Evolution, 4th Edition

SummARY

■■Evolutionary developmental biology (EdB) seeks

to integrate data from comparative embryology

and developmental genetics with morphologi-

cal evolution and population genetics, and to

determine how changes in genes are expressed

as changes in phenotypes. understanding the

molecular mechanisms by which genes, together

with environment, produce phenotypes helps us

understand how phenotypes evolve. mutational

changes in the genes that produce a develop-

mental pathway may cause advantageous altera-

tions of the phenotype, so both the phenotype

and its underlying genetic network evolve. The

mechanistic proximal causes of phenotypes com-

plement the ultimate causes of allele frequency

change, such as natural selection, in understand-

ing the evolution of form.

■■m any differences among species are due to

heterochronic and allometric changes in the

relative developmental rates of different body

parts or in the rates or durations of different life

history stages. Some characteristics have evolved

by heterotopy, expression at a novel location on

the body. The modularity of morphogenesis in

different body parts and in different develop-

mental stages facilitates such changes.

■■The vast diversity of multicellular eukaryotes is

largely due to diverse uses of a toolkit of genes

and developmental pathways that are conserved

across wide phyletic ranges.

■■d evelopmental pathways include signaling

proteins, transcription factors, cis-regulatory

elements and structural genes. Evolutionary

change in the regulatory connections among

signaling pathways and transcription factors, and

between transcription factors and their targets,

is believed to underlie much of the phenotypic

diversity seen in nature. morphological variation

within and among species may be caused by

changes in either regulatory or protein-coding

sequences, although regulatory changes may

play a larger role.

■■A gene may have many cis-regulatory elements

(enhancers) that bind different transcription

factor proteins and can be expressed in diverse

tissues or at different times in development.

Some cis-regulatory elements have originated

from transposable elements, but most of them

have evolved by mutation in their sequence.

Changes in their interactions with transcription

factor genes can alter the time and place of their

activity. Evolution of the coding sequence of a

transcription factor can change its developmental

function.

■■d uring evolution, genes and developmental

pathways have often been co-opted, or recruit-

ed, for new functions, a process that is probably

responsible for the evolution of many novel

morphological traits. This process results from

evolutionary changes in functional connections

between transcription factors and cis-regulatory

elements.

■■m odularity among body parts is achieved by

patterning mechanisms whose regulation is often

specific to certain structures, segments, and life

history stages. modularity helps different parts of

the body develop divergent morphologies (e.g.,

differences among segments). Pleiotropic effects

of genes that affect functionally interacting char-

acteristics may evolve, resulting in the evolution

of functional modules (phenotypic integration).

■■Genetic and developmental constraints can make

some imaginable evolutionary changes unlikely

to occur.

■■Based on changes in the expression of certain

genes and developmental pathways in response

to environmental signals, a single genotype may

be expressed as an array of different pheno-

types, the genotype’s norm of reaction. Reaction

norms are genetically variable, and so can evolve

by natural selection. Especially if the environment

varies, phenotypic plasticity may evolve. Con-

versely, selection for a constant phenotype can

result in canalization. Genetic assimilation is the

genetic fixation of one of the states of a pheno-

typically plastic character. it is not known how

important genetic assimilation is in evolution; nor

is it known if adaptation may occur first by a non-

genetic phenotypic change that later becomes

genetically fixed by natural selection.

TERmS And ConCEPTS

allometric growth

(allometry)

alternative spliicing

canalization,

canalized

cis-regulatory

element

co-option

constraint

DNA methylation

enhancer

evolutionary

developmental

biology (EDB)

evolvability

gene regulatory

network

genetic assimilation

genetic toolkit

15_EVOL4E_CH15.indd 396 3/22/17 1:30 PM