Natural Selection The Mechanism of Evolution

Natural Selection – Darwin’s and Wallace’s Big Idea Both Darwin and Wallace proposed natural selection as an explanation for how evolution occurs - Why is Evolution by Natural Selection so strongly associated with Darwin? - He published The Origin of Species a lengthy and well written explanation of natural selection

The Foundations of Natural Selection Observations of the natural world – the Beagle voyage The writings of Thomas Malthus- population grows faster than food – leads to competition Uniformitarianism – proposed by James Hutton - the processes that shape the earth are constant and ongoing at a slow uniform rate - they are too slow to be seen in a human lifetime but add up to major changes over geologic time

The Foundations of Natural Selection continued Artificial selection- people have chosen variations in animals and plants to emphasize and produced plants and animals that are beneficial to humans

Artificial Selection

Genetic Variation: a key to natural selection Mutation Occurs randomly -1 in 100,000 replications - creates new alleles - must occur in sex cells to be passed on - can be harmful, helpful or have no effect

Genetic variation the key to evolution - continued Migration – new individuals join a population and bring in new genes Genes can move into or out of a population by migration

Variation a key to selection Recombination Meiosis-crossing over Sexual Reproduction

Natural Selection All species have genetic variation The environment provides challenges to an individual ‘s ability to reproduce More individuals are born than can survive creates competition to survive Individuals who are better suited to the environment produce more offspring The traits of individuals who are better suited to the environment increase in a population

Natural Selection in a Nutshell Differences in phenotype leads to differences in the success rate of organisms to reproduce. Those phenotypes which are better suited to their environment, have more reproductive success Natural Selection acts on phenotype to change the gene pool

Natural Selection all of the genes in a population are referred to as the population’s gene pool Over time, the selection of phenotypes, changes the frequency with which alleles occur in a population’s gene pool - the alleles for advantageous traits become more common, while alleles for harmful traits become less common

Natural Selection Leads to Adaptations Adaptations are traits that enhance the survival and reproductive success of an organism

3 Types of Natural Selection: Directional Selection Selects for one of the extreme phenotypes - directional selection occurs in response to a change in the environment that gives a competitive advantage to a particular phenotype

Examples of Directional Selection Bacteria developing resistance to antibiotics The beak sizes of ground finches on Daphne Island

3 Types of Natural Selection: Stabilizing Selection Selects for the average phenotype and against the extreme phenotypes Occurs when the environment is stable for long periods Example birth weight of babies.

Stabilizing Selection

3 Types of Natural Selection: Disruptive Selection Selects against the average and selects both extremes. Example: African Seed Crackers

African Seed Crackers Birds feed on two types of seeds one large, one small. Birds with average size bills can’t eat either type efficiently And so aren’t common in the population

Genetic Drift: Another Force for Change Genetic Drift = Changes in a population’s gene pool that happen by chance - its effect is most profound in small populations - it reduces genetic variation in these - this could reduce a population’s ability to survive environmental change

Genetic Drift Continued An organism can be well suited to its environment can be removed by chance The result: Organisms that aren’t as well adapted can survive to reproduce by chance - this shifts the gene pool of the population

Genetic Drift Once an allele is removed from a population, it is unlikely to return

Genetic Drift – The Bottle-neck effect Something happens to catastrophically reduce a population’s size for at least one generation Dramatically changes allele frequencies in a small population

Bottle necked species Elephant seals hunted to near extinction – 20 individuals - which 20 survived? The best adapted or the least desireable? Was their survival random luck? -now number 30,000 but all individuals are descended from the 20 survivors -little genetic diversity this population has a reduced ability to survive an environmental challenge

Founder Effect Occurs when a small population migrates to a new area or becomes isolated Limited gene pool – over time some generally rare traits become more common - Huntington’s disease in South Africa - Amish communities FE most commonly recognized in genetic disorders – occurs in other traits as well When the Affrikans setteled S.Africa their original set of genes had a disproportionate number of Huntington’s alleles – since it was a small isolated population Huntingtons is much more common there than eleswhere

Genetic Drift as a force for change In small isolated populations - genetic drift could allow otherwise rare alleles to become common - this could cause the isolated population to diverge from the main population and over time become a new species