Chapter 15 Evolution in the Fast Lane
Read about Manatee’s • Group of mammals called sirenians • Closest living relative- dugong • Four living species of sirenians • Fifth species, Steller’s sea cow is now extinct due to hunting in 1770’s • Manatee were in danger but not protected by federal and state government • Listed as endangered species
Natural Selection • Define • What are the effects on small population? • What is you have very limited diversity?
The effect of small, isolated populations on genetic diversity • Small, isolated populations tend to have less genetic diversity than larger populations made up of many interbreeding individuals.
The effect of small, isolated populations on genetic diversity • Limited genetic diversity can seriously impair a population’s evolutionary success in the face of a changing environment.
Populations and Genetic Diversity • From a genetic perspective, each population of an organism has its own particular collection of alleles. The total collection of alleles in a population is the population’s gene pool.
What is the total gene pool in the class? • How many students? • Each student has how many chromosomes? • What is our gene pool?
Populations and Genetic Diversity • Within the gene pool, each allele is present in a certain proportion, or allele frequency, relative to the total number of alleles. • Example • Homozygous dominant allele for brown hair (BB) • Any brown heads in the class??? • How many total alleles for brown hair? • What is the allelic frequency in the class? • What is the genotypic frequency in the class?
Populations and Genetic Diversity • When alleles change frequency over time, a population evolves. • Evolutionary changes in a gene pool can have good, bad, or neutral lasting consequences in a population. • The gene-pool-altering force that results in adaptation is natural selection.
Populations and Genetic Diversity • Natural selection is not the only force that alters allele frequencies: i.e. mutations, etc. • Random and does not along cause adapation. • A change in allele frequency that does not by itself lead a population to become more adapted to its environment is nonadaptive evolution.
Nonadaptive evolution is caused by mutation, genetic drift, and gene flow. • All contribute to evolution in some way but alone do not lead to a population becoming adapted
Genetic drift • Genetic drift is a change in allele frequencies between generations that occurs purely by chance. • By simple chance, some individuals just happen to survive and reproduce, whereas others do not. • Those that pass on their genes were not necessary more fit or better adapted, just lucky
Genetic drift • Because only a subset of the population (with a subset of the total alleles) reproduces, only a subset of alleles is represented in the next generation. • Over time, genetic drive decreases the genetic diversity of a population.
Genetic drift • Genetic drift tends to have more dramatic effects in smaller populations than in larger ones, because in a population with few individuals, any single individual that does not reproduce could spell the loss of alleles from the population.
Genetic drift: The founder effect • The founder effectis a type of genetic drift in which a small number of individuals leave one population and establish a new population; by chance, the newly established population may have lower genetic diversity than the original population.
Genetic drift: The founder effect • When the founders begin to reproduce and populate a new area, the genetic diversity of the new population reflects the reduced diversity of the founders.
Genetic drift: The bottleneck effect • The bottleneck effectis a type of genetic drift that occurs when a population is suddenly reduced to a small number of individuals and alleles are lost from the population as a result.
Genetic drift: The bottleneck effect • When a population is cut down sharply, and often suddenly, there’s a good chance that the remaining population will possess a greatly impoverished gene pool.- become extinct
Genetic drift: The bottleneck effect • Bottlenecks can also occur from natural causes like an extremely cold winter that causes half the population to die.
Genetic drift: The bottleneck effect • Populations forced through a genetic bottleneck contain only a small fraction of the original starting diversity in the population.
Why is genetic diversity important? • A diverse gene pool gives a population more flexibility to survive in a changing environment • The more genetically diverse a population is, the more ways it has to adapt.
How do biologists measure the genetic health of populations? • Population biologists compare the gene pool at a given time to a population that is known not to be evolving. • If the two populations differ, the population is evolving, and you can begin to investigate why.
How do biologists measure the genetic health of populations? • Allele frequencies in a non-evolving population behave in a predictable way: they do not change over time. • In a non-evolving population, genotype frequencies remain unchanged from one generation to the next, a condition known as Hardy-Weinberg equilibrium.
Hardy-Weinberg equilibrium • The Hardy-Weinberg equilibrium provides a baseline from which to judge if a population is evolving or not. • For a gene with one dominant and one recessive allele, pand q, this formula can be written as: p2 +2 pq + q2= 1 • p2is the frequency of homozygous dominants • 2 pq is the frequency of heterozygotes • q2is the frequency of homozygous recessives
Hardy-Weinberg equilibrium • By definition, a population is not evolving (and is therefore in Hardy-Weinberg equilibrium) when it has stable allele frequencies and, therefore, stable genotype frequencies from generation to generation.
Hardy-Weinberg equilibrium Five conditions must be met for a population to be in Hardy-Weinberg equilibrium: • No mutation introducing new alleles into the population • No natural selection favoring some alleles over others • An infinitely large population size (and therefore no genetic drift) • No influx of alleles from neighboring populations (i.e., no gene flow) • Random mating of individuals
Hardy-Weinberg equilibrium • In nature, no population can ever be in strict Hardy-Weinberg equilibrium because it will never meet all five conditions. • In other words, all natural populations are evolving. • By describing the pattern of genotypes in a non-evolving population, Hardy-Weinberg equilibrium provides a baseline from which to measure evolution.
Hardy-Weinberg problems • Handout in class
Inbreeding • Inbreeding is mating between closely related individuals. Inbreeding does not change the allele frequency within a population, but it does increase the proportion of homozygous individuals to heterozygotes.
Inbreeding • Because closely related individuals are more likely to share the same alleles, the chance of two recessive alleles coming together during a mating is high. • When that happens, homozygous recessive genotypes are created, and previously hidden recessive alleles start to affect phenotypes.
Inbreeding • The negative reproductive consequences for a population associated with having a high frequency of homozygous individuals possessing harmful recessive alleles is an inbreeding depression.
Gene flow between populations increases genetic diversity • Isolated populations have limited genetic diversity because they only share alleles with themselves, and the likelihood of inbreeding goes up.
Gene flow between populations increases genetic diversity • Gene flowis the movement of alleles from one population to another, which may increase the genetic diversity of a population.
Gene flow between populations increases genetic diversity • Like genetic drift, gene flow is a type of evolution that does not lead to adaptation. Unlike genetic drift, gene flow increases the genetic diversity of a local population by introducing alleles from its neighbors.
What is a species? • The term species comes from the Latin word for “kind” or “appearance.” Evolutionary biologists define a species using thebiological species concept, which defines a species as a population of individuals whose members can interbreed and produce fertile offspring.
What is a species? • Members of different species cannot mate with each other because their populations are reproductively isolated. Reproduction isolation occurs as a result of mechanisms that prevent mating (and therefore gene flow) between members of different species.
What is a species? • Reproductive isolation can be caused by a number of factors: • Ecological isolation • Temporal isolation • Behavioral isolation • Mechanical isolation • Gametic isolation • Hybrid inviability • Hybrid infertility
What is a species? • Ecological isolation occurs when species live in different environments and therefore never encounter one another.
What is a species? • Temporal isolation occurs when species display mating behavior or fertility at different times.
What is a species? • Behavioral isolation occurs when species have different mating activities or behaviors.
What is a species? • Mechanical isolation occurs when mating organs are incompatible.
What is a species? • Gametic isolation occurs when gametes cannot unite.
What is a species? • Hybrid inviability occurs when gametes unite, but viable offspring cannot form.
What is a species? • Hybrid infertility occurs when viable hybrid offspring cannot reproduce.
Speciation • Speciation is the genetic divergence of populations owing to a barrier to gene flow between them, leading over time to reproductive isolation and the formation of new species.
Speciation • To study ancestry, researchers often rely on sequences of mitochondrial DNA.
Speciation • Speciation that occurs because of geographic or ecological separation is known as allopatry.