Natural Selection Chapter 15

Natural Selection? On the following three slides, you will read statements based on student ideas about now natural selection works. Use your clickers to vote for whether you think that the statement IS or is NOT what scientists mean by “natural selection.”

How Selection Works

1. Variation Exists All populations vary as the result of the accumulation of small, random mutations over many generations.

2. Inheritance of traits Inheritable traits (those coded for by genes) are passed directly to the offspring from the parents through genetic information.

3. Differential Survival More offspring are born than can survive. Many offspring die young. Those with traits best suited to the environment are more likely, though not guaranteed, to survive.

4. Differential Reproduction Some survivors fail to reproduce. Some have traits that better insure reproduction than others.

5. Differential Inheritance Survivors that reproduce pass some of their traits on to their offspring. Those with favorable traits may pass those favorable traits on — or not.

Natural Selection in Action Peppered Moth simulation: http://www.echalk.co.uk/Science/Biology/ PepperedMoth/PepperedMoth.htm

Natural Selection in Cats Traits that ensure a domestic cat’s survival among humans: Endearing qualities: purring, snuggling, being playful, cuteness. Helpful qualities: rodent control.

When domestic cats are abandoned: Most die within a week or two from starvation, accident, or predation. The traits that help them in a domestic setting are useless in the wild. The few that survive have the strongest feral instincts. Av. feral lifespan: 2-4 years

Selection’s Effects Generation 2 Generation 3 Generation 1

As time passes... With each generation, domestic traits are selected against, while feral traits are favored. After several generations, even kittens that are captured young can be challenging to tame.

Genes and Evolution Genes are the units of heredity. Genes code for proteins, which result in our set of traits. Genes are passed from parent to offspring through the sex cells.

Different “versions” of genes are alleles. Dominant alleles are expressed in the phenotype (expressed trait) even if only one copy is inherited. Recessive alleles are expressed only if two copies are inherited. “Genotype” is a description of the alleles for a given trait in an individual: BB, Bb, or bb

The Gene Pool Concept The “gene pool” of a population is the entire collection of alleles for a given trait throughout a given population. The word for all genes for all traits in an individual or population is genome.

Antibiotic Resistance Antibiotic resistance has been an increasing problem since the 1970s. How does natural selection contribute to the rise in antibiotic-resistant bacteria? (Note it is bacteria that become resistant, not people. Bacteria do not become “immune” — they do not have immune systems.)

Helicobacter pylori is a bacteria that causes stomach ulcers. Most H. pylori bacteria, when encountering an antibiotic, metabolize it with an enzyme that turns it to a toxin. A few H. pylori bacteria have a mutation that interferes with the production of the enzyme. The mutant bacteria aren’t affected by antibiotics.

Hardy-Weinberg Equilibrium Allele ratios in a gene pool will not change from generation to generation (that is, no evolution) only if all these things are true: No mutation Large population No migration No selection Random mating

Mutations Small mutations appear randomly in populations. The appearance of mutations changes allele ratios by “breaking” functional alleles (as in genetic disorders) adding new alleles

Population size In large populations, random events have a very small effect. In small populations, because fewer individuals have any given trait, random events can have a larger effect. Changes in gene ratios caused by random events are called “genetic drift.”

A population bottleneck is genetic drift.

Founder effect is genetic drift.

Migration Migration into and out of a population can change gene ratios. Immigrants can bring in new mutations, or a different ratio of alleles. Emigrants may take away a high proportion of a certain allele. Small population are more affected than large populations.

Selection Selection may increase or decrease the frequency of certain alleles: Directional selection: favors one end of a range over another. Disruptive selection: disfavors the midrange. Stabilizing selection: favors the mid-range.

Mating Behavior Mate choice among most organisms is selective, not random. Sexual selection may favor traits that are in conflict with natural selection. For example, bright-colored male guppies attract more females, but are also more visible to predators.

Evolution Happens Because perfect Hardy-Weinberg equilibrium is never met with in nature, all populations experience small shifts in gene ratios with each generation. Gene ratio shifts may fluctuate with cyclical changes in climate. Long-term changes in habitat (such as global climate change) can shift the gene ratios far enough to bring about speciation.

Recap Natural Selection is a phenomenon that can be studied directly. Natural Selection causes change in the genetics of a population over generations (evolution). Other factors that can change genetics of a population include migration, sexual selection, mutations, and effects of random events in small populations.