The Evolution of Populations

Darwin’s Proposal Individuals are selected; populations evolve. Individuals are selected; populations evolve.

Darwin’s Proposal Although he had combined many different scientific ideas, knowledge of genetics was lacking at the time Although he had combined many different scientific ideas, knowledge of genetics was lacking at the time  Blending hypothesis was the only idea at that time

Darwin’s Proposal His “change in species over time” definition of evolution could not be explained with the current genetic knowledge His “change in species over time” definition of evolution could not be explained with the current genetic knowledge

Darwin’s Proposal When Gregor Mendel proposed the idea of genes, discrete inheritable units, it provided an explanation When Gregor Mendel proposed the idea of genes, discrete inheritable units, it provided an explanation  (half a century later…)

Modern Synthesis Rediscovery of genetics actually refuted Darwin’s ideas because it did not explain the continuum of traits Rediscovery of genetics actually refuted Darwin’s ideas because it did not explain the continuum of traits –E.g. fur length, running speed, height, hair color –Because Mendel’s theory spoke only of either/or traits (i.e. one allele or another) The understanding of multiple genetic loci helped pull together Mendel and Darwin’s ideas The understanding of multiple genetic loci helped pull together Mendel and Darwin’s ideas

Modern Synthesis Led to the foundation of Population Genetics Led to the foundation of Population Genetics –The study of how populations change genetically over time. This then led to modern synthesis This then led to modern synthesis –Integrated: statistics, inheritance, rules of natural selection, genetics, biogeography, botany…. –Focused on populations

Populations A localized group of individuals that are capable of interbreeding and producing fertile offspring. A localized group of individuals that are capable of interbreeding and producing fertile offspring. All of the alleles present at all of the loci in all of the individuals makes up that population’s gene pool. All of the alleles present at all of the loci in all of the individuals makes up that population’s gene pool.

Populations Within the gene pool each allele has a frequency (proportion) Within the gene pool each allele has a frequency (proportion) –denoted as “p” and “q”  p = the frequency of the dominant allele  q = the frequency of the recessive allele –The sum of all allele frequencies at one loci must be 1  p+q = 1  E.g. :  p= 80%  q= 20%

Populations Understanding the gene pool helps us to be able to study microevolution Understanding the gene pool helps us to be able to study microevolution –The change in the frequency of alleles in a population over time

Studying Microevolution Hardy-Weinberg Theorem Hardy-Weinberg Theorem –Describes how genetic variation is retained in the population  (which subsequently would allow natural selection to act) –If only Mendelian segregation and recombination of alleles at fertilization are involved…  The gene pool will remain constant from one generation to the next

Studying Microevolution E.g. E.g. In a population of wildflowers the pink allele is A and white allele is a. In a population of wildflowers the pink allele is A and white allele is a. –500 total plants in the population, 20 are white and 480 are pink (320 AA and 160 Aa) How many total alleles for flower color are there in this population of 500 flowers? How many total alleles for flower color are there in this population of 500 flowers? –1000 What is the frequency of A allele and a allele? What is the frequency of A allele and a allele? What is the frequency of A allele and a allele? What is the frequency of A allele and a allele?

Studying Microevolution What is the frequency of A allele and a allele? What is the frequency of A allele and a allele? What is the frequency of A allele and a allele? What is the frequency of A allele and a allele?

Studying Microevolution In order for this to remain the same In order for this to remain the same –That is, in order for variation to be maintained (Hardy- Weinberg Equilibirum), the population must be non- evolving –If this is the case, allele frequencies will remain the same

Studying Microevolution Conditions for non-evolving population: Conditions for non-evolving population: –Very large population size –No migration –No mutations –Random mating –No natural selection

Studying Microevolution If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then p 2 + 2pq + q 2 = 1 –And p 2 and q 2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype  p 2 = the frequency of AA 2pq= the frequency of Aa q 2 = the frequency of aa

Studying Microevolution Practice: Practice: –In a population of 200 mice:  98 are homozygous dominant for brown coat color (BB)  84 are heterozygous (Bb)  18 are homozygous recessive (bb) –The allele frequencies of this population are  ____________B allele __________b allele –The genotype frequencies of this population are  _______BB __________Bb _________bb

Studying Microevolution Practice: Practice: –In a population of 200 mice:  98 are homozygous dominant for brown coat color (BB)  84 are heterozygous (Bb)  18 are homozygous recessive (bb) –The allele frequencies of this population are  (98*2) + 84 = 280 : B allele (18*2) + 84: b allele –The genotype frequencies of this population are  98/200= 0.49: BB 84/200= 0.42: Bb 18/200= 0.09: bb

Studying Microevolution Practice: Practice: –In a population of 200 mice:  Using what we know about the allele frequencies:  280/400 = 0.7: B allele (p) 120/200 = 0.3: b allele(q)  What do you think the genotype frequencies would be in the next generation p 2 + 2pq + q 2 = (2*0.7*0.3 ) = 1 BB= p 2 =0.49 Bb= 2pq =0.42 bb= q 2 =0.09 These are the same as the last parental generation

Studying Microevolution The Hardy-Weinberg theorem describes a hypothetical population The Hardy-Weinberg theorem describes a hypothetical population In real populations, allele and genotype frequencies do change over time In real populations, allele and genotype frequencies do change over time

Studying Microevolution We can use the Hardy-Weinberg equation We can use the Hardy-Weinberg equation –to estimate the percentage of the human population carrying the allele for an inherited disease –to understand how a certain population is evolving (how the allele frequencies are moving away from those predicted be HWE)