Ch. 23 Evolution of Populations (cont.)

Causes of Microevolution Microevolution – a generation to generation change in a population’s frequencies of alleles Two main causes of microevolution are genetic drift and natural selection Genetic drift – change in population’s allele frequencies due to chance

An Example of Genetic Drift Bottleneck Effect Disasters such as earthquakes, floods, droughts, and fires may reduce the size of a population dramatically The small surviving population may not be representative of the original population’s gene pool Example – cheetahs

An Example of Genetic Drift Bottleneck Effect

An Example of Genetic Drift The Founder Effect Occurs when a few individuals from a larger population colonize an isolation island, lake, or some other habitat The smaller the sample size, the less the genetic makeup of the colonist will represent the gene pool of the larger population they left Example – Population of Amish in Lancaster County, PA Original pop. size of 30; current pop. size is 12,000. Frequency of extra fingers and toes is 1 in 14 rather than 1 in 1,000 in the US pop.

Causes of Microevolution Natural Selection – alleles passes on from one generation to the next due to some variants leaving more offspring than others Gene Flow – Genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations) Mutations – Causes a change in DNA. Can only lead to evolution and natural selection if the mutation is in the gametes

Genetic Variation Polymorphism – When 2 or more forms of a discrete character are represented in a population (applies only to discrete characteristics – not those on a continuum)

Variation Between Populations Geographic Variation – differences in gene pools between populations or subgroups of populations; can occur in different populations or within a population Cline – a graded change in some trait along a geographic axis

Variation Between Populations

What Generates Genetic Variation? Mutations - Only mutations that occur in cell lines that produce gametes can be passed along to offspring (it is rare that this occurs) Only those mutations that allow an organism to be better suited to its environment will exist in a pop. Mutations are more likely to be present in high numbers when the environment is changing and mutations that were once selected against are now favorable under the new conditions

What Generates Genetic Variation? Sexual Recombination Sexual reproduction recombines old alleles into fresh assortments every generation During meiosis, crossing over and the law of segregation provide for an 1 in 8 million chance of identical sperm and a 1 in 8 million chance of identical egg cells

Evolutionary Fitness Darwinian Fitness – the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals Relative Fitness – the contribution of a genotype to the next generation compared to the contributions of alternative genotypes for the same locus Survival alone does not guarantee reproductive success

Modes of Selection Directional Selection – shifts the frequency curve for variations in some phenotypic character in one direction or the other by favoring what are initially relatively rare individuals that deviate from the average for that character Diversifying Selection – occurs when environmental conditions are varied in a way that favors individuals on both extremes of a phenotypic range over intermediate phenotypes Stabilizing Selection – acts against extreme phenotypes and favors the more common intermediate variants

Modes of Selection

Natural Selection Favors Sex Sexually reproducing individuals generate genetic variation that natural selection works on Sexual dimorphism – differences in male and female animal species that are not directly associated with reproduction, but increase an animals chance of reproducing

Natural Selection Favors Sex

Four Reasons Why Natural Selection Cannot Produce Perfection Evolution is limited by historical constraints – evolution does not scrap ancestral anatomy and build each new complex structure from scratch. Ex. Back problems in humans Adaptations are often compromises – seals could walk on rocks easier with legs, but could not swim as fast

Four Reasons Why Natural Selection Cannot Produce Perfection Not all evolution is adaptive – not all alleles fixed by genetic drift in the gene pool of the small founding population are better suited to the environment than alleles that are lost. Selection can only edit existing variations – Natural selection favors only the fittest variations from the phenotypes that are available, which may not be the ideal traits. New alleles do not arise on demand.