EVOLUTION Crash Course – Natural Selection Crash Course – Speciation

Evolution Change in a population over time Change in genetic make-up (allele frequencies) Results in new species Species are defined by their genetic characteristics. Each species has a typical number of chromosomes and a particular set of genes. Within a species, however organisms differ. Because individuals have different alelles (versions of a gene), they have different traits. As species evolve over time, some traits (and the alleles that cause them) become more common and others disappear entirely.

Population A group of organisms of the same species that share a geographical area and breed with each other

Adaptation Any trait or behavior that makes an organism likely to survive in its environment

Can you spot the adaptation?

Mechanisms of Evolution “Descent with Modification”

Mechanisms of Evolution Natural Selection = “survival of the fittest” organisms most suited for survival will reproduce and pass genes for the beneficial trait Alleles for a trait that give an advantage increase Ex: giraffe neck length

Mechanisms of Evolution Genetic Drift = random changes in allele frequencies due to chance not based on adaptations or fitness Ex: landslide

Mechanisms of Evolution Migration = movement of individuals into and out of a population Individuals bring their alleles with them More likely to impact smaller populations

Mechanisms of Evolution Bottleneck Effect = when an entire population is reduced to just a few individuals Leads to decreased genetic variation Population susceptible to extinction EX: overhunting of cheetahs

Mechanisms of Evolution Founder Effect = formation of a new population when a few individuals leave a larger population New gene pool can be very different from the original EX: island species

Speciation through Isolation Formation of new, genetically distinct species from populations of existing species Isolating mechanisms prevent mating and breeding between two species Geographical isolation = physical barriers Behavioral isolation = differences in courtship rituals Temporal isolation = differences in mating times

Evidence for Evolution “Descent with Modification”

Evidence for Evolution Fossils = remains or traces of organisms that once existed on Earth Support the ideas that life on Earth has changed and that new species arise from existing species EX: fish existed before four-limbed creatures

Evidence for Evolution Anatomy and Development Homologous structures = similarities in structure due to evolution from common ancestor EX: limbs Analogous structures = function similarly because they are adaptations to similar conditions EX: wings Vestigial structures = structures that have lost their original function EX: leg bones in whales

Homologous Structures Forelimbs of mammals

Analogous Structures Wings built very differently

Vestigial Structures Leg bones in whales Tail bone in humans

Evidence for Evolution Genes and Molecules (genetic codes) Genetic code is nearly identical in all living things The more similar two gene sequences are the more closely they are related Can be used to create a lineage

Patterns in Evolution “Descent with Modification”

Convergent evolution Evolution toward similar characteristics in unrelated species due to similar environments Analogous structures

Divergent Evolution When closely related species evolve in different directions Homologous structures

Coevolution Evolution of one species drives the evolution of another

Effect of Natural Selection on Trait Distribution “Descent with Modification”

Normal Distribution Most individuals fall in the middle with few outliers Ex: human height Evolution by natural selection can change the distribution of traits ib a population so the “shape” of the graph can change or the direction can shift

Stabilizing selection The “middle” trait is selected for; more individuals fall in the middle and even fewer fall at either end

Directional Selection one of the extremes will be selected for and become the new normal; direction of distribution shifts

Disruptive selection “middle” trait selected against so that both extremes become more common

Keystone Prep EVOLUTION

Keystone Prep Which of the following is a result of cheetahs having gone through a population bottleneck? The cheetah species is more likely to become extinct Mutations occur more frequently in the cheetah genome Individual cheetahs have different alleles for many genes The cheetah population cannot increase past a certain size

Keystone Prep Which is one way that natural selection differs from genetic drift? Natural selection causes the frequencies of alleles in a population to change Natural selection affects smaller populations more often than larger populations Natural selection depends on some traits helping individuals in their environment

Keystone Prep A particular gene has two alleles, G and g. Each allele has a frequency of 50% in a population. Which is most likely to preserve these allele frequencies? The population doubles in size over several generations A number of individuals with one allele migrate from the population One allele results in a phenotype that better enables organisms to reproduce

Keystone Prep Recently, bears have been found in the Arctic Circle that are hybrids between grizzly bears and polar bears. Which type of isolating mechanism usually keeps these species from producing offspring? Mating and breeding in different habitats Production of a nonviable embryo or fetus Inability of sperm to reach the egg during mating Failure of he fertilized egg to implant in the uterus

Keystone Prep Under which of the following conditions would a dominant allele that coded for a fatal disease most likely remain in a population? The allele is acted on by natural selection The gene is expressed only in members of one sex The gene is expressed in individuals later in life The allele mutates to a nonfatal form during an individual’s lifetime

Keystone Prep Imagine that DNA replication and cell division were completely error-free and that cell DNA were completely protected from substances that cause mutation. How would natural selection be affected? It would become more efficient since it removes mutations from the population It would become ineffective since it depend on variations within populations

Keystone Prep

Keystone Prep

Keystone Prep

Keystone Prep

Keystone Prep