Population Genetics Chapter 11 The study of allele frequencies in populations

Reminder: Populations A group of the same species living in an area No two individuals are exactly alike (variations) More Fit individuals survive & pass on their traits

Reminder: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

Modern Synthesis Theory of Evolution TODAY’S theory on evolution 1. GENES are responsible for the inheritance of characteristics 2. POPULATIONS, not individuals, evolve due to natural selection & genetic drift 3. SPECIATION usually is due to the gradual accumulation of small genetic changes

What causes populations of organisms to EVOLVE???

There are 5 factors that can lead to or cause EVOLUTION!!! 1. Genetic Drift 2. Natural Selection 3. Gene Flow 4. Mutation 5. Sexual Selection (non-random mating)

see chapter 11.3 Causes of Evolution Genetic Drift - the change in the gene pool of a small population due to chance

Factors that Cause Genetic Drift Bottleneck Effect a drastic reduction in population about 10,000 years ago Reduced genetic variation Smaller population have a hard time adapting

Factors that cause Genetic Drift Founder’s Effect Genetic drift that occures after a small number of individuals colonize a new area

Causes of Evolution Natural Selection - success in reproduction based on heritable traits results in selected alleles being passed to relatively more offspring (Darwinian inheritance) - Cause ADAPTATION of Populations

Causes of Evolution: Gene Flow Changes in the gene pool of a population because of the introduction of genes from another population by migration. Example: the gene pool in southeast Asia was changed when U.S. Soldiers had children with Vietnamese women during the Vietnam war.

Causes of Evolution: Mutation a change in an organism’s DNA Mutations can be transmitted in gametes to offspring Non-random mating - Mates are chosen on the basis of the best traits

Having a lot of genetic variation within a population is GOOD. Chap 11

The Gene Pool Chapter 11.1 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

Allele Frequencies Different combinations of alleles in a gene pool can be formed when organisms mate and have offspring. The Allele Frequency is the measure of how common a certain allele is in the population. Allele Frequency = the number of times an allele occurs the total number of alleles for that gene

Let’s calculate the allele frequency for: You have a population of mice. There are 30 mice total and 15 mice are black (BB), 10 mice are brown (Bb) and 5 mice are white (bb). What is the allele frequency of B? What is the allele frequency of b? You can track the frequency of an allele over time to see the changes in populations

Answer this!! If a certain trait’s allele frequency is 100%, describe the genetic variation of that allele in the population.

2 main sources for variation in allele frequencies 1. Mutation 2. Recombination through Meiosis Q? Why aren’t mutations in somatic cells sources of genetic variation??

Evolution Through Natural Selection Modes of Natural Selection 1. Directional Selection Favors individuals at one end or the other end of the phenotypic range Most common during times of environmental change or when moving to new habitats example, colored animals like mice. At times black is favored, other times white is favored

Modes of Natural Selection 2. Disruptive selection Favors both extremes over intermediate phenotypes Occurs when environmental change favors an extreme phenotype Can be a pathway for species formation WITHOUT geographic isolation

Modes of Natural Selection 3. Stabilizing Selection Favors intermediate over extreme phenotypes Reduces variation and maintains the cureent average Example: Human birth weight

How does Natural Selection Affect Allele Frequency?? 1. Mutations provide the raw material on which natural selection acts 2. Evolution depends on variations because it is the only way that differences among organisms are created 3. Acts on Populations not individuals by changing the % of alleles in the population

How Do New Species Occur? ISOLATION!!!!! (nobody likes to be alone!!)

New Species Evolve in Patterns: Convergent Evolution: Different species must adapt to similar environments. Evolution towards SIMILAR characteristics in UNRELATED species. ex: Dolphin/Shark Divergent Evolution: Closely related species evolve In different directions. Evolution towards DIFFERENT characteristics in RELATED species. ex: Kit fox/Red fox Co-Evolution: When 2 or more different species evolve in response to changes in the other. ex: Acacia plant/ants

Repeating Patterns of Speciation Throughout the history of EARTH Punctuated Equilibrium A pattern of BURSTS of evolutionary activity followed by long periods of stability. EX: Adaptive Radiation The diversification of one ancestral species into many descendent species. EX: mammals

Genetic Variations, (changes in allele frequencies) within Populations creates Evolution of Species!!