Chapter 1 – Adaptation by Natural Selection
Explaining Adaptation Before Darwin
●Organisms are complexly adapted to their environments to help them survive and reproduce
●Human eye:
○Allow us to move thru environment, locate resources, avoid dangers
○Many parts:
■Light enters thru transparent opening
■Passes thru the iris, which regulates amount of light entering the eye and allows it to function in many different lighting conditions
■Light passes thru lens that projects an image onto the retina on the back surface of the eye
■Many light-sensitive cells turn the image into nerve impulses, send to brain
○Humans + terrestrial mammal eyes work a lot like camera lens; other animals’ eyes work differently
●Pre-Darwin, people believed adaptations were divine creation
○Clearly the eye is designed to see, so there must be a designer
Darwin’s Theory of Adaptation
●Darwin was supposed to be a doctor, but eventually pursued natural history
●Darwin’s theory was rejected by most scientists and considered heresy
Darwin’s Postulates
●Struggle for existence
○Ability of a population to expand is infinite, but the environment has a finite ability to support populations
○When food is plentiful, populations will grow until their numbers outweigh food supply
●Variation in fitness
○Organisms within a population vary
○This variation affects individuals’ ability to survive and reproduce
●Inheritance of variation
○Variation in individuals is passed on to offspring
○If a trait is advantageous, that individual will better survive → more offspring → that trait will become more common
●The competition for resources was referred to as “struggle for existence”
●Coined the term natural selection referring to artificial selection practiced by animal/plant breeders An Example of Adaptation by Natural Selection
●Darwin’s finches were crucial to the development of his theory
●Later ecologists studied finches on one of the Galapagos Islands, called Daphne Major
○Medium ground finch – small bird that eats seeds
●A few years into the study, there was a severe drought → plants produced fewer seeds → finches quickly ate the small, soft seeds and only large, hard seeds were left
●The drought displayed all 3 of Darwin’s postulates:
○Food supply was not enough to sustain the entire population
■Many finches did not survive the drought
○Beak depth/size varied among the finches on the island, and it affected survival
■Large beaks → easier to eat large, hard seeds
○Parents and offspring had similar beak sizes
■Average beak depth of all birds before the drought was much smaller than after the drought
●Morphology: Organism’s size, shape, and composition
○The morphology of the bird population changed as birds became adapted to the environment
●Over 2 years, beak size increased 4%
●This change will not happen forever
○Disadvantages to larger beaks: less likely to survive juvenile period (require more
food)
●Evolutionary theory predicts that over time, avg beak length will increase until the costs of a larger beak exceed the benefits
○Finches with the average beak size will thrive; larger/shallower beaks will be disadvantaged
○This is called an equilibrium, and the process that puts a trait at equilibrium is called stabilizing selection
●Populations are affected other processes that cause traits to change unpredictably
●Populations do not remain static over time unless selection is operating to keep it that way
●Natural selection has no foresight; can only adapt organisms to current environment
○Environments change over time → selection traces these changes
○In the study, an unusually wet season a few years later favored small beaks, and reversed the changes in beak size
●Essentialism: pre-Darwinian view that species were fixed in nature
○“Adding a side to a triangle doesn’t make it a new triangle, it makes a rectangle. They believed the same about species – if you changed a finch, it was no longer a finch”
●Both stasis (staying the same) and change result from natural selection
Individual Selection
●Selection produces adaptations that benefit individuals ○Often leads to changes that allow population to compete with other species
●Sometimes selection increases reproductive success of individuals but decreases average reproductive success of the group
●Almost all organisms produce more offspring than are necessary to maintain the species
○Exhibits conflict b/n individual and group interests
●The offspring who don’t survive are considered a waste of resources for the rest of the group
○Too many offspring → overexploitation of resources
○Species as a whole is more likely to survive if all females produced fewer offspring, but natural selection among individuals favors females who produce many offspring
●Fecundity: ability to produce offspring
●EX: most of a group of monkeys produce 2 offspring, but have a few high-fecundity females that produce 10 offspring
○High-fecundity females have high-fecundity daughters → proportion of high-fecundity females increase in the next generation
○As time goes on, proportion of high-fecundity females increases rapidly
●Natural selection results from competition among individuals, not competition among species
Evolution of Complex Adaptations
●Accumulation of small variations by natural selection gives rise to complex adaptations
Why Small Variations Are Important
●Continuous Variation: trait that has infinite amounts of variation between two extremes
○Ex: human height
●Discontinuous Variation: trait that has several distinct types with no intermediates
○Very rare in nature
●Many thought that new species arose as discontinuous variants; Darwin thought it did not play an important role in evolution
○Believed (correctly) that continuous variation is essential for complex adaptations
■Very unlikely that the eye evolved all at once
●Selection is a cumulative process
Why Intermediate Steps Are Favored by Selection
●Argument against small steps: unlikely for all changes necessary for a complex adaptation to each be adaptive
○“An eye is usefully only after all parts of the complexity have been assembled; until then, it is worse than no eye at all”
●Darwin’s response was that it wasn’t worse than no eye at all!
●Many organisms display the likely stages of the process:
○Many invertebrates have a simple light-sensitive spot ■Photoreceptors have evolved many times from normal surface cells
■Favored when info ab light intensity is useful
○Light-sensitive spot forms a depression
■Light does not hit all cells at the same time → provides info about the direction the light is coming from
■Mobile organisms may need better info about light direction than mobile ones
○Depression deepens in a series of steps
■Each deepening depression → better directional selection (all of the steps needed for this are adaptive)
○If depression gets deep enough, can form images on light-sensitive tissue
■Selection could favor neural machinery necessary to interpret the image in certain settings/organisms
○Transparent cover forms over depression
■Protects eye interior from parasites and damage
○Lens evolves thru gradual modification of the transparent cover or internal structures
●“Evolution produces adaptations like a tinkerer, not an engineer”
○New traits/organisms are created thru small changes to existing ones
●Chance does play a powerful role in evolution
○If even a single step of a complex trait were not favored by selection, would be difficult to get the trait
○However, cumulative adaptations are very powerful
●Convergence: evolution of similar adaptations in unrelated groups of animals
○Proves how strong selection can be
●EXAMPLE: marsupial mammals vs placental mammals
○Most of the world’s mammals are placental mammals , which have long pregnancies and nourish young within the uterus
○Australia and South America became separated from Pangaea before placental mammals evolved
■They have marsupials that rear their young in external pouches
○Some of these marsupial mammals looked very similar to placental mammals elsewhere
■South American marsupial saber-toothed cat had many of the same traits and adaptations as the North American placental saber-toothed cat
■Last common ancestor of the two mammal types was a nocturnal, insectivorous shrew-like creature from 120mya
●EXAMPLE 2: aquatic eyes
○Spherical gradient lens is adaptive for aquatic organisms
■Better light-gathering ability and sharper image over full 180 degree visual
field for underwater lighting
■These eyes have evolved independently 8 times in only distantly related aquatic species
Explaining Adaptation Before Darwin
●Organisms are complexly adapted to their environments to help them survive and reproduce
●Human eye:
○Allow us to move thru environment, locate resources, avoid dangers
○Many parts:
■Light enters thru transparent opening
■Passes thru the iris, which regulates amount of light entering the eye and allows it to function in many different lighting conditions
■Light passes thru lens that projects an image onto the retina on the back surface of the eye
■Many light-sensitive cells turn the image into nerve impulses, send to brain
○Humans + terrestrial mammal eyes work a lot like camera lens; other animals’ eyes work differently
●Pre-Darwin, people believed adaptations were divine creation
○Clearly the eye is designed to see, so there must be a designer
Darwin’s Theory of Adaptation
●Darwin was supposed to be a doctor, but eventually pursued natural history
●Darwin’s theory was rejected by most scientists and considered heresy
Darwin’s Postulates
●Struggle for existence
○Ability of a population to expand is infinite, but the environment has a finite ability to support populations
○When food is plentiful, populations will grow until their numbers outweigh food supply
●Variation in fitness
○Organisms within a population vary
○This variation affects individuals’ ability to survive and reproduce
●Inheritance of variation
○Variation in individuals is passed on to offspring
○If a trait is advantageous, that individual will better survive → more offspring → that trait will become more common
●The competition for resources was referred to as “struggle for existence”
●Coined the term natural selection referring to artificial selection practiced by animal/plant breeders An Example of Adaptation by Natural Selection
●Darwin’s finches were crucial to the development of his theory
●Later ecologists studied finches on one of the Galapagos Islands, called Daphne Major
○Medium ground finch – small bird that eats seeds
●A few years into the study, there was a severe drought → plants produced fewer seeds → finches quickly ate the small, soft seeds and only large, hard seeds were left
●The drought displayed all 3 of Darwin’s postulates:
○Food supply was not enough to sustain the entire population
■Many finches did not survive the drought
○Beak depth/size varied among the finches on the island, and it affected survival
■Large beaks → easier to eat large, hard seeds
○Parents and offspring had similar beak sizes
■Average beak depth of all birds before the drought was much smaller than after the drought
●Morphology: Organism’s size, shape, and composition
○The morphology of the bird population changed as birds became adapted to the environment
●Over 2 years, beak size increased 4%
●This change will not happen forever
○Disadvantages to larger beaks: less likely to survive juvenile period (require more
food)
●Evolutionary theory predicts that over time, avg beak length will increase until the costs of a larger beak exceed the benefits
○Finches with the average beak size will thrive; larger/shallower beaks will be disadvantaged
○This is called an equilibrium, and the process that puts a trait at equilibrium is called stabilizing selection
●Populations are affected other processes that cause traits to change unpredictably
●Populations do not remain static over time unless selection is operating to keep it that way
●Natural selection has no foresight; can only adapt organisms to current environment
○Environments change over time → selection traces these changes
○In the study, an unusually wet season a few years later favored small beaks, and reversed the changes in beak size
●Essentialism: pre-Darwinian view that species were fixed in nature
○“Adding a side to a triangle doesn’t make it a new triangle, it makes a rectangle. They believed the same about species – if you changed a finch, it was no longer a finch”
●Both stasis (staying the same) and change result from natural selection
Individual Selection
●Selection produces adaptations that benefit individuals ○Often leads to changes that allow population to compete with other species
●Sometimes selection increases reproductive success of individuals but decreases average reproductive success of the group
●Almost all organisms produce more offspring than are necessary to maintain the species
○Exhibits conflict b/n individual and group interests
●The offspring who don’t survive are considered a waste of resources for the rest of the group
○Too many offspring → overexploitation of resources
○Species as a whole is more likely to survive if all females produced fewer offspring, but natural selection among individuals favors females who produce many offspring
●Fecundity: ability to produce offspring
●EX: most of a group of monkeys produce 2 offspring, but have a few high-fecundity females that produce 10 offspring
○High-fecundity females have high-fecundity daughters → proportion of high-fecundity females increase in the next generation
○As time goes on, proportion of high-fecundity females increases rapidly
●Natural selection results from competition among individuals, not competition among species
Evolution of Complex Adaptations
●Accumulation of small variations by natural selection gives rise to complex adaptations
Why Small Variations Are Important
●Continuous Variation: trait that has infinite amounts of variation between two extremes
○Ex: human height
●Discontinuous Variation: trait that has several distinct types with no intermediates
○Very rare in nature
●Many thought that new species arose as discontinuous variants; Darwin thought it did not play an important role in evolution
○Believed (correctly) that continuous variation is essential for complex adaptations
■Very unlikely that the eye evolved all at once
●Selection is a cumulative process
Why Intermediate Steps Are Favored by Selection
●Argument against small steps: unlikely for all changes necessary for a complex adaptation to each be adaptive
○“An eye is usefully only after all parts of the complexity have been assembled; until then, it is worse than no eye at all”
●Darwin’s response was that it wasn’t worse than no eye at all!
●Many organisms display the likely stages of the process:
○Many invertebrates have a simple light-sensitive spot ■Photoreceptors have evolved many times from normal surface cells
■Favored when info ab light intensity is useful
○Light-sensitive spot forms a depression
■Light does not hit all cells at the same time → provides info about the direction the light is coming from
■Mobile organisms may need better info about light direction than mobile ones
○Depression deepens in a series of steps
■Each deepening depression → better directional selection (all of the steps needed for this are adaptive)
○If depression gets deep enough, can form images on light-sensitive tissue
■Selection could favor neural machinery necessary to interpret the image in certain settings/organisms
○Transparent cover forms over depression
■Protects eye interior from parasites and damage
○Lens evolves thru gradual modification of the transparent cover or internal structures
●“Evolution produces adaptations like a tinkerer, not an engineer”
○New traits/organisms are created thru small changes to existing ones
●Chance does play a powerful role in evolution
○If even a single step of a complex trait were not favored by selection, would be difficult to get the trait
○However, cumulative adaptations are very powerful
●Convergence: evolution of similar adaptations in unrelated groups of animals
○Proves how strong selection can be
●EXAMPLE: marsupial mammals vs placental mammals
○Most of the world’s mammals are placental mammals , which have long pregnancies and nourish young within the uterus
○Australia and South America became separated from Pangaea before placental mammals evolved
■They have marsupials that rear their young in external pouches
○Some of these marsupial mammals looked very similar to placental mammals elsewhere
■South American marsupial saber-toothed cat had many of the same traits and adaptations as the North American placental saber-toothed cat
■Last common ancestor of the two mammal types was a nocturnal, insectivorous shrew-like creature from 120mya
●EXAMPLE 2: aquatic eyes
○Spherical gradient lens is adaptive for aquatic organisms
■Better light-gathering ability and sharper image over full 180 degree visual
field for underwater lighting
■These eyes have evolved independently 8 times in only distantly related aquatic species
