ESR 173 Unit 7 Lecture

• Biodiversity

• Evolution

• Species Interactions

Biodiversity

• Degree of variation of life-forms in a given habitat or ecosystem

• Measure of the health of the ecosystem– Biodiversity-stability hypothesis: greater

biodiversity means greater resilience to disturbance

• Varies geographically; some areas very rich in diversity and other areas much less so

Plants

Animals

Fungi

Protists

Bacteria

Archaea

1.7-1.9 million named and identified (to science) different unique kinds of livings organisms

The Theory of Biological Evolution

• Charles Darwin’s theory of evolution by means of natural selection explains the unity and diversity of life as “descent with modification”– Parents pass traits to their offspring (heredity)

– Individuals within a population vary in those traits (variation)

– Some individuals have traits that are better adapted to particular circumstances and conditions than other (fitness)

– Those more fit have greater reproductive success than those with less fit, increasing the proportion of “fit” genes in the population through each successive generation

– Heritable traits + variation + environment = differential reproduction

• All living organisms are related to one another in a common tree of descent

Charles Darwin

Primary Characteristics of Natural Selection

1. Inheritance of traits from one generation to the next and some variation in these traits—that is, genetic variability

2. Environmental variability acts on the genetic variation within the population favoring some traits over others

3. Differential reproduction that varies with the environment … those individuals with more favorable traits leave more offspring on average increasing the fitness of the population to the environment

• Observation #1: For any species, population sizes would increase exponentially if all individuals that are born reproduced successfully

• Observation #2: Populations tend to be stable in size, except for seasonal fluctuations

• Observation #3: Resources are limited

Natural Selection based on 3 Observations and 2 Inferences

• Inference #1: Production of more individuals than the environment can support leads to a struggle for existence among individuals of a population, with only a fraction of their offspring surviving– More offspring are born than survive to

reproduce in natural populations

• Observation #4: Members of a population vary extensively in their characteristics; no two individuals are exactly alike

Variation In Snails

Variation In Human Height

Variation in Human Height

Variation In Human Skin Color

• Observation #5: Much of this variation is heritable (passed from parent to offspring through reproduction); in other words, many of the traits an individual has are due to their inherited genetic makeup

• Inference #2: Survival depends in part on inherited traits; individuals whose inherited traits give them a high probability of surviving and reproducing are likely to leave more offspring than other individuals

• Inference #3: This unequal ability of individuals to survive and reproduce will lead to a gradual change (in genotype and phenotype frequencies) in a population, with favorable characteristics accumulating over generations

Summary of Natural Selection• Natural selection is differential success in

reproduction from interaction between individuals that vary in heritable traits and their environment

• Natural selection produces an increase over time in adaptation of organisms to their environment

• If an environment changes over time, different traits are favored resulting in a change in the frequency of the newly favored traits– Salt n’ pepper moth and industrial pollution– Blue eyes and UV-b radiation– Frizzle feather gene in chickens and temperature

Natural Selection

Geographic Isolation and the Origin of New Species

A single population is divided into two due to a geographic barrier to reproduction and over time, due to differences in the environment selecting for different traits becomes 2 different species.

Spotted Owl Species Have Genetic and Physical Differences

Genetic Diversity

• Genetic variation within a population depends on genetic diversity

• Genetic diversity is the degree of difference between the genes of individuals in a population

• Without genetic variation, a population cannot adapt to changes in the environment and so is at a greater risk of extinction

• Genetic variation is a result of mutations occurring in the replication of DNA

Habitat Diversity

Variation in the type of environments organisms can live in. More habitat diversity allows for greater species diversity.

Habitat Diversity Affects Species Diversity

Habitat Diversity

Species Diversity

• Species richness: the total number of different unique kinds of organisms (species) living in a habitat or ecosystem

• Species evenness: the relative percentage each species makes up of the total community

• Species dominance: a single species has a very large population relative to all of the other species populations

• Two communities, each with the same species richness have unequal species diversity if one has greater or lesser species evenness (has species dominance)

Is Richness important?• How does species richness affect the entire living system

– Experiments link total system productivity to richness (probably because resources include lots of different things [sunlight, different minerals] and the entire spectrum of resources are more efficiently used by a group of species, each most efficient with a particular resource)

– There may be a link between richness (diversity) and the stability of ecosystems – Rich, diverse systems may be less susceptible to invasion by foreign species ･ diversity

may reduce the total amount of unused resources, preventing a new species from gaining a foothold

– Diverse communities may be less susceptible to changes brought about through disease– Rich communities have lower average species density, so many species may be below their

threshold density of susceptibility, preventing disease from establishing itself

• Importance to us– Many individual species provide benefits to us, and the argument is that we do not know

what is out there, but we do know that losing a species is the loss of potential human benefit– aesthetic considerations (not to be discounted in the face of short-term economic benefit)– ecosystems provide many services (fresh water, cleaning the air, etc.), and there may be a

link between the diversity of an ecosystem and its ability to provide those systems (here, more diversity means better services)

Species Diversity

Stabilizing selection

• is a form of selection in which both extremes form an array of phenotypes are eliminated– the result is an increase in the frequency of

the already common intermediate phenotype– for example, human birthweight is under

stabilizing selection

Forms of selection found in nature

Disruptive selection

• is a form of selection in which the two extremes in an array of phenotypes become more common in the population– selection acts to eliminate the intermediate

phenotypes– for example, beak size in African blackbellied

seedcracker finches is under disruptive selection because the available seeds are only large or small

Forms of selection found in nature

Directional selection

• is a form of selection that occurs when selection acts to eliminate one extreme from an array of phenotypes– for example, the enzyme lactate

dehydrogenase has a cold-adapted form that is more common in northern latitudes

Forms of selection found in nature

Three kinds of natural selection

Symbiosis• Living together

• A relationship in which ones organism (the symbiont) lives on or in another organism (the host)

• Parasitism: the symbiont benefits (+) and the host is harmed (-)

• Mutualism: both the symbiont and the host benefit (++)

• Commensalism: the symbiont benefits (+) and the host is neither benefited nor harmed (0)

Parasitism: + -

Mutualism: + +

Mutualism: + +

Commensalism: + 0