Unit 11: Evolution

Fossils

the preserved/traces of remains of ancient organisms

Absolute dating

process of determining age of remains based on content & decay rate of radioactive isotopes

Relative dating

process of determining age of remains based on position in rock strata

Law of Superposition

in any undisturbed sequence of rocks deposited in layers, the youngest layer is on top & the oldest on bottom

Index fossil

• widely distributed fossil

• of narrow range in time

• regarded as characteristic of a given geological formation

Biogeography

geographic distribution of organisms on Earth indicates evolution & movement of tectonic plates over geological time

Comparative anatomy

• homologous structures

• analogous structures

• vestigial structures

Homologous structures

• structurally similar features in different organisms suggesting common ancestry

• structures perform different functions

• may suggest divergent evolution

Analogous structures

• similar features found in unrelated organisms that have evolved to perform the same function

• structurally dissimilar

• no common ancestry

• suggest convergent evolution

Vestigial structures

an anatomical feature that no longer seems to have a purpose in the current form of an organism

Comparative Embryology

• study of similarities & differences in development of embryos of different species

• similarities in embryos are evidence of common ancestry

Molecular evidence

comparing DNA sequences & protein sequencing between species to determine relatedness

Molecular clock

using the # of changes in sequences of biomolecules (mutations) to deduce time in prehistory when 2 or more life forms diverged

• mutation rate is relatively constant, therefore history can be inferred

Ideas about how

species change over time

LaMarck believed that evolutionary changes were caused by

organisms actively adapting themselves to environmental conditions (disproven)

Law of Use and Disuse (disproven)

the more a particular structure is used → the more prominent and well-developed it will become (& vice versa)

Inheritance of Acquired Characteristics (disproven)

belief that traits an organism developed can be passed down

No genetic link to explain how

developed traits are heritable

Malthus realized populations tended to increase

geometrically

• limited supply of resources could not keep up (competitive situation)

Darwin had 3 major points in his theory of “descent with

modification” through natural selection

1. Species over-

reproduce

2. Competition for

limited resources occurs

3. Variations exist among individuals -

• some are better at competing for limited resources

  • ultimately reproduce more (offspring skews the gene pool)

  • leads to evolution

Adaptation (favorable genetic variation)

makes an organism more likely to survive & reproduce

Fitness (measure of reproductive success)

how many surviving offspring are produced

Speciation

accumulation of favorable adaptations over time

• result in formation of new species

Genetic equilibrium is a condition in which

allele frequencies in a population don’t change from 1 generation to the next

With genetic equilibrium, the rate of trait occurrence

remains constant (no evolution occurs)

Evolution

disruption of genetic equilibrium

Directional selection

extreme phenotype becomes a favorable adaptation

Directional selection is usually caused by

a change to the environment/migration to new habitats

Stabilizing selection

average phenotypes become more favorable → extreme phenotypes become more unfavorable

Stabilizing selection usually inhibits the

rate of evolution bc of a narrowed range of variation

Disruptive selection (rare form of natural selection)

extreme phenotypes become more favorable than average phenotypes

• creates 2 separate subpopulations

Hardy-Weinberg Principle

outlines conditions necessary for genetic equilibrium in a population to be maintained (no evolution)

Allelic frequency

term used to describe how often a particular allele occurs in a population

Gene pool

all of the possible alleles that exist in a population

There are 5 conditions necessary to

maintain genetic equilibrium

1. No

mutations

2. Individuals cannot enter

nor leave a population

Emigration =

leaving

Immigration =

entering

Gene flow

movement of genes from 1 population to another

3. Large population

(prevents genetic drift)

Genetic drift

change in the allelic frequency of a small population due to chance

2 important causes of

genetic drift

Founder’s Effect (reduces genetic variation)

populations started by a few pioneering individuals moving into a new region

Bottleneck Effect (reduces genetic variation)

small group of surviving members of a population breeding together

4. Individuals mate randomly;

no selective breeding

5. No natural selection

(equal survivorship)

Since these conditions can never be met, genetic equilibrium can’t

possibly exist (evolution must occur)

Evolution

• allelic frequencies are never in equilibrium

• gene pools change over time

• phenotype frequencies are changing

Hardy Weinberg mathematical formulas were

used to predict

• allele & genotype frequencies in a population

• (can predict) occurrence of hidden genotypes

Formula for freq. of dominant & recessive alleles in population

p + q = 1

Formula for freq. of genotypes in population

p² + 2pq + q² = 1

Disruption of genetic equilibrium may lead to evolution of existing species but

may not result in formation of new species

Formation of new species may occur over

many generations

Formation of new species requires

isolation of subpopulations

1. Geographical isolation

new land/water barriers form

• e.g. change in the course of a river; new highway built across a field

1. allopatric speciation (apart)

species arise in separate settings

2. Reproductive isolation

inability of formerly interbreeding organisms to mate & produce fertile offspring

2. sympatric speciation

species arise in the same setting

2a. Prezygotic (prevents breeding)

species evolve adaptations that prevent mating

• e.g. different breeding times/mating calls

2b Postzygotic (do breed)

though species interbreed, their offspring don’t go on to make other offspring

• mating is unsuccessful

• e.g. tigers & lions produce ligers; ligers are sterile

Various definitions of species exist, generally:

• members of a species share identical traits

• live in the same geographic area

• have the opportunity to mate

• can mate successfully to produce fertile offspring

Coevolution

joint change in 2 or more species in close interaction

• e.g. predators/prey or plants/pollinators

Convergent evolution

unrelated species become more & more alike in appearance as they adapt to similar environmental pressures

• e.g. different plants displaying similar adaptations to desert climate

Divergent evolution

process by which 2 related species become more dissimilar over time as they adapt to separate environments

Adaptive radiation (type of divergent evolution)

process by which a single species develops simultaneously into many different species

Gradualism

proposes evolution is a slow, gradual & continuous change

• fossils show slight changes in organisms between rock layers

Punctuated equilibrium

proposes that species have long periods of genetic equilibrium interrupted by geologically brief periods of rapid evolutionary change