of the Sierra Nevada mountains gradually reward. If the decreases with increasing elevation. To eliminate the effect of environmental differences at different does not detect a moth elevations, researchers collected seeds from various on either screen, it pecks altitudes and planted them in a common garden. They the green circle to continue then measured the heights of theresulting plants. to a new of images (a Mean height (cm) new feeding opportunity). RESULTS The average plant sizes in the common garden were inversely correlated with the altitudes at 0.06 which the seeds were collected, although the height Experimental group sample differences were less than in the plants’ natural environments. 0.05

Phenotypic diversity 0.04 Sierra Nevada Great Basin

Atitude (m) Frequency- CONCLUSION The lesser but still measurable clinal Range Plateau 0.03 variation in yarrow plants grown at a common independent control elevation demonstrates the role of genetic as well as 0.02 environmental differences. collection sites 0 20 40 60 80 100 Generation number Figure 23.11 Plain background Patterned background

EVOLUTION OF POPULATIONS Sexual Selecon

• Genecs & Variability • Natural Selecon (NS): differenal • Non‐Adapve Evoluon reproducon due to differenal survival. • Adapve Evoluon: Natural Selecon • Sexual Selecon (SS): differenal reproducon – Modes of Selecon due to increased Reproducve (RS) despite possible decreased survival. • Sexual Selecon

Sexual Selecon Sexual Selecon • Even though some variaons may increase • Even though some variaons may decrease survival, health, compeve success, etc., … survival, health, compeve success, etc., … • they will not increase in frequency in the gene • they will increase in frequency in the gene pool if they are not also associated with pool if they are also associated with increased increased reproducve success! reproducve success!

Heyer 10