Evolution and Population Genetics Chapter 3 Evolution and Population Genetics

Evolution generates biodiversity Species = a population or group of populations Whose members share characteristics They can breed with one another and produce fertile offspring Population = a group of individuals of a species that live in the same area Evolution = means change over time Biological evolution: genetic change in populations over time Genetic changes lead to changes in appearance, functioning or behavior over generations

Natural selection Evolution may be random Or directed by natural selection Natural Selection = traits that enhance survival and reproduction Are passed on more frequently to future generations Than those that do not

Evolution by natural selection It is one of the best-supported and most illuminating concepts in all science It is the standpoint of modern biology We must understand it to appreciate environmental science Knowing ecology and learning the history of life Evolutionary processes influence pesticide resistance, agriculture, medicine, health, etc. In 1858, both Darwin and Wallace proposed natural selection as the mechanism of evolution

Natural selection shapes organisms Premises of natural selection: Organisms struggle to survive and reproduce Organisms produce more offspring than can survive Individuals of a species vary in their characteristics due to genes and the environment Some individuals are better suited to their environment and reproduce more effectively Natural selection acts on genetic variation

Genetic variation Adaptation = the process where, over time, characteristics (traits) that lead to better reproductive success Become more prevalent in the population Adaptive trait (adaptation) = a trait that promotes reproductive success Mutations = accidental changes in DNA that may be passed on to the next generation Non-lethal mutations provide the genetic variation on which natural selection acts Sexual reproduction also leads to variation

Natural selection acts on genetic variation Natural selection changes characteristics through: Directional selection = drives a feature in one direction Stabilizing selection = favors intermediate traits Preserving the status quo Disruptive selection = traits diverge in two or more directions

Environmental conditions affect selection Environmental conditions determine the pressures natural selection exerts These pressures affect who survives and reproduces Traits evolve that allow success in that environment But traits that promote success at one time or place may not do so at another Natural selection weeds out unfit individuals It also elaborates and diversifies traits that may produce new species

Selective pressures influence adaptation Related species in different environments Experience different pressures Evolve different traits Convergent evolution = unrelated species may evolve similar traits Because they live in similar environments

Evidence of natural selection is everywhere It is evident in every adaptation of every organism Artificial Selection = the process of selection conducted under human direction Producing the great variety of dog breeds and food crops

Evolution generates biological diversity Biological diversity (biodiversity) = the variety of life across all levels of biological organization Species Genes Populations Communities Scientists have described 1.8 million species Up to 100 million species may exist Tropical rainforests are rich in biodiversity

Speciation produces new types of organisms The process of generating new species from a single species Allopatric speciation: species formation due to physical separation of populations The main mode of speciation Populations can be separated by glaciers, rivers, mountains Each population gets its own set of mutations

Another type of speciation Sympatric speciation = species form from populations that become reproductively isolated within the same area Feed in different areas Mate in different seasons Hybridization between two species Mutations

Speciation results in diverse life forms How do major groups diverge over time? Phylogenetic trees (cladograms) = show relationships among species, groups, genes, etc. Scientists can trace how certain traits evolved

The fossil record Fossil: an imprint in stone of a dead organism Fossil record: the cumulative body of fossils worldwide The fossil record shows: Life has existed on Earth for at least 3.5 billion years Earlier types of organisms evolved into later ones The number of species has increased over time Most species have gone extinct There have been several mass extinctions in the past

Extinction Extinction = the disappearance of a species from Earth Species last 1-10 million years Biological diversity is now being lost at an astounding rate This loss of species is irreversible Number of species = speciation - extinction

Extinction is a natural process, but … Humans profoundly affect rates of extinction Biodiversity loss affects people directly Food, fiber, medicine, ecosystem services

Some species are more vulnerable to extinction Extinction occurs when the environment changes rapidly Natural selection can not keep up Many factors cause extinction: Severe weather, climate change, changing sea levels New species, small populations Specialized species Endemic species = a species only exists in a certain, specialized area Very susceptible to extinction These species usually have small populations

Many U.S. amphibian species are vulnerable Many U.S. amphibians have very small ranges They are vulnerable to extinction The Yosemite toad, Houston toad, Florida bog frog 40 salamander species are restricted to areas the size of a typical county Some U.S. salamander species live on top of single mountains

Earth has had several mass extinctions Background extinction rate = extinction usually occurs one species at a time Mass extinction events = killed off massive numbers of species at once Occurred five times in Earth’s history 50-95% of all species went extinct at one time Cretaceous-Tertiary (K-T) event: 65 million years ago Dinosaurs went extinct End-Permian event: 250 million years ago 75-95% of all species went extinct

The sixth mass extinction is upon us Humans are causing the sixth mass extinction event Resource depletion, population growth, development Destruction of natural habitats Hunting and harvesting of species Introduction of non-native species It is 100-1,000 times higher than the background rate and rising Amphibians are disappearing the fastest 170 species have already vanished It will take millions of years for life to recover

Ecology is studied at several levels Ecology and evolution are tightly intertwined Biosphere = the total living things on Earth And the areas they inhabit Community = interacting species living in the same area Ecosystem = communities and the nonliving material and forces they interact with

Levels of ecological studies Population ecology = investigates the dynamics of population change The factors affecting the distribution and abundance of members of a population Why some populations increase and others decrease Community ecology = focuses on patterns of species diversity and interactions Ecosystem ecology = studies living and nonliving components of systems to reveal patterns Nutrient and energy flows

Each organism has habitat needs Habitat = the environment where an organism lives It includes living and nonliving elements Habitat use = each organism thrives in certain habitats, but not in others Results in nonrandom patterns of use Habitat selection = the process by which organisms actively select habitats in which to live Availability and quality of habitat are crucial to an organism’s well-being Human developments conflict with this process

A specialized frog Epiphytes grow on trees for support Obtaining water from the air They collect pools of rainwater and pockets of leaf litter Frogs lay their eggs in these rainwater pools

Habitats vary Habitats vary with the body size and needs of species A soil mite vs. an elephant Species have different habitat needs at different times Migratory birds use different habitats during migration, summer and winter Species use different criteria to select habitat Soil, topography, vegetation, other species Water temperature, salinity, prey Species survival depends on having suitable habitat

Organismal ecology: niche Niche = an organism’s use of resources Along with its functional role in a community Habitat use, food selection, role in energy and nutrient flow, interactions with other individuals Specialists = have narrow niches and specific needs Extremely good at what they do But vulnerable when conditions change Generalists = species with broad niches They use a wide array of habitats and resources They can live in many different places

Population characteristics All populations show characteristics that help scientists predict their future dynamics Population size = the number of individual organisms present at a given time Numbers can increase, decrease, cycle or remain the same

Population characteristics Population density = the number of individuals in a population per unit area Large organisms usually have low densities They need many resources and a large area to survive High densities make it easier to find mates But increase competition and vulnerability to predation Increased transmission of diseases Low densities make it harder to find mates But individuals enjoy plentiful resources and space

Population characteristics Population distribution (dispersion) = spatial arrangement of organisms Random = haphazardly located individuals, with no pattern Uniform = individuals are evenly spaced Territoriality, competition Clumped = arranged according to availability of resources Most common in nature

Population characteristics Sex ratio = proportion of males to females In monogamous species, a 1:1 sex ratio maximizes population growth Age distribution (structure) = the relative numbers of organisms of each age in a population Age structure diagrams (pyramids) = show the age structure of populations In species that continue growing as they age Older individuals reproduce more (i.e. a tree) Experience makes older individuals better breeders

Birth and death rates Survivorship curves = the likelihood of death varies with age Type I: more deaths at older ages Type II: equal number of deaths at all ages Type III: more deaths at young ages

Four factors of population change Natality = births within the population Mortality = deaths within the population Immigration = arrival of individuals from outside the population Emigration = departure of individuals from the population Births and immigration add individuals Deaths and emigration remove individuals Crude birth (death) rates: number of births (deaths) per 1000 individuals per year

Population growth rate Natural rate of population growth = (Crude birth rate) – (crude death rate) Population change due to internal factors Population growth rate = (Crude birth rate + immigration rate) – (Crude death rate + emigration rate) Net changes in a population’s size/1000/year Growth rate as a percent = Population growth rate * 100% Populations of different sizes can be compared

Exponential population growth Exponential growth A population increases by a fixed percent Graphed as a J-shaped curve Exponential growth cannot be sustained indefinitely It occurs in nature with a: Small population Low competition Ideal conditions

Limiting factors restrain population growth Limiting factors = physical, chemical and biological attributes of the environment They restrain population growth Environmental resistance = all limiting factors taken together Stabilizes the population size Space, food, water, mates, shelter, suitable breeding sites, temperature, disease, predators Aquatic systems: salinity, sunlight, temperature, etc.

Carrying capacity Carrying capacity = the maximum population size of a species that its environment can sustain Limiting factors slow and stop exponential growth An S-shaped logistic growth curve Many factors contribute to environmental resistance and influence a population’s growth rate and carrying capacity

Population density impacts limiting factors Density-dependent factors = limiting factors whose influence is affected by population density Increased density increases the risk of predation and competition for mates Results in the logistic growth curve Larger populations have stronger environmental resistance Density-independent factors = limiting factors whose influence is not affected by population density Events such as floods, fires, and landslides

Perfect logistic curves aren’t often found

Carrying capacities can change Environments are complex and ever-changing Changing carrying capacities Humans lower environmental resistance for themselves Increasing our carrying capacity Technologies overcome limiting factors By increasing carrying capacity for humans We have reduced the carrying capacity for countless other organisms Calling into question our own long-term survival

Reproductive strategies vary among species Biotic potential = an organism’s capacity to produce offspring K-selected species = species with long gestation periods and few offspring Have a low biotic potential Stabilize at or near carrying capacity Good competitors r-selected species = species which reproduce quickly Have a high biotic potential Little parental care, populations fluctuate greatly

Costa Rica’s species were in danger Few people saw the need to conserve biological resources Until it became clear they were being rapidly lost

Conclusion The fundamentals of evolution and population ecology are integral to environmental science Natural selection, speciation and extinction help determine earth’s biodiversity Understanding how ecological processes function at the population level is crucial to protecting biodiversity