EVOLUTION

NUMBER OF SPECIES ON EARTH

• Described by scientists: 1.5-1.8 million• Estimate of total #: 13-20 million• How did we get so many different

species on earth?

EVOLUTION

• Evolution-change over time

• Biological evolution: change in a population’s gene pool over time

• Gene pool-all of the genes present in a population (gene-sequence of DNA that codes for a particular trait)

• Leads to changes in frequency of an appearance or behavior from generation to generation

BIOLOGICAL EVOLUTION• Example of change in trait (gene) frequency:

• Today: 40% of mice in a population are brown and 60% are tan

• The next generation: 28% brown, 72% tan

MECHANISMS OF BIOLOGICAL EVOLUTION

• Mutation

• Migration

• Genetic drift

• Natural selection

MUTATION• Changes in DNA

• The change needs to occur in a sperm or egg cell (for evolution to occur) to be passed on to offspring

MIGRATION• Half the population has one trait (striped) and

the other half are solid

• Immigration into or emigration out of a population could change the proportion of solid to striped fish

• Over generations, the gene pool will change

GENETIC DRIFT• Sometimes a natural disaster kills a large

number in a population, leaving only a few individuals behind to reproduce

• This population would have a different gene pool from the original population happening by chance

NATURAL SELECTION• The process by which traits that improve an

organism’s chances for survival and reproduction are passed on more frequently to future generations than those that do not have the trait(s)

NATURAL SELECTION• Conditions of natural selection:• Organisms produce more offspring than can survive

• Individuals in a population vary in their characteristics

• Individuals vary in their fitness

• Survival of the fittest

ORGANISMS PRODUCE MORE OFFSPRING THAN CAN SURVIVE

• If every individual in a population reproduced to its full potential, the population would grow exponentially, BUT in nature there are things that limit pop. Size (not enough resources, predators, etc)

• Because of these limiting factors, a struggle for existence (competition) between the same species is created

INDIVIDUALS OF A SPECIES VARY IN THEIR CHARACTERISTICS

• Not all individuals in a pop. are the same

• Variations are due to both genes and the environment

• These variations must be HERITABLE (able to be passed from parent to offspring)

INDIVIDUALS VARY IN THEIR FITNESS

• Variation can sometimes be harmful or helpful (most have little effect)

• Individuals with helpful variations are better suited to their environment than individuals without them, organisms better suited to their environment are more likely to survive and reproduce

• Fitness-describes how reproductively successful an organism is

• A heritable trait that increases an individual’s fitness is an ADAPTATION

SURVIVAL OF THE FITTEST

• An individual with high fitness produces more offspring and passes on its genes more frequently than an individual with low fitness

• Therefore, the next generation will have a higher proportion of individuals with that trait (or carrying that gene)

• **fitness is always defined by the context of an organism’s environment and environments can change

• A trait that is adaptive in one location or season may not be adaptive in another (an organism that is “fittest” in one place and time may not be the fittest forever

3 TYPES OF NATURAL SELECTION

• Directional• Stabilizing• Diversifying

DIRECTIONAL NATURAL SELECTION

• Individuals with traits at one end of the normal range become more common than the midrange

• It pays to be different• Periods of environmental change

STABILIZING NATURAL SELECTION• Favors individuals with average genetic

make-up while eliminating individuals on both ends

• “it pays to be average”• Little environmental changes, species

are well-adapted

DIVERSIFYING NATURAL SELECTION• Eliminates individuals with normal

genetic make-up while favoring individuals at both extremes

• “it does not pay to be normal”

TYPES OF NATURAL SELECTION

MALE AND FEMALE GUPPIES

• If nature works on a population equally, why are there differences in males and females in certain species?

SEXUAL SELECTION

• This occurs when females choose their mates based on some trait that signifies “fitness”

• In this case, females (not the environment) are determining which traits are passed on to future generations

ARTIFICIAL SELECTION

• The process of selection conducted under human direction

• Examples: dogs-breeders maintain varieties by by allowing only like individuals to breed

SPECIATION• The process by which new species are generated

• There are many mechanisms of speciation, but we will focus on geographic isolation (allopatric speciation)

• Allopatric speciation:

• A population is somehow broken up geographically (by a river, mountain range, etc.) so the isolated populations cannot interbreed anymore (no longer share a gene pool)

• Mutations that arise in the population of one group cannot spread to the other population (not interbreeding) so each population develops their own set of mutations, so much so even if they ever come into contact again they WON’T be able to interbreed

Allopatric Speciation

EXTINCTION• The disappearance of a species from earth

• 99% of species that have ever lived have gone extinct

• Average life span of a species is 1-10 million years

• In general, extinction occurs when the environment changes rapidly or severely enough that a species cannot adapt

• Background extinction: when extinction occurs gradually, one species at a time

• Mass extinction: huge number of species at once, there have been 5 during earth’s history-the last mass extinction occurred 65 million years ago and the dinosaurs were wiped out along with approx. 70% of species at the time

EXTINCTION• Current extinction rates are 100-1000x the natural

background rate

• Are humans causing the next mass extinction?

NICHES

• Can be described as either generalists or specialists

GENERALIST SPECIES

• Can live in many places, eat wide variety of food, and tolerate a wide range of environmental conditions

• Examples: mice, rats, deer, cockroaches, channel catfish

GENERALIST SPECIES

SPECIALIST SPECIES

• Live in only one type of habitat, one of very few types of food, tolerate narrow climatic and environmental range

• Examples: spotted owls, giant pandas, tiger salamanders

SPECIALIST SPECIES